US2841726A - Apparatus for producing thermallycool charged particles - Google Patents

Description

July 1, 1958 R. c. KNEcHTLl APPARATUS FOR PRODUCING THERMALLY-COOL CHARGED PARTICLES 2 Sheets-Sheet 1 Filed June 25 1957 NN mw vxwsm A mm A Nw we INVENTOR. RONALD C. KNEcHTLl TURA/EY July 1, 1958 R. c. KNECHTLI 2,841,725

APPARATUS FOR PRoDucINc THERMALLY-coor. CHARGED PARTICLES Filed June 25, 1957 ffy.;

2 Sheets-Sheet 2 INVENTOR. l IONALD C. K NECHTLI Ronald C. KnechtlLCranbnry, N. J., assigmlyihyV mesne assignments, to the United States ofAmerica as represented `by the Secretary of the Army i.

lApplication June 25;, 1957, SerialfNo.ft67p,9l43

9 claims. (ci. ais- 4)` r;

vThe present `invention relatesA to improved apparatus for producing low temperature charged particles. in high vacuum. i g

`When electrons are emitted from a` thermionic'emitter or cathode they leave, not at a uniform velocity, `but with a thermionie velocity spread. Since suchla velocity spread is. typical of aheated gas, the'electron stream may Ibe said to possess a temperature.. Thist'emperature is substantially equal to the temperature of the emitter. The factors determining the noise ligure (a measure of the noise introduced by the device) of a microwave beam amplifier device, such as a traveling-wave tube, are mainly the current and velocity uctuations in the beam. The current uctuations are independent of, the` temperature. However, the mean-.square value ofthe velocity fluctuations is proportional to the emitter temperature. Thus, if one could find some way to lreduce the temperature of the emitter, or ofthe electron beam itself, a new way to reduce the noise iigure of microwave amplifierswould be available. Reducing the temperature of the emitter below a certain point is not possible, becauseof the way in which thermionic emitters operate, and because of the non-existence of emissive materials with very low` work function. One method of reducing the; thermal velocity spread of the electrons in a beam is that of velocityselection, e. g. utilizing magnetic deflectionrflnvelocity selection, only electrons within a narrow part of the Maxwellian" velocity distribution are' used.` Also, space chargeelects defeat attempts to realize a velocity selectoi" capable of yielding appreciable currents or current densities. Typicalorders of magnitudeof room tempera- Y ture electron currents obtainable with velocity selectors are of the order of 10-10 ampere.

A eopending application of Walter R. Beam and myself (RCA 44,239), led concurrently herewith, discloses and claims an apparatus for reducing the thermal velocity spread of a high temperature electron stream in which the electrons are cooled :by means of thermalizing collisions with the molecules of a cool gas, such as ammonia or hydrogen, inthe presence of positive ions to neutralize the space charge of the electrons., The electrons drift from the coolingregion, which may have a pressure between l05 and 10T1 mm. of Hg, `through a differential pumping system, inwhich the gas pressure is reduced along the path of the stream to a highrvacuum, of the order of l0s mm. of Hg, or less, at the output end of the apparatus where the electron beamis utilized. An axial magnetic eld is provided along the path of the stream to confine the electrons and ions to the stream. Preferably, the electrons and ions are produced at the same end of the apparatus by a plasma emitter in the form of a heated tungsten plate which emits electrons thermionically and produces positive ions by contact ionization of atoms of an alkaline metal vapor, such as cesium, potassium or rubidium.` The electrons are drawn through theA cooling and pumping regions vby a weak electric lield, While the positive ions are carried by the ilow of the cool- 2,841,726 ijatented July 1, 1958 ICC ing gas at a lower velocity. Where a low temperature electron beam is desired, rather than a plasma stream, means are provided at the high vacuum end of the apparatus for separating the ions and electrons and for uniformly accelerating the electrons as a beam along a desired path.

The differential pumping system shown in Fig. l of said copending application comprises a series of constrict-` ingltubular members aligned along the path of the plasma stream beyond the cooling region and spaced apart axially to form pumping chambers each of which is connected to a high vacuum pump. In this pumping system, the gas ow velocity is substantially reduced, in the axial direction, across each pumping chamber. Since the positive ions are carried along the path of the stream by the flow of the gas, the variation in gas ilow velocity at each pumping chamber results in a variation of ion How velocity. However, in order to minimize the ion transit time through the differential pumping system and the consequent recombination losses, the ion ilow velocity should be kept uniformly high throughout the system. In order to assist in maintaining a uniform flow of ions through the apparatus, a steady supply of alkaline metal vapor atoms to the plasma emitter is desirable. Previously known evaporators available for supplying the cesium, or

`other alkaline metal Vapor, for the Contact ionization process are single stage evaporators characterized `by erratic or unsteady evaporation of the metal into vapor form.

An object of the present invention is to provide an irnproved differential pumping system.

Another .object of the invention is to provide a distributed pumping system capable of producing substantially constant gas ilow velocity therethrough.

A further object is to provide an improved apparatus for producing a low temperature electron beam in high vacuum.

Still another object is to provide a metal evaporator i capable of producing a steady stream of metal vapor.

The improved dilerential pumping system according to the present invention comprises elongated perforated tu- -bular means and means for evacuating` the space sur-` rounding the tubular means. The perforated tubular means is preferably made up of a series of aligned perfo- `is a two-stage evaporator wherein the bulk of the metal is located in a container heated to a high enough temper` ature to produce evaporation at a higher rate than ulti mately desired, and the metal vapo-r passes through a cooler control region the temperature of which can be controlled to smoothly control the rate of ilo-w of the metal vapor therethrough.

In the annexed drawing:

Fig. l is an axial sectional View, partly schematic, of an improved apparatus for producing a low temperature electron beam in high vacuum according to the present invention;

Fig. `2 is an axial sectional View of a two-stage evaporator according to the invention;

Fig. 3 is a top plan view of a structural embodiment of the apparatus schematically shown in Fig. l

3. Fig. 4`is a transverse sectional view taken on the line 4-4 of Fig. 3; and

Fig. 5 is an exploded perspective View of the tubular enclosure and pumping chamber unit forming parts of the apparatus of Fig. 3.

. Fig. 1 shows an electron cooling apparatus according to the invention comprising an elongated tubular enclosure or housing 1, preferably a glass tube of precision inner diameter, containing, from left to right, a gas inlet and plasma emitter assembly 3, a cooling chamber 5, a distributed gas pumping system 7, an electron extractor structure 9, and a high vacuum output chamber 11 adapted to be vacuum sealed to a device for utilizing the low temperature electron beam produced by the apparatus.

The assembly 3 comprises an evacuated inner tubular member 13 closed at the right hand end by a tungsten plate 15 which is adapted to 'be heated by electron bornbardment by a cathode 17 mounted within the member 13. The inner member 13 is surrounded by two tubular members 19 and 21, having converging conical extensions 19a and 21a, forming two annular passages 23 and 25 for conducting gases to the cooling chamber 5. The outer passage is connected to a source of cooling gas, such as ammonia or hydrogen, for example. An evaporator 27, shown in detail in Fig. 2, is mounted at one side of the enclosure 1 for producing and directing a stream of atoms of lan alkaline metal vapor, such as cesium, potassium or rubidium, to the front surface of the hot tungsten plate 15 for producing positive ions by contact ionization.

The cooling chamber S is formed by the forward portion of the outer extension 21a as shown in Fig. 1. However, it will be understood that some cooling will occur in the subsequent pumping system 7.

' The initially hot electrons from the plate 15 are drawn through the cooling chamber 5 and pumping system 7 at a'drift velocity substantially less than their initial mean thermal velocity, by means of a weak electric field extending from an accelerating electrode in the extractor structure 9. This drift velocity is typically of the order of 105 cm./sec. The positive ions will usually have suicient initial velocities due to their temperature to reach the cooling chamber 5. Preferably, the cooling gas is one that can effectively transport the ions through the cooling and pumping regions and also one that does not produce dissociation products at the hot tungsten plate that are harmful, in which case only the cooling gas need be used. However, the structure shown in Fig. l may be used with an inactive second gas, such as nitrogen or argon, for example, which is supplied to the inner passage 23 and ows past the plasma emitter plate 15, carrying the ions through the cooling and pumping regions and also preventing any appreciate amount of the cooling gas from diffusing back to the plate 15.

The pumping system 7 of the present invention comprises at least one elongated perforated tube 31 mounted within the enclosure 1 and forming an extension of the member 21. Three such tubes 31 are shown in Fig. 1, axially separated by small gaps. Each tube 31 is attached to an apertured disc 35 at the left end and a supporting spider or wheel 37 near the right end to forma pumping chamber unit 39 (shown also in Fig. 5). The discs 35 have a diameter to snugly lit within the precision glass tube enclosure 1 for accurate alignment of the tubes 31. The discs 35 of adjacent units form, with the enclosure 1, a pumping chamber 41 surrounding each perforated tube 31. Each of the pumping chambers is evacuated through at elast one hole 43 through the enclosure 1, a connecting member 45 surrounding the enclosure 1V in sealed relation, and a cold trap 47 by a high vacuum pump 48, as shown in Figs. 3 and 4.

As shown in Figs. 3 and 4, the three cold traps 47, and the attached pumps 48, which may be conventional diffusion type pumps, are supported Von four I-beams 49 on a supporting base 51, and the enclosure 1 is supported by the connecting members 45 which are carried by the cold traps 47. Each member 45 may beprovided with a butterfly valve 53 to control the pumping speed.

A series of electromagnets 55, surrounding the enclosure 1 on each side of the connecting members 45 and supported on the I-beams 49 by brackets 57 as shown, may be used to produce the desired axial magnetic eld along the plasma stream for conlining the electrons and ions to the stream. A magnetic field producing means is indicated schematically in Fig. 1 by the arrow B.

Fig. 3 shows the high vacuum end of the enclosure 1 connected bya exible bellows coupling 58 to a noise measuring cavity tube 59 mounted within a solenoid'60 for utilizing the low temperature electron beam.

Referring again to Figs. 1 and 5, each of the tubes 31 is provided with a multiplicity of small apertures or perforations 61. The distribution of the apertures 61 around and along the tube 31 may be either uniform or random. However, the distribution or spacing of the apertures 61 in the axial direction, and/or the size of the apertures, may be chosen so that the total aperture area per unit length increases in the direction of gas and ion travel, to

obtain'substantially constant gas ilow velocity. The distribution of the apertures 61 in the walls of the tubes 31 required to obtain constant gas velocity along the axis of the system is given by the following equation:

A=area of the apertures, in cm.2

z=axial distance from the beginning of the tube 31, in cm.

tT=rate of change (derivative) of the area of the apertures with the distance z, in cm2/cm.

A0=cross section arca of tube 31, in cm.:l

`w=average gas flow velocity, in cm./sec.

T=gas temperature, in degrees K.

M :molecular weight of the gas pc=gas pressure in the pumping chamber 41 outside the tube 31 p=gas pressure .in the tube 31 at distance z.

B is a design parameter which determines the change of pressure p with distance z according to the following law:

P=Poe"Bz (2) where p0 is the gas pressure at z=0 (high pressure end, or beginning of tube 31). B is given by:

kB gl log 9 y L PL 3) where The pressure pu throughout each pumping chamber 41 is substantially the same as the pressure within the tube 31 at the gap 33. For example, pc in the rstchamber is approximately equal to p2.

The tubes 31 are spaced from each other and from the emitter assembly 3 to permit` the application of diterent potentials thereto, if desired. If the gaps 33 between adjacent tubes 31 are short compared to the length L of each tube, lno appreciable effect on theion flow velocity is produced by this spacing. Analternative would be to bridge the gaps 33 by apertured tubular insulating members. The application of positive potentials, V1, V2 and V3, of the order of hundredths of a volt, with respect to the emitter 15 increasing in the direction of stream flow, to the tubes 31 will help to prevent the ions from being collected by the tubes. If it is not required to apply different potentials to diiierent parts thereof, the perforated tubular means can consist of a single tube provided with perforations or apertures distributed along each pumping chamber substantially in the manner described above.

After passing through the last stage of the pumping system 7, the low temperature plasma stream enters the electron extractor structure 9. This structure may comprise an annular ion collector 62 operated at a negative potential of .1 to volts relative to the emitter 15, and an annular electron accelerating electrode 63 operated at a positive potential of the order of 50 volts.

Fig. 2 shows the details of the improved, two-stage evaporator 27, constituting a feature of the present invention. The evaporator 27 is made up of an oven member 67 containing a capsule 68 containing the metal to be evaporated, a tubular control member 69, a heat insulating tubing 71, e. g. of thin stainless steel, connecting the members 67 and 69, a heat conducting tubular extension 73 of the control member 69 adapted to extend in spaced relation through the member 21 of the electron cooling apparatus, and means including a heat conducting sleeve 75 and a ange 76 for mounting the member 69 on the member 21 in gas-tight relation. The members 67 and 69 are provided with coils 77 and 79, batteries 81 and 83 and rheostats 85 and 87 for heating the members to desired temperatures. A cap 89 with a properly oriented hole 91 is slipped over the upper end of the extension 73 to direct the stream of metal vapor toward the tungsten plate (Fig. 1). The cap can be easily removed to insert or replace the capsule 68 in the evaporator. The ange 76 may be sealed to the member 21 by means of an O-ring 93 as shown.

In operation of the evaporator in the electron cooling apparatus of Fig. 1, the oven member 67 and capsule 68 are maintained at a temperature suiiicient to produce abundant evaporation of the alkaline metal in the capsule. The temperature in the control member 69 is kept below that of the oven member 67. In the case of cesium, typical temperatures are about 250 C. for the oven and from 100 to 150 C. for the control member. The integral extension 73 maintains uniform temperature up to the top of the evaporator. Part of the vapor from the hot oven member 67 condenses in the control member 69. The vapor pressure and the rate of flow of vapor to the hole 91 are primarily controlled by the temperature of the control member 69 and its extension 73. Thev iiange 76 is kept near room temperature by thermal contact with the member 21. When the control member 69 is not heated its temperature is maintained by the heat conducting sleeve 75 at the temperature of the flange. This is made possible by making the heat ow resistance of tubing 71 much larger than that of sleeve 75. However, the heat ow resistance of sleeve 75 is made large enough to heat the control member by its heater coil 79 to the temperature above room temperature required to produce the desired rate of evaporation. By changing the heating power to the control member 69 only, the rate of evaporation can be easily controlled over a range of more than two orders of magnitude without the undesirable bursts characteristic of conventional single stage evaporators.

The evaporator 27 is particularly advantageous for ionization inthe elctroncooling apparatus ofthe present invention. `However,'it will be" understood that the evaporator can be'used'forithecontrolled evaporation of rna-` terialsv other than cesium. p ,Y

While the invention-in both the pumping system. and the evaporatorhasv been disclosed, in the preferred embodiment, as incorporated in an apparatus in which the ions and electrons are produced by a single emitter, it will be understood that the electrons could be produced by a separate emitter located at the same end of the apparatus as the ion source.

What is claimed is:

1. A distributed pumping apparatus, for reducing the gas pressure along a constricted passageway between two regions, comprising elongated tubular means constituting said passageway, said means being provided with a multiplicity of apertures -therethrough distributed around and along ythe length thereof, wall means forming at least one annular pumping chamber surrounding said tubular means, and means for evacuating said pumping chamber, lwherelby the gas pressure in said tubular means is progressively reduced along its length by evacuation through said apertures.

2. A pumping app-aratus according to claim 1 wherein said last-named means comprises a high vacuum pump coupled to said pumping chamber through a plurality of apertures in the outer wall thereof.

3. A pumping apparatus according to claim 1, wherein the total area of said apertures per unit lentgh of said tubular means increases in the direction of diminishing pressure.

4. A distributed pumping apparatus, for reducing the gas pressure along a constricted passageway between two regions, comprising elongated tubular means constituting said passageway, said means comprising a plurality of aligned elongated tubes each provided with a multiplicity of apertures therethrough dis-tributed around and along the length thereof, wall means forming a like plurality of annular pumping chambers each surrounding and substantially coextensive with one of said tubes, and separate means for evacuating each of said pumping chambers, whereby the gas pressure in each tube is progressively reduced along its length by evacuation through said apertures.

5. A pumping apparatus according to claim 4, wherein said wall means comprises an elongated cylindrical tube having an inside diameter much greater than said tubular means, and each of said apertured tubes is coaxially mounted Within said cylindrical tube by means of a disc with an aperture tted to one end of said apertured tube and a spider near the opposite end thereof.

y6. A pumping apparatus according to claim 4, Wherein the total area of the apertures per unit axial length increases along each of said apertured tubes in the direction of diminishing pressure.

7. In an apparatus for producing a beam of low temperature plasma wherein a stream of high temperature electrons and lions is cooled by collisions with the molecules of a cool gas in a cooling chamber at a pressure between 10*5 and 101 mm. of Hg; the combination with said cooling chamber of an elongated tubular means communicating at one end with said cooling chamber and adapted to receive said stream ang g-a-s therefrom, said tubular means having a multiplicity of apertures therethrough distributed around and along the length thereof, means forming at least one elongated evacuating chamber surrounding said tube, and pumping means communicating with said evacuating chamber for reducing the gas pressure therein.

8. A plural-stage evaporator structure comprising an oven member adapted to receive the material to be evaporated, means for heating said oven member to a tempera ture suicient to produce abundant evaporation, elongated ubljllr mQanS fipr QOIldlwtilg the. Vapor fromA said oven Y 9V.` An evaporatorl structure. according ,.to. claim 8, member to the region where the vapor is. tov'be used, said wherein said temperature controlling means comprises tubular means comprising temperature control member means` for heating said controlmember, and means for isolated thermally from said oven memfber by a heat inmounting said cont-rol member on a' -cool support includsulating tube, and means for cont-rolling the temperature 5 ing a moderately heat conducting member. i of said control member independently of said oven mem- `ber. l No references cited.

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