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    Beam power absorption devices 707and a positron, are released. The energyof the photon appears primarilyaskinetic energy of the electron and positron. The process is called pairprod uction." Both se condary particles are stil l highly energetic and canliberate more photons by bremsstrahlung. These photons , in turn , canmaterialize in pair production. Thus, the population of electrons, photons,and positrons increases rapidly at the expense of the average energy of theparticles. This phenomenoniscalledthed eve lopmentof an electromagneticcascade shower."After traversing a certain amount of matter, the particle populationreachesa m axim um which is referred to as the shower m axim um . This is alsothe location of peak energy deposition in matter , as this quantity is propor-tional to the showe r m ultiplicity.As th e energyof the photons decreases to the range of 1 to 5MeV, theyinteract m ainly by elastic scattering with the o rbital electrons o f theatom;thephoton is scattered and the electrons recoil. After several elastic scatteringcollisions th e photon has los t much energy and the probability of its beingabsorbed in a collision with an atom ic electron is greatly increased. W hen thishappens,an elec tron is ejectedfrom th eatom and the photon disappears. Thisphenomenonisrefer redto as photoelectricabsorption. Thus,as theradia-t ion shower t raverses beyond the shower maximum, the total number ofelectrons, positrons, and photons decreases; the shower is expo nentiallyattenuated.Equations tocom putethetotal num berofelectronsorphotons in ashowerdue to an incident electron or photon, respectively, have been given.1 At theshower maximum ,th enumber ofelectrons,H *,due to an incident electronof energyE0 is

    - 2 (20-1)and the number of photons, H x, due to an incident photon of energyE0 is

    - 2 (20-2)where 0 is the critical energy of the target material, i.e., the average energylossdue toionization (orradiation), per uni tradiation length.The longitudinal distance in radiation lengths to the shower maximumcan be computed from

    (20-3)L \c'o Jand

    r /F..\ 1 (20-4)

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    708 D. R. Walzet a/.whereone radiation length,X0, isdefinedas the distance(of matter traversed)in which an electron's energy is reduced by radiation to l/e of its originalvalue.T he radiation shower developsnot onlyin the direction of the incidentelectron beam but also radially. At the beginning of the shower, where th eaverage energy of the particles is still high, scattering angles of the showerparticles are smal l ,and secondary electrons, positrons, and photons are alsoemitted at small angles.Thus, the shower develops mainly in the forward orlongitudinal direction. As the average energy of the shower particlesdecreases, scattering and emission angles become larger, and a significantradial shower development results. This has a profound inf luenceon lateraldimensions of energy-absorbing equipment.Evaluation of cascade showers is a very difficult mathematical task.Approximat ions and simplifications have to be introduced to make calcula-tions practical . Relatively few simplifications are ma d e in Monte Carlocalculations, using differential and total cross sections for the various pro-cesses described above. The longitudinal shower development has beentreated in detail analytically.1 More recent ly a Monte Carlo method to cal-culate th e three-dimensional shower has been deve loped .2 '3 This methodwas used to s tudy th e effectiveness of an a luminum collimator in a 20-GeVelectron b ea m .4 The results are based on a single incident electron, i.e., apoint source; they are thus not particularly useful for practical applications.Assumption of apointsource results inunrealistically high energy densitiesnear the origin.Figure 20-1 Longitudinal show er deve lop-ment of a20-GeV, 2.18-MWe lectron beaminwater,aluminum, an dcopper.

    DEPTH (RADIATION LENGTHS)

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    Beampowerabsorptiondevices 709Point source results of the three-dimensional shower in semi-infinitemediaofcopper,aluminum, andwater have been transform ed into phys icallyrealizablefinite-sizedbeams assuming a Gaussian radial beam distribution.5Threed ifferent incident beam prof iles hav ing s tandard deviat ions ofab=0.1,0.3, and 1.0 cm w ere treated. All values are for rad ially sym me tric, 20-GeV,

    incident electronbeams witharepetitionrateof 360pulses/sec,a pulse lengthof 2.1 x 10~6 sec, and an average beam pow er of 2.18 MW . These beamdataallow conversion of the energy densities obtained in the Monte Carlo cal-culationsinto power densities. The longitudinal power deposition in Cu, Al,andH2Oisshown in Fig. 20-1. The curvesare forab=0.3 cm, a beam sizeclosely resembling expected operating conditions. Figure 20-2 presents th ecorresponding radial power distribution at the shower maximum.

    Power deposition and temperature riseA close examination of the relationships given in Figs. 20-1 and 20-2 revealss o m e interesting facts. The energy deposition rates at the shower maximumand at the origin, r =0, are veryhigh. Using radiation length values fromTable 20-1, volum e heat sources ofS = 198, 16.4,and 2.2k W /c m3for copper,a l u m i n u m , and water, respectively, can be computed . The resulting localtemperature rise,A T,assum ing uniform heat sou rce distribution in the vo lum e

    Figure20-2 Radialshower deve lopmentof a 20 -G e V ,2.18-MW e lectronbeam atth e shower max imum ( for 0 7 = 0.3 cm )in water, a lum inum ,andcopper .

    o iRADIUS (cm)

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    710 D. R.\Na\zetalelement under consideration, is spectacular. It can readily be calculatedfrom

    AT=pc(PRR) (20-5)where S is the heat source from Fig. 20-2in wattsper cubic centimeter,p and c are the specific gravity and specific heat of the material in grams percubic centimeter an d watt-seconds per gram-degree centigrade, respectively,PRR is the pulse repetition rate in pulsesper second, an d AT is in degreescentigrade per pulse.For theheat source value satr =0, as presented above,Eq. (20-5) gives for copper A rm a x= 158C/pulse, for aluminum 18C/pulse,and for water 1.5C/pulse.Maximum power densities and calculated peak temperature rises perpulse have been presented6 and an abstract is given in Table 20-1for twostandard deviations, ab=0.1cm andob=0.3 cm. For other materials, themaximum power deposition can bedetermined fromn m a x p A V P d E

    AE0 dx (20-6)whereIImax can beevaluated from either Eq. (20-1) or Eq. (20-2),PAV is theaverage incident beam power,E0 is the energyof the incident beam, A is thebeam-spread area at shower maximum, and dE/dx = &0IX 0 is the ene rgy lossof the shower electrons. A num ber of potentially useful materials for energyabsorber applications, including important material constants, have beengiven.6 An expanded, slightly altered version is presented in Table20-2.Calculated valuesarebasedonE0=20GeV,PAV= 2.2 MW, andA =1cm2.Table20-1 Maximum powerdensities andpeaktemperaturerisesat the shower maximum incopper, alum inum, and water

    Radiat ionMaterial lengths0.0- 0.5

    Cu 6.0- 6.59.5-10.00.0- 0.5

    Al 5.0- 5.59.5-10.00.0- 0.5

    H20 4.5- 5.09.5-10.0

    Depth

    cm0.0- 0.7168.59- 9.31

    13.60- 14.320.0- 4.52

    45.2 - 49.785.8 - 90.30.0- 18.65

    167.8 -186.5354.3 -373.0

    a b0.1cmS(kW/cm3)

    57.2920.0328.8

    14.456.614.24.57.11.0

    Ar(C/pulse)45.1

    725.5259.3

    15.761.715.53.04.70.7

    a b= 0.3 cmS(kW/cm3)6.6

    198.186.1

    1.916.44.40.732.20.33

    Ar(C/pulse)5.2

    156.267.9

    2.117.94.80.51.50.22

    Fo rEQ=20GeV.PAv= 2.18MW.

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    Tabfe2O-2 Propert ies and character istics of potentially useful materials for energy absorberapplicationsMaterials

    CharacteristicsZo 'cyft

    *max

    n ;xdE

    ~dxP

    dEcpckT'meltSAra-10 6E-10-6OLE

    (MeV)(g/cm2)(radiation-lengths)

    (MeV/(g/cm2)(g/cm3)(MeV/cm)(W-sec/g-C)(W-sec/cm3-C)/ W \\cm2-C/cm/(C)(kW/cm3)(C/pulse)d/ C)(psi)(psi/C)

    Be4

    110.066.0

    4.2425.6

    1.671.853.081.783.31.68

    1278.08.77.3

    12.344.0

    540.0

    CPyrolytic6

    79.043.34.57

    34.51.822.03.640.971.940.0252.0

    3600.013.819.7+0.06-0.8

    4.4+0.26-3.5

    H207.23

    72.835.74.66

    37.02.031.02.034.224.220.006

    8.35.5

    Al1340.024.35.28

    64.01.642.704.450.942.542.39

    659.031.134.125.010.0

    250.0

    772224.015.15.80

    103.01.594.507.170.542.420.17

    1800.081.5

    102.08.7

    15.0130.0

    Fe2620.613.9

    5.93118.0

    1.487.87

    11.60.483.790.63

    1530.0152.0112.0

    12.129.0

    350.0

    Cu2918.813.0

    6.06131.0

    1.448.95

    12.80.3853.453.9

    1 083.0186.0150.0

    16.617.0

    282.0

    Ta73

    8.26.96.85

    277.01.19

    16.619.8

    0.1292.140.63

    3000.0601.0772.0

    6.727.0

    181.0

    W74

    8.16.86.86

    281.01.18

    19.322.70.1342.581.46

    3380.0696.0748.0

    4.351.0

    220.0

    Pb82

    7.46.56.98

    305.01.14

    11.3512.90.131.480.083

    327.0431.0807.0

    29.42.3

    68.0Eo =20GeV,PAY=2.2MW, andA = 1cm2.* Most values from 0. I.Dovzhenkoand A. A.P omanskii,J. Exptl. Theoret.P hys. (U.S.S.R.)45,268-278 (1963).cA llvalues take density effec tintoaccount.

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    712 D. R.Walz et a l.I t should be noted that using A const , does not ad equa te ly accou nt for therad ia l shower deve lopm enta s a func t ionof Z; the re fo re , thet em pera tu re r iseand power deposi t ion as computed f rom Eqs. (20-5) and (20-6) are high forlow-Z mater ia ls , approximate ly cor rec t for me d i u m- Z mater ia ls , and lowfor high-Zm ater ia ls .

    The va lues shown in Tables 20-1 and 20-2 indicate clearlythatm e d i u m -and high-Z mate r i a l s are not usefu l fo r appl i ca t ions r equ i r ing co n t inuo usexposure to the beam. Energyisdepo si ted a t a m u c h h ig h e rrate than can behand led by t h e r m a ldiffusion fo r prac t ica l geom etr ies . The loca l t em pera tu reincreases dram atica l ly wi th each successive pulse , and fa i lure due to grossgeometr ica l c h a n g e s or m e l t ing r e su l t s fo r m o s t o f these mater ia ls wi th in afraction of 1 sec. No te that the pro du ct ofp c, i.e., the specific heat capacity, isessential ly constant for al lm e ta ls .Fur the r examina t ion o f Table 20-2 indicates that low-Z materials offersome promise for successful application in the con stru ction of energ yabsorbers fo r high power densi ty beams. In low-Z mater ia ls th e power isdissipated in a much larger volume, i .e . , th e heat source is less intense.Heat transfer problemsIn theprev ious sec t ionit wasshownhow to ca lcula tera teso f heat depositionand local tem peratu re r isesper pu l se .Thenext stepin thecour seof anene rgyabsorber analysis i s the de term inat ion of the te m peratu re d ist r ibut ion resul t -in g f rom th e beam power deposi t ion and boundary cond i t ions imposed o nthe sys tem. Heat t rans fer be tween two ad jacent par t ic les of m atter i s thet ransfer o f the rm a l (hea t ) ene rgy by v i r tue o f a t em pera tu re difference f romthe hot te r to the co ld er .Inany so l id energy absorber , heat i s t ransfer re dsole lyby thermal conduct ion. In an i so tropic body the law of heat conduct ion canbe stated as

    dTq =-k (20-7)onwhere q is the hea t flux in a d i r ec t ion / ? , and k is the t h e r m a lco nduc t iv i ty .UsingEq. (20-7), the m ost ge nera l equ at ion des cr ibing the tem pe ratu re d ist r i-bu t ion in any sol id can be de r ived . If the t h e r m a l c o n d u c t i v i tyis a s s u m e dto becons tan t the heat equat ion (Four ier ' s law of conduc t ion) can be statedas

    ^T 1kV2T + S =pc (20-8)01where V 2 is the Laplac ian ope rato r , S is the rate o f heat generat ion per un i tv o l u m e ,and T is the t i m e ;S may be a funct ion ofspace and time. For steady-state condi t ions wi th no internal heat generat ion, Eq; (20-8) reduces to thefamil iar Laplace equat ion. The heat equat ion has been so lved for a widevariety ofapplications.7

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    Beampower absorption devices 713Even w h e nlow-Z materials are use d, so that h eat sou rce intensi t ies are

    grea t ly r e d u c e d , th e r e s u l t in g h e a t f luxes f ro m a su r f ace are very h igh fo rprac t ica l geomet r ies . Cons ider th e case of an a l u m i n u m s lab o f t h i c k n e s s< = 0.5 cm p l a c e d n o r m a l to the beam d i rec t ion at the s h ow e r m a x i m u m .A s s u m e that one s ide of the s lab faces a vacuum and the other s ide is watercoo led . To a f i rs t a p p r o x i m a t i o n the hea t sources a re u n i f o r m l y d i s t r ibu tedt h ro ug h t he cyl indrica l v o l u m edefined by the t h i c k n e s s of the s lab and theeffective d i a m e t e r of the inc ide nt par t ic l e beam. T h is vo lu m e e l em ent for a1-cm d iamete r beam is 0.39 c m 3 , and the v o l u m e h e a t so u r c e is S z z 12k W / c m 3 ( f rom Fig. 20-2 assuminga beam of 20-GeV energy and 2 .18-MWaverage power) . The power diss ipated in th is space is then /*AV = 4 . 7 kW.Neglect ing r a d i a l c o n d u c t i o nfor the t im e being, it is f o u n d t h a t th er e s u l t a n ts teady-s tate local heat f lux in the beam direct ion f rom th e s lab and into th ewat e r is q 6.0 k W / c m 2 . T h is is a very high if not excessive heat flux.A so lu t ion to Eq. (20-8) wil l read i ly y ie ld the temperature r ise across theslab as

    which w ould be appro xim ate ly 630C. C ons idera tion o f rad ia l cond uc t ion7will reduce th is to about 500C, a temperature still excessive for practicalapplication. Examination of Eq. (20-9) showsthat d is the o nly variable sinceq = const . f ) d ) . For the case of uni fo rmheat sources , d enters l inea r ly intoq ; the re fo re an increase o f 6 by a factor o f 2 increases th e hea t f lux by afac tor of 2 and the t e m p e r a t u r e difference by a factor of 4.

    The high energy deposi t ion rates and resul t ing large instantaneous tem-pera t u re rises as g iveni n Table 20-2 dictated th e select ion of low-Z materialsfor beam abso rpt ion devices . Sim ilar ly , h ighheat f luxes force the adopt ionof s m a l l wal l th ick nesse s and resu l t in a special mode of heat t ransfer . Forheat t ransfer surfacesa t r o o m t e m p e r a t u r e o r sl ightly above, heatf luxes up toappro x im at e l y 0.25 W / c m 2 can be h a n d l e d by natural convect ion in air andt h e r m a l radiat ion. The S tefan-B ol tzm ann law for heat t ransfe r byradiationbetween tw o surfaces separated by a v a c u u m is given as

    q =a(C2T*-C{Tt) (20-10)w he r e a = 5.77 x 1 0 ~1 2 W / ( c m 2K4) is the S te fan-Bol t zmann cons tant, andQ andC2are cons tant s dependingon the orientat ion, dis tance, absorpt ion,and ref lec t ion propert ies of the two surfaces .For heat f luxes 0.25

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    714 D. R.\Na\zetal.At water velocit ieso fabout 1.5m/sec andheat f luxes o fm o r e thanabout65W/cm2, another heat t ransfer mechanism becomes important . It is com-

    mo nly re fe r red to asboi l ingheat t ransfer . Extensive li teratur e isavailable,9'10and only basic mechanisms will be reviewed here.Air or wate r vapor bubblestrapped increvices of the metal surface beginto grow in s ize as the metal surface temperature and, therefore, the tempera-t u r eof the the rm al bou ndar y layerisincreased to apoint where bubble growthcan besu s ta ined . Depending on the veloci tyand t e m p e r a t u r e of the coolant ,the bubbles wil l grow to a size such that they are swept away by viscousdrag f rom the fluid, or they detach themselves due to their buoyancy. Someofthe water vapor staysin the cavityand is the nucleus for the next bubble.Once in the main s t ream the bubbles wi l l col lapse more o r less rapidly,depending on the subcooling, i .e. , the t empera ture difference between theboi l ing point and the b ulk f luid tem pera ture . In highly subcooled l iquids thebubb l e s c an col lapse without ever leaving the surface. This has impor tantimpl icat ions as will be shown later.The vapor bubbles carry large amounts of energy away f rom the surface,th rough the the rmal boundary l ayerand out into the main bulkfluid.Ratherhighheatf luxes can behandled in this fashion. Moreover, th e moving bubblesstirup the thermal boundary layera nd f resh bulkf lu idreaches the ho t surface.This mechanism isreferredto as nucleateboiling and occursin all energyabsorbers discussed below.

    As the heat flux is st il l further increased, the number of bubbles and then u m b e r o f nucleation sites increase until adjacent bubbles start to interferewith each other's g rowth and mot ion. This is the condition o f m a x i m u mheatf lux,also referred to ascrit icalo r burnout hea tflux. Any fu r the r increaseof the sur face t em pera ture wil l result in the formation of a stable vapor filmwhich , in turn, prevents the bulk l iquid f rom reaching the surface and, thus ,acts as an insulator. This condition iscal led " f i l m boiling." It usual ly resul t sin a rapid rise of the metal surface temperature to the point o f dest ruct ion.

    U nf o r t una t e l y , the large numbe r of variables , such as surface geo m etryand condit ion, f lu id veloci tyand t empera ture , f lu id properties, and radical lyvarying heat t ransfer mechanisms for different regimes, make a single heatt r a ns fe r corre lat ion and predict ion o f b u r n o u t a very difficult task. N oun i v e r sa l lyacc ep table co r re l a t ionye tex ist s. Fur thermore , mo s t exper imenta ldata repor t ed in the l i t e ra ture are for the case o f u n i f o r m l y heated largesur face areas. They do no t adequate ly rep resent the local heat transfer con-di t ions e x p e c t e d f r o m i m p i n g e m e n t o f high-intensi ty part ic le beams intosol ids. Typical valueso f b u r n o u t h e a tf luxes for moderate water veloci t iesa rein the ne ighborhood of 1 k W / c m2 .

    High local heat f luxe s from ele ctron-bom barde d water-cooled targetsha ve been repo r t ed .1 ' T h e v a l u e sarem u c h h i g h e r t h a n th e u s u a lb u r n o u t h e a tf l uxes .Boi l ing hea t t r ans f e r exp er im ent s were a lso per fo rm ed a t S L A C1 2 toestabl i sh permiss ib le heat f lux value s f rom a local hot spot fo r d i f ferentmaterials . Flat plates of varying thickness were bombarded with 15-keV

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    Beampowerabsorption devices 717The first signs of onset of thermal fatigue are an elongation of certaincrystals and a slip at the grain boundaries. Under continued cyclical load, th eslipeventu ally develops into individual po res, andcracksappear. Ifadditionalexternal constraints exist, f racture of the structure may result. The magnitude

    of the stress necessary to cause failure in the structure diminishes withincreasing number of cycles. The fatigue or endurance limit of the material isdetermined by the nu m ber of cycles requ ired to g enerate cracks at the givenelevated temperature, as described above.Electron accelerators can typically produce 1010 pulses /yr , and long-termfatigue values have to be selected accordingly.

    Superposition of cycling and steady-state stresses and prediction of theexpected lifetime can be accom plished using, for exam ple, a Goo dm an dia-gram, described in more detail inanother section below.

    Thermal shockA number of accelerators exist that can pro du ce bu rs ts of very high-powerdensity over short periods of time. The high rates of energy dissipation d ur ingthe pulse give rise to very steep tem pe ratu re gradients in space and time, resu lt-in g in proportional thermal stress gradients. The latter can cause formation oft h e r m a l shock waves which,in t u r n , m ay r esu l t in f rac ture and spal lat iono fmaterials, regardless of how well th e part is cooled.An exper imentto de te rminetheimpor tance ofthiseffect on ana l u m i n u mco l l imator module is described in more detail below. The rate of energydeposi t ion during th e pulse in the m o d u l e w a l l w as 600,000 k W / c m3 . N odamage was observed. This may be explainedby the re lat ively high du cti l i tyof the a l u m i n u m ; another , more br i t t le mater ial m ay have fai led.

    20-2 High-power beam dumps (DR W)This section discussesthe important design criteria and features of the high-p o w e r A-beam dump D-ll (see Fig. 17-1) and beam d um p eas t D-400.Emphas is is g iven to the three impo rtant d um p component s :the w indow, thevortexflowregion,and thep late c ompar tment .Theauxi l iarybutimpor tantproblems of radiolysis in water , th e evo lu t ion of free hydrogen and its dis-posal, are also treated in detail.CriteriaThe high-power beam dumps are to be capable of continuously absorbingandd issipatingthe fu l l beam power p roduced by the SLAC linac overawiderange of energies. This includes conditions arising from radiators placed inth e beam transport sys tem up stream of a du m p. Table 20-3 gives a su m m aryof the most important criteria.

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    Beampowerabsorptiondevices 719

    W A T E R **RETURN

    Figure 20-3 Artist'sconceptionof a2.2-MW( 0= 11-25GeV) beamdump.

    occur. Formation of a gas space would reduce th e effective Z drastically; itwould shiftthe shower maximum downstream and could resultindestructionof th e pla te compartment in the rear of the dump. Hydrogen formed in theradiolysis of water could come out of solution and might present additionalcomplications. Thus, th e exposure of a volume e lement of water to the hotcore of the beam has to be l imited to a fewpulses at mos t .Severa l methods could be applied to exchange th e water continuously inth e cyl inder .In order to economizeon the flowrate and to reduce th e beamexposuret imeof av o l u m e e l e me nto fwater, it wasdecided to use a vortexflow. An inlet flow header located at the periphery of the shell induces th evortex f low. Water is injec ted throu gh a series of holes, equally spaced overthe 10 radiation leng ths. The wate r then flows spirally towa rd the center ofth e vessel where th e exit manifold is located. The velocity normal to thenominalbeam center l ine(at R = 30 cm) was set at approximately 100cm/sec.For this velocity nov o lum e e l ementof water equal in size to the hot coreo fthe beam isexposed to more than about 4pulses.Knowledge of the quantitative radial velocity distributionis essential toguarantee safe operation. Seven different flow regions have been identifiedand are listed as fo l lows:1. The boundary layer at the cylinder wall2. The rotational flow as a result of the submerged nozzle3. The potential vortexflow region (perturbe d by the effectof the geometricdisturbance due to the inlet nozzle)4. The interface between regions 2 and 35. Re gions 4, 2, and 1 for the ou tlet nozzle in the center.

    Losses in the system are mainly frictional losses in the various boundarylayerregionsdu e toshearing forcesandedd y m ixing losses fromjetsand wakes.

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    720 D. R. W alz efa /.Viscous forces are small compared to kinematic forces in thepotentialvortexflow region and the flow is to first approximation frictionless. The mathe-matical m ode l describing all regions sim ultane ou sly is com plicated and itssolutionverydifficult if notimpo ssibletoo btain.O n theother hand, individualregions can readily be analyzed and solutions are possible.

    In order to answer the m ost pertinent question concerning the performanc eof an actual beam du m p, name ly, what is the inlet jet velocity required toguarantee the magnitude of the velocity across the beam set forth in thecriteria, two scale models were built and velocity distributions and pressurelosseswere measured .15 The tests resulted in the following conclusions:1. The ve locity is highest at the center, then decreases to a flat m inim um andincreases again as theperiphery is approached.2. Theratioof the velocityat thebeam to the inletjet velocityis, to firstapproximation, independent of the Reynolds number over a wide rangeof NRe (onlyth e velocitywasvaried, howev er).3. The velocity ratio varies linearly with the ratio of nozzle diameter tonozzlespacing.4. A central outlet nozzle has a stabilizing influence on the flow and itssymmetry.

    Conclusion 1indicates thatthe flowpattern is not just a simple potentialvortex,givenby V r= constant, but contains a term fo r solid angular rota-tion. This was to be expected due to the presence of other flow regions asdescribed above. On the other hand, angular momentum is conserved to alarge degree.

    Conclusion 4 deserves some fur ther comments : for the potential vortexund er c onsideration the sink is located at the c enter of the vessel. In principle,no outlet nozzle has to be present along the center. Omission of the outletpipe and creation of an ou tlet nozzle at the end of the vessel wo uld change thetwo-dimensional vortex into a three-dimensional one. This is analyticallypredictable and was experimentally verified. In the case of a stable flowpatter n the re sults indicated littledifference in the veloc ity at the beam lo cationbetween presence and absence of the outlet nozzle. Presence of the pipe,however, stabilized the flow appreciably and reduced asymmetry due toth e perturbation causedby the geometric disturbance of theinlet nozzle.Theaxis of the inlet manifold coincides with a horizontal plane throughth e rotational sym m etry axis of the vessel. This location gave best resultsatthe beam axis. The diameter of the dump cylinder was determined byradial shower development considerations.The 20-radiation-length plate compartment will now be described. Sincespaceis at apremiumand largepiecesof equipment areexpensiveto fabricateand difficult to handle, it is not economical to attenuate fully the cascadeshower in water alone. Solid copper plates are introduced downstream of theshower maximum, at a location where the cascade shower is sufficientlyattenuated to cause only moderate heatfluxes and temperature rises.

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    Beam powerabsorptiondevices 721Atotal of nineteen plates are contained in the compartment. They aregraduated in thickness according to shower attenuation to maintain th edesign heatflux and temperature criteria and, thus, optimize th e total lengthof the vessel. The first plate is 0.32 cm thick, the last one 3.8 cm. Theplates are equ ally spaced and water-cooled at about 200 cm/sec water

    velocity. The maximum heat flux anywhere in the system is 2 kW/cm2 (forPAV =2.2 MW).Thedum p vesselismountedon am obile f rametoallow remote placementorrem ova l, since high levelsofinduc ed radioactivity willin du etime resultininaccessibility of the area.Window and window removal systemIt has already been mentioned that the beam enters the dump through a thinwindow. The use of a window is necessary because the stainless steel vesselshell, whichisapproximately 1 cm thick, is not capable of dissipating all thepower deposited in it. The size of the window is determined (a) by the maxi-m um possible beam deviations f rom th e nominal center line,(b) by the aper-ture of a protection collim ator upstream of the d um p, and (c) by the m om entumspread of the residual electron beam due to a0.01-radiation length target.Theeffective window area normal to the beamisdefined by a 15-cmdiametercircle.High-purity copper (with thin layerso fnickeland hard chromium platingas discussed below) was selected as windowmaterial. It iscompatible withthe rest of the system, which contains only copper and stainless steel. Basedon peak power deposition, aluminum or ti tanium would have been superiorto copper (see Tables 20-1 and 20-2). However, in an aqueous system alu-minum wouldnot be compatible withthecopper usedin theplatecompart-ment, and ti tanium presents more fabrication problems than copper. Thedisadvantage of copper due to its high Z is offset, in part, by its excellentthermalcondu ctivity.The wind ow thickness was chose n to be 0.127 cm (0.050in.).The windowseparateswatera televated pressurefrom th e beam transport vacu um sys tem.To minimize the stress level, a hemispherical shape was adopted. The powerdeposited in the window can be readily calculated from a modified formofEq. (20-6):

    dEP=1.6 x lQ-19N8p (20-15a)\A>J\t

    ordEP= / A v < 5 p (20-15b)

    where T V is the num be r of electronsp er second, < is the window thickness, pthe specific gravity, and 7AV is the average beam current.

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    722 D. R. \Na\zetaf.For "0= 11GeVand PAV= 2.2 MW, the total power deposited in thewindow isPw =0.325kW. The minimum expected beam size at the A-beamd u m p(D-l1 ) is approximately0 .1cm2.The resulting heatfluxintoth ewateris about 2 kW/cm2 (this includes consideration of lateral conduction in thewindow). Assuming steady state and the proper boundary conditions, solu-

    tion o f Eq. (20-8)yieldsa maximum tempera ture difference across the windowof about 32C.T he thermal stressesar emoderate (forawater-cooled window,takingintoaccountth etemperature difference acrossth e interface) bu twouldbe evere if such a sm all beam cross section existed co ntinuou sly at full poweroperation. Aseparate window-coo ling mechanismwas developed. Individualjetsofwater impingein the area of high heat flux andpreventdevelopment ofburnout conditions.The window is expected to be the weakest i tem in the beam dumps. Theproduction costs of a d u m p are high and lead times are long. For thesereasons, it was decided to m a k e th e window exchangeable. Two seals arerequired:one between water and air , and the other one between vacuum andair. In the high radiation environment, only all-metal joints are useful fo rextended service. After extensive evalu ation of variou s all-metal joints, theknife-edge-type o f vacuu m joint was chosen for both the vacu um and thewater side of the window. It employs copper for the window, which alsoserves as the gasket, and stainless steel for the knife-edge flange. These arethe same materials as are used in the restof thed u m p . Theknife-edge gaskethas the advantage that it will seal vacuum tight, even with shallow scratcheson the knife-edgeor the gasket. Thus, neither th e knife-edge nor thegasketiscritical for a successful seal.

    The expected high levels of induced radioactivity will result in inaccessi-bili ty of the d um p, and, consequ ently, the w indow mu st be rem otely exchange-able. For this purpose a hydraulic-pneum atic remo te window -remo valmechanism was developed and successfully tested. In order to reduce th ecomplexity of this mechanism, only two bolts are used to tighten the flangesand m a k e th e seal. The flanges are, therefore, very stiff to achieve successfulsealing. The bolts are turned by means of a hydraulically manipulated,pneumatic impact wrench, and windows are exchanged by employing ahydraulical ly operated mechanism. Detailed instructions for removal of thewindowunit have beenprepared.16A full-size prototype of a window was tested in the National Bureau ofStandard's linac.17Tem peratures were m easured with thermocou ples attachedto the air sideo f thewindow.Thehighest heat transferratefrom th ewindowto the water wasapproximately 1.25kW/cm2 in the area of beam impinge-

    ment. The highest tem perature recorded was 315C. This thermocoupledid not, however, coincidewith the beamcenter line,andtemperaturesmayhave beenashigh as 350C. Furthermore, th edataare not corrected for beamexposure of the thermocouples. This would lower th e values. The windowshowed no spallation effects due to thermal shock, and the hard-chromiumplating on the water side appeared to be undamaged .

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    Beam powerabsorptiondevices 723

    Material selectionand fa bricationThe du m p vessels, includ ing all thepiping, are fabricated from stainless steel,Type 316-L. This material was selected because of its superior corrosionresistance. It is a fully austenitic, low-carbon steel which is Mo-stabilized.Carbide precipitation in the m ulti tu de of welds is negligible and corrosionresistance in these areas is very good. Furtherm ore, Type 316-L appears to beless susce ptible tha n type 304 to stress corr osion cracking, frequently th eswift destroyer of stainless steel structures. Allwelds were mad e by the tung ste n inert gas arc me thod (TIG) to highestwelding quali ty standards. Only low-carbon, Type 316-L welding rod wasused. Full penetration and fusion were required, since the lack of either maycause crevice corrosion, subsequent stress-corrosion cracking, and pitcorrosion. Allwelds we re inspected u singx rays and dyepe netrants .The plates located in the rear of the vessel are oxygen-free high-conduc-tivitycopper, g radu ated in thickness as previously described. W ithou t surfacetreatment th e cavitation erosion problem would be a serious limitation onthe expected lifetime of a beam du m p. E xperim ents have indicatedthat hardmaterials are much less subject to cavitation erosion than soft ones. Goodresults were achieved by electroplating the copper surface with a sandwich ofnickel and hard chromium. The soft nickel layer is approximately 0.0025 cmthick, th e hard chromium layer about 0.001 cm . Thereare two reasons forthe nickel substrata. First, it seals th e copper surface an d protects it fromvarious corrosion mechanisms; this is important, since hard chromium issusceptible to cracking. Second, th e soft nickel layer acts as a stress-absorption buffer. Significant differences in the coefficient of linear thermalexpansion betweencopper and chromium would cause large thermal stressconcentrations at the interface duringoperation.T hechrom ium would crackand m ight evenflakeoff .Thenickel layer,inturn,issoft, canyield easily,andhas a thermal expansion coefficient which is intermediate to the coefficientsof copper and ch romium. It is of importance to mention that th e hardchromium has to be plated above 55C, otherwise continuous microcracksare formed and the plating is porous .The be a m d um p prior to installation is shown in Fig. 20-4. The right-hand side of this front v iew show s the water inlet m anifold at the top , thenthe water out le t manifold,and a drainage line at the bottom. Also shownaspart of the water outlet manifold is a Venturi for removal of air during thefillingproc edu re and for c ontinuo us venting of gases formed in the radiolyticdecomposition of water.The water system and corrosion problemsThe radioactive cooling-water loops are described in detail elsewhereinthistext, and a brief treatment will suffice here. The water quali ty of the primaryradioactive water loop is monitored daily. A resin ion-exchanger located in

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    724 D. R.\Na\zetal

    Figure 20-4 Frontviewof2.2-MWbeam dump(D-400).

    abypassloop servesto keepth ewaterat 1m e g o h m -c m ,orbetter.The pH iskept slightly acidic, approximately 6.2-6.5, from carbon dioxide dissolvedin the aerated water. Particular attention is givento chlorides in the water,since theyare a major factor in initiating stress-corrosion cracking in stain-less steels. Chloride concentrations a re < 1ppm . B asedon extensive expe ri-mental results from other laboratories, it is felt that chloride concentrationsshould not exceed0 .1ppm for safe,long-term operation. Significant a m o unt sof hydrogen peroxide, H2O2, are fo rmed in the radiolysis of water. It is anoxidizing agent and, therefo re, caused conce rn abou t th e corrosion resistanceof th e materials used in the radioactive loops. A s tudy ofH2O2chem is tryindicated that no problems should exist in the water loops. Stainless steelsaswella saluminum alloys derive their superior corrosion resistance from thepresence of a dense,more-or-lessinsoluble, oxide film on the surfa ces of thesematerials . Aluminum containers are often used to store H2O2 and keep itfrom decomposing. It is felt that th e presence of H2O2 in the radioactivewater loops either enhances formation of even denser oxide fi lms or has noinfluence at all on the corrosion behavior of these system s.

    A major concern during maintenance work has been the radiation fromcuries of 7B e(54dayshalf-life), adaughter nuclideof16O.It isfo rm edin no tnegligible qu antitiesin thebeam dum ps under high-power operation. Measure-ments revealedthat essentiallya ll 7B e istrapped in the ion exchangers, thuslocalizing th eradiation problem and making it easy to shield against.The tritium(3H) bui ldup in the radioactive water systemsis also moni-tored. Thetotal vo lumeo fwater contained in the A-beam dump radioactivewater loop is approximately 12,000 liters. It wasestimated thatabout 5 M W -hr were dissipated in this volum e dur ing the first 9m onths of operation. Atthe end ofthisperiodthe3Hlevelwas 3 x 10~4^Ci/cm3orabout4 mCi for

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    Beam powerabsorption devices 725the total system. The maximum permissible concentra t ion (MFC) for dis-posal of water into sewers is 10~3 ^Ci/cm3. Thus, drainage of the water atregular intervals is an inexpensive solution to minimize health hazards.Operational experienceTo date (July 1967),after 12 m onths of co ntinuou s operation, no failures haveoccurred . The highest average power deposited and dissipated in the A-beamd u m p so far is 240 kW at 17.5 GeV (on March 28, 1967).Radiolysis andra dioactivity in the waterLarge quantities of water are used in the beam dumps as the coolant andprimary energy absorbant. In the high-power beam dumps, approximate ly90 % of all the ene rgy enter ing the du m ps is dissipated directly in the w ater,with th e rem aining 10% dissipated in the copper plates, and avery smallpartlost from th e system due to radiation. For electrons with energies EQ >0 .5MeV, th e linear energy transfer, i.e., th e amount of energy lost per unitdistance traveled is (dE/dx)p x 2 MeV/cm. This energy is lost in discreteam oun ts averaging about 100 eV per eve nt. The e nergy usually convertswithin lessthan 10~1 2 secintoheat. Water moleculesareexcitedin the regionwhere this energy transfer occur s ; such a region iscalled a spur. The energydeposition reaches a m a x im u m at the end of the track of a charged particleand results in format ion o f a high-temperature region, cal led a thermalspike. For this case, spurs will be an average of 5 x 10~5 cmapart.18

    During th e pulse, high concentrations of free radicals such as H and OHare formed in the spur. Some of these radicals willreact with one another ;others diffuse out of the spur into th e bulk water. The reactions within th espur will yield H2 , H2O, H2O2, and others. They can be symbolical lyrepresented asH2O->H,H2 ,OH,HO2,H2O2,H2O,e.' (20-16)

    where ea~ is the hydrated electron.The radicals diffusing into th e bu lk can increase in concentration to a pointwhere they no t only react with th eH2 ,H2O, and H2O2, bu t also with o neanother to f o r m mo r e H2 and H2O2. Since th e solubility o f hydrogen inwater is only about 0.8 x 10~3moles/l iter, after a certain t ime there will be anet evolution of free hydrogen. Also, once equil ibrium concentration isreached, hydrogen peroxide is expected to decompose due to radiation andother c hem ical reactions,and a netev olut iono fO2can be expected. Theoreti-cal quantitative prediction of evolution rates is a complicated task. It isthoughtthata t least fou rtee n rate e quations hav eto beso lved simu l taneously.Measurements have been m a d e1 9 to de termine the amount of free hydro-gen produced using an e lec t ron linac giving pulses of approximately 7 x 104g-rads and energiesEQ x 15Me V .Thedoseintotheeffective irradiated volume

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    Beam power absorption devices 727only 10 m in to reach th e safe limit at PA V = 150 kW . Unfo rtunate ly, even atlo w average pow er levels of, for exam ple, 50 kW the am ount of radioac tivityreleased into the atmosphere is prohibitive for any long-term venting opera-tion.The SLAC Health Physics G r o u p has analyzed th e gases on top of thesurge tank during several experiments and identified 15O (2 min) andnC(20.5 min) as major c ontribu ting isotopes; both are dau ghte r nuclide s of16O. A drying column failed to remove th e 15O, suggesting that it is ingaseous form (O2) ra ther than in the H2O molecu le s of the water vapor.Chemical removal of CO and CO2 indicated tha tvir tual ly allnC is inCO2.Theradiation levelat a distance of 100 cmfrom the top of thesurge tankwas^60R / h o u r for steady-state dissipationof 170 kW in the d u m p .Two solutions have been proposed for removal of evolved hydrogen: achemicalCO2 removal-storage-venting system and a catalytic recombinationsystem. In the first system gases coming off the surge tanks are diluted toachieve concentrations o f 1m Ato satisfy them i ni m u m requ i rement sfo r proper functioningof the acceleratorphasing system and the beam position mo nitors. Second, it should also accepta tune-up beam at f u l l energy and f u l l cu rrent (50 mA for Stage I). For adump of reasonable size and cost , this requires a reduction in the beam

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    728 D. R.\Na\zetaf.repetition rate to approximately 10pulses/sec. W ith th e inclusiono f a safetyfactor, th e maximum average power absorption capacity was set at 60 kW.Altho ug h the pow er absorption specifications for the central beam aresomewhat lowerthan for the tune-up beam, i t was decided to duplicate thetune-u p beam d um p for this application and th us red uce engineering costs .

    The B-beam dump islocatedin the Btarget room and servesto absorband dissipate th eelectron beam and as the^-mesontarget. Its power absorp-tion capacity is 120 kW.Table20-4 shows a summary of the important cri teria of the low-powerbeam dumps.Designfeatures, ma terials,and fabricationIn the tune-up and central beam du m ps, beam pow er isdissipated in aseriesof water-cooled, high-conductivity copper plates. They are graduated inthickness according to shower developm ent and a ttenua tion, thu s optim izingTable20-4 Low-powerbeamdump criteria

    CharacteristicsMaximum average beampowerIncidentbeam energyMinimum beamsizeEntrancewindowsizeThicknessTotal lengthDump d iameterWidthHeightDump shel l material

    Tune-up a ndcentral beamdumps60 kW

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    Beam powerabsorption devices 729

    Figure 20-5 Plate com partment of 60-kW tune-up beamdump (D-10) .

    th e dump length. Figure 20-5 shows a partial assembly of a tune-up dump.Al l plates are covered with electrodeposited layers of nickel and hard chro-mium for cavitation-erosionprotection asde scribed in detail in the previoussection. The plates are contained in a Type 316L stainless steel shell.The lateral dump dimensions were selected to allowa reasonable marginfo r possible beam excursions, taking into account radial power escape andheat transfer and thermal stress in the stainless steel sidewalls.The window is a portion of a cylindrical surface to minimize th e wallthickness and, consequently, th e power absorption. For fabrication reasons,stainless steel has been selected in preference to copper as the w indow m aterial .Thewindowha sbeena rcwelded (TIG)to thestainless steel sidewalls. Copperwould have required a more expensive brazing operation. The sm al le r wal lthickness combined with th e capacity fo r operating at much higher thermalstresses partially offsets the disadvantage of stainless steel because of its lowthe rmal conductivity compared to that of copper. Figure 20-6 showsthe com-pleted tune -up dum p assembly m ounted unde r it s support f lange and readyfor instal lation in the divergent vacuum chamber.The B-beam d u m p is circular in cross section. It has a flat plate copperwindow, furnace-brazed to a stainless steelflange. The latter iswelded to thestainless steel plate compartment. Other featuresarevery sim ilar to the tune-up dump design and are, therefore, no t described further.

    Operations to dateExperience during the first year of operation of the beam dumps has beenflawless. Them aximum average powers deposited in the tune-up beam dumpand th e centra l beam dump were about 40 and 45 kW, respectively. TheB-beam dump has dissipated up to 75 kW .

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    730 D. R.Walzetat

    Figure 20-6(D-10).

    Assembly of 60-kW tune-up beam dump

    20-4 Collimatorsandslits (DRW)General considerations and definitionsA list of important components in any beam transport system almost alwaysincludes collimatorsandslits.Thefunctionsofthese devicesmay bedescribedas fo l lows: collimators are used to define the spatial extent of the particlebeam, to form aperture stops for beam transport systems, and to protectmagnets and other equipment f rom physical damage by the beam; slits aresimilarlyused to provide stopsthatdefinemomentum transmission in a beamtransport system containing dispersive elements.In the followingsections three basicallydifferent typesofcollimators andslitswill betreated indetail:(1) thevariable-aperture, high-power collimatorand slit; (2) the variable-aperturehigh-Zcollimators and slits; and (3) thefixed-aperture protection collimators. Either variable or fixed-aperturecol l imatorsare employed depending on their function in the beam transportsystem.For example,at thebeginningof thebeam switchyardand thebeamtransport system,a set ofvariable-aperture collimators definethebeam crosssectionat thecenterof thepulsed magnetsPM-1through PM-5. (For locationsseeFig. 17-1.)

    Under normal operation the powerabsorptionin the collimators shouldbe relatively low compared to the total beam power. However, mis-steeringof the beam, beam breakup at the end of the accelerator, or misalignment ofthe collimators can cause abnormally high-power densities to be absorbedby these devices, which must, therefore, be designed to operate in this con-ditionfor an extended period of time without suffering physical damage.

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    Beam power absorption devices 731Most components in the beam transport system must be protected fromexcessivepow er deposition or excessive exposu re to primaryradiation. Theirlocation and aperture are usual lyfixed and, consequently, they m ay be pro-tected by fixed-aperture collim ators installed in f ront of them.The slits are o f variable aperture to satisfy the varying dem ands on maxi-

    m um al lowable momentum spread of the beam for different high-energyphysics experiments.A relatively high-power absorption m ay occur when anarrow momentum spectrum of the e lectrons i s desired and significantmo me n t u m components must be removed, or when the output of theaccelerator is unstable.Sinceone of the prim e o bjectives of slits and collim ators is the rem oval ofparticleso f unwanted mome nta f rombeam transport systems, it is of utmostimportance to select a proper geometryand/or suitable materials. A poorchoice of the latter m ay resul t in excessive multiplicity along th e beam-definingedges and in the introd uc tion of a new, significant m om entumspreadinto the beam. This could reduce or even nullify th e benefits of momentumselection in the beam transport system.It would seemthat opt imum resul ts can be achieved by using ahigh-Zmateria land a short ph ysical length. This statem ent co ntains som e element ofspeculation and m ore expe rimental wo rk is needed to determine the effect ofphysical length on slit scattering. In the paragraph on shower development inSection20-1,i t has been de m onstratedthat high-Zmaterials are not useful forcont inuous dissipation of afull-power SLAC beam. Therefore a compromisehas to be m a d e on Z which results in an increase in physical length ofth eslits.

    Ideally, th e length of a slit should bezero, i.e., th e slit should coincidewith th e image of the centerof the pulse magnet group(PM-1-5) formed bythe quadrupole doublet (Q-10, Q-l l or Q-30, Q-31). The size of this imageand co nse qu ently the res olu tion at the slit is a func tion of the particle beamsize at the center of the pulse magnet group. After passing through thequadrupo les (Q-10 and Q-ll), the beam is bent a total o f 12 by the firstbending magnet g roup in the A-beam transport system (BIO through B13),and the beam is thus dispersed for momentum analysis at the sli t . Thedispersions at the A-beam slit (SL-10) and at the B-beam slit (SL-3.0) are0.177%/cm and 0.343%/cm, respectively. Only a horizontal slit is neededto remove dispers ive components of the beam, since th e beam spectrum isdisplayed as a func t ion of m om entum in the hor izontal plane.

    20-5 High-pow er col limator andslits (DR W)In th e fol lowing section the high-power collimator (C-l) and slit (SL-10) arediscussed. For locations see Fig. 17-1. An analysis of a slit is presentedapplying the principles and equations given in Section 20-1. Corrosionproblems arising in a stainless steel-aluminum-water system aretreated in

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    732 D. R.Walzet al.Table20-5 High-power collimator andslitcriteria

    Maximum average beam powerFo rminimumbeam sizeForaverage expected beamsizeIncidentbeam energyMinimumbeamsize (forAp/p %%;angulardivergence h.d10~5radian)At collimator C-1At sit SL-10Atslit SL-30Averageexpected beamsizeAt thecollimatorAtslitSL-10Atslit SL-30Maximum aperture openingCollimatorSlitsSL-10and SL-30TotallengthTotal usable heightMaterialsModules

    WaterpipingVacuum shellsMaximum water pressureWaterflowvelocity in prime heat transfer areasMinimum flow rateFor 2.2 MW

    For1 MWMaximum water inlettemperatureOperational vacuumT heaperturesareremotely adjustableduringbeam

    1.0 MW2.2 MWx0.79 cm;ab,y= 0.21 cmbxby= 4 cmbx15cm(&p/p= 2.66and5.13%, respectively)30radiation lengths7.5cmAluminumalloy6061-T6andwaterStainlesssteel Type316-LStainlesssteel Type304-L75 psia>150cm/sec (x 5ft/sec)950liter/min(250gal/min)750liter/min(200gal/min)40C

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    Beampowerabsorption devices 733For multigigaelectron-volt, multimegawatt electron beams, entirelynewconcepts must be fo rmula ted and applied. At SLAC, various collimatorconcepts wereanalyzed.24It wasd emonstrated that a water-cooled, rotating-drum-type collimator was feasible in principle as an aluminum structure forup to2.2-MW average beam pow er, at leaston thebasisofevaluationofheat

    transfer, thermal stress, strain, and fatigue problems. The same geometry,usingOFHC copper as the material , could operate safely for average beampowers up to 500 kW. However , th e rotating drum-type collimator presentssome very difficult problems associated with the operation of bearings anddynam ic seals in a vacuum under high radiation doses (up to 1014 ergs/g/yrare expected),and anonrotatingdevicehasobviousadvantages.In a quasi-stationary slit, evenmedium-Z materials must be ruled out,sincethe thickness of pow er-absorbing walls becomes too small for practicalapplications. Walls cannot be made infinitely thin since they separate thevacuum systemfrom the coolant under pressure, i.e., the device is a pressurevessel. The optimum geometry for a given pressure is a hollow circularcylinder, a tube. The feasibility of a modular a r ray of tubes or a tubefo res t" as the basic collimator element hasbeendemonstrated.12'25

    Efforts to fabricate prototype collimator eleme nts of the tube-forest mod elwere no t fully successful. The mult i tude ofwater-to-vacuum joints presentedproblems.Amo dificationof thetube-forest conceptwas, therefore, developed .Theideaw as to let thespacing forpum p-out betweentheoutside diametersofindividual tubesin thearray shrinktozero, i.e., tobringalltubesinto physicalcontact with alladjacent tubes. A solid block of material, perforated with anarray ofholes having adiameter identical to the inside diameter of the tubes,closely approaches this concept as shown in Fig. 20-7.The number ofwater-to-vacuum joints in oneplane is thus reduced to one at the periphery.

    Figure 20-7 Geo m etric configuration ofhigh-pow er sl it modules.

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    734 D. R. Wa lz et al.The he at transfer behavior of this new g eom etry is to first approximationstillrepresented by the ho llow circular cylinder geom etry. For awallthicknesswhich is small in comparison to the particle beam size, one can assume uni-form heat source distribution as a result of the beam power deposition. Tofirst approximation, axial conduction can be neglected. The temperature

    distribution is described by Fourier's law of conduct ion. Fo r steady-state,uniform heat source distribution, Eq. (20-8) may bew ritten inpolar coordi-nates asd2T IdT S ,-rT--r+7=0 (20-17)dr2 r dr k

    withbo und ary condi tionsT(r f) = T t (20-18a)

    and( T T I VJ = ;r(rg-r?)S (20-18b)

    where q ' is the heat transfer rate per unit length across the metal-liquidinterface.The solution tothis set of equations is(r2- r ? ) ( 2 ( M 9 )

    and the maximum temperature difference across the cylinder wall isAT=\2rlln( -(r?- r?)] (20-20)

    The heat source S can either be calculated as outlined earlier or it can beextracted from curves such asgiven in Fig. 20-2. In order to make computa-tional results realistic, an effective beam size has to be defined. Peak powerdissipation occursat theorigin, r= 0.Values obtained inthis fashion wou ld,however, result in unrealistically high temperature gradients. Therefore, an"effective" beam size equal to the "hot core of the beam is used.It wasarbitrarily defined as the space limited by a radius, rhc,at which the localpow er depo sition is 80 % of the peak value occ urring at theorigin.The powerdeposition is then averaged within this volume element and results in thevalue for the heat source. For the case of an incident beam of standarddeviation ab=0.3 cm , the corresponding valueof the hot core at theshowermaximum isrhc =0.26cm . Knowledgeof the radial power deposition distri-bu tion in just a fe w dep th locations issufficient to construct a curve express-ing therelationshipof the percentofpower deposited(withinrhc )per unit

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    736 D. R. \Na\zetaf.The maximum possible principal thermal stress in a ful ly restrainedsystem due to a temperature gradient may be calculated from Eq. (20-12)using Eqs. (20-20) and (20-21),

    < 7 t h = - a A T t o t (20-23)where th e effective temperature d if fe rencefor the thermal stress developmentconsists of two components,

    AT to t = ATf l Im + ATm e t a l (20-24)Note, this stress is a compressive stress for a positiveAr, i.e., a temperatureincrease.Numer ica l example : Assume a lu m inum a l loy 6061-T6 as material ,Ar= r0- rt=0.127cm (=0.050 in.), ab=0.3 cm,PAV =1 MW. At a depthof 5 radiation lengths,o ne obtains from Fig. 20-1,

    P'= 14.3kW/cm(= J27rrP(r)dr\Approximate ly 9.5% ofP' is deposited within the hot core for rhc =0.26cm ,t h u s S =6 .4kW/cm3. Equations (20-20) and (20-21) yieldAt =35.5C andq = 0.88 k W /c m 2 ; if axial co ndu ction is consid ered , Eq. (20-22), AT1reducesto 32C. Correction by a factor of 2 due to leakage of part ic les through th eboundary yie ldsa temperature gradient across th e wall , from th e vacuum toth e water, ofA7m e t a l = 16C (o r 35C forPA V=2.2 MW ) .Assuming a bulk water temperature of 40C and a water pressure of 10psig, A T ^ i i n ,w 74C. The effective total temperature gradient for t h e r ma lstress considerations, Eq. (20-24), isArtot = 90C, and the ma x i mu m me t a ltemperature at the vacuum interface is 130C. Equation (20-23) yields atherm al s tress valueof c r t h= 22,500psi for a f u l l y rest ra ined system.Th eyieldstrength of alloy 6061-T6 at 135C is ffYT = 30,000 psi and the endurancelimits fo r 106 and 1010 cycles are 18,000 and 12,000 psi, resp ec tive ly. Theinstantaneous temperature r ise due to 1 pu l se is AT" 4C/pulse, whichresul ts in an additional cyclical therm al stress o a i t m a x 1000 psi. Sup erp osi-tion of the cyclical stress and steady-state stress can be accomplished bydefining a me a n effective stress am as

    m=ifaot+O=K2ffth + ffait) (20-25)For this example ,am = 23,000 psi. Next a Good man diagram is const ruc ted .Let th e ordinate represent th e cyclical stresses and m a r k GE= 12,000 psi for10 10 cyc les .T he abscissa is to represent th e steady-state stresses and crrr =30,000 psi isma r k e d . The aE and aYT are connected by a straight line.Thecalculated sample point is located, with cral t as the ordinate value and amas the abscissa value. Any combinat ion of am and < 7 a lt which results in apoint located in the t r iang ular area def ined by the origin, < T , and aYT hasunlimited life expectancy. For this example, i .e ., PA V = 1 MW, no fa t iguefailure is expected. It can also be shown that a 2.2-MW beam can be safelydissipated forab>0.3 cm .

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    Beampower absorption devices 737

    Material selection and corrosionproblemsAn ev a lua t ion o flow-Zfabricat ion ma terials resul ted in se lection of a lu m inu mal loy 6061-T6 for the sl it modules. This al loy has good corrosion resistance(as will be shown below ), s t rength , and therm al cond uc t ivi ty . I t has beensuccessful ly applied fo r u r a n i u m fue l e lement c laddings in nuc l ear reac to rswhere it experiences similar operating conditions. The s t reng th - t empera turerelat ionship is flat and f avorable up to 150C.For higher temperatures th e s t rength decreases rapidly and s m a l l t e m -perature changes resul t in large s t rength variat ions . Design in th is region isno t r e c o m m e n d e d u n l e s s one can pinpoint the t e m p e r a t u r e a c c u r a t e l y . T hea l loy i s qu it e im m une to rad ia t ion- induc ed l a tt ice s t ru c tu re changes fo r dosesup to 1016 ergs/g . Format ion o f vacanc ies and inters t ices sl ightly increasesthe yield and u l t imate t ens i l e s t reng th . For higher doses a rapid increase ins t rength is record ed; the m ater ial becom es brit t le . Some o f the al loy ing con-s t i tuents are m o r e l ikely to be displaced from their proper lat t ice site andchem ica l p roper ty changes in addit ion to mechanica l ones may be o fimportance. The low-Z of the slit m od ule mate r ia ls a l so he lps to keepd o\induced radioact ivi ty , an impor tant f ac to r in accessibil ity to slit andl imator locat ions .Stainless steel Type 316-L w as selected for the water piping fo r reasonsstated in the p rev ious sec t ion . Im po r tan t l inksin the water systema reb e l l o w sw hic ha l low sup p lyand r e t u r n o fwate r to the ad jus tab leslitj aws . Incone l6 00was selected as the mate r ia l fo r th is appl icat ion. It does no t appear to besuscept ib le to s t ress-corrosion cracking and exhibits good al l -aro und cor-rosion res is tance. This is m ost im por tant for s t ressed c om pone nts with th inwal ls(0.03 cm). Inco nel 600 and s tainless s teel Type 316-L can readi ly bejo ined by TIG welding.

    All external mater ials exposed to the beam switchyard atmosphere werecare fu l ly selected to wi ths tand th i s env i ronm ent . High levels o f radioact ivi tyneart hepower absorbers wil lr e s u l tinionizat iono f the swi t chyard a tmosphere .Oxygen and nit rogen ions wil l com bine to fo rm nitrogen oxides , wh ich, int u r n , will for m nit r ic acid with the water vapo r in the air . Signif icant am ou ntsof HNO3are expected to be f o r m e d at high-power operation27 accord ing to

    2H2O+2N2 + 5O2-+4HNO3 (20-26)One molecu le o f HNO3 is formed for each 35 eV absorbed.28 It has beenes t ima ted 2 9 that at f u l l power ope ra t ion , approxim ate ly 600 W are absorbedby the air in the ent ire beam switchyard. This resul t s in format ion of thee qu iva le n t of a p p r o x i m a t e l y 1 l i ter /day of c o m m e r c i a l , 70% concentratednitricacid. At this time it is st il ldifficult to estimate for what percentage ofthe t ime one has to expect f u l l power opera t ion in the fu tu re . M o r e o v e r , theswitchyard is not herm et ica l ly sea led , and s ignif icant amounts of air areexchanged in the course o f normal da i ly a tmospher ic p res sure changes . T heswitchyard air is also vented i r regular ly af ter an appropriate cooling-off

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    738 D. R.Walzeta l.period for maintenance work. Nevertheless, the net amount of HNO3 leftmight turn out to be significant and warrants careful selection of materials.Theslitand coll imatorv acuu m shells ,flanges, bellows,andother importantstructu ral items are fabricated from stainless steel Type 304-L. This materialalso exhibits less outgassing than mild steel, and pump-down time is short-ened. The support structures are very massive, and the use of stainless steelwould have been prohibitively expensive. They were fabricated instead frommild steel.Nitric acid corrosion is not expected tojeopardize seriously th eproper functioning of these structures.The usual rus t layer forme d onm ild steel surfaces m ay, howe ver, presenta health hazard.The rust is continuously being irradiatedas is the rest ofthe eq uipm ent, and induc ed radioactivity could mak e maintenance workdangerous. Radioactive rust particles could be picked up on shoes andclothing. The seriousness of radiation through th e soles of the feet is seldomtreated with sufficient concern. There isalso th e remote possibilitythat underdry conditions rust particles may become airborne and could be inhaled.All support structures were, therefore, painted with a highly radiation-resistant, corrosion inhibiting paint. For highest resistance th e paint has tobe baked onto the surface. Thisoperation was combined with a stress-reliefoperation to increase dimensional stability. Materials painted in this fashionhave been exposed to a total dose of 2.4 x 1013 ergs/g. They showed noindications of damage, and adhesion was not impaired.30A cause for concern was the simultaneous presence of a l um i num andstainless steel in the slit and collimator radioactive water loops. Stainlesssteeliscathodica nd a lum inum wi thit shigh anodic po tentialreactswi th mostcathodic materials. A li terature survey was carried out31 on the corrosionbehavior ofaluminum al loysin a closed-loop aqueous systemin the presenceof stainless steel, nuclear radiation, varying water purity, temperature, andheat flux.Several corrosion mechanisms are active.1. Aluminum reacts with oxygen-containing water by thermochemical ordirect corrosion according to the equat ion

    4A1+ 3O2 + 6H2O->2 A12O 3 3H2O (20-27)This hydrous aluminum oxide is called bayerite. It adheres well to thea luminum surface and is primarily responsible for the favorable corrosionbehavior o f alu m inum . At elevated tem peratures a denser oxide fi lm isfo rmed, A12O 3 H2O, k n o w n as boehmite. The solubility of bayerite inhigh-purity w a t e r3 2 is a function of pH and is at a minimum at a pH ofapproximate ly 5. At this level of acidity, stainless steels suffer some attack.2. The presence of cathodic materials, particularly copper, causes electro-chemicalcorrosion. Copperionsin the aqueous system willplateout on thea l u m i n u m surface and they will cause pitting of the aluminum. Stainlesssteel and chromium result in only slight electrolytic attack unless in directcontact, even though both have a cathodic potential. N o problem exists ifhigh purity of the water is maintained.

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    Beampower absorption devices 7393. Pitting corrosion or fo rmat ionoflittle pitsor cavitiesisbrough taboutby a local cell action on a very small scale. Chemical (for example, copperions) and geometrical discontinuities on the surface are responsible for localdes truction of the passivating oxide film, fol lowed by format ion of a cell.Highlocal currents and corrosion rates are the resul t . At e levated tem peratures ,

    above 100C, pittingis not a ser ious problem.4. Failures due to s tress-corrosion have been encountered. High puri tywater and the use of cer tain tempered (heat-treated) a lu m inum al loys with afine,random grain pattern are good protection against stress-corrosion.5. The previously described cavitation erosion phenomenon associatedwith high heat transfer rates results in accelerated loss from the a luminumsurface. This is partly due to increased solubil i ty in the boiling heat t ransferzones and par t ly due to spallation of the brittle oxidefilm .6. Aluminum and most of i ts a l loys react favorably with respect to cor-rosion while irradiated in aqueoussystems.33 Observations oflower corrosionrates than for unirradiated sys tems were made. The autho r bel ieves that thedifference in the corrosion behavior is due to form ation of hydrog en peroxideand oxidizing free radicals in the radiolysis process in water. The oxidizingagents, in turn, will cause formation of an even denser protective oxide f i lmon the a lum inum sur face which re ta rds fur thercorrosion.Aluminum a l loy 6061-T6 satisfied most of the fabrication, strength,fatigue and corrosion criteria and was selected as module material. Rigidcontrol of water purity isessential for long-term operation. The water shouldbe kept acidic; whereas a pH of 7 would f avorthe stainless steel,a pH of 5would be preferable for the a l u m i n u m . The systems at SLAC show a pH of6.2 to 6.5 and resistivity is better than 1megohm-cm.Finally, one should mention the t ransi t ion f rom a luminum to stainlesssteel which exists in various locations. Commercially available brazed transi-tion pieces we re used. In that application, a luminumis in direct contact withstainless steel, except for the brazing material interface, and a potentialhazard exists.Designfeature highlightsBoth th e collimator and the sli t have variable aperture openings. The col-l imator is form ed by placing two slits in series with the second rotated 90 withrespectto the firstabout the comm on axis .A slitis 30radiation lengths(^5meters) long, so each one had to be bui l t up f rom modular sect ions forfabrication and alignment reasons. Typical modules are shown inFigs. 20-8and 20-9.

    Atotalof eleven modules are assembled to astrongbackto forma jaw,and two opposing jaws form a slit. Figure 20-10 showsapa r t ia l ly completedhorizontal s li t . It isde sirable to build these devices as shor t as possible sincespace is at a p remium. The length can be optimized if the governing wallthickness between adjacent holes as well as the vacuum interface is tailored

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    740 D. R. Walz et al

    Figure20-8 Front view of high-power slit modules.according to shower development and attenuation. This was done foreconomical reasons in discrete steps rather than c ontinuou sly. Thus , th eratioofa luminum to water and, therefore, th e effective Z isincreased dow n-stream from the shower maxim um in favor of a luminum according to showerattenuation.Figure 20-9 Topviewofslitmodules.

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    Beam power absorptiondevices 74 1

    Figure 20-10 Horizontal slit module-strong-back assembly (SL-10) with vert ical slitassembly(C-1-V)inbackground.

    Each module floats axially with respect to the strongback on linear ballbushings; this allows for thermal expansion between strongback and power-absorbing modules,and the strongback can properly maintain the straightnessof the be am -de fining edge. The strongback is fabricated from stainless steelType 316-Las a hollow circular cylinderfor maximum torsional rigidity.It isused as water inlet manifold and its temperature remains essentially constantduring operation. Onlythe last mo dule isrigidly co nnected to the strongbackvia th e water inlet pipe and all modu le s are in series in the water loop.A double pantograph assembly (paral lelogram linkage) isused to providecenter-line stability. The link between tw o opposing pantographs is madefo r zero backlash with high-strength, low-stretch aircraft cables, Fig. 20-11.Spherical ball-journals connect pantograph arms and strongback. Theyallow differential gap opening between f ront and back of the slit jaws wh ichm ay be used to accommodate angular divergence of the beam. Properassembly of two opposing jaws al lows nesting of the module convolutions,i.e., no line-of-sightispossible, and a slitcan beusedas a beamstopper (forprinciple, see Fig. 20-7). In order to protect equipment downstream of thecoll imator or slit from excessive radiation and to minimize th e possibility ofhalo (penumbra) formation behind th e beam-defining edge due to the low-Zaluminum-water combination, a set of9.5-radiation-lengthcopper modules

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    Beampower absorptiondevices 74 3

    Figure 20-12 Front view of high-powerslit (SL-10).Slitassembly placed in the vacuum tank, with secondary-emission monitorfoils.

    Modules and their fabricationAs already indicated above, eleven individual modular elements make up aslit jaw. The first six modules are identical and the water-to-vacuum wallthickness is Ar = 0.127 cm (0.050 in.). Note that for 20-GeV electrons th eshower m aximu m occurs at the beginning of module No. 4. Modules No. 7and 8 are identical and Ar = 0.305 cm (0.120 in.). Module No. 9 has Ar =0.760 cm (0.300 in.), and the last tw o modules ar e again identical withAr= 1.520c m(0.600 in.).The modules were manufactured from high-quali ty aluminum blockswhich were forged, stress-relieved, and tempered to a T-6 condition. Eachm o d u l econsists o f 3parts:a30-cm-high m odu le bodyand two flowheaders.The m ach ining of 30-cm long holes of 1.6 cm d iameter presented m anydifficult problems. Tolerances on hole position and parallelism had to beheldvery closely to satisfy tem per atu re, Eq. (20-20), and corrosion criteria.Different techniques were investigated, including gun drilling, electric-discharge milling (EDM), and electrochemical milling (ECM). The problemwas finally successful ly solved with a tape-controlled, deep-hole drillingtechnique.The flow headers were welded to the m odu le body w ith an electricarc (TIG) method for thin walls and with th e electron beam welder for thickwalls. Full penetration wasrequired to minimizecrevicecorrosionandnotchsensitivity problems. Total leak rates in excess of 10~9s tandard-cm3 He/secwerecause fo r rejection.The reasons for the height and d epth o f the mo dules are as follow s:radiation damage due to high-energy electrons penetrating th e a luminumis

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    744 D. R.Walzet al.not yet wellknown. For doses inexcesso f 1016 ergs/g, significant embrittle-ment isknownt o resultf romdislocations, vacancies, a nd interstitial atoms.Moreover, the aluminum oxidefilm may spallin the areaofhighheatflux,and cavitation erosion will fur ther reduce the wal l thickness.A n increase inheight of themodule body combined with provision f o rvertical adjustmentof the slit with respect to the nominal beam center line allowsfo r variousbeam exposure locations and, thus,f o rdistributionof theradiation dose overa large volume. The slits are adjusted at 1010 ergs/g. Center-line stabilityduring this operation is maintained by two guideposts mounted on themain support frame.The module transverse depth isdeterminedbyradial shower developmentconsiderat ions,maximum transverse beam excursion, andbeam momentumspread. A maximum transverse f lux of 1W/cm2 leaving th e module planeadjacent to thestrongback was set ascriterion.Themodules were made 16.5cm deepto satisfy this condition.

    A small prototype of an aluminum slit module w as tested fo r thermalshock17 in the Astron accelerator at Lawrence Radiation Laboratory inLivermore . This high-intensity machine can produce bursts of very highpower density over short periods of time. The peak power density w asachievedby abeam ofE0 =3.8MeV,7pe ak= 90A,pulselength= 0.3 x 10~6sec, pulse repetition rate= 5 pulses/sec, and beam diameter= 1 cm. Theresulting local heatflux is lowcompared to thedesigncriterion.Much morespectacular is therateo f energy deposition during th epulse, which gives riseto very steep temperature gradients in space and time, resultinginpropor-tional thermal stress gradients. For this run the rate of energy depositionduringthepulsein themodule wallwas600,000kW/cm3.Although very highincomparison with such values from other accelerators, this power depositionw asnothigh enoughtodamage thealuminum. No effecton themodule wallwas visually detectable after several hours of beam exposure. Thus, evenfor awell-focused SLAC beam, no thermal shock and spallation problems(beyond spallation of oxidefilms) are expected for the high-power slit andcollimator.

    Fabrication andalignmentThe slits were assembled in the SLAC fabrication facilities.The internal slitassemblies were built in a clean room, using methods which are consideredgood practice for handling of vacuum equipment. Particular attention wasfocused on thealignmento f thebeam-defining plane formedby theconvolutedfaceof all themodules.The latter were optically alignedflat to 0.015cm.As was mentioned earlier, the dispersion at the slitdue to the first bendingmagnet group is0.15%/cm.Thecompleted slit assemblyw astested for align-ment hysteresis under vacuum. Hysteresisup to 0.013cm was recorded.

    Thehorizontal and vertical slitsof the collimator are tied to laser beamstations. The relationship between the slit center line and the laser beamcenter linewasestablishedin theshop.AFresnel target allowspositioningof

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    Beampower absorption devices 745th e slit such that its center line coincides with the nominal electron beamcenter line. This is discussed in more de ta i l in Chapte r 22.Assurance conc er ning the ac tual lo cation of each slit and its center lineis obtained in three different w a y s :1. Second level optical survey in the switchyard is used to position each slit

    with respect to an external reference. Tooling balls and m irror stages(see,fo r example ,Fig.20-13), rigidly m ou nted to the external structure of theslit, were set during assembly and have known distances and rotationswith respect to the slit center line . Precision alignm ent jack s allo w adju st-ment s to 0.003 cm accuracy.2. The two sl its of the coll im ator (as well as thehigh-Zcoll imator) arecon-nectedto lase r beam target stations asdescribed above. Alaser surveycandetec t deviations of0.001 cm .3. Theelec tron beam can beused as asurve y tool to ascertain such informa-tion as rotation of the slit about an axis normal to the beam center lineand comparison of center-line location of high-Z and high-power col-l imators or sl its. This inform ation is obtained using tw o steering magnetsto deflect th e beam across th e front face of the slit. The beam currenttransmission is then m easured as a func tion of mag net curre nt. Differencesof 0.005 cm can be detected.

    Figure 20-13 High-Z collimator (C-0)with high-power collimator (horizontalslit C-1-H) as installed in the beamswitchyard.

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    746 D. R. Walzet a /.

    Operations to dateA l t h o u g h p o w e r levels encou ntered to date (Ju ly 1967) have been low , opera-tions of high-power sli ts and co l l imator s for the first 12mo nths have beenessential ly faul t less . Up to a ppro xim ately 50 kW of average beam power hasbeen dissipated. Reproduc ibi l i tyof aper ture w idth and center lineof a 5-meterlong slit of 0.03 cm has been achieved.The pr imary radioactive water sys tem is s imilar to the one for the high-power beam dump and will not be discussed in f u r t he r detai l . Hydrogenrecom biners are p lanned for ins tal lat ion in the near fu ture .The qu estion of wh ether or not the low-Zand large physical length of theseslits have a negativeeffect on beam analysis and definit ion has not yet beeninvestigated. However ,the l imi ted num ber ofphysics experiments cond uctedso far has not uncovered any deleter ious effects.20-6 High-Zslits and col limators (WSS, DR W)Variable-aper ture , low-pow er,high-Z sli ts and collimators were designed andbuilt to serveasbackupdevicesfor the high-power units.The slit material iscopper and proper ly only medium Z, but the term highZ was used todistinguish these devices from the high-power units described in the previoussection. Tw o col l imators and two slits are now in use : a collimator (C-0)immediately upstream of the high-power col l imator (C-l), a photon beamTable 20-6 High-Z coll imator and sl it cri teria

    Maximum average beam powerForminimum beam size,ab< 0.3 cmFor average expectedbeamsize,a6>0.3 cm

    Incident beam energyPulserepetition rateBeam sizes (see Table 20-5)Maximum aperture openingTotallengthMaterialsModules

    WaterpipingVacuum she l ls

    Maximum water pressureWater flowvelocity in prime heat t ransfer areasMinimum flow rateforfullpowerMaximumwaterinlet temperatureMaximum temperature r iseOperat iona l vacuumAperture isremotely adjustable during beam operation

    20 kW40 kW150cm/sec (x 5 ft/sec)34liter/sec(10gal/min)40Cx25C10~4torror better

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    Beampower absorption devices 747co l l im a t o r(C-10), and two slits (SL-11, SL-31) fo r momentum definition inth e two beams servingthe major research areas. The important criteria ares u m m a r i z e d in Table 20-6.Design featuresThe high-Z collimator and photon beam collimator (which are in mostaspects identical) are formed, as is the high-power collimator, by two slitsin series withthe second rotated 90with respect to the firstabout the com-mon axis. Boththe horizontaland vertical slitsof a collimator are installedinonevacuumtank because of theirsmall physical size.TwoopposingOFHCcopper modules form a slit. Figure 20-14 pictures a set of modules for theco l l im a t o r (C-0). They are 10 cm deep, 20 cm high, and 50 cm long. Thev o l u m e closeto thebeam-defining edgeisperforated witha row ofhole pairsfo r propercooling. Thewaterpassagesareconnectedin seriesbymeans ofsquare flow cavities mi l led into the copper and closed with a cover platebraced to the module.The cavity reversesthe flow direction and promotesgood mixing . T he minimum wal l thickness between the collimating surfaceand the flow passagesis 1.0 cm . The lengthof themodulesisthus35radiationl engthsup to atransverse depthof 1.0cm, and itdecreasesto aminimumof15.5 radiation lengthsat adepth o f 1.8 cm ,coinciding with th e centers of thefirstrow o fholes.Inorder to minimizethe possibilityo fpenumbra formationint hecaseo f theslitsand thephoton beam collimator,the f lowchannels werealtered downstream f rom the shower maximum. At an axial shower depthsuch that power deposition hasdecreased to a lowlevel, the twoparallelf low

    Figure20-14brazing.

    High-Zslitcopper modulesbefore furnace

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    748 D. R. Walz et a /

    Figure 20-15 Double pantograph assem bly with slitmodules attached.

    passages were combined into one, which is of somewhat larger diameterto keep th e velocity constant. Its center is located midway between th etwo smaller passages. This provides an increase in Z downstream from th eshower maximu m in the transverse region extending from 1.0 cm to the flowpassages.The modules are mounted to a four-pivot point o r double pantographassembly a sillustratedin Fig. 20-15.T hepivot points aremountedontaperedroller bearings to al low adjus tme nt for zero backlash. The two halvesof thepantograph assembly are cross-connected with low-stretch, high-strength,flexible aircraft cables. The cables provide easy zero-backlash adjustment.The pantograph arrangement maintains the collimating faces of the modulesin a parallelattitude and at anequaldistance from th e center line.Themax-imum total variation for full aper ture adjus tment betweenany twoopposingmodules was measured to be about 0.006 cm . Aper ture adjus tments can bemade only as parallel translations about a common center line, i.e., no

    accommodation to angular divergenceof the beam ispossible.An interesting featureis the coil-spring water manifold,seeFig. 20-16.The m axim um po ssible ape rture opening is 15 cm , i.e., 7.5 cm transla tion foreach jaw. Small s troke and other physical limitations make bellows a poorchoice for a flexiblelink in the water supply and return lines. Thus, th e coil-spring water manifold wasdeveloped as an alternative tobellows.Each slit isoperated by one actuator consisting of a precision jack witharotationrestrictor and a bellows vacuum seal. Rotary motion is supplied toth e input shaft of thejack for 100 : 1 reduction and creation of the linearmotion to movethe modules .

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    Beam power absorption devices 74 9

    Figure 20-16 High-Zcollimator (C-1, C-10);vacuum tank installation with view of coil-spring water manifold.

    Fabrication andmaterial selectionThe modules were machined f rom solid forgings.T oassure soundness in thefurnace-brazed joints and prevent hydrogen blistering, OFHC copper w asselected. The coil-spring water manifolds connecting the external piping toth e movable modules were fabricated from extruded electrical conduit. Themateria lisEverdur 1015,ahigh-purity silicon bronze.It wasselectedfor itshigh strengthandgood corrosion behaviorinwater.Thetube work-hardenedand achieveditshigh strengthand elasticityin the coil-bending process.Thecoils were hand-brazed to the module to confine annealing to the brazingjoint. After allbrazing w ascompleted, th ecoil-module assemblies were shotpeened with small glass beads (0.002-0.007 cm ) propelled by 60 psig airpressure .Brazed areas received prolonged exposure to the impinging beadsto assure restoration o fstrength and fatiguelife. The resto f thewater pipingis stainless steel, Type316-L.

    The pantographs were fabricated as welded structures from aluminumalloy 6061, solution heat-treated and tempered to the T-6 condition afterwelding.To maintain stability the maximum allowable design stress in thepantograph assemblywas set at3500 psi. This compares very favorably withayield strength of 40,000 psi for alloy6061-T6. The actuator vacuum seal

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    750 D. R. Wa lz et a /.bellows are of the weldeddiaphragm type, fabricated from 0.015-cmthickInconel 600.

    The vacuum she l lwasm a d eby rollingand welding0.8-cm thick stainlesssteel Type 304-L plate to forma circular cylindrical vessel. Dished heads werewelded on each end. The vacuum shel ls and slit assemblies were vacuumleak checked withahel ium mass spectrometerat various stagesof fabrication.N o leaks in excess of 10~8 standard-cm3 He/sec were allowed and all finalassemblies showed no leaks at the sensitivity limit of the spectrometer,10 10standard-cm3 He/sec.All units exceptthephoton beam col l imator are mou nted on the corre-sponding high-power sli tand col l imator support f r ames ; seeFig.20-13. Theywere aligned at the time of mount ing to be coincident with th e high-powerequipment. Small la teral adjus tments of + 2.5 cm can be made wi thoutdis turbing th e lateral position of the high-power devices. These adjustmentsarem a n u a l and semiremote f rom th e upper tunnel housing.

    The photon beam col l imator was installed on a special framethat pro-v ides r emote powered ad jus tment of +3.75 cm in both the vertical andhorizontal directions.The high-Z devicesare tied into the m agnet cooling-water (LCW) system, described in Chapter 24.Operational experienceFunctioning of all the high-Z units hasbeen faultlesstodate (July 1967).Thesignificant difference be tweenthe Z of the high-power and high-Z slitsaswel las a large difference inphysical length(10 : 1)should makeit possibleto partially answer questions concerning the influenceof these parameters onslit scattering and beam ha lo fo rmat ion.Magnets were used to deflect th e beam across th e f ront of the par t ia l lyopened high-Z B-beam slit (SL-31) and beam current t ransmiss ion was mea-suredas afunct ion of magnet cur rentinorder to obtain beam spectra. Figure8-17 show s the spectru m for a 6 GeV be am (obtained on Feb ruary 1, 1967).20-7 Coll imator actu ation anddr ive system (LRL, DR W)Two types of ad jus tment s can be m ad e on each s l i t and co l l im ator : (1) re-motely co nt ro l led motor ized ad jus tm ents dur ing beam o pera tion, such asopening and c los ing th e slit aper ture , changing the co l l imator aper ture , o rchanging the beam expo sure location on the mo du les ; and (2) sem iremote andm anual ad ju s tme nts on maintenance days , pe r fo rmed in the tu nne l hous ing t his covers all al ignment operations and is done solely by mechanical pre-cision jacks.In case of a m ajor dis tu rbanc e, such as a severe ear thqu ake, i t i s poss ibleto add or r emo ve spacer sup to +15c m .Table 20-7 sum ma r izesthei m p o r t a n tcr i ter ia w ith identi f icat ionof indiv idua l equipm ent .

    All r emote ly cont ro l led ad jus tments are accompl i shed f rom the DataAssembly Bui lding (DAB). A g r o u p o f panels in the DAB houses th e

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    Beam power absorptiondevices 751Table20-7 Actuationandalignmentsystemcriteria

    VariablesGross manua l positioning ofsupportf rames,t ransversetothe beam direct ionSemiremote, manual a l ignmentof sup port frame s transverseto the beam directionSemiremote, manual a l ignmentof high-Z support framesinhorizontal planeMaximu m poss ib le aper tureopening (remotely, motor-ized)

    DeviceC-0, C-1SL-10,SL-11

    SL-30,SL-31C-0, C-1SL-10,SL-11SL-30,SL-31

    C-0SL-11SL-31C-0SL-10, SL-11C-10C-1

    M a x .translation

    15cm

    7.5 cm

    2.5 cm

    15cm3.8 cm

    Max.tolerance0.3cm

    0.05cm

    0.010cm

    Reproducibi l i ty o f aper tureandcenter line (including a llto le rances) All Dev ices

    Remote, motorized al ignmentof photon beam collimatorsupport frame C-10

    Remote, motorized change of C-1beam exposurelocation (radi- SL-10at ion damagedistributio