US2798181A - Pumping ion source - Google Patents

Description

July 2, 1957 J. s. FOSTER,A JR 2,798,181

PUMPING ION SOURCE Filed March 26, 1954 ATTORNEY.4

rui/irme roN SOURCE John S. Foster, ir., Livermore, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Application March 26, 1954, Serial No. 419,121

8 Claims. (Cl. 313-161) The present invention relates to an improved ion source adapted not only to produce copious quantities of ions but also to evacuate by the process of ionization the ion source itself and attached separation or analyzing chambers.

lon sources, which are widely employed in both industry and research, are evaluated upon a value of factors, and one of these factors which is of paramount importance in certain ion source applications is the ratio of ions to gas molecules that are expelled or escaped from the source proper into the attached apparatus operating upon the ions. Conventional ion sources have present therein a certain amount of gas either produced during the ionizing process or directly introduced into the source as the material to be ionized. Substantially all applications of ion sources include an evacuated chamber wherein electromagnetic, electrostatic or other means are employed to utilize the ions produced by the source, and this utilization is universally accomplished in a vacuum. Particularly in the instance where ion beams of very large currents are employed difiiculty arises in maintaining an adequate vacuum in the chamber wherein the ions are utilized, for as the ion production is increased, likewise the amount of gas escaping from the ion source increases. lt has been shown that for conventional ion sources producing high current ion beams, the ion current is proportional to the ion source opening through which ions are extracted from the source so that an increase in ion beam current for any particular source appears necessarily to increase the volume of gas flow from the source through this opening. The present invention entirely overcomes this above-noted limitation upon ion sources and not only is it possible with the present invention to increase the ion current from an ion source without increasing gas fiow therefrom, but also the present invention operates to extract gas moleculesfrorn the chamber into which the ion beam is directed so that there is actually produced the reverse of the abovenoted ion-gas relationship.

The present invention provides an ion source producing an ion beam of readily controllable current wherein this ion beam actually evacuates the chamber into which it is directed. Rather than expelling gas molecules from the source, the present invention removes gas molecules from the attached chamber so as to consist not only of an ion source, but also of an ion pump. The relative magnitude of ion production and ion pumping may be readily varied by controlling the various parameters of the herein described ion source.

lt is an object of the present invention to provide an improved ion source.

it is another object of the present invention to provide an improved pumping ion source.

lt is a further object of the present invention to provide a combined ion pump and ion source.

It is yet another object of the present invention to provide an improved high current ion source of the oscilnited States Patent O N' Patented July 2, 1957 ICC 2 lating electron type capable of establishing by the ion production mechanism a vacuum whereby attached chambers may be evacuated.

Various objects and advantages of the present invention may become apparent to those skilled in the art from the following description taken together with the accompanying drawings of a preferred embodiment thereof, wherein:

Figure 1 is a longitudinal center section of the ion source;

Figure 2 is a simplified wiring diagram of the source of Fig. l; and

Figure 3 is an embodiment of the source adapted for use in a particular accelerator, such as a cyclotron.

Considering now the structural details of the present invention and referring to Fig. 1 of the drawing, the ion source 1 includes an electrically conducting anode cylinder 2 having an open end 3 and at the opposite end thereof an end wall 4 provided with a central aperture 6. Attached to the cylinder 2 exterior of the end wall 4 is an open ended tube 7 which mates with the aperture 6 in end wall 4 and extends therefrom in axial alignment with cylinder 2. Attached about the open end 3 of cylinder 2 is a thickwalled flange S which extends outwardly therefrom and slanted away or back from the end of cylinder 2. About cylinder 2 and attached tube 7 there is provided a solenoid winding 9 which upon energization provides a magnetic field through cylinder 2 and tube 7 axially thereof. A pair of small pipes 11 and 12 are connected to tube 7 in communication with the interior thereof; tube 11 being adapted for connection to a source of gas to be ionized and tube 12 being adapted for connection to an evacuation system whereby a continual fiow of gas through pipe 11, across tube 7 and out pipe 12 may be attained. Adjacent the free end of tube 7 and in alignment therewith there is provided an electron emissive filament 13 and adjacentA the free or open end 3 of cylinder 2 there is provided a plurality of electrodes 14 which may be arranged in a circular configuration with the diameterv greater than that of cylinder 2.

Considering the electrical connections of the ion source, and referring to Fig. 2, it will be seen that there is provided a magnet power supply 16 which is connected across solenoid Winding 9. A filament current supply 17, shown for convenience as a battery, is connected across filament 13 to provide heating current therefor and electrical connection is made between filament 13 and electrodes 14. A high potential source 18, also shown merely for purposes of illustration as a battery, is connected between cylinder 2 and the connection between filament 13 and electrodes 14. Thus electrically, filament 13 and electrodes 14 are maintained at the same potential and cylinder 2 is maintained at a highly positive potential with respect thereto.

Considering the operation of the embodiment of the invention described above and illustrated in Figs. l and 2 with reference only to the production and extraction of ions from the source, energization of the filament 13 by the filament current supply 17 causes this filament to become electron emissive so that electrons are freed from the surface thereof. The anode cylinder 2 and attached tube 7 are maintained at a positive potential with respect to the filament 13 by the power supply 18 so that electrons emitted from the filament13 are attracted toward the adjacent tube 7 by the relatively positive potential thereof. With the solenoid 9 energized by the power supply 16, there is established axially of the anode cylinder 2 and tube 7 a uniform magnetic field and electrons attracted from the filament 13 are constrained by this magnetic field to substantially traverse the lines of force thereof.

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ere'drrtafrre nerja ef fhf sectie-aes" 1 4: r"h ei'rrifs or at least a'Ina'j'crity thereof, arrepelled back into t he anode cylinderrand thence reverseltheir path until theyl are agrari repen-ary trennrarrs.- There a thrfs'eab; lished by the present invention angoscilllating velectron-.dts-

I'o'ns` are produced within'. the' 'source hy`` tl 1`e` diriisiori of gas" therein, and this' is accomplishefdt l llconnected to the tub'e'7ail cent the til praetice it'has 'been' found advantageous tpro'vide a continual flow ofgas through' thisl'pojrtion' of thesouilce" so that a gas return path ispi'ovi'ded' by the p'ip'e 12 al`s:o connected to the tube whereby a 'g as" flli wl is'produced directly across, the ilarrrent;l 1 3 within the tube 7. Gas" entering tube 7 through' pipe 11"is ionized'by' the 'tis'cilltl ing electron discharge; this.' ionization changesth'e characteristic of the diseharge to' thatof 'anja're'plasma'21-l- The filament 13 extends substar'itiallyover'the'entire'end' of the tube 7 so that electrons are emitted'ov'er' substantially the entire cross section of tube 7,'and thusrthe'wre sultir'ig arc discharge or arc plasma"ha-s'substantially the same cross section as'the tube 7. This arc discharge extends from the vicinity of lamen't V13 through the tube 7 an'd anode cylinder 2. Atthe open env'd of 'anode cylinf der 2 the arc reaches the'vic'inity ofthe acceleratin'g'elec# trodes 14 whereat'the relatively negativp''t Vin'tial thereof repels the electrons in 'the' arc'pla'sra atl-:a'ttra'c'tsthe ions," therein so that the arc looses its unity aty thisp'o'iirt" and the ions thereof are accelerated away from' the'iori source. The' arc21 may be controlled at the' open-f end' of" the' anode cylinder 2 in the' manner'illu'strated in Figi." 1 By' expanding the magnetic' tield 'at' this' point to' 'expandthe' arc, as' illustrated. The provision of" a low reluctr' path in' the form of the flange 8 slanted frrn the 'open of the anode cylinder Z'baek toward the solenoid'vin'd; ings 9 causes the magnetic lines of force to diverge at the open end of the cylin'de'r 2, and, as the" arev tends' to follow the magnetic lines of forceit, too, is expanded; Particu'# lar advantages may be obtained from this type' of' arc control and in this respect reference is made tmy co:-Y pending applicationfor a Coasting Arc Ion Source, Serial No. 297,629, led July 8, 1952. The expanded' ion beam may be operated upon in'any desired manner either to refocus it or' to utilize it in'its expanded form a's dictated by the one practicing the invention.

In addition to the production ofcopious quantities of ions in the above-noted manner, the ion source of' the present invention is further adapted to evacuate the vchamber with which it communicates.v As thearc discharge 21 is a ,strong ionizing-medium, substantially all gasmolecules therein are ionized so that the pressure at the arc. is lowered thereby and gas m'olecules from the ext'eriorfof the source' drift into the open end `thereof toward'the low pressure region therein.' Pressure within the ,sourcejcylinl der 2 is determined by the'number of ions' that strike the cylinder walls andthe end wall 4v whereat 4they are nerrv trlized arid. become. gas. moleeuleaeainr.,.Ieamasneti eldestablished axially through thec'ylinderzby windlvssnsr ing 9 restricts ion movementradially of the cylinderl so titouf ythe -iti-iis escaping from theA arc withinthe cylinder 2, the great majority, will travel generally longitudinally thereof. These ions in the main thus emerge from the open end of the cylinder 2 or impinge upon the end wall 4 and only the latter group pick up electrons to become neutral gas molecules for establishing the base pressure of the source. Gas molecules neutralized at the end wall 4 leave same with la random motion so that with an adequatel'ength L to diameter D of the cylinder 2, a desired proportion of Same', will reenter the arc 21 and become re-ionized prior to leaving the open end of the cylinder. It is preferred that L D 5 for high vacuum within the source and attached chamber, although higher ratios may be employed and 'even' lower ratios may be used but with decreased pumping effects. A further important criteria is the ratio of arc diameter d to the cylinder diameter D. Anincrease Ain arc diameter places the outer arc surface nearer th ylinder wal l s o that a greater proportion of ions prngthe a'rcimpinge upon the cylinder walls` to increasethe basepreeslrewithin the source; however, this saine increase in arc diameter also provides a greater probability. of gas molecules fr om the end wall falling back into the arcfor re-io'nization'before leaving the open cylinder 'e'rrd. r"lZ balance of these opposite effects dictates aufiiiter'riiediate value .for the ratio of arc diameter to ,ic'ytifrdraiifa frd'if is" preferable that this ratio d/D-:z'f 3j.

At t e closed end or filament end of the source the i ithin Athe zii-'c s'trike the `filament by virtue of the a'ttr ting 'electr'o` sttie -field thereof and ions there pick er riroelectr'oiis to'becme neutral gas molees. 'O g to the r'ndoiir' motion of gas molecules 'etendfto pa'fss axially along the source away ent 'rid this is 'increased by the relatively gasl pressure, tilcc'a't caused by the ions neutralized at thetilanrnt.' The'sernoleeufle'sare prevented from passing'I 'directly iri i the cyli'rider 2 by' the tube 7 which retri'cts f hel'penmg i the cylinder 2 and the rilarrient' 13 to the afredr eter. A's the arc substantially ii'lls th'extirb'e 7, there remains' no free space for gas moleculesv to' Ytrai/'el' therealn'g without passing 'through the arc;Y Consequently, gas molecules can no't travel throght'he'tub 7 withoutpa'ssi'ng through the arc wherein they are ted' toftlie ionizing effect thereof. By providing 'a sriliicient'ly long tube' 7 substantially all gas moleulesenteringlsarrie' are ioriized and none enter the cylinder to-raise" theprssiure therein.' This condition is p'rodueed by `relating the tube'length l to the tube and arc diameter d vin substantially the same relation' as similar proportions of the cylinder 2, i. e., l/dS.

Summari'zingfthe pumping action of the source it will be A'seerr that the" a'ic 2'1 io'nies gas molecules entering same, and als that despite the magnetic eld some ions drift outl of the'arc sides". With L/DS the base pressure produced by' ion 'drift from the arc is minimized so that gas molecules move into the source through the open en'd thereof. The expanded arc adjacent electrode 14/ioniz'es`-sor`n`e of tlies'e molecules and the ions then travel into the cylinder 2 wherein they enter the arc itself or travel to the end wall 4 where they are again neutralized. Molecules leaving the end wall 4 mainly reenter the arc plasma `proper by virtue of the L/D and d/D ratios and are again ionized. Ions within the arc generally move" 'axially 'thereof in both directions and those moving toward the filament 13 eventually strike same and are neutralized. vGas 'molecules so formed must' pass through' the ar'cV within the tube 7 where they are re-ionized so that substantially no gas molecules as such enter the' cylinder 2' from the filament 13. The above mechanism provides :Lone way path for gas molecules from the chambenZZ' into the source and thus the chamber 22 is eva cuated by this pumping action of thel sorircgmtnthefilament the gas molecules formed may be removed by a pumping system attached to the pipe 12; however, all gas molecules formed at the iilament will chemically combine with the source elements in the vicinity of the filament in the absence of pumping means attached thereto so that none is required and a high pumping speed may yet be attained by the present invention.

The pumping ion source of the present invention may be employed in a variety of ways, as for example, as a pump alone, and may as an ion source be employed in connection with numerous different types of apparatus. There is shown in Figure 3 a modiiied form of the source 1 adapted for use in a particle accelerator of the cyclotron type. The cylinder 2 and attached tube 7 are aligned with the magnetic lield H of the cyclotron and at a distance from the cylinder 2 in axial alignment therewith is disposed a second cylinder 23 having a closed end 24 adapted to terminate the arc discharge 21 extending through both cylinders. The second cylinder 23 may or may not be provided with a restriction tube like tube 7 and ion accelerating electrodes 26 are disposed adjacent the arc plasma 21 between the cylinders for drawing ions radially from the arc at this point and directing same into desired orbits within the cyclotron. The cylinders 2 and 23 are electrically connected together as are the iilament 13 and the end plate 24 to establish an oscillating discharge through the cylinders 2 and 23 as explained above. Ion production and ion pumping are the same as previously explained with the main dierence being in the manner of ion extraction.

What is claimed is:

1. A pumping ion source comprising an open ended tube, a cylinder Idisposed in axial alignment with said tube and having a diameter at least twice the diameter of said tube, an annular wall connecting adjacent ends of said cylinder and tube, means establishing a magnetic field axially of said cylinder and tube, a rst electrode disposed adjacent the free end of said tube and having an electron emissive surface facing and substantially covering same, a second electrode disposed at the free end of said cylinder, and power supply means electrically connecting said iirst and second electrodes and maintaining same at a negative potential with respect to said cylinder and tube for establishing an ionizing oscillating electron discharge through said cylinder and tube of substantially the same diameter as said tube whereby gas within said cylinder is ionized and said cylinder is evacuated thereby.

2. A pumping ion source comprising a cylinder having an open end and a tubular axial extension of reduced diameter at the other end thereof, means establishing a magnetic eld having lines of force axially through said cylinder, a filament disposed adjacent said small cylinder end with an electron emissive surface adjacent same and substantially coextensive therewith, an ion accelerating electrode disposed adjacent the large open end of said cylinder, power supply means maintaining said filament and electrode at a negative potential with respect to said cylinder for establishing an ionizing oscillating electron discharge through; said cylinder, and means supplying gas to said source adjacent said lament whereby said electron discharge ionizes said gas to produce an arc plasma through said cylinder.

3. A pumping ion source as defined in claim 2 further characterized by the diameter of said tubular extension and of said arc plasma being at most one-half the diameter of said cylinder and the length of said cylinder excluding said extension being at least ive times the cylinder diameter whereby said cylinder is evacuated by said arc discharge.

4. A pumping ion source as claimed in claim 2 further dened by the length of said tubular cylinder extension being at least iive times the diameter thereof whereby substantially all gas molecules entering said tubular eX- tension are ionized by the arc filling same.

5. kA pumping ion source comprising means establishing a magnetically collimated oscillating electron discharge and including an `anode cylinder through which said discharge extends, means introducing gas to said discharge for producing an arc discharge therefrom, Va tube of smaller diameter than said cylinder disposed at one end thereof about one end of said arc discharge and being substantially lled therewith, said tube having a length-to-diameter ratio of at least live whereby substantialrly all gas molecules entering said tube are ionized, and ion extraction means adjacent said arc for removing ions therefrom.

6. A pumping ion source comprising a cylinder having a lateral opening near the center thereof and at least one end with a reduced diameter of substantial axial extension, `a pair of electrodes disposed one at each end of said cylinder with at least one electrode being electron emissive, power supply means maintaining said cylinder at a positive potential with respect to said electrodes for establishing an electron discharge, means establishing a magnetic field axially through said cylinder for collimating said discharge to pass through said cylinder and oscillate between said electrodes, gas supply means connected to said cylinder adjacent one end there of for introducing gas therein whereby same is ionized to produce an arc plasma through said cylinder, and electrodes disposed adjacent the central cylinder opening for attracting ions therethrough from said arc plasma, said cylinder having a diameter that is substantially oneiifth the cylinder .length from the central opening in each Idirection for the extent of the large cylinder diameter.

7. A pumping ion source comprising a cylinder having one unrestricted end andone restricted end with an aperture in the latter axially of the cylinder and at most one-half the cylinder diameter, a tube extending axially from said cylinder about said `aperture and having a diameter not more than one-half the cylinder diameter, an electron emitter disposed Iadjacent the outer end of said tube in alignment therewith, means maintaining said cylinder and tube at a positive potential with respect to said emitter whereby electron discharge occurs from the latter toward the former, and means establishing a collimating magnetic lield through said electron emitter and axially through said cylinder and -tube for confining electron discharge to a path through said tube and cylinder with discharge diameter in said cylinder limited to tube diameter.

8. A pumping ion source comprising an elongated hollow anode having one restricted end, a tube extending from said restricted anode end of lesser diameter than said anode and communicating therewith, a cathode disposed across the outer end of said tube for establishing a -discharge of substantially the diameter of said tube, and means establishing =a magnetic field through said tube and anode for collimating said discharge to a substantially constant diameter therethrough.

References Cited in the file of this patent UNITED STATES PATENTS 906,682 Birkeland Dec. 15, 1908 2,182,736 Penning Dec. 5, 1939 2,232,030 Kallmann Feb. 18, 1941 2,282,401 Hansell May 12, 1942

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