US2636990A - Ion source unit - Google Patents

Description

April 28, 1953 J. D. Gow Em 2,636,990

ION SOURCE UNIT Filed Dec. 14, 1949 llllllfll lfllllll K INVENTORS O2 JAMES D, Gow

A BY JOHN 5. FOSTER, ./r.

ATTORNEY.

Patented Apr. 28, 19.53

UNITED STATES PT'NT OFFICE -ION 'SOURCE 'UNIT James D. Gow, Richmond, `and John fs. Foster, Albany, Ualif., vassignors to the United 'States o'f America as represented by the United States Atomic :Enel-gy Commission application Dec-emper- 14, 194s, Serial No. '132,922

6 Claims.

The present invention pertains to 'source units in general and relates particularly to im provements in ion sources of fthe type -eniplo'yi ing a magnetic eld to inuence electron -discharge therein. Ion source units off this general type are commonly referred to as PIG ion sources in 'view -o'f the early work 'done in lthis field by Penning and la'ter exploited by the Philips Company under Ethe hame of Philips In Gage. Ion Gage have since been applied to ion sources and particular sources of this type are disi closed in Patents Nos. 2,499,288 and 2,499,289 issued February 28, 1950 Lto John G. ABackus for Vacuum Analyzer and Ion Generator, respectively, and Patent No. 2,499,320 'issued February 28, 1950 to Robert Lovein'ger for Ion Generator. The present invention comprises an improved PIG ion source unit wherein the output is maxi-- mized.

In recent years great advances have been made in the eld of electromagnetic :separation and particle acceleration and many new machines have been developed in 'this .elda One of the difficulties attending such progress has been the provision of a sui'cient quantity of ions to properly operate these machines. Numerous im'- provements upon PIG sources and variations thereof have been advanced as a solution of the diiculties attendant the removal of ions from the source; however, to our knowledge, it has been possible thus far to remove only a small fraction of the ions actually formed within the source. By employing an entirely different method of removal the present invention is capable of expelling 'a large percentage of the -ions lformed within the sourc'e.

It is therefore an object of the present invention to provide a new and improved .ion source unit.

It is another object of the present 'invention to provide an improved -ion source of the -P-IG type.

It is also an object of the present invention to provide a novel method and apparatus for the removal of ions from an lion source.

It is a further object 'of 'the present invention to provide a highly -effective apparatus for the removal of ions 'from the center of ion density in a PIG ion source.

Further objects and advantages will become apparent from a study of the following disclosure and attached drawings, of which Figure 1 is a sectional view of the ion source;

Fig. 2 is a schematic wiring diagram of the invention; and

The principles lexl'r'i'p'loy'ed in the Philips 3 is a sectional view of one embodiment of the cathode structure.

Considering the 'drawings in general, it Will be note'd that the 'elements 'of our improved -source include 'a cylinder -I having therein an laxially apertured anode 6 and a pair o spaced cathodes -l and i8 which are disposed adjacent to open ends of 'the anode and 'equidistant therefrom. A gener-ally `conical and centrally bored prohe I'4 is provided adjacent one 'off the cath- -odes 8.

Communication between the interior o'f anode '6 and the exterior o'f the cylinder I is provided through one of the `apertures 'of the latter, through 'an opening ,in the cathode 8.. and through the central bore in probe I4, thereby providing an exit path for ions formed within the anode interior. Ions are urged along this path by means of appropriate potentials applied to the 'aforementioned elements of the source.

Referring now to Fig. 1 in more detail, it will be noted that cylinder I is suitably mounted upon 'a face plate or other stationary member 2. One end of cylinder I is sealed by a plate 3 extending thereacross and such plate may be either removably or permanently secured thereto. Plate 3 is apertured to receive a tube '4 which provides lcommunication between the interior of the cylinder and a suitable external gas system. Such system (not shown) may advantageously lcomprise means to evacuate the cylinder and further means to introduce ain atmosphere of some desired -gas or vapor into the 'cylinder i. Within cylinder 'i there is provided an anode -6 and cathodes I and '8. VAnbei-e 8 comprises a hollow cylinder of lappropriate axial and diametral proportions having 'end walls provided with small openings `9 preferably in axial alignments. Also, the anode cylinder is so disposed that its axis is coincident with that of cylinder I and is maintained in this position by 'an annular insulator II snugly surrounding the anode and in turn, closely surrounded and supported by the cylinder I. Cathode 'l is 'disposed between the anode 6 and the cylinder end plate 3 and is of approximately the same diameter as that 'oi cylinder -I. Also, 'cathode 'I has a transverse opening I2 which provides a continuation cf the passageway of tube 4 'into the central portion ci the cylinder I whereby the entire cylinder interior may be evacuated 'and filled with a suitf able gas or vapor through the tube il. Cathode 8 is disposed at the opposite ene"` of anode d from cathode 1 and like cathode has a diameter of approximately the same size ais tinc internal diameter of cylinder I. Cathode S is provided with a transverse, central aperture I3 for the passage of ions therethrough, as is explained more fully below. In a preferred embodiment of the invention, such as is exemplied in Fig.' 1, cathode 8 is a solid ofV relatively large axial dimension and is provided with aperture I3, which viewed from the right of Fig. 1 ares outwardly almost to its periphery as a conical surface of relatively large area.

Thus the cathode 8 viewed from the anode side appears as a disc with an aperture I3 in the center, while the same cathode viewed from the opposite side reveals a disc having a conical depression in the center thereof tapering toward a small opening in the opposite side.

It will of course be appreciated that the aperture I3 is quite small in order not to interfere with electron discharge from the anode-presented face of the cathode 8 as set forth below. In

an ion source of approximately the dimensions illustrated, the aperture I3 may have a diameter of the order of 0.030 inch.

A probe I4 is provided adjacent the cathode 8 on the opposite side thereof from the anode 6. The probe I4 may be cylindrical in shape with one end open and the other end comprising conical walls I6 tapering to a small aperture I1 about the axis of the probe cylinder I4. The angle of the probe cone and the angle of the conical depression in the cathode 8 are identical and both the cathode 8 and the probe I 4 are symmetrically disposed about the axis of the cylinder I so that the conical end wall of the probe I4 is parallel to the conical wall of the cathode 8 which it faces. The probe I4 may -be maintained in position by the use of an annular insulator I8 which snugly engages the exterior of the probe I4 and bears upon the interior wall cf the cylinder I. For particular applications of the invention, it may be advantageous to provide an extension I 9 upon the probe I4 which may extend beyond the face plate 2 and such extension may be joined to the probe proper by any suitable means such as by cooperating threads as shown in Fig. 1.

The spaced axial relationship between the elements of the source may be conveniently maintained by the use of cylindrical spacers 2|, 22, and 23. These spacers 2I, 22, and 23 may be inserted between the end wall 3 and the cathode 1, the cathode 'I and the anode insulator II, and the anode insulator I I and the cathode 3, respectively, as shown in Fig. 1. The entire assembly may then be secured in position by the use of hollow plug 24 which is secured to the cylinder by means permitting of longitudinal adjustment, such as bolts or threaded connection with the cylinder I, as shown. In assembling the apparatus the anode, cathodes and probe assembly are inserted in the cylinder I with spacers of the proper size separating them and the hollow plug 24 is then inserted and tightened up until no slack remains in the system and all elements are secured in position.

With regard to the electrical potentials of the various elements of the source, reference is made to Fig. 2 in which the relative potentials are indicated. The cathodes 1 and 8 are maintained at a negative potential with respect to the anode 6, and the probe I4 is maintained at a negative potential with respect to the cathodes. It will of course be apparent that many types of potential supply sources would be suitable for use in providing the required potentials and thus no specific description of such auxiliary apparatus of this type is included. Only relative potentials have been recited and no limitation is intended upon their absolute value. In fact in most instances it is advantageous to operate the cathodes at ground potential and the anode positive and the probe negative; however, only the relative potentials need be maintained to remain within the scope of the invention, and it will of course be appreciated that the provision of further insulation means enabling the cylinder I to remain grounded while the cathodes 'I and 8 are maintained at some other potential is also well within the spirit of the invention. The anode potential may be provided by connecting means, as depicted in Fig. 1, including a wire 3| which connects to the anode, passes through an insulator 32 inserted in the cathode 1, and is connected to an insulating seal 33 having a conductor through the center thereof and passing through the end wall 3 to connect with a suitable power supply (not shown). By this or any other suitable means the anode potential may be obtained without hazard to the vacuum within cylinder I.

A magnetic field is maintained through the ion source by any suitable means, such as the solenoid 34, shown in Fig. 1, or the magnetic eld which is produced by certain types of particle accelerators or separators with which the source may be used. As illustrated in the drawing, the magnetic eld is acting perpendicular to the electron emission surfaces of the cathodes and axially of anode 6. The electrostatic iield within the source also acts perpendicularly to the cathodes and considering the effective electron emission area lying in the projection of anode apertures the electrostatic lines of force curve from the cathode to the edges of the anode apertures. Thus at the ends of the anode the electrostatic eld is directed at least in part normal to the magnetic eld; which condition produces a maximized electron path. Elongation of the electron discharge path increases the amount of ionization and 'of course maximum ionization is obtained when the electron path is oscillatory, as is set forth below, and thus the magnetic field may be advantageously maintained at a ninety degree angle to the electric field in part as shown in order to produce such a path.

Considering now the operation of the ion source, it should be noted that an electric eld is established between the cathodes and the anode and a magnetic field is established through the anode and perpendicular to the cathode surfaces. The cathodes are formed of electron emissive material so that establishment of the above noted electric field causes electron discharge from each cathode. The electrons discharged from the cathodes are influenced by the magnetic field to travel along the lines of force of the magnetic field. Thus a part of the electron discharge from each cathode impinges the outside of the anode while the rest of the ldischarge passes through the openings 9 in the end walls of the anode 6. The electron discharge entering the anode is constrained from impinging on the anode by the action of the magnetic field and thus passes through the anode until suiciently close to the opposite cathode to be repelled by the negative charge thereon. Thus electron discharge which does not initially impinge on the exterior of the anode enters the anode and is subject to the successive attracting and repelling forces of the anode and cathode potentials while directed by the magnetlc field so that an oscillatory phenomenon results with a large number of electrons oscillating within the anode between the cathodes.

Through the medium of an external gas systern described above and the connecting tube 4 an ionizablc atmosphere is maintained within the cylinder I. The oscillatory electron discharge within the cylinder causes ionization of the atmosphere and produces a high concentration of positive ions along the axis of the anode. These positive ions in turn cause further ionization and also mpinge on the cathodes with sufficient energy to produce an appreciable secondary electron"emission which further increases the amount of ionization within vthe source. A

In order to remove the ions formed within the source, opening I3 is provided together with probe I4 which is situated adjacent thereto, as described above. A relatively large negative potential is applied to the probe I4 which establishes an electric iield that attracts the ions formed in the source so that a large portion of such ions pass through the opening I3 in the cathode S and hence, by virtue of their velocity, pass through the opening I'I in the probe Il'. and on into whatever device is being utilized in connection with the source. With the described invention these ions may be expelled in a high velocity beam of small cross section which is particularly desirable for use in most types of particle accelerators.

It will be appreciated that electrons emitted from thesurfaces of the cathodes are constrained to travel in a directionperpendicular to these surfaces by the magnetic field which acts normal to the cathode surfaces. Thus all of the electrons which are emitted from the area of the cathodes Within the projection of the openings 9 in the anode anode. Electrons emitted from the remainder of the surface of the cathodes travel directly to the anode. As the pressure within the source `is maintained at a low value, the mean free path of electrons in the source is large and thus no self-sustaining ionization results from electron discharge directly between the cathodes and anode.

The effective distance between the anode and cathodes is materially increased by the employment of a magnetic field perpendicular in part at least to the electric fields, as described above. This lengthening of the electron path increases the ionization produced by the electrons and, consequently, materially increases the ion density within the source. The ion density is a maximum along the axis of the anode and thus it is desirable to remove ions from this region if ion output current is to be maximized. This is accomplished by the eiects of the electric iield on the probe I4 which attracts the ions from their region of maximum density through the aperture I3 in the cathode. It will of course be appreciated that the diameter of the aperture I3 is quite small. If the diameter of the aperture I3 is too large the electron discharge will be extinguished; however, a number of additional apertures in close proximity to aperture `I 3 may be employed to increase the output without materially affecting the electron discharge.

The output current may be further increased by constricting the electron discharge in order to decrease the cross section thereof and increase the ion density along the axis of the cylinder I. rlhis may be accomplished by means, such as illustrated in Fig. 3, wherein the anwill enter the 1 ode and cathode elements are similar to .and are maintained in the same position as the elements shown in Fig. 2, and described above. As in the previously described embodiment, the cathodes Ia and 8a are preferably formed of a good electron emitting material, such as aluminum; however, the anode-presented area of the cathode surfaces is decreased by masking elements 4I and 42 which are placed over the electron emission surfaces of the cathodes 'Ia and 3a, respectively. The masks 4I and 42 are formed of a material having very poor electron emission qualities, and axially aligned transverse apertures in these masks expose a limited area of the cathodes 'Ia and 8a to the anode 6a so that the electric eld established between the anode and cathodes produces electron emission only from the exposed portion of the cathodes la and 8a. This reduces the electron discharge cross section and increases the ion density along the axis of the anode 6a so that a larger quantity of ions are in position to pass through the aperture ISa in the cathode 8a thereby increasing the ion output from the source unit.

With regard to the removal of ions from the source, it is of interest to note that the anode produces a focusing action upon ions passing through the openings 9 therein as a result of the positive potential applied to the anode. This focusing action causes a constriction oi the ion stream leaving the anode and thus increases the density of the ions at or about the axis of the source, thereby increasing the ion output current passing through the aperture I3 which is symmetrical about the axis of the source.

With regard to the probe structure and position, it will of course be appreciated that the current output of the source is a function of the distance and voltage drop between the cathode E and probe I4. Thus, in order to maximize the output current it is necessary to maximize the 'folta-ge drop and minimize the separation. It may'bc further noted that for certain applications of the invention it is advantageous for the ion beam produced from the source to have a small cross section. The illustrated embodiment of the probe l is particularly adapted to the production of such a beam. As may be noted from a consideration of the electric eld about the probe i4, the majority of ion acceleration provided in a region of converging lields wherein the ion stream is focused and thus the subsequent defzcusing elect of the eld inuences the ion stream to a lesser extent. The result is that the probe field acts to consti'ict the ion stream and produce an intense beam oi small cross secion. In order to satisfy the requirement conica-l probe configuration and small probe-cathode separati-on the cathode is conical shaped, as described a'oovc, and the probe is nested therein. as shown in l. Maximum focusing of the ion stream is obtained with a cone angle of ninety degrees; however, such angle is not critical and nominal lvariations from ninety degrees do not produce a detectable change in the beam. It will of course be appreciated that the invention is not limited to a conical probe configuration which has been illustrated as only a preferred embodiment. Ions may be attracted equally well from the source by electrodes or probes of other shapes and actually the shape of the pobe is only of importance in defining the beam after its exit from the source proper.

The present invention clearly overcomes the previous problems attendant the removal of ions 7 v from a PIG ion source, that is, it is capable of producing an extremely large quantity of ions even in a low pressure area and is further capable of removing a large proportion of these ions from the source by attracting them directl from the center of ion concentration.

It will of course be apparent that numerous modifications and variations of the ion source are possible within the spirit and scope of the invention, and thus the invention is not to be limited to the exact details disclosed except as may be defined in the following claims.

What is claimed is:

1. An ion source unit comprising a pair of cathodes each having a fiat front surface, said cathodes being disposed with fiat surfaces adjacent and parallel, a hollow cylindrical anode disposed between said cathodes and spaced therefrom with the axis thereof normal to the fiat surfaces of said cathodes, said anode having openings in each end thereof about the axis of said anode cylinder, one of said cathodes having an inverted conical surface in the back thereof with an aperture for-med from the point of said cone through said cathode, said aperture being symmetrical about an extension of the axis of said anode cylinder, means establishing a magnetic field through the source parallel to the axis of said anode cylinder, a probe electrode having a conical surface with an aperture through the probe at the point of said conical surface, said conical probe surface mating with the conical surface of said cathode in close proximity therewith and parallel thereto, said probe aperture and said cathode aperture being in axial alignment, and means maintaining said probe electrode at a negative potential with respect to said apertured cath-ode whereby ions are attracted from a region about the axis of said anode and expelled from the ion source.

2. An ion source unit comprising in combination an envelope enclosing an ionizable atmosphere, a pair of parallel spaced cathfodes, an anode having an opening therethrough disposed between said cathodes and spaced therefrom, potential supply means establishing` an electric field between said anode and cathodes whereby electron discharge from said cathodes is established, means establishing a magnetic field through said source and directed in part at an angle of approximately ninety degrees to said electric field whereby said electron discharge is constrained to oscillate through said anode, discharge limiting means defining the area of electron discharge from said cathodes, and ion removal means adjacent one end of said electron discharge whereby ions are attracted directly from the region of said electron discharge.

3. An ion source unit as defined in claim 2 wherein said discharge limiting means comprises a pair of masks, one covering the surface of each of said cathodes, said masks being formed of a. material having poor electron emission qualities and each having an aperture therein, thereby limiting the area of said cathodes available for electron emission.

4. An ion source unit comprising means including a cathode establishing an ionizing arc discharge, said cathode having a conical depression in the back thereof and an aperture therethrough from the apex of said conical depression, a conical electrode outside said source proper and having an aperture through the apex of said cone, said electrode nesting into the conical depression in said cathode, the surface of said electrode being parallel to the adjacent surface of said cathode and in close proximity therewith, and means impressing a negative potential upon said electrode with respect to said cathode whereby ions are attracted from said source, the angle of said electrode cone being approximately ninety degrees whereby ions attracted from said source are focused into a beam.

5. An improved ion source unit comprising a hollow cylindrical envelope, means establishing a magnetic field axially through said envelope, a first electron emissive cathode disposed within said envelope in closing relation to one end thereof, a cylindrical anode spaced from said first cathode within said envelope and having an aperture therethrough about the axis of said envelope, a torroidal insulator disposed about said anode cylinder in bearing relation to the exterior thereof and to the internal surface of said cylinder, said insulator mounting and insulating said anode cylinder, a second electron emissive cathode disposed within said envelope on the opposite side of said anode from said first cathode and spaced from said anode, said second cathode being in closing relation to the other end of said envelope from said first cathode, said first and second cathodes having flat parallel front surfaces facing said anode with said surfaces normal to the axis of said envelope, said second cathode having an aperture therethrough about the axis of said envelope and a concave back surface with the concavity being cone-shaped and said aperture being at the apex thereof, and a conical ion ejector having an aperture therethrough about the axis of said envelope, said ion ejector cone having the same angle as the conical concavity of the back of said second cathode and being disposed adjacent thereto and insulated therefrom.

6. An ion source comprising an evacuated cylindrical envelope formed of electrically conducting material and having a closed end, a first disc-shaped cathode Within said envelope adjacent the closed'end thereof, said first cathode being disposed about the axis of said cylinder normal thereto and in electrical contact with said envelope, a second cathode disposed within said envelope in electrical contact therewith and spaced from said first cathode with adjacent cathode faces being parallel, said second cathode having a concave surface away from said first cathode and an aperture therethrough at the point of maximum concavity, a hollow cylindrical electrically conducting anode disposed within said envelope equidistant between said adjacent cathode surfaces, a rst torroidal insulator about said anode cylinder in bearing relation thereto and to said envelope and maintaining said anode in axial alignment with said envelope, a plurality of insulating cylindrical spacers disposed one between the closed end of said envelope and said first cathode and one on each side of said first torroidal insulator to preserve the above spacings, a probe electrode within said envelope and having a convex surface mating with the concave surface of said second electrode and disposed adjacent thereto, a second torroidal insulator disposed about said probe electrode in bearing relation thereto and to said envelope and said second cathode to maintain said probe electrode in axial alignment with said envelope, said probe electrode having an aperture axially therethrough in alignment with the aperture in said second cathode, andl a hollow plug threadably 9 engaging the open end of said envelope and bearing upon said second torroidal insulator whereby the aforementioned elements are maintained n spaced relation within said envelope.

JAMES D. GOW. JOHN S. FOSTER.

References Cited in the le of this patent 10 Number Name Date 2,499,320 Loevinger Feb, 28, 1950 2,507,652 Smith Mar. 16, 1950 OTHER REFERENCES

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