US3500122A

US3500122A - Gas discharge ion source with multiple anode structure and a.c. supply means

Info

Publication number: US3500122A
Application number: US405156A
Authority: US
Inventors: Gordon Sohl
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1964-10-20
Filing date: 1964-10-20
Publication date: 1970-03-10
Anticipated expiration: 1987-03-10

Description

.March 10, 1970 G. SOHL 3,500,122 GAS DISCHARGE ION SOURCE WITH MULTIPLE ANODE STRUCTURE AND A.C. SUPPLY MEANS Filed Oct. 20, 1964 I l3 l2 I0 15 1 s INVENTOR GORDON 50H! ATTORNEY:

W/ll/AM A. KEMMEZ JR.

United States Patent U.S. 'Cl. 315-111 9 Claims ABSTRACT OF THE DISCLOSURE A gas discharge ion source having a chamber containing an ionizable gas, a cathode, and at least two anodes with an alternating current power source connected between them and a constant magnetic field adapted to increase the length of the electron path between them.

In general, the present invention relates to an ion source for a device such as an ion engine. More particularly, the present invention involves a gas discharge ion source adapted to use alternating current directly while maintaining continuous arc operation with high mass efficiency.

At present, much research and development effort is being expended in the area of electrical propulsion systems for space vehicles because of the very large velocities they can produce. One electrical propulsion system is the ion engine utilizing a gas discharge ion source such as described by Speiser, et al. in Studies of a Gas Discharge Cesium Ion Source, American Rocket Society Annual Meeting, (November 1962), and Test Results for a Cesium Electron Bombardment Ion Motor, Institute of the Aerospace Sciences Meeting, (January 1963). As disclosed in the aforementioned Speiser, et al. papers, the process of ion production from a gas discharge ion source is due almost entirely to collisions between gas atoms and electrons with a kinetic energy greater than the ionization potential level. Thus, a continuous flow of electrons from the cathode to the anode must be maintained, i.e., an arc must be maintained, in order to maintain a high level of ion production. If, for example, the gas atoms escape in a neutral form, they are not accelerated and thus reduce the mass efficiency of the ion engine. Because of such reasons, a gas discharge ion source for an ion engine invariably utilizes direct current to maintain the are between the cathode and anode. However, the power for operating the ion engine is available as a practical matter only in alternating current form. Consequently, at present, it is necessary to convert the alternating power to direct power when operating the gas discharge ion source for the ion engine. Such conversion not only entails a power loss but also requires additional electrical equipment which adds weight and operation problems to the total system. For this reason, it has been attempted in the past to operate the gas discharge ion source for an ion engine on alternating current using the conventional construction illustrated in the Speiser et al. papers. However, it was found that the use of alternating current with such equipment caused the arc to be extinguished during the negative half cycle of the alternating current which in turn caused neutral gas atoms to escape from the ion source and to reduce the mass efficiency of the ion source.

Consequently, an object of the present invention is a low voltage gas discharge ion source adapted to use alternating current directly.

Another object of the present invention is a gas discharge ion source comprising a chamber containing a body of gas adapted to be ionized by bombardment with 3,500,122 Patented Mar. 10, 1970 electrons. The chamber has an inlet for admitting said gas and an outlet for emitting positive gas ions. In the chamber is a cathode and at least two spaced anodes substantially spaced from the cathode. The ion source also includes means for maintaining a magnetic field in the chamber adapted to increase the length of the electron path between the cathode and anodes. The method of the present invention involves forming the aforementioned device and then connecting the lines from an alternating current power source to the primary winding of a transformer. Then, the aforementioned anodes are connected to the end taps of the secondary winding of a transformer while the cathode is connected to an intermediate tap of the secondary winding of such transformer. Finally, a discharge is initiated in the gas between the cathode and the anodes.

In order to facilitate understanding of the present invention, reference will now be made to the appended drawings of a preferred specific embodiment of the present invention. Such drawings should not be construed as limiting the invention which is properly set forth in the appended claims.

In the drawings:

FIG. 1 is an axial cross-section of a semi-schematic diagram of a gas discharge ion engine incorporating gas discharge ion source of the present invention.

FIG. 2 is a cross-sectional view of FIG. 1 taken along lines 2-2 FIG. 1.

FIG. 3 is a schematic wiring diagram of the present invention illustrated in FIG. 1 with its connection to a source of three-phase alternating current power.

As illustrated in FIGS. 1-2, the present invention is utilized in a gas discharge ion engine 10 having an accelerating electrode 11 with a plurality of apertures 11' for ejecting the positive ions generated in the gas discharge ion source 12. The gas discharge ion source 12 includes a cylindrical chamber 13 having a side wall 14, a first end wall 15, and a second end wall 16. The chamber 13 contains a body of gas adapted to be ionized by bombardment with electrons. Preferably, the gas is composed of materials such as cesium or mercury which are adapted to be ionized by bombardment with low energy ions. Also, such gas is maintained at a low pressure in order to support the discharge between the cathode and anodes. Thus, the gas pressure is preferably in the range of about 1 10- to 1 10- mm. Hg when cesium is used. The first end wall 15 has an inlet 17 for admitting the gas to the chamber 13. The second end wall 16 has a plurality of outlets 18 aligned with the accelerating electrode apertures 11' for emitting positive gas ions. Thus, the end wall 16 forms a screen electrode. In the chamber 13, adjacent to the end wall 15 is a cathode 20 which adjoins the inlet 17. The cathode 20 comprises a cover 21 mounted on the end wall 15 and connected to a gas inlet conduit 22. The cover 21 encloses an emitter coil 23 of refractory metal grounded to the cover 21. Covering the emitter coil 23 and adjoining the conduit 22 is a baffle plate 24 for directing the gas flow over the emitter coil 23. As disclosed in the above Speiser, et al. papers, the surface of the emitter coil 23 has a low work function maintained by bringing the cesium through it to form a cesium coating on the surface. The cathode 20 also includes means for heating it to initiate a discharge between it and the anodes 30. Such means comprises a resistance coil 25 coaxially mounted within the emitter coil 23 and separated therefrom by an insulating sheath 26. However, as noted in the abovementioned Speiser, et al. papers, by proper design, the cathode can be heated solely by power delivered by the bombardment of the positive ions from the gas onto the cathode. Such self-heating cathode operation is achieved by the combination of introducing the gas into the chamber 13 by the cathode as noted above and mechanically enclosing the cathode such as illustrated in FIG. 1 wherein the opening 17 be tween the cathode 20 and the chamber 13 is substantially smaller than the cathode cover 21. In this way, the gas pressure adjoining the cathode is increased to promote self heating operation. Around the exterior of the side wall 14 is a magnetic coil 40 which provides means for maintaining a substantially constant magnetic field in the chamber 13 adapted to increase the length of the electron path between the cathode 20 and the anodes 30. The magnetic coil 40 and resistance coil each have a power input lead and common leads grounded to the walls of the chamber 13. The

walls

14, 15, and 16 of the chamber 13 as well as the cover 21 in turn are electrically connected to a common output lead 27. Thus, both the cathode 20 and chamber 13 are maintained at the same voltage with respect to the anodes 30.

In the chamber 13, substantially spaced from the cathode 20 are at least two spaced anodes 30. As illustrated in FIGS. 1 and 2, the chamber 13 is cylindrical and there are equally spaced eighteen anodes extending parallel to the axis of the chamber 13 adjacent to the chamber side wall 13. However, as illustrated in the schematic wiring diagram in FIG. 3, the anode 30 are electrically connected in groups of threes with each member of the group being spaced apart by 120 degrees so that there is the electrical equivalent of at least six anodes equally spaced around the circumference of the side wall 14. For example, the electrodes 30a are connected together to a common lead. Each anode 30 is supported by an insulating post 31 and is connected by a lead 32 through an insulating sleeve 33 to an external source of power.

To operate the gas discharge ion source 12, the chamber 13 is constructed as set forth above with the cathode 20 at one end of the chamber 13 and eighteen anodes 30 spaced from the cathode 20 and around the side wall 14 in the chamber 13. A substantially constant magnetic field is formed in the chamber substantially parallel with the anodes by activating the magnetic coil 40. The lines from the power source then are connected to the end taps of the primary winding of a transformer. Specifically, as illustrated in FIG. 3, the lines of three-phase alternating current power are connected to the primary delta winding taps 51 of a three-phase transformer. The six anode leads 32 are connected to the six secondary winding taps 53 of a center-tap star output of the transformer 52. The cathode 20 is connected to the center tap 54 of the secondary winding of the transformer 52 by the output lead 27. The emitter 23 is then heated by connecting the resistance coil 25 to an appropriate source of power and gas such as cesium vapor is admitted to the chamber 13 through the inlet line 22 and inlet 17. Under such conditions, a discharge is initiated in the gas between the cathode 20 and the anodes 30. When such discharge has been initiated, a portion of the positive ions produced will bombard the cathode 20 to generate sufficient heat to maintain its operation so that the power input to the emitter 23 may be cut off.

A gas discharge ion source was constructed and operated as set forth above. Specifically, an arc current of about 33 amps was set up under a voltage difference of about 7 volts with a magnetic field of about 8 gauss. Under such conditions, it was found that a beam current of 325 ma. was produced with the emitted positive ions having an energy of 3500 ev. A mass utilization efficiency of 87% was attained at an are energy expenditure of 764 ev./ ion. With such ion source, a thrust of 7.35 X 10" lbs. was achieved for a power to thrust ratio of 200 kw./lb. Thus, the power efi'iciency was 79% and an over-all engine efficiency of approximately 69% was obtained.

The rectification of the alternating current power by the discharge ion source of the present invention appears to depend on the relative mobilities of the ions and electrons in the ionized gas plasma in the chamber. Thus,

. 4 where the anodes are maintained at about 7 vo 'lts above the cathode, the ionized gas plasma has the voltage of about 1 volt above the anodes and forms a double sheath such as discussed in the above Speiser, et al. papers. Under such conditions, electrons emitted by the cathode can pass freely from the gas plasma to the anode; however,

' when the voltage difference is reversed, the low mobility of the ions substantially prevents them from traveling to the then negative anode. Consequently, with a single anode, the'discharge is extinguished during the negative half cycle of the alternating current power. However, with atleast two anodes, an anode is positive with respect to the cathode at all times so that sufficient electron current will be drawn to maintain the discharge. In addition, by utilizing at least six anodes in conjunction with three-phase alternating current power, the positive voltage difference between the anodes and the cathode is always maintained above the forward voltage drop necessary to maintain the are between the cathode and the anode and a relatively smooth arc current is maintained.

Many other-specific embodiments of the present invention will be obvious to one skilled in the art in view of this disclosure. Thus, for example, a variety of anode and cathode configurations may be utilized as long as a magnetic field may be utilized between the cathode and anode to hold the electrons sufficiently long therebetween to generate a supply of positive ions.

There are many features in the present invention which clearly show the significant advance the present invention represents over the prior art. Consequently, only a few of the more outstanding features will be pointed out to illustrate the unexpected and unusual results obtained by the present invention. One feature of the present invention is that large alternating currents may be supplied to the device of the present invention at low voltage and the power is used directly with high efficiency to form a gas discharge ion source. Thus, the need for the normally required conditioning equipment is eliminated. Another feature of the present invention is that a multiple-phase alternating current power can be supplied to a single gas discharge ion source without the need for rectifying the individual current phases. Still another feature of the present invention is that the rectification of the alternating current power occurs in the device which is resistant to radiation unlike the present widely-used solid state rectifiers.

It will be understood that the foregoing description and examples are only illustrative of the present invention and it is not intended that the invention be limited thereto. All substitutions, alterations, and modifications of the present invention which come within the scope of the following claims or to which the present invention is readily susceptible Without departing from the spirit and scope of this disclosure are considered part of the present invention.

What I claim is:

1. A simple, efficient gas discharge ion source adapted to use alternating current directly while maintaining continuous arc operation with high mass efficiency comprising:

(a) a chamber containing a body of gas adapted to be ionized by bombardment with electrons, said chamber having an inlet for admitting said gas and an outlet for emitting positive gas ions;

(b) a cathode in said chamber and at least two spaced anodes in said chamber substantially spaced from said cathode;

(c) means for maintaining a substantially constant magnetic field in said chamber substantially parallel to the axis between said inlet and outlet adapted to increase the length of the electron path between the cathode and anodes; and,

(d) means'fore supplying alternating current power to said cathode and anodes, connected to said cathode and anodes.

2. A gas discharge ion source as stated in claim 1 wherein said gas is cesium vapor.

3. A gas discharge ion source as stated in claim 1 wherein said gas is maintained at a pressure in the range of about 1X10 to 1X10 mm. Hg.

4. A gas discharge source as stated in claim 1 wherein said cathode is adjacent to one end wall of said chamber and adjoins said inlet so that the incoming gas contacts said cathode.

5. A gas discharge ion source as stated in claim 1 wherein said chamber is cylindrical with said anodes extending parallel to the axis of said chamber and being positioned adjacent to the chamber side wall and said cathode is positioned adjacent to one end wall of said chamber.

6. A gas discharge ion source which, as stated in claim 4, includes means for heating said inlet cathode to initiate a discharge between said inlet cathode and said anodes.

7. A gas discharge ion source as stated in claim 5 wherein at least six anodes are equally spaced around the circumference of the side wall.

8. A method of using alternating current directly from a power source to operate a gas discharge ion source with continuous arc operation and high mass efliciency comprising:

(a) containing in a chamber a body of gas adopted to be ionized by bombardment with electrons;

(b) contacting said body of gas with a cathode and at least two spaced anodes substantially spaced from said cathode;

6 (c) forming a substantially constant magnetic field in said chamber extending substantially parallel to the axis of said chamber, adapted to increase the length of the electron path between the cathode and anodes. (d) connecting the lines from said power source to the primary winding of a transformer; (e) connecting said anodes to the end taps of the secondary winding of said transformer and said cathode to an intermediate tap of the secondary winding of said transformer; and, (f) initiating a discharge in said gas between said cathode and anodes. 9. A method as stated in claim 8 wherein the discharge between said cathode and anodes is initiated by heating said cathode.

References Cited UNITED STATES PATENTS 3,113,427 12/1963 Meyer 313-6-3 3,243,954 4/1966 Cann 315l11 3,283,205 11/1966 De Bolt 315--111 JAMES W. LAWR'ENCE, Primary Examiner P. c. DEMEO, Assistant Examiner US. 01. X.R.