IEEE Transactions on Nuclear Science, Vol. NS-32, No. 5, October 1985

THE TEVATRON BO LOW BETA SYSTEM

K. Koepke, E. Fisk, G. Mulholland, H. Pfeffer*Fermi National Accelerator Laboratory

Batavia, Il 60510

Abstract

A low beta insertion has been installed at the BOstraight section of the Tevatron, the location of theCollider Detector Facility. The focus is achievedwith four superconducting quadrupole doublets. Theengineering design and performance of this systemexclusive of beam behavior will be described includingthe magnets and their power supply, quench protectionand cryogenic subsystems.

Introduction

The BO low beta insertion consists of four nestedhigh field superconducting quadrupole doublets thatbracket the BO straight section. The outermostdoublet (Ql circuit) replaced the quadrupoles locatedat A48 and B12 of the normal magnet lattice. Theother doublet quadrupoles - circuits Q2, Q3 and Q4 -

were inserted at the start and end of the BO straightsection. In addition to the low beta quadrupoles, 8correction dipoles, 16 beam position detectors, 6flying wire detectors and 9 cryogenic vessels wereadded to the tunnel.

The low beta doublets are powered with separatepower supplies. The Ql circuit operates atapproximately half the ramp current during beamacceleration and is programmed thru a ±6 kA rangeduring a low beta squeeze. The other doublets arepowered after the particle beams have reached theirpeak energies and as the low beta squeeze starts.During a low beta squeeze cycle, the existingquadrupole, skew quadrupole and sextupole correctionmagnets in the Tevatron are reprogrammed to maintainstable circulating beams. Small orbit distortions(±.040") caused by low beta quadrupole misalignmentsare corrected with additional dipole correctors andthe local dipole correction magnets close to the BOstraight section. Each of the Q2, Q3, Q4 quadrupoleshas motor driven mounts which can be adjusted tocorrect for larger orbit distortions. A more detaileddescription of the low beta lattice and the requiredlow beta controls are given in accompanying papers atthis conference.' 2

Fig. 1. Low Beta Quadrupole Cross-section

*Operated by Universities Research Assoc., Inc., undercontract with the U.S. Department of Energy

Low Beta Quadrupoles

A cross section of the low beta qyadrupoles isshown in Fig. 1. Except for changes in theircryostats and overall lengths, the quadrupoles aremechanically identical to the Tevatron quadrupoles.3The cable copper to superconductor ratio has beenchanged from 1.8/1 to 1.3/1 to allow these magnets tooperate at currents in excess of 6 kA. At 6 kA, thequadrupoles have a gradient of 26*kG/in. The maximumrequired gradient to achieve a B of 1 m at a beamenergy of 1 Tev is 25.4 kG/in. All of the quadrupoleswere cold tested for maximum operating current andfield quality prior to their installation in thetunnel and all achieved their design fields withminimum training. Additional quadrupole parametersare given in Table 1.

Table 1 Low Beta Quadrupole Parameters

Cable Cross Section (in)Strand Diameter (in)No. Strands/CableFilaments (um)Copper/Supercond. (nom.)Cable Short Sample

.044 x .055 x .307

.026823201.3/1 by volume6290 A, 5.79 T, 4.6 K

Inner coil Outer Coil

Inner Radius (in)Outer Radius (in)Key Angle (deg)No. Turns

1.7502.06730.119414

Yoke Inner Radius (in)Yoke Dimensions (in)

2.0882.40530.783920

4.0009.75 x 15.478

Q1 Q2 Q3A,B

Coil Length Actual (in)Magnetic (in)

Yoke Length (in)Slot Length (in)Ind./mag.(mH)

69.66.163.

1 29.7.1

182.9180.176.9195.19.3

146.9144.140.9159.15.5

Q4

146.9144.140.9159.15.5

The low beta doublets are assembled out of 10quadrupoles. The Q3 doublet magnets (288" magneticlength) were split into separate 144" magnets tofacilitate fabrication and handling. The Ql magnetassembly is the most complicated. Its length was setby the available space (128.972") at the B12 latticelocation. In addition to the 66.1" quadrupole, thisassembly contains 40" concentric wound horizontal andvertical correction dipoles, two 6 kA and four 50 Apower leads, a vertical beam detector, cold bypassbuses to allow the accelerator magnet bus to bridgethe gap at this location, relief valves for the singlephase LHe, two phase LHe and LN, carbon resistorthermometers and voltage taps for quench detection.The A48 magnet is identical to reduce the number ofspare magnets required. Field polarities are adjustedby reversing the magnet current polarities. The otherquadrupoles are simpler, containing single phase LHerelief valves because of their relatively long lengthsand voltage taps for quench protection. The Q2, Q3,Q4 magnet strings are mechanically symmetric inrotation around the center of the BO straight sectionexcept for the cryogenic connection points to theoriginal magnet lattice. This simplified themechanical layout while again reducing the requirednumber of replacement parts.

0018-9499/85/1000-1675$01.00( 1985 lEEE

1675

B12-I LOCATION

LVvMAAJ HEATER

-,IDAQQIr 7.1 m H

Fig. 2. Power Circuit for the Q1 Doublet

Power Circuit

The low beta circuits operate on separate powersupplies which are programmed with microprocessorcontrolled waveform generators'. The power circuitfor the Ql doublet is shown in Fig. 2. The powercircuits for the other doublets are similar. The Qlcircuit contains a thyristor current reversingswitch.5 This allows the use of identical powersupplies for all the circuits. The Q3 circuitcontains two dump circuits - one next to the powersupply, the other centered in the inductive load - toreduce the voltage-to-ground during a dump by a factorof 2. All of the power supplies and dumps are locatedin the BO service building. Warm electricalconnections are made with 2" square, 1.125"? innerdiameter, water-cooled bus. The helium cooled powerleads are rated at a constant 6 kA.

The power supplies installed at present are

temporary units initially intended for beam linemagnets. They are 12 pulse, 150 kW dc, currentprogrammable supplies which are capable of a

continuous 5 kA at 30 V and have a current regulationtolerance of .05%. The low beta supplies on orderhave a regulation tolerance of .001% and a continuousoutput current of 7.5 kA at 50 V. The temporarysupplies limit low beta operation to a beam energy of800 GeV.

None of the power supplies are voltage inverting.The required current regulation with negative currentrates is possible due to the voltage drop across theresistive elements in series with the superconductingmagnets. The Ql circuit has additional diodes in the

current reversing switch which allows -300 A/s currentrates down to zero output current. Thesuperconductiong magnets quench at current rates inexcess of 300 A/s. Therefore, a minimum of

approximately onq minute is required to squeeze thebeam size to a 8 of 1 m.

The regulation requirements can be separated intothree segments: the beam acceleration cycle duringwhich only the Ql circuit is powered; the programmedsegment during which the beam 8 function changes; andthe final segment at low beta and constant current.

The most stringent regulation condition occurs duringthe final constant current mode during which themachine tune shift tolerance is estimated to be .0003.This tune shift implies that the low beta circuitsmust be current regulated (drift and ripple) toapproximately .001%. No additional filters arerequired except for a passive filter in the Ql circuitwhose inductance of 14 mH is insufficient to guaranteeuniform slow spill during fixed target operation.

700 T - I600_500- * 6 kA

0- 2 kA400 + ADIABATIC CALCULATION

300 _ +_

w 200_ +

+ 0. 0

I100 _+ 0

+ 0

2 50 - r O0I'- 50 0

o 2 3 4 5 6

MIITS (106A2s)

Fig. 3. Cable Temperature versus Miits

Quench Protection

The reduced copper content and high operatingcurrent of the low beta superconducting cable relativeto Tevatron cable6 makes quench protection somewhatmore difficult. The measured low beta cabletemperature as a function of the integral fI2dt (106A2s equal Miits) is shown in Fig. 3. Fig. 4 shows ameasurement of Miits versus quench current for a Qlmagnet with its terminals shorted during the quench.

10

U,

0

C,)-

87654

3

2

.5

J.5.5MAGNET

I 1. hO

2 3 4 5 6 78910

CURRENT (kA)

1.0 uLuJ.8 (f

.6

.5 <

.4 :0

.3 >

.2 zzL.s

0

0

.05 -

-J

LuJ

Fig. 4. Miits and quench delay versus magnetcurrent for heater initiated quenches.

The primary quench protection for the low beta

magnets are the resistive dumps. Table 2 shows the

dump resistances that, for magnet currents up to 6 kA

and a quench detection level of .25 V, limit the peak

conductor temperatures to 500 K. The advantage of

external dumps is that they absorb most of the

magnet's energy, thereby minimizing the refrigeratordisturbance and the likelihood of a secondary quenchin an adjacent Tevatron magnet.

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A48-I LOCATIONLUl, HEATER

5OV7.5 KA

0

01

/o

- RIGHTI

SCALEi@\

UR I.

Circuit ParametersDump

Resistance Voltage at(mQ) 6kA (V)

71 426193 1158

* 310 * 1860155 930

*Total for two dump circuits

Redundancy in quench protection is obtained withquench heaters as the dc contactors used in theTevatron dumps7 are too slow in this application. Thequench heaters are energized simultaneously with thedump(s). Below a magnet current of 1 kA, the quenchheaters become increasingly unreliable in starting aquench. In this case, the series resistance of thecircuit is sufficient to protect the magnets as longas the power supply shuts off.

A standard Tevatron Quench Protection Monitor(QPM)7 with modified software is used to monitor themagnets for quenches and the power leads foroverheating. The QPM compares four coil voltages todetermine the presence of a quench. The quenchdetection level is 0.25 V. Noise or electronic driftrelated false quenches have not been a problem. In thecase where only two magnets are in the circuit, thevoltages are obtained with magnet center taps. Thedump circuits, heater power supplies, voltage tofrequency converters, etc., are either standardTevatron components or sufficiently similar tosimplify operation and maintenance.

Cryogenics

Although electrically independent, the low betaquadrupoles are cryogenically in series with theTevatron magnets. This eliminates the need forseparate refrigeration systems but increases the 4.6 Kheat load for the A4 and Bi refrigerators by as muchas 115 W each. In addition , each refrigerator hasto supply LHe cooling for eight 6 kA power leads (16.81/hr per lead at 6 kA dc).

The Ql magnet assemblies replace smaller magnetsin the Tevatron magnet lattice. The Tevatron powerbus now bypasses this location inside a 4.6 K transferline. The remaining low beta magnets are attached tothe Tevatron magnet cryogenic system at the pointswere the cryogenic system ended at the start and endof the BC straight section. The modified cryogenicarrangement - upstream and downstream are identicalis shown in Fig. 5. The original cryogenic turn-around boxes were replaced with a power feedcan forthe Q2 magnet, followed by the Q2 magnet, a spacer

piece which also contains two beam position monitors,the Q3A and Q3B magnets, a modified turn-around boxand finally, the Q4 magnet.

The two Q3 magnets closest to the beaminteraction point and both Q4 magnets had to bemounted on support beams which project into the BOCollision Hall. The solid angle coverage for highenergy physics relative to the beam collision pointwas enhanced by minimizing the size of the originalcryogenic turn-around box and by moving it inwardbetween magnets Q3B and Q4. The Q4 magnets aretherefore only cooled with direct return subcooledLHe. A return path for the LN was provided by addinga second concentric shield to this magnet's cryostat.An inherent cool-down time increase of a factor of twowas accepted for this magnet.

In addition to the Joule-Thomson valve whichseparates the single phase and two phase LHe, eachmodified turn-around box also contains the He cool-down line, cryogenic relief valves, cryogenicinstrumentation, two beam position detectors and theelectrical connections for the Q3 and Q4 magnets. Thepower leads for these magnets are 30 ft back insidethe tunnel. A transfer line, cooled only by the powerlead LHe flow, transports the superconducting leadsfrom the power leads to the turn-around box.

Acknowledgements

Many people have contributed to the Low Betaeffort. In particular, we want to acknowledge thesubstantial contributions of our colleagues:D.Augustine, H.Barber, J.Carson, R.Ducar, H.Edwards,J.Edwards, R.Ferry, J. Fritz, H.Fulton, T.Groves,D.Johnson, R.Johnson, H.Jostlein, F.Krzich, S.Lackey,F.Lange, E.Larson, G.Lee, J.Lockwood, W.Marsh,D.Mizicko, C.Moore, A.Oleck, S.Reeves, C.Rode, G.Tool,F.Walters, F.Willeke, D .Wolff.

References

1. D. E. Johnson, 'The BO Low-Beta Design for theTevatron', submitted at this conference

2. D. Finley, R. Johnson, F. Willeke, 'The Control andInitial Operation of the Fermilab Low S Insertion',submitted at this conference

3. W. E. Cooper, E. Fisk et al., IEEE Trans. onMagnetics MAG-19, 1372, (1983)

4. C. J. Rotolo, IEEE Trans. on N.S., NS-30, 2959,(1983)

5. K. Bourkland, J. Dinkel, IEEE Trans. on N.S., NS-26, 4063, (1979)

6. K. Koepke, P. Martin, and M. Kuchnir, IEEE Trans.on Magnetics MAG-19, 696, (1983)

7. R. Flora, J. Saarivirta, G. Tool, D. Voy, IEEETrans. on N.S., NS-28, 3289, (1981)

LN SPLIT SHIELDCONNECTION

I co10

TI - VPTDP- DIFFERENTIAL PRESSUREC - CARBON THERMOMETERPt- PLATINUM THERMOMETERF - FLOW METER

Fig. 5. Low Beta Cryogenic Arrangement

Table 2 DumpCircuit

Number Inductance(mH)

Q1 14.2Q2 38.6Q3 62.Q4 31.

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