US2933630A - High voltage ion source - Google Patents

Description

April 19, 1960 J. 5. LUCE HIGH VOLTAGE ION SOURCE 2 Sheets-Sheet 1 Filed Aug. 14, 1959 INVENTOR.

John S. Luce ATTORNEY April 19, 1960 J. s. LUCE HIGH VOLTAGE ION SOURCE 2 Sheets-Sheet 2 Filed Aug. 14, 1959 mm mm an ffi vm ion INVENTQR. John S. Luce mi 2% mi ATTORNEY United tews Pa ifl T HIGH VOLTAGE ION SOURCE John S. Luce, Oak Ridge, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission Application August 14, 1959, Serial No. 833,895

5 Claims. (Cl. 313-63) The present invention relates to production of relatively large beams of highly energetic molecular ions, and more especially to a method of and apparatus for forming intense energetic beams of energetic molecular ions including an ion source and means to accelerate ions from this source through grid electrodes to a desired region of focus.

This invention is an improvement over the system set forth in the application of John S. Luce et al., Serial No. 598,725, filed July 18, 1956, and entitled, Ion Acceleration System. In that application, ions are accelerated from an ion source through an acceleration grid placed in close proximity to the source grid in such a manner that the ions do not pass through any small focal point until they reach a suitable collector which is placed less than 180 from the arc chamber or accelerating region. With such a device, intense normally-emergent ion beams may be produced from the source and focused at a desiredlocation without first passing through a common focal. point, so that no beam blow up occurs due to coulomb repulsion. By providing a grid across the. ionexit slit, the arc is strengthened in the center and shielded from external fields, so that relatively large currents may be'obtained with relatively low accelerating voltages.

The grid-type ion source of the above application, due

to the high output ion current and large surface area,

has been found useful also as a source of molecular ions. The effect of the large surface area of this source facilitates some recombination at the walls of the ion source are chamber. application was constructed from graphite. Although the high output ion current and the large surface area were efiective in producing some molecular ions, the number of such ions were limited.

In the device shown in my co-pending application Serial No. 728,754, filed April 15, 1958, and in that shown in the application of Albert Simon, Serial No.

The source box in the aforementioned ice which they can see, an excess of these electrons will 732,770, filed April 28, 1958, an energetic, high current source of molecular ions is desired to eflect the destruc tion or burnout? of the neutral particles in order to be able to form a thermonuclear plasma. My aforementioned application Serial No. 598,725, however, is not primarily concerned with providing a source of molecular ions, and particularly not concerned with providing a molecular ion source having a large output current of at least 80 milliamperes and with an accelerated energy of the order of 600 kv.

- Attempts to accelerate ions to high energies such as 600 kv. by simply applying the desired accelerating voltages to focusing electrodes proved unsuccessful, however. Electrons in the vicinity of a high positive potential and in a strong magnetic field will oscillate along the magnetic field. Unless these oscillating electrons are materially suppressed, they will produce more free electrons by ionizing collision with other particles, and the presence of alarge number of free oscillating electrons is deleterious to the proper operation of the device. Since electrons drain to the point of highest positive potential cessive electricaldrain current from the equipment, and/ or excessive outgassing and loss of vacuum, any or all of which are likely to result in complete shut down of the equipment.

With a knowledge of the difiiculties encountered in prior attempts to provide a high current, high energy molecular ion source, it is a primary object of this invention to provide means for increasing the recombination rate of the grid-type ion source to thus produce maximum molecular ion density.

It is another object of this invention to provide means for not only producing larger density molecular ion beams, but also increasing the energy of such beams by a novel means of applying a very large acceleration voltage to accelerate ions in such beams without any serious electron drain.

These and other objects and advantages will become apparent from a consideration of the following detailed description and the accompanying drawings, wherein:

Fig. 1 shows a schematic view of the source box and accelerating grid which illustrates the focal system of this invention.

Fig. 2 shows a longitudinal cross-sectional view of the high-voltage system for use with the ion source of Fig. l, and

Fig. 3 shows a cutaway isometric drawing voltage system of Fig. 2.

The above objects have been accomplished in the present invention by generating the maximum number of molecular ions, providing an ion source and source grid of metal which is water cooled, to effect maximum recombination of atomic ions to molecular ions, applying a very high accelerating voltage to withdraw and accelerate the ions, and dumping the excess electrons at the lowest possible potentials. Maximizing the ratio of molecular to atomic ions is accomplished by maintaining the pressure in the ion source chamber at a low value, so that the average energy of the electrons in the ion source plasma is increased. It has been found that such energetic electrons are also required, in addition to large cool metal surface areas for producing a large molecular ionoutput. In order to accelerate the molecular ions formed in the source, an accelerating grid is placed adjacent to the source grid, a slotted, grounded accelerating eletrode is placed adjacent to the accelerating grid, a potential of about 35 kv. is maintained between the source grid and accelerating grid, and a potential of about 600 kv. is maintained between the accelerating grid and accelerating electrode. In order to keep at a minimum the large number of oscillating electrons which are created when such high voltages are employed in the vicinity of a strong magnetic field, a plurality of high voltage cascaded shields are employed, with a conventional electron dumping system being employed between each shield, so as to dump the of the highelectrons at the lowest possible potential, rather than at 600 kv.

Refer now to the drawings which show one embodiment in which the principles of this invention may be carried out. In Fig. l, a rectangular box 1 defines an arc chamber 2 in which is established an arc plasma 3 comprising positive ions and electrons. A flat source grid 13, constructed from tantalum, forms the front Wall of the arc chamber and is provided with a plurality of apertures 4 to divide the ion beam into a pluralityof discrete, small beams. The rest of box 1 may be con structed from graphite. The grid is maintained at the same potential as the arc chamber box 1. Spaced closely adjacent to the grid 13 is an accelerating grid 14 provided with a plurality of apertures 8 in alignment with apertures 4 in grid 13. The central grid portion of grid V issuedlannary 18, 1955.

14 is slightly convex in curvature. It may be fiat, if desired, but it is preferably made convex to obtain a better focus of the 'ion beams. An accelerating potential for positivcions is applied between the grid 14 and thegrid 13. and between the grid14 and a slottedelectrode, as shown in Fig. 2, to form an ion beaml and to accelerate it into a magnetic field whichis orientedin a direction parallel to the arc discharge 3, and the beam follows along the paths illustrated into a focus along the focal line 6, where a collector may be positioned, orthe beam may be used in a device for dissociating, and/ or ionizing .themolecular ion beam such as disclosed in -myv afor=- mentioned co-pending application Serial No. 728,754. A vapor-entry slot, not shown, is provided in the bottom of box 1 and receives vapor such ashydrogen or deu-..

tritium, as described in my prior; Patent No. 2,700,107,

It will be noted that the focal line 6. is no longer 180 "from the are chamber or from the accelerating region as it is in the normal calutron operation. Rather the curvature of the accelerating grid causes ions to be focused at a point less than 180 from the accelerating region,

as shown. This focusing arrangement may be betterunderstood by considering that a virtual focus is formed at a point behind the arc chamber box 1 along the line 7, displaced 180 from the focal line 6. It may be seen that by virtue of this apparatus, the actual ion beam never goes through a real focal point until it reaches the collector or other device for utilizing .the beam, so thatthere is not now a space-charge imposed limitation on the amount or. vcurrent.density'of the beam going through a saddle point, as fully discussedin the aforementioned application, SerialNo. 598,725.

A filament electrode, notshown, is'mounted'at one end. of box 1 for providing a source of electrons for arc.

discharge 3. These electrons areaccelerated from the filament through an opening in oneend of box 1, through thechamber 2, and through an opening in the other end of'box 1 andjstrike an anode, not shown. A potential of 50-300 volts between box 1 and the filament maybe provided to accelerate electrons into the chamber. The spacing between grids 13 and 14 is made very small, but may very from about to The apertures 4 in source grid 13 are about x 5 and are spaced abontlfig" apart, while the apertures 8 in accelerating grid 14 are about x and-arespaced about A apart. It should be noted that apertures Sin grid l-ff aresli htly longer than apertures 4 in grid 13 to allow for alignment of the apertures in the grids;

The vapor entering box 1 is ionized by the accelerated electrons between the filamentand anode, and an arc discharge 'with' an arc voltage of about 110 volts and an arc'cu rrent of about 5 amperes is thus formed therebe tween. The molecular ions formed fromtheions in the discharge by recombination at the tantalum grid 13 and asa function of the pressure in box 1, are accelerated out of box 1 by the accelerating grid and electrode asdiscussed above.

I have found that by controllingat-least two primary cond1tions,l can insure a largemolecular ion output from the above described device. These conditions are the; v averageenergy of ,the electrons injected into the. ion

produce only atomicions'or neutrals. No numerical fig n -e. an bcgstatcd forth: optimum vpresnrre,.valve; how:

' measuring the molecular ion'output.

. shield 35. Conductor 24 is connected at one endby.

are chamber walls and face are constructed.

ever, a criterion for optimum operation has been established and is as follows. The operating pressure is found by adjusting the flow of gas; to the arc chamber while Optimum pressure is achieved at the point where maximum molecular. ion output is obtained and yet the arc in the chamber extinguishes when the ion accelerating voltage is removed. The accelerating voltage affects the pressurein the arc.

' Expressed in a different form, the pressure is lower than that required to provide a mean energy of electrons in the E ion source plasma of 15 ev. andican be. determined-for each particular source, accelerating voltage, andinput gas. The second controlling condition, i.e., recombination rate, is affected by the structural materials. of the arc chamber, particularly the material from which the I have found that metal surfaces such as copper, stainless steel, and tantalum are superior to all othermaterials, such as previously utilized carbon chambers, in effecting the high: Cooling the metal surfaces, as;

est recombination rate. by water cooling coils further aids recombination. Cooling coils of a conventional type are provided to cool the surfaces of box 1, Fig. 1.

The long dimension'of the apertures in grids 13 and 14 may be either vertical or horizontal, that is, normal or parallel to the direction of the magnetic field. Itis, preferred to use vertical apertures because better focused,

beams may be obtained.

As set forth above, a magnetic field, is provided, by. conventional means not shown, and ,it is oriented in a, direction parallel ,to thedirection of the arc discharge ,3

in' Fig. 1. The magnetic field ismaintained ,at afield. strength of about 3000 gauss, for example.

Referring now to Fig; 2, there is illustrated a high voltage sytem which is used with -the source box. of Fig. 1 for accelerating ions therefrom. A. plurality 1 ofv cyli n-. drical conductors 16, 18, 20, 22, and 2 4 am. separatedby cylindrical insulators 17, 19, 2 1, 23, and 25, as 1111151. trated, and they all pass through a vacuum system'face plate 15. Insulator 25 has a flange which is. held against:

plate 15 in sealing engagement therewith by a clamping element 26 and screws-27. 7

Inner conductor 16 is connected to a source of potential of about 635 kv. in aterminal box 28. Conductor 16 is also connected to ion sourcebox 1 and source grid 13., Filament electrode, 40 is connected by leads, not

shown, to a source of potential slightly higher then that connected to-source box 1 to provide for the acceleration of electrons into source box 1, as discussed above.

Conductor 13 is connected at one end by a connection 29 to a source of potential of about 600 The other end of conductor lli is-connectcd to a high voltage shield 37 'whichterminates with the accelerating grid-14, It

canbe seen that accelerating grid 14 is maintained eta;

potential about 35 kv. lower than the potential on source grid 13.

Conductor 20 is connected at one end by; a connection 30 to a source of potential of about 450 kv.- The other:

end of conductor 20'is connected to a high voltage shield .36. Conductor 22 is connected at one end by a-con m=.c;.-Z

tion 31 toa source-of potential of about 390 kv. The. other end of conductor 22 is connected to ahigh voltage a connection 32 to 'a source of potential'of-about kv.; The other end of conductor24 is connectednto'a high voltage shield 34.

' A shield 33 which is grounded by lead39 enclosestall the other shields and terminates in an acceleratingslot 38 in front oftheaperturesjS' inv accelerating; grid 14. Thus shield 33 serves as an accelerating electrode .for. the molecular ions which are given an'initial acceleration by grid-'14.- Itcan be seen that thereis a 600 kv.- voltage drop to ground between grid 14 andgroundcd-shieldGS;

Aconventional'chevron typeelectron dumping system is used between-grounded shield.33 and shicld:3.4,.bctweca assessesshields 34 and 35, between shields 35 and 36, between shields 36 and 37, and between shield 37 and source box 1, as illustrated in Fig. 2. Fig. 3 shows a better perspective of a portion of the various dumping systems. From Fig. 2, it can be seen that there is a potential gradient between each of the shields of 150 kv., except between shield 37 and source box 1. In the latter instance there is a potential gradient of 35 kv., since electron drain is greatest near the ion source.

The use of a single chevron type electron dumping system is set forth in US. Patent No. 2,713,640, issued July 19, 1955, to H. W. Savage, to which reference is made for a more detailed understanding of the principles of operation of this type of system. In that application, the electron dumping coupling has relative conformations and are so positioned that the electric field therebetween has directional components at an angle between and 90 to the direction of the magnetic field and so that the resultant of the effect of the magnetic field and said components of the electric field on electrons has a direction toward the highest potential body causing electrons to dump thereon. Each of the electron dumping couplings employed in the present invention are also positioned in a similar manner with respect to the magnetic field. The use of a single dumping system, however, would be inadequate for the present invention where much higher voltages are employed. By using a cascaded series of dumping systems, as set forth in Fig. 2, it is possible to use the high voltages employed and still su-ppress the damaging electron oscillations that are characteristic of devices operating with high voltages that accelerate electrons within a strong magnetic field. In Fig. 2 there are provided a series of shields of increasing higher potentials so that each shield is coupled by a chevron type dumping system to an adjacent shield of higher potential. The electrical field in each of the couplings is in such a direction that the resultant of the electric and magnetic field acting on electrons exerts a force on them in a direction towards the higher potential body so that the electrons will be dumped thereon, and the conformation of each of the dumps being such that an electron traveling along any line of magnetic flux is confronted by a high potential surface. By using a plurality of such dumps, each with a portion of the overall potential between them, all electrons are dumped at the lowest possible potential.

As can be seen in Fig. 2, there are a pair of electron dumping systems between each of the shields. Shield 34 has a pair of chevron type members 41, 43 mounted on its top surface, and a pair of chevron type members 56, 58 mounted on its bottom surface. A member 42 mounted on shield 33 extends into and along the V groove formed by members 41, 43. A member 57 mounted on shield 33 extends into and along the V groove formed by members 56, 58. Shield 35 has a pair of chevron type members 44, 46 mounted on its top surface, and a pair of chevron type members 59, 61 mounted on its bottom surface. A member 45 mounted on shield 34 extends into and along the V groove formed by members 44, 46. A member 60 mounted on shield 34 extends into and along the V groove formed by members 59, 61.

A pair of chevron type members 47, 49 are mounted on the top surface of shield 36, and a pair of chevron type members 62, 64 are mounted on the bottom surface of shield 36. A member 48 is mounted on shield 35 and extends into and along the V groove formed by members 47, 49. A member 63 is mounted on shield 35 and extends into and along the V groove formed by members 62, 64. A pair of chevron type members 50, 52 are mounted on the top surface of shield 37, and a pair of chevron type members 65, 67 are mounted on the bottom surface of shield 37. A member 51 is mounted on shield 36 and extends into and along the V groove formed by members 50, 52. A member 66 is mounted on shield 36 and extends into and along the V groove formed by members 65, 67.

pair of chevron type members 53', 55 are mounted on the top surface of source box 1, and a pair of chevron type members 68, 70 are mounted on the bottom surface of source box 1. A member 54 is mounted on shield 37 and extends into and along the V groove formed by members 53, 55. A member'69 is mounted on shield 37 and extends into and along the V groove formed by members 68, 70.

It should be understood that the source and the various shields are enclosed within a vacuum chamber, one wall 15 of which is shown in Fig. 2. The pressure in this chamber, external to the source, is substantially 10- mm. Hg. A strong magnetic field is provided and is maintained at a strength of about 3000 gauss, and is oriented in a direction parallel to the direction of the arc discharge which is formed in source box 1 by the ionizing action of the injected electrons upon the gas introduced into the source box from a source, not shown. Conventional coils, not shown, are also provided for cooling the surfaces in source box 1, since this also aids in the recombination of atomic ions to form molecular ions which are then accelerated out of the source box by accelerating grid 14 and the accelerating electrode formed by the front surface of shield 33. The molecular ions will then have an accelerated energy of about 635 kv. and will have a current density of about 2.5 amperes. The high density, high energy, accelerated molecular ion beam may then be used in any device where such a beam is desired. For example, it may be used in a device such as disclosed in my aforementioned co-pending application, Serial No. 728,754, or in the aforementioned application of Albert Simon, Serial No. 732,770. In the Simon application, in order to achieve destruction or burnout of neutral particles in the system disclosed, it is necessary to provide an energetic, high current molecular ion source. The source, as set forth in the present application, is one satisfactory means for providing the molecular ion source necessary in the Simon application to achieve burnout. High energy ion beams are also used to bombard target materials to study the interaction of such ions with matter, as in other types of particle accelerators.

This invention has been described by way of illustration rather than limitation, and it should be apparent that the invention is equally applicable in fields other than those described,

What is claimed is:

l. A device for producing a high current, high energy molecular ion beam within a strong magnetic field comprising a container, a metal ion source box means disposed within said container and defining an arc chamber having an ion exit aperture, means for establishing an are parallel to the direction of said field through said chamber, means for feeding vapor at a controlled rate into said are where said vapor is ionized, a first multiapertured, Water-cooled metal grid member disposed across said ion exit aperture, means for accelerating ions from said are through said field comprising a second, multi-apertured, convex grid member disposed closely adjacent to said first grid member, respective grid aperture being aligned to allow ion passage therethrough, and a slotted accelerating electrode disposed adjacent to said second grid member with the slots in said electrode aligned with the aperture in said second grid member, a source of high potential having a series of taps to provide intermediate potentials and both negative and positive terminals, said positive terminal being connected to said arc chamber and said first grid member, a grounded shield member terminating with said accelerating electrode, a plurality of spaced, overlapping high voltage shields disposed Within the area bounded by said grounded shield and disposed in a cascaded arrangement with respect to said are chamber, means connecting said shields to respective taps for establishing a potential gradient between each of said shields, the shield having masses heehi he t. poten al w thr pe t .s ndb in fismfir nated-with said second grid member, said first grid member-being maintainedat apotential higher than said second grid; member, and electron oscillation suppressing means for said highvoltage shields comprisinga pair of chevron type electron dumping systems coupled between each of'said shields and between said highest potentialshield and said arc chamber.

2. The device set forth in claim 1, wherein the number of said high voltageshields is four, the gradient'between shields is at leastl50kilovolts, the pressure in said container -is l0 mm. Hg, the magnetic field is 3000 gauss, the arc volt-ageis- 110 volts, the arc current is 5.

amperes, and the accelerating voltage is 635 kilovolts, tothereby produce a molecular ion current of about 2.5 amperes.

3;; The device setforth in claim- 1, wherein the means for, establishing a potential gradient between each of said shields; and between-saidhighest potential shield and said are chamber comprises a plurality of concen-V tric cylindrical conductors separated byicylindrical insulators, said conductors and insulators extending through one wall of said container, the innermost cylindrical conductor being connected to said are chamber and said first grid memberand to said high potential source, means for connecting each of the other conduc tors-to each of said high voltage shields, respectively, a

plurality of high voltage sources, the lowest voltage source being about 150 kilovolts and each succeedingv voltage souree being 150 kilovolts higher than the preced'in voltage source, and means for connectingeach of said plurality of voltage sources to each of said. other cylindrical conductors, respectively.

a 4. A device for producing a high current, high energy-,

molecular ion beam in astrong magneticfield compris-V ing a container, an arc chamber disposed within said container and having an ion exit aperture, means for ac celerating electrons into said chamber to form an arc,

, means for feeding vapor into said are at ac ontrolled rate, said are ionizing said vapor, means for establishing a strong magnetic field having a direction parallel to said arc, a first multi-apertured metal grid disposed across.

said ion exit aperture and contacting said arc, a second, multi-apertured, convex, acceleratingggrid disposed closely adiacent to said first grid andaligned with the apertures therein, a slotted accelerating electrode disposed adjacent 'to said second grid with the slot in said electrode aligned with the apertures in said second grid, a plurality of spaced, overlapping, high voltage shields disposed in a cascaded arrangement with respect to said are chamber, means connected to said shields for establishing a potential gradient between each of said shields'of. at least 150 kilovolts, a grounded shield enclos-- ing said high voltage shields and terminating with said accelerating electrode, said second grid being connected to the high voltage shield that has the highest potential with respect to ground, and means connected to said arc chamber and said first grid for establishing a potential gradient between said first and said second grid of at least 35 kilovolts.

5. The device set forth in claim 4, wherein the number of said high voltage shields is four, the pressure in saidcontainer is about 10- Hg, the magnetic field is about 3000 gauss, the arc voltageis about 110 volts,

the arc current is about 5 amperes, and the accelerating voltage isabout 63$;kilovolts. I

No references cited.

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