US2925504A - High-vacuum pumps for high-voltage acceleration tubes - Google Patents

Description

Feb. 16, 1960 R, w, CLOUD ETAL 2,925,504.

HIGH-VACUUM PUMPS FOR HIGH-VOLTAGE ACCELERATION TUBES Filed June l7. 1957 2 Sheets-Sheet 1 W T y T 21 I! k 1 I II-J 6 :H W/T- 2 Ill: l I! III! I lllllllll i 1 II Fig. 3

Feb. 16, 1960 R. w. CLOUD ET AL 2,925,504

HIGH-VACUUM PUMPS FOR HIGH-VOLTAGE ACCELERATION TUBES Filed June 17. 1957 z Sheets-Sheet 2 HIGH-VACUUM PUMPS FOR HIGH-VOLTAGE ACCELERATION TUBES Robert W. Cloud, Lexington, and John G. Trump, Winchester, Mass, assignors to High-Voltage Engineering Corporation, Burlington, Mass., a corporation of Massachusetts Application June 17, 1957, Serial No. 666,150 ls Claims. (Cl. 313--7) Thisinvention relates to anew type of high vacuum pump which employs an active metal getter such as barium in combination with an activating agent such as ionization. In particular,.the invention comprehends a getter-ion pump which forms an integral part of a particle acceleration tube. In one embodiment of the invention, heat for the evaporation of a barium getter is provided from a heat source outsidethe accelerationtube tobe evacuated, In. another embodiment-of the invention, the amounts. of bariumavailable for evaporation are in excess ofT'IO grams High-vacuum pumps constructed according f totheinvention are capable of maintaininga high vacuum in sealed-off accelerationtubes of the megavolt class. These tubes, in a sense, represent a difi'icult vacuum problem because'they contain not only the gas ingredients of glass and metal, b'ut'also the'vap'orsof the hydrocarbons used to form strong glass-'to-metal joints betW'eeh'the multiple glass andmetal materials.

Activemetal getters such as evaporatedb'arium do not absorb many organic materials so that active metals are not effective. in'maintaining a high vacuum in systems which" are sealed with or contain organic materials. However, the combination 'ofsuch a getter with an activating agent, such as ionization or heat, is effective in removing fronran enclosed region, not'only the common gettera'ble gases, such as hydrogen, nitrogen, oxygen,.and waterfv'apor, but also the previously non-getterable gases including the noble gases (helium, argon, krypton, etc'.) and ceitain'molecules suchas hydrocarbon vapors. In the" case of the ungett erable compoundmolecules, the mechanism probablyinvolves first their dissociation into the simple getterable ingredients. It,may also involve,

ast'itd'oes in the case of the noble gases, the activation of the molecules without dissociation. Suchactivation, even though to a sub-ionization level, would. tend to render these-molecules"capable'of attachment to an active metal getter.

Itha's long been known that many substancessuch as barium; potassium, thorium, magnesium, and zirconium are capable of absorbingconsiderable quantities of ordi nary'gasmolecules. The useofbariumandother' getters is a common device in the manufacture of radio tubes and other"permanently sealed-off systems. These gen-- of getter material,

erally'employ very small quantities generally less than 10' or 100 milligrams.

It haslike'wise'beenunderstood for many years that thepr'esen'ceof ionization in an evacuated enclosure re sulfsiin the absorption of residual gas, includingfeven the noble gases'which arenot absorbed bya'n' active metal getter alone. Such absorption ofresidual gas was noted in the old gas-filled X-ray tubesemploying cold cathodes.

These X-raytubes derived their electron current through positive ion bombardment of the cathode, the positive ions beingformed by collisions of theelectrons in the beam with the residual gas molecules. In such tubes ssaryto provide a source of gas toprevent the, gradual cleanup ofthe tube into a non operative high vacuum condition. Such cleanup'in the presenceof United States Patent "ice ionization was also noted by many investigators using closed systems containing ionization gages as a means of measuring high vacuum pressure.

- It is not believed, however, that the prior art contains descriptions of getter ion pumping devices particularly. adapted to the evacuation of sealed-oif'systems wherein the rate of evolution of gas exceeds 10" millimeter-liters per second, whether, due to the large size of the system or to gasket material or to organic cements or to other causes. Such systems include, but are not limited to, multiple-electrode high-voltage acceleration tubes having;

organic seals, which cannot be heated to conventional outgassing temperatures, and linear accelerator tubes, which mayfemploy a type of metal gaskctseal having a melting point too low' to permit conventional outgassing techniques. Thereforewhile in the followingdetailed description the invention is described with particular reference to a multiple-electrode high-voltage acceleration" tube, the invention is not limited thereto, but includes other types of sealed-off systems wherein the rate of evolution of gas may exceed 10- millimeter-liters per second. Onesuch system is the so-called tandem particle accelerator, wherein negative ions are accelerated towards a high-voltage terminal and then, after passing through a gaseous-stripper? region, are accelerated away from the terminal; the stripper region continuously evolves gas which must be pumped.

Vacuum devices. may be divided into twocategories: (1)those which are permanently sealed-01f for the life of the device, and-(2) those which are continuously pumped to maintain the highvacuum. The use of getter material has generally been confined to permanently sealed- 0E devices which have been very carefully outgassedand. pumped down to a high vacuum prior to seal-off, and the function of the getter has been confined to absorbing the residual gases which remain after such outgassing, pumping, and seal-oh. These preliminary steps of outgassing and pumping have been thought essential in-connection with sealed-01f devices, with the getter playiuga subsidiary role. Both processes must be carried; outlwith great. care, and are particularly difi'icult in the case of-systems containing electrodes at high potential, more specifically, in excess of 100 kilovolts. Such a high voltage device is hard to construct. They must maintain high vacuum over'extended periods and greatcare is required in their manufacture. Metal for the electrodes often, must be oxygen free and the electrodes often are outgassedprior to installation in the tube. assembly. The assembly is subjected, to vigorous heat treatment andoperation' prior to seal-oil using high speed diffusion.

. pumpsand-traps to collect the emitted gases. Furthermore; in. order to free such substances of organic vapors, painstaking developments have been made of glass and metal which are mechanically compatible, which can-:be joined and stillwithstand the high temperature cycling required 'in the former ou'tgassing procedure. The introductionof any source of vapor is scrupulously-avoided, and even so the life of suchtubes is often limited by gassiness, Which'may result from-inadequate outgassing: procedure.

Devices which cannot be outgassed for one reason or another have alway'sbeen continuously pumped, presumably because the ideaof a continuous evolution of gas Within the container to be evacuated suggests a continuous pumping, system. Accordingly, in the initial development of high-voltage acceleration tubes, wherein the highenergy charged particles release gases by bombardment of the-solid surfaces Within the tube, continuous pumping. systems were always used. Inthe sealed-01f. acceleration tubes'whichwerelaterdeveloped, it was found necessary. tojtakeeXtremely elaborate steps to outgas and pump. down" the acceleration tubes prior to seal-'ofi. Such vigorous treatment reduces the evolution of gas within these acceleration tubes down to or below that prevalent in small radio tubes and accordingly either no getter is used, or only a small quantitysuch as that used in conventional radio tubes.

The idea of using an adequate amount of getter material coupled with an independent source of excitation and ionization forthe permanent evacuation of unoutgassed high-voltage acceleration tubes has not been considered prior to this invention. The reasons for this include, in addition to those hereinbefore mentioned, the undesirable effects resulting when the getter material is exposed either to high energy charged particles or to high voltage gradients. Moreover, if this idea had been proposed earlier, it would have been rejected as unworkable. It has been well known in the art that if the cases normally entrained in an unoutgassing cubic centimeter of copper are released, the quantity of gas thus released will amount to hundreds of cubic centimeters at atmospheric pressure. In accordance with the invention, a sufficiently large quantity of getter material is used to absorb gases during the life of the evacuated device, and the getter material is shielded from the high energy charged particles and from intense electric fields. Moreover, the getter material is not permitted to become overheated, whereby the tendency of the getter to migrate is minimized. In a preferred embodiment of the invention, the getter material is vaporized so as to form an extended surface by a heat source external to the chamber to be evacuated.

r In accordance with the invention, it is possible to combine materials without excessive regard for their outgassing properties. Glass and materials may be chosen for their electrical characteristics and may be joined with an organic seal or low-melting-point solder or metal gasket. The tube need not be subjected to any temperature cycling or operated prior to seal-off. Before the pump of the invention is used, the tube need be evacuated only with a mechanical pump to a pressure of 10- millimeters of mercury or thereabouts. Even this use of a mechanical forepump may be eliminated, since the pump of the invention is capable of operating even at atmospheric pressure. Moreover, the invention is applicable to tubes characterized by very high gradient requirements such as acceleration tubes, operating at a total voltage of the order of several million volts and with electrical gradients lengthwise of 5.00 kilovolts per foot or more. The invention may best be understood from the detailed description thereof having reference to the accompanying drawing in which:

Fig. ,1 is a side view, partly in longitudinal central section and partly broken away, of a multiple-electrode high-voltage acceleration tube incorporating a high vacuum pump in accordance with the invention;

. Fig. 2 is a perspective view of a longitudinal central section of a portion of the acceleration tube of Fig. 1; and

Fig. 3 is a side view similar to that of Fig. 1 but showing the evaporation device by means of which the getter is evaporated in accordance with the invention.

Referring to the drawings, there is shown in Figs. 1-3 an acceleration tube of the general type disclosed in United States Patent No. 2,460,201 to Trump and Cloud and comprising a multiplicity of alternating insulating rings 1 and apertured electrode disks 2 terminating at one end in a cathode 3 and at the other end in an extended anode 4. V Such an acceleration tube may be adapted to accelerate various types of charged particles, such as positive ions, negative ions or electrons, and merely by way of example the acceleration tube shown in Fig. 1,in an X-ray tube wherein electrons emitted at the cathode 3 are accelerated onto a gold target 5 for the production of X-rays. The tube is assembled in a vacuum-tight manner and is then evacuated through a-copper tube 6 in the cathode 3 of the acceleration tube by a mechanical pump a issues (not shown). For example, the tube may be pumped down in this manner for 3 days. The tube is then sealed ofi' by pinching the copper tube 6 and 7 so as to form a cold-welded seal. The tube is then evacuated to a high vacuum by means of the getter ion pump which has been made integral with the acceleration tube in accordance with the invention and which will now be described. in detail.

The getter ion pump comprises in combination an ionization gauge 8 and a getter chamber 9, both of which form part of the extended anode 4 and which are therefore at ground potential except as noted hereinafter; The ionization gauge 8 operates on well-known principles whereby electrons are accelerated towards an electrode, but are hindered from reaching said electrode by the small dimensions of the electrode and by the deflecting action of a magnetic field. In the drawing, the electrode toward which the electrons are accelerated comprises a filamentary ring 10 which is supported by four'short rods 11 of .030-inch stainless steel wire attached. to an apertured electrode disk 12. This filamentary ring 10 is supported in the magnetic field which exists between two ring magnets 13. Some mild steel pieces 14 provide a path for the return flux and shield the charged particles being accelerated by the acceleration tube from the mag netic field. The magnets 13 are at ground potential and a positive potential of 2800 volts is applied to the fila mentary ring 10 by a suitable power supply 15. Eleo-' trons between the ring magnets 13, whether produced by field emission from the surface of the magnets 13 or by. ionization due to cosmic rays or otherwise, 'are accelerated towards the filamentary ring 10; but the magnetic field, which may be of the order of 10 gausses, causing the electrons to travel in helical paths so that they do not strike the filamentary ring 10 until after they have traveled great distances. As a result, the device acts. as an ionization gauge. That is to say, the device ionizes the gas in its vicinity, and the resultant current between the magnets 13 and the filamentary ring 10 is a measure of the'gas pressure within the acceleration tube and may be read on the meter 16.

Below the ionization gauge 8 is provided a grounded getter chamber 9. Pieces of barium 17 are placedvin the bottom of the chamber 9, and the top is enclosed by an aluminum plate 18 which is provided with a series of apertures 19 and to which is attached one or more baflles 20. Each bafiie 20 may comprise, for example, a SO-mesh stainless steel screen which is spot welded in tubular. form. The upper wall 21 of the chamber 9 is surrounded by a thick tube 22 of mild steel which is permanently welded to the chamber 9. The lower wall 23 vof the. chamber 9 is thinner than the upper wall 21'and the mild steel ring 24 which surrounds it is removed during evaporation of the getter. The purpose of the mild steel members 22 and 24 is to shield the charged particles being accelerated by the acceleration tube from external magnetic fields such as those produced by the drive motor of an electrostatic accelerator in which the tube might. be placed. a

, Referring now to Fig. 3, during evaporation of the barium getter, the lower wall 23 of the chamber9 is surrounded by an oven 25, while the upper wall 21 of the chamber 9 is surrounded by a water-cooled jacket 26. The barium 17 is heated to evaporation temperatures, such as 820 centig rade, in the oven and the barium vapor is condensed principally on the inner surface of the upper wall 21 of the chamber 9 but also on the baflles 20. Surfaces on which the barium getter is deposited are shielded from later being struck by the beam of charged particles by providing cylindrical tubular members 27 through which said beam travels. It will now be seen that the apertures 19 in the top plate 18 are necessary in order that any gas within the acceleration tube may traveltherethrough and be trapped by the getter;

As stated hereinbefore, an importantfeatu reof;

invention is the provision of heat for the evaporation of the getter from a heat source outside i the acceleration tube tobe evacuated. In accordance with this; aspect of the invention, it is only necessary that-the actual heat source be external to the evacuated region and not exposed thereto. Thus, for example, the external heat source of the invention may comprise a hollow metal tube which is introduced into the evacuated .region through a vacuum-tightseal and within which a heating elements provided. The heat source is thus external to the evacuated region and-is separated therefrom by the hollow metal tube, which is grounded and vacuum tight.

In our work wehave employed an active metal getter, preferablybarium, in combination with ionization to pump on the complex gaseousingredientsiof high voltage acceleration tubes. We have preferentially used barium as the active metal getter and have included relatively large amounts of barium in these pumps in order to give them extended pumping times ranging into years. Even a small pump of this type for example is commonly given between and 100 grams of barium, and pumps are now being designed which contemplate an initial loading of pounds of barium.

In one embodiment of the invention the pump is of the intermittent type. The barium is evaporated onto a cold surface which subsequently absorbs over an extended period of time the gas molecules which come in contact with it. On a typical acceleration tube of the megavolt class, a single evaporation, for example, may subsequently maintain a high voltage-insulating vacuum within the tube for a period rangingfrom 1 month to 1 year. This pump, however, is capable of repeated evaporation cycles whenever further fresh gettering surfaces become necessary.

The invention also comprehends a container loaded with getter-coated surfaces and provided with a source of ionization. Such containers could be bolted on any system requiring the maintenance of a high vacuum and then connected to it by opening a valve. This would in effect be a getter-ionization pump in which the evaporation has been accomplished in advance.

Having thus described the method of the invention, together with a preferred embodiment of apparatus for carrying out the method, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense and not for purposes of limitation, the scope of the invention being set forth in the following claims.

We claim:

1. In combination with a sealed-off system containing electrodes at high potential, means for exciting and ionizing the residual gas within said sealed-off system, a metallic getter within said sealed-off system, and means for shielding said excitation-ionization means and said getter from the electric fields associated with said high potential.

2. In combination with a sealed-off system containing electrodes at high potential, means for exciting and ionizing the residual gas within said seal-oil system, a metallic getter within said sealed-off system, and a heat source external to said sealed-off system and adapted to evaporate said getter.

3. In combination with a sealed-off system containing electrodes at high potential, means for exciting and ionizing the residual gas within said sealed-off system, an amount of barium within said sealed-elf system at least of the order of 10 grams, and a heat source external to said sealed-off system and adapted to evaporate said barium periodically.

4. In combination with apparatus for the acceleration of charged particles including a tube traversed by a stream of charged particles, the construction of said tube being such that the rate of evolution of gas therein may exceed 10 millimeter-liters-persecond, a getter-ionization pump integral with-said--tube.

, 5. In combination with apparatus for the acceleration of 'charged particles including a tube traversed by a streamof chargedparticles, the construction of said tube within said tube having an extensive surface area, and

means for shielding said ionization means and said getter from the beam of charged particles accelerated by said tube during operation thereof.

7. Apparatus in accordance with claim 6 wherein said getter comprises barium.

8. Apparatus in accordance with claim 7 wherein said getter comprises barium in an amount at least of the order of 10 grams.

9. In combination with a multiple-electrode acceleration tube comprising a series of alternating insulating rings and apertured electrodes cemented by a synthetic resin and closed off at one end by an end piece and at the other end by a tube extension, a getter-ion pump integral with said acceleration tube.

10. In combination with a multiple-electrode acceleration tube comprising a series of alternating insulating rings and apertured electrodes closed olf at one end by an end piece and at the other end by a grounded tube extension, a getter-ion pump integral within said grounded tube extension.

11. A multiple-electrode acceleration tube comprising in combination a series of alternating insulating rings and apertured electrodes cemented by a synthetic resin and closed ofi at one end by a cathode and at the other end by an anode extension, a pair of axially aligned ring magnets supported within said anode extension, a filamentary ring supported between said ring magnets in axial alignment therewith, means for imparting a positive voltage to said filamentary ring with respect to said ring magnets, a getter within said anode extension having an extensive surface area, and at least one tubular shield supported axially within said anode extension so as to shield said ionization means and said getter from the beam of electrons accelerated by said tube during operation thereof.

12. A method of assembling a high-vacuum sealed-off system without outgassing the same, which method comprises the following steps: assembling a system and hermetically sealing the same with the exception of an aperture for pumping, said system having therein an amount of metallic getter at least of the order of 10 grams. and ionization means, said assembly being performed Without outgassing so that the rate of evolution of gas within said system may exceed 10 millimeter-fliers per second, evacuating said system to fore-vacuum pressure through said aperture, hermetically sealing said aperture so as to seal off said system, evaporating said metallic getter by means of a heat source external to said sealedotf system, and cooling an extensive surface area within said sealed-off system so that the evaporated getter condenses thereon.

13. A method of assembling an acceleration tube which method comprises the following steps: cementing together a multiplicity of alternating insulating rings and apertured electrodes; sealing to the end thereof an end-piece having a small tube therethrough and a tube extension, respectively, said tube extension having therein a large amount of barium and ionization means; evacuating said accelera- '7 tion tube to fore-vacuum pressure through said small tube in said end-piece; pinchingofi said small tube so as to seal ofli said acceleration tube; evaporating said barium by means of a heat source external to said acceleration tube; and cooling an extensive surface area within said tube extension so that the evaporated barium condenses thereon.

14. A method in accordance with claim 13 wherein the amount of barium evaporated is at least of the order of 10 grams.

15. In combination with a sealed-off system for the acceleration of charged particles whose envelope includes materials which do not permit the application of temperatures substantially above 200 C. for outgassing purposes, a getter-ionization pump integral with'said systern.

' References Cited in the file of this patent UNITED STATES PATENTS 2,332,428 Atlee et a1. Oct. 19, 1943 2,540,647 Bienfait Feb. 6, 1951 2,636,664 Hertzler Apr. 28, 1953 2,640,948 Burrill June 2, 1953 2,656,489 Ford Oct. 20, 1953 2,796,555 Connor June 18, 1957 Yoder June 10, 1958 OTHER REFERENCES Evapor-Ion Pump, by H. G. Herd, R. H. Davis, A. S. Divatis and D. Saxon, Physical Review, 2nd series,

15 vol. 89, No. 4, page 897, February 13, 1953.

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