Conferences and Symposia

The Second International Congress of The International Organization

for Vacuum Science and Technology and The Eighth National Vacuum

Symposium of The American Vacuum Society, Washington, October, 1961

THE largest vacuum congress that has ever been held took place at Washington during the four days from 16th to 19th October, 1961. About 1800 people were present and, in a total of 22 sessions, 204 papers were read. The object of this report is to discuss the proceedings in a way which will point to the main trends in development and research.

The subject is considered here under the following headings, arranged alphabetically for convenience : (1) adsorption and gettering ; (2) components ; (3) gauges ; (4) general topics; (5) getter-ion pumps, cryopumps and diffusion pumps ; (6) mass spectrometry ; (7) mechanical pumps and sorption pumps ; (8) space simulators ; (9) sputtering ; (10) thin films ; (11) ultra-high vacuum ; (12) vacuum metallurgy ; (13) vacuum systems.

It must be stressed that parallel sessions of two or three were held which, combined with the sheer physical effort of attending lectures all day for four days, made it impossible to appreciate more than about fifty of the papers presented. To that extent this account is biased ; further, to avoid an over-long report, review papers receive less attention than those reporting new work.

(I) Adsorption and gettering B. M. W. Trapnell surveyed the chief features of the

experimental and theoretical work on chemisorption and physical adsorption ; recent measurements on the velocities and heats of chemisorption were described and an appeal was made for further work on carbon. Adsorption of semi-conductors, especially in relation to the effects of charge transfer between an adsorbing particle and the semi- conductor, was discussed by J. T. Law. The preparation of initially atomically clean single crystal surfaces, including those of semi-conductors, in ultra-high vacuum was described by J. A. Dillon. The interaction of gas atoms with surfaces and molecular processes in adsorption on metals were the subject of papers by G. D. Kalsey and G. Ehrlich respectively. J. P. Hobson reported work on physical adsorption in which pressures of the order of lo-20 Torr were achievable by cooling ultra-high vacuum systems to liquid helium tem- peratures. Sticking probability measurements were described by D. Lee, H. Tomaschke and D. Alpert. In a study of the desorption of gas by photons, W. J. Lange and H. Riemersma found that about 10-s gas particles were released per photon (of energy a few electron-volt) for the system carbon monoxide on nickel. A new method of recording and displaying by

electronic means the variation of temperature with time of an initially flashed tungsten filament was described by A. E. Saunders and J. Yarwood in connection with work on the measurement of accommodation coefficients. The gettering efficiency of titanium was shown to be adversely influenced by carbon impurity because of the formation of hydrocarbon vapours and carbon oxides when hydrogen or oxygen are being sorbed respectively (L. Holland, L. Laurenson and P. G. W. Allen). The extraordinary efficiency of bulk getters consisting of an alloy zirconium with 16 per cent aluminium at about 300°C was the chief feature of the exten- sive investigation of getters from metals of the 4th A group and thorium by P. della Porta, T. Giorgi and S. Origlio.

(2) Components K. S. Balain reported his work on epoxy resin seals for

high and low temperature work. Epoxy resin Stycast 285GT (Technical Bulletin 7-2-7, Emerson and Cuming, Inc. Canton, Mass., U.S.A.) was chosen for copper potentials leads out of high vacuum at liquid helium temperatures ; epoxy resin Stycast 2762 was successful for lead-ins in high temperature (up to 250°C) vacuum systems. W. R. Wheeler and M. A. Carlson compared four seal designs for ultra-high vacuum flanges and concluded that the so-called “Conflat” sealwas the best: this consists of conical sealing surfaces and a flat metallic gasket. R. A. Haefer described an “ ion baflle ” for reducing back-streaming of oil from a mechanical or vapour pump : a cold cathode magnetron or inverted magnetron electrode arrangement is used in the pump inlet line ; the decomposed hydrocarbon molecules have an extraordinary tendency to recombine to solid polymers, preferentially at the electrodes. J. Hengevoss and W. K. Huber described ultra-high vacuum experiments in which residual gas analysis was by a cycloidal type mass spectrometer. A special study was made of the liquid nitrogen trap : in ultra-high vacuum, it was shown that the action consists of adsorption of permanent gases at well baked-out surfaces of trap.

(3) Gauges An ultra-high vacuum gauge of better sensitivity than the

Bayard-Alpert type and based on the omegatron electrode structure (but without mass spectrometer action) was described by A. Klopfer. Though having a very low X-ray limit, the complex electrode structure would be difficult to

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de-gas readily. A. van Oostrom described a design of Bayard-Alpert gauge with an extremely thin ion collector wire and a special grid structure with optimum potential distribution for which the X-ray limit was about 6 x lo-13 torr. A hot-cathode magnetron ionization gauge was described by J. M. Lafferty. In the most recent development, the low pressure limit has been extended below 4 x lo-14 torr by use of an electrostatic .lens system that focuses ions on the first dynode of an electron multiplier. A special Geiger- Mtiller tube which counts U.V. photons from electron-excited residual gas was used as a vacuum gauge by I. Alexeff ; this should theoretically measure pressures down to lo-13 torr. Susceptibility to cathode contamination would probably lead to considerable sensitivity variations. H. Schwarz described a “ resonance vacuum gauge ” of which the operation depends on the damping of a light vibrating membrane by the surrounding gas ; it is alleged to cover the range from 760 to 10-s torr.

(4) General topics

Papers presented included subjects of vacuum spectroscopy (B. Rosen), an ultra-high vacuum U.V. monochromator (L. Marton, J. A. Simpson, J. A. Suddeth and L. B. Leder), apparatus for measuring gas evolution rates (M. L. Hill), distillation (E. Heunecke and W. Nerge), molecular dis- tillation of rice oil (K. Sakutai), steam sterilizers (T. W. G. Rowe, R. Kusay and E. Skelton) and reactions of silica glass with metals (H. Okamoto).

(5) Getter-ion pumps, cryopumps and diffusion pumps One of the most interesting contributions was that of

K. C. D. Hickman on “ high vacuum with the polyphenyl ethers “. These consist of chains of benzene radicals inter- bonded by oxygen and have exceptional stability and resis- tance to radiation damage. The isomer mixture, bis (phenoxy-phenoxy) phenyl ether has a vapour pressure of about 10-g torr at 25°C and yields vacua approaching 10-g torr in the unrefrigerated condensation pump. This fluid can also be applied to gaskets, valves, joints, and as a coating to large inside metal surfaces. Tests of this fluid on modified commercial vapour pumps were described ; new designs of pumps to take full advantage of these fluids were called for and their desirable features outlined.

J. Grobman reported on “ a catalytic method for cryo- pumping hydrogen “. In research on electric thermal rockets, hydrogen has been proposed as a propellant. Vacuum chambers for testing these rockets could be reduced in size and cost if condensers for hydrogen could be incorporated. Large-scale helium refrigeration is possible but very costly. An alternative is a catalytic surface reaction whereby hydro- gen is chemically converted into water which will condense readily on a liquid-nitrogen cooled surface. Catalyst studied was palladium-coated alumina pellet. Catalyst support geometries were considered.

New work on cryopumping was also reported in the

session on space simulators, and is included under that heading here.

The object of the research reported by N. Milleron and L. L. Levenson was to achieve the highest possible pumping speed for hydrogen and helium compatible with negligible oil contamination rate in the optimization of oil diffusion pump systems for ultra-high vacuum. A new diffusion pump was designed with Hickman’s new oils in mind. Quick-acting boiler heaters were designed to produce super- heated oil vapour whilst maintaining relatively low-tempera- ture liquid in the boiler, to supply each jet of a diffusion pump with an independent vapour source, to minimize oil cracking, to maximize efficiency of conversion of electrical energy to vapour energy at jets and to eliminate eruptive boiling. The oil boiler designed was so effective in producing a directed stream of vapour that the same basic design was used for the top jet. The heater used was nichrome ribbon 0.001 in. thick and 1 in. wide, corrugated transversely and coiled into spiral with glass tape separating turns.

A. R. Hamilton presented a paper on “ some experimental data on parameter variations with a triode ion pump “. This consists of two cathodes with a central “ egg-box ” shape anode ; the cathodes are at -3500 V and the anode at +3500 V with the collector surfaces and pump casing earthed; a magnetic field is provided perpendicular to the plane of the electrodes. The search for a new sputter cathode material or combination of materials which might be more efficient than titanium was unsuccessful. Other points brought out were (i) thorough baking is essential to remove water vapour; (ii) new gas releases molecules of previously pumped gas; (iii) foreign gases are produced in the pump as a result of outgassing and re-emission of previously pumped gas, (iv) as gas composition is unknown, especially during first hour of pumping on a specific gas, error in ionization gauge readings is considerable.

An interesting survey consequent on their “ one year operating experience with 70 getter-ion pumps ” by C. L. Gould and R. A. Dryden indicated the advantages of these pumps for producing “ dry ” vacua and that troubles were reduced as personnel became more trained and experienced. H. Adam described a getter-ion pump of the evaporation type having a speed of 1000 l./sec for nitrogen ; the chief new addition to this pump (which had been first described two years previously by Reich and Noller) is a semi-automatic supply unit controlled by a multi-point switch so that a favourable sequence of pre-heating and titanium evaporations is ensured. A new, but not radically new, design of titanium evapor-ion pump was also described by J. Sarrau. R. A. Clausing gave details of the interesting results from his

‘large-scale getter pumping experiments with vapour deposited titanium films : extremely high speeds were obtained in the sorption of hydrogen, deuterium, oxygen, nitrogen, carbon oxides and methane.

New results obtained for the barium getter-ion pump were reported by R. W. Cloud, H. Milde and S. F. Philp : high pumping speeds are attainable for hydrogen and oxygen but the speed for noble gases is low. N. Milleron gave preliminary data of his experiments with Penning type discharges supported on condensable metal vapours.

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(6) Mass spectrometry

A helium mass spectrometer with a two-stage sector field analyser of one inch radius for leak detection described by M. Doctoroff, S. S. Grossel and D. W. Oblas had improved sensitivity resulting from an axially arranged source of positive ions in which the electrons (produced thermionically) and the ions created in the gas were accelerated in the same direction, in contrast to the typical Nier source. G. Reich and H. G. NGller compared the omegatron and topatron as partial pressure gauges where the latter is a high frequency mass spectrometer of robust construction and allows also the recording of the total pressure. The main advantages of the topatron are its robustness, that no magnet is needed, its total pressure range can be extended to 10-J torr and it is not so much affected by impurities as the omegatron. On the other hand, the omegatron has the great advantage that practically all ions produced reach the ion collector corresponding to a transmission of one as compared with only about 10-3 for the topatron. The resolving power of the omegatron is mass-dependent with practically complete separation up to a mass of 45 ; the resolving power of the topatron is more or less independent of mass but is smaller ; also, the topatron exhibits the higher background current, but developments designed to reduce this by means of metallic screens around the ion source and collector were described.

A rapid scanning r.f. mass spectrometer developed for application in vacuum coating with a recording time of 20 set over the mass range from 2 to 350 a.m.u. and a resolving power of 100 was described by W. E. Dahlke, H. W. Ehlbeck, J. Ruf and H. J. Schuetze. The quadrupole mass spectro- meter was the subject of the paper by K. G. Guenther and W. Halenlein. W. K. Huber and E. A. Trendelenburg gave details of their mass spectrometer investigations in ultra-high vacuum systems using a cycloid type instrument. This instrument was chosen as it is more compact than the sector- field mass spectrometer and has double-focusing properties, so neither energy nor angle of ions need be sharply defined. Detection limit of several lo-12 torr is possible and partial pressure analysis in range of several lo-10 torr. Instrument could be operated in the presence of condensible materials by raising its temperature to 200°C : thus usable, for example, to analyse mass spectrum of DC704.

(7) Mechanical pumps and sorption pumps

A new mechanical pump was described by J. R. Boyd with which theoretical pressure ratios of 100 : 1 are possible at atmospheric pressure as compared with about 3 : 1 for Roots pumps. The power requirements of oil-sealed mechanical pumps, where the power peaks in the range 200 to 300 torr, were considered by E. A. Winzenburger. A bakeable molecular pump having a steel alloy rotor suspended and rotated magnetically inside a stainless steel vacuum chamber, capable of a speed of 10 1. per set and an ultimate of 4 x IO-10 torr was described by C. E. Williams and J. W. Beams.

The use of molecular sieve and other materials in sorption pumps was discussed under ultra-high vacuum (see that

section of this report) as well as in fore-pump application. F. T. Turner and M. Feinleit contributed a paper entitled “ performance criteria for sorption pumps “. Using molecular sieve 5A, adsorption isotherms were obtained for nitrogen at - 195°C and at 20°C and also for neon and helium at -195°C. Measurements were with a small cell designed to minimize temperature gradients and to ensure rapid attainment of temperature equilibrium. The cell was first heated to 300°C and pumped to 10-h torr. Gas was then admitted, the cell chilled and pressures recorded. At -195”C, nitrogen and oxygen are sorbed much more than neon or helium. Sorbent saturation for nitrogen and oxygen is apparent above about 10-z torr. The quantity sorbed at 20°C is several orders less than at - 195 “C. Commercial sorption pumps use v/w values between O+)Ol and 0.1 where v is the volume of the system in litres and w is the mass of the sorbent in grammes. Mass spectrometric analysis of the residual gases was reported. Also, multi- staged sorption pumping was studied ; the combination of a mechanical and pre-pumped sorption pump was more effective than two sorption pumps in cascade. Activated charcoals, with their higher thermal conductivity, give higher initial pumping rates than zeolites but the latter have greater capacity for gas, less “ dusting ” and need much shorter outgassing times.

(8) Space simulators

Great interest was evinced in the American work on space simulators, especially as little work has been done in this field in Europe. D. C. Spence dealt with the “ design and operating criteria for space environmental vacuum chambers”. A 120 ft3 chamber made of 304 stainless steel was pumped by 50,000 c.f.m. diffusion pump using DC704 with 485 c.f.m. roughing pump and 6 c.f.m. holding pump. A 32 in. isolation valve above a chevron baffle was used between the diffusion pump and the chamber port. A refrigeration system utilizing

Freon-13, Freon-22 and ethylene supplied temperatures of -200°F to cold wall located on interior surface of chamber. Best performance was achievement of IO-6 torr in 60 min. Second space simulator was also described for a micro- meteorite satellite demanding temperature cycling from +3OO”F for 32 min to - 100°F for 48 min with two 20 min transition periods.

D. L. Dowling, D. B. Herrick and W. E. Ross related their experience on the use of Roots pumps to evacuate large chambers, and envisaged need to evacuate chambers as large as 200 ft in diameter to 10-4 to 10-s torr. Speeds of 50,000 to 150,000 ftJ/min will be demanded. A. D. Le Vantine in a paper on “ space thermal environment simulation facilities ” argued that pressures of lo-5 torr were necessary to achieve thermal transfer simulation. Gas evolution from a space-craft is difficult to assess. Pumping systems most suitable are those using oil diffusion pumps and those with cryogenic pumps. With the latter (more expensive), size and location of pumping surfaces in chamber are important and they must be protected by shielding with additional liquid nitrogen cooled baffles from radiant heat. Details were given of suitable arrangements of pumping surfaces,

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comparison of use of liquid helium, hydrogen and nitrogen for cryopumping, mathematics of space heat-sink and simu- lation of this sink by use of flat black epoxy paints with emissivities of 0.94 in solar range and 0.92 in infra-red range ; also fin arrangements on inner wall of simulator. For solar simulation, a mercury-xenon or carbon arc is used. Two chambers built were described.

Two papers were presented by E. A. Richley, R. J. Cybulski and T. A. Keller. The first dealt with plant for sustaining pressures of 10-s to lo-6 torr for experiments on electric propulsion devices ; cryopumping with a liquid-nitrogen cooled surface condenses rocket exhaust, oil diffusion pumps handle permanent gases. An 80 ft x 25 ft electric pro- pulsion facility is under construction. The second paper considered that, in the large cryogenic condensers being used for electric propulsion research, the temperature of liquid nitrogen is not reached over the entire condenser surface so liquid helium is desirable as all gases except helium, hydrogen and neon then have negligible vapour pressures. A toroidal liquid helium cooled copper cryo- pump is recommended. For a 16 ft x 5 ft pilot chamber, 100 1. liquid-helium Dewar flask and double-walled stainless steel tube transfer line is used. Liquid-helium cooled copper toroid has mean diameter of 31.5 in. and surface area of 6.46 ftz. On introduction of liquid helium into prepumped (i.e. by oil diffusion pump set-up with liquid nitrogen cooled surfaces) chamber there is an almost immediate drop of the pressure by a half-decade into the 10-s torr range. The first 100 1. of liquid helium is consumed in a few minutes ; the second 100 1. in about 2 hours. For an 80 ft x 25 ft tank an estimated 1030 ft2 of liquid-helium cooled surface is necessary, assuming initial pressure of 10-6 torr and final one of 10-7 torr. Omegraton analysis shows largest relative change of mass numbers 7, 14, 28 and 32 on introduction of liquid helium.

(9) Sputtering From the points of view of presentation of material and

comprehensiveness of survey, one of the best sesssions was on sputtering. This commenced with an excellent review by G. K. Wehner including the following details : sputtering yields for silver for H+, D+ and He+ ions over energy range from 0 to 12 keV ; for copper with Xe+, Kr+, A+, Ne+ and N+z over the range from 0 to 70 keV ; Ta, Cu and Ag for ions in mass range from 10 to 80 a.m.u. at 45 keV ; sputtering yields and collection saturation values for Al, V, Cu, Nb, Ag, Ta and Au under 45 keV Krf-ion bombardment ; sputtering yields and collection saturation values for a (111) Cu surface under 45 keV Kr+ ion bombardment as a function of angle of incidence ; sputtering yields for Ag, Pd, Rh, Ru, MO, Nb and Zr under A+-ion bombardment in range from 0 to 600 eV ; for Ge bombardment by A+, Kr+, Ne+, Xe+ and Ag+ over range 0 to 60 eV ; and further data on effects of incident ion energy, angle of incidence, effects of crystalline and polycrystalline structure. Models of the sputtering process were also considered.

Wehner’s survey seemed to leave little to be said ; however, 0. Almen and G. Bruce reported their first-class experimental

and theoretical work with monoenergetic beams of noble gas ions. R. V. Stuart and N. Laegreid then gave details of experimental methods of measuring sputtering yields at medium and low energies, including results for polycrystalline and semiconductor targets under normally incident mercury and noble gas ion bombardment. D. E. Harrison discussed a “ theoretical model of the sputtering process ” and two other papers followed entitled respectively “ ion beam apparatus for the study of angular distribution of sputtered material ” (R. D. Knight) and “ the sputtering of Cu by Nfz and N+ as a function of pressure and temperature ” (T. W. Snouse and M. Bader).

(IO) Thin films This subject received greater attention-as determined by

the number of papers presented-than any other branch of vacuum science and technology. The interest in this field has been greatly stimulated in recent years by applications such as the preparation of thin dielectric film capacitors and thin film metallic resistors for microcircuitry, vacuum- deposited thin ferromagnetic films with a future in digital computer storage devices, films of the semiconductor materials ; studies of thin films by the techniques of electron diffraction, X-ray diffraction and electron microscopy are resulting in new data on structure, adhesion properties etc. The deposition of thin films in ultra-high vacuum is of con- siderable current interest ; new developments in plant include those for the controlled deposition of thin films whilst recent studies of vaporizing sources tend towards those in which electron bombardment heating is used.

Within this province, noteworthy papers were those by H. J. Degenhart and I. H. Pratt on capacitors formed by thin films of silicon monoxide in as many as seven stacked layers separated by aluminium electrodes, by K. H. Behrndt on thin ferromagnetic films, including data on the influences of using ultra-high vacua, A. Neugebauer’s study of the magnetization of nickel films of less than 10-h cm thickness, the examination of stresses in various dielectric films by H. Blackburn and D. S. Campbell and also in silicon monoxide films by J. Priest and H. L. Caswell. Electron-beam evaporation of germanium in ultra-high vacuum together with an extensive report of its electrical properties was undertaken by W. Reichelt and P. Mueller ; electron bom- bardment heating techniques for vaporizing sources were also reported by G. Siddall, B. A. Probyn and P. Duval.

T. K. Lakshmanan presented data on the resistivity, temperature coefficient of resistance, Hall effect and stability of evaporated nickel-chromium alloy films where the filament (40 cm long by 0.08 cm dia.) was directly heated to about 1000°C. M. Beckerman and R. E. Thun reported on the “ electrical and structural properties of dielectric-metal mixtures ” with the expectation that such mixtures would yield resistor properties superior to those of alloys ; the main conclusion of this work was that chromium-silicon monoxide films give a stable and reproducible film resistor material surpassing the performance of known alloy film resistors.

Conferences and Symposia 165

K. Taylor read a paper concerned with the technical details of the preparation by controlled vacuum deposition methods of a transistorised 4 kc/s phase-shift oscillator contained on a t in. x + in. glass slide. The resistors were of nichrome and the capacitors of silicon monoxide but the transistors were conventional. “ Evaporated integrated microcircuitry ” was also the subject of D. S. Moore’s paper.

M. S. P. Lucas, C. R. Vail, W. C. Stewart and H. A. Owen have developed a modification of the vacuum-arc process for the rapid vacuum deposition of tantalum, niobium, vanadium and iron. An arc source was also used for the production of unsupported thin films of carbon by B. J. Massey and H. L. LaNieve.

Instruments for controlling deposition rates in thin film production were described by W. Steckelmacher and J. English, who used a modulated-beam photometer for measure- ment of the film reflectance or transmittance and also by M. H. Perkins who used a modified Bayard-Alpert gauge as a sensing device. This gauge has electrodes which can be heated to about 1000°C to prevent metal deposition on them and is placed in the vacuum coating plant to measure both the gas pressure and the partial pressure due to ionized evaporant atoms.

Structure studies on thin films were reported by G. G. Via and R. E. Thun who used a combined evaporator and electron diffraction apparatus, F. Schossberger and F. Ticulka who employed X-ray diffraction, C. J. Calbick who made electron microscopy observations on glass and quartz substrates and J. K. Lee who has developed a radioactive tracer technique using sulphur 35 and silver 110 m for the analysis of vacuum deposited films of the phosphor, silver activated zinc sulphide.

(I I) Ultra-high vacuum Pressures below 10-s torr are now commonplace but

whereas only two or three years ago interest was centred chiefly around small glass vacuum systems, the more ambi- tious development of larger plant made of stainless steel and with metal gasketry is currently receiving most attention. The small-scale techniques originated by D. Alpert and his followers and utilizing ionization gauge pumping in small chambers isolated from the conventional pumps by a bakeable valve have given place to the use of conventional metal pumps with bakeable stainless steel liquid nitrogen traps.

The first session on ultra-high vacuum was concerned with limitations in the attainment of this desirable condition and limiting parameters. Here an interesting survey in which experimental and theoretical data spread through the litera- ture was collated, was given by A. Venema who was primarily concerned with the factors deciding the quasi-equilibrium formed between pumped and released gas, analysis of the residual gas, and a plea for more work of a fundamental nature on sorption, desorption, sticking coefficients and binding energies. A. J. Norton reviewed his own work on the permeation of gases through glasses, polymers and metals. F. Flecken and H. G. Nijller reported results on the outgassing of different metals in high vacuum using a specially

designed plant in which careful attention was paid to error elimination.

Within the subject of ultra-high vacuum-but it should probably have been included under a different title-F. W. Markley gave outgassing data on several epoxy resins and rubbers in connection with the design of the 12.5 BeV proton- synchrotron at the Argonne National Laboratory. Results were included comparing usefully the outgassing of stainless steel, cold rolled steel, aluminium, Teflon, Convaseal, neo- prene and various solid and liquid epoxy resins, also AF-31 nitrite phenolic adhesive film used to bond stainless steel members.

L. L. Levenson and N. Milleron described their experiments on the absorption capacities of artificial zeolite and activated alumina as room temperature trap materials in a 10 in. diameter trap above a 7 in. oil diffusion pump.

As regards ultra-high vacuum plants, these are included here under “ vacuum systems ” (Section 13).

(12) Vacuum metallurgy Metals and alloys unknown outside the research laboratory

a decade ago have now been made available in commercial quantities as a result of developments in the important technology of vacuum metallurgy and, moreover, familiar metals like copper and steels and some alloys are now pro- ducible in a state of purity giving valuable electrical and mechanical properties.

The techniques of melting casting, brazing and sintering metals in vacuum which make use of resistance, induction and arc heating have now become established, though many problems remain. The chief innovation is the use of electron- beam heating. Five and more years ago this was almost the preserve of the technician who wished to de-gas the electrodes of a valve or ionization gauge by electron bombard- ment ; now we have reports of work on electron beam melting furnaces in the megawatt range. Such is the rate of progress. Indeed, of a total of 20 invited papers on vacuum metallurgy, nearly one-half were concerned with electron- beam heating.

J. W. Meier read a paper entitled “ high power density electron beam welding of several materials ” in which the source was rated at 3 kW with normal accelerating potentials between 60 and 150 kW and a beam diameter at the work piece of about 0.01 in., welding speeds up to 120 in./min and a vacuum of 5 x 10-S torr. Experience was reported on welding of some stainless steels, columbium, titanium, aluminium and molybdenum. C. B. Sibley and M. L. Torti described their “ Experience with an electron beam melting furnace ” in which two basic methods used were (i) a ring cathode in which the ring filament electron emitter is directed to a consumable electrode and/or to the ingot top located in a retractable stool type cold mould ; (ii) a remote gun with the source of electrons in a separate vacuum chamber several feet from the molten pool and partially isolated by apertures from the melt chamber.

H. R. Smith, C. d’A Hunt and C. W. Hank gave details of the first megawatt class electron beam furnace to be built.

N. Beecher and J. L. Ham described the development of a

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vacuum arc skull furnace for the recovery of titanium scrap in which high pressure helium gas cooling is used instead of water-cooling because of the serious danger of explosion

with the latter. A skull furnace was also described by C. Hayashi, S. Kashu

and T. Tsuchiya for casting uranium and uranium alloy fuel rods for nuclear reactors. Eight 150 x 12 mm rods could be cast at each pouring in a vacuum chamber pumped at 4500 l./sec and with an arc supply of 40 V, 8000 A. Centri- fugal casting was incorporated with a bottom-pouring crucible.

The composition and pressures of gases in a vacuum arc furnace were studied by E. W. Johnson, R. Itoh, R. L. Readal and M. L. Hill ; two vacuum gauges, an Alphatron and thermocouple type were used for pressure measurement whilst gas analysis was by means of a mass spectrometer. R. Lesser described “ a new vacuum fusion analyser for high purity metals ” ; an outline of theoretical ideas lead to following desirable features of the analyser (with special consideration for Ti, Zr, V, Nb, Ta, MO and W) : high temperature of crucible ; high vacuum over metal ; quick transmission of the crucible carbon to the sample ; gas quantity pumped away from apparatus without a new sample being in hot crucible must be in reasonable relation to the gas quantity released from the sample to be analysed. Achieved by h.f. induction heating and the use of a graphite crucible standing free without thermal graphite insulation. Gas analysis was by a gas chromatograph with beta-ray ionization detector.

E. W. Hoyt gave a lecture on the consolidation into useful shapes by vacuum hot pressing within a shaped die of refrac- tory materials such as borides, carbides, nitrides, oxides and silicides prepared as powders. Control of grain size, density and final shape were possible. Pressing chamber consisted of inner Vycor tube and outer acrylic tube. Pressing is done in vertical metal or graphite dies, inductively heated from outside the chamber, and with double-acting upper and lower punches.

Vacuum brazing was the subject of the paper by M. M. Schwartz. For brazing columbium and its alloys and also Fansteel No. 82, titanium was used as a filler at a brazing temperature of 330 “F. A resistance-heated vacuum furnace operable up to 4500 “F at pressures of 10-4 Torr was described for this purpose.

0. Korn and J. Moll presented the design of a single-crystal pulling vacuum apparatus intended for the manufacture of semiconducting metals for transistors.

(13) Vacuum systems

In one session the topics were systems, leaks and gas flow ; in the other, the emphasis was on ultra-high vacuum systems.

G. L. Munday and G. Zinsmeister related experience with the pumping equipment for the CERN 25 GeV proton synchrotron. Seventy individual pump stations each based on an oil diffusion pump are used for the evacuation of the vacuum chamber which consists of stainless steel tube of elliptical cross-section with axes 140 x 70 mm interspersed with 25 cylindrical tanks (900 x 350 mm) for beam sensing

devices and four larger tanks housing inflector electrodes. The proton orbit diameter is 200m. The total weekly maintenance time on the 70 pump stations from beginning of 1961 has been an average of 2 man-days with the machine running continuously for 88 hours per week. Contaminated vacuum gauges were a source of difficulty ; Penning gauges have to be cleaned after continuous operation for 6 to 8 months.

H. Adam described a commercial ultra-high vacuum pump set based on oil or mercury diffusion pumps. Oil diffusion pumps used DC 704 with heater power increased above normal to give higher pumping speed for hydrogen, often a predominant gas at the ultimate pressure. The 500 I./set diffusion pump has water-cooled baffle surmounted by baffle cooled either by freon or liquid nitrogen. Above this is a stainless steel, bellows-sealed, hand-operated isolation valve with Viton sealing for the seat, bakeable to 200°C. Viton gaskets are used for all flanges on the high vacuum side. The protective gear was described in outline. Electrical heating strips are used for bake-out. Total bake-out times of 90 hr were usual ; ultimates of 2.5 x IO-9 Torr were attained.

R. E. Honig also described a “ portable, high-speed, ultra-high vacuum system ” alleged to yield analysis of residual gases at lo-11 Torr level. To avoid oil contamination the system employs gas sorption on zeolite at liquid nitrogen temperature and Penning getter-ion pumps of sputter type instead of diffusion pumps. Demountable flanges employ gold wires or copper washers. To attain lo-10 Torr or lower, a liquid helium trap of speed 700 l./sec for nitrogen is used. To attain 10~10 Torr, bake-out times of the order of 50 hr were necessary where volume of stainless steel chamber was 51.

0. Winkler and E. A. Trendelenburg dealt with “ the problem of evaporation in ultra-high vacuum “. L. Holland and S. Bateman described a stainless steel system for vacuum deposition work in a 19 in. diameter chamber ; metal or Viton A gaskets are used. A pressure of 10-s Torr is attainable quickly without high temperature bake-out. B. D. Power, N. T. M. Dennis and D. J. Crawley considered those factors to which a mercury diffusion pump is sensitive but which are of less importance with oil diffusion pumps.

(14) Other applications

Apart from the topics mentioned in the above survey, other applications of vacuum technology received little attention. J. M. Dunoyer lectured on new work in lyo- philization, J. D. Mellor and also U. Hackenberg and G. W. Oetjen reported their recent investigations on aspects of freeze-drying and M. Beyer with W. Reulecke presented a paper on the drying and impregnation of high voltage cables.

An excellent exhibition on display throughout the con- ference of commercial vacuum equipment was a most successful feature ; 45 manufacturers from America and Europe took part. The considerable number of new American firms marketing vacuum apparatus were most noteworthy ; the majority of these firms-unlike the few