US3308621A

US3308621A - Oscillating-electron ion engine

Info

Publication number: US3308621A
Application number: US334972A
Authority: US
Inventors: Edward A Pinsley
Original assignee: United Aircraft Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1963-12-30
Filing date: 1963-12-30
Publication date: 1967-03-14
Anticipated expiration: 1984-03-14

Description

FINEBZ March 14, 1967 E. A. PINSLEY OSCILLATING-ELECTRON ION ENGINE Filed D60. 50, 1963 3 Sheets-Sheet 1 March 14, 1967 E. A. PINSLEY 3,308,621

OSCILLATING-ELECTRON ION ENGINE Filed Dec. 50, 1963 3 Sheets-Sheet 2 K/M/ /M March 14, 1967 E. A. PINSLEY n 3,308,621

OSCILLATING-ELECTRON ION ENGINE Filed neo. so, 196s s sheets-sheet :s

United States Patent O 3,308,621 OSCILLATING-ELECTRON ION ENGINE Edward A. Pinsley, Glastonbury, Conn., assignor to United Aircraft Corporation, East Hartford, Conn., a

corporation of Delaware Filed Dec. 30, 1963, Ser. No. 334,972 20 Claims. (Cl. 60-202) This invention relates to an electrical propulsion system. More particularly, this invention relates to an improved oscillating-electron type of ion engine.

In the oscillating-electron ion engine, positive ions are produced by electron bombardment of neutral gas molecules. These ions are accelerated by self-generated potential gradients maintained in an essentially space-charge neutral plasma. The basic principle of the operation of the engine involves the establishment of an axially symmetric electrostatic potential well on which electrons are axially trapped `and from which ions are axially accelerated. The radial drift of electrons is inhibited by the presence of a confining magnetic field.

The oscillating-electron ion engine utilizes many of the features of the Penning discharge. A discussion of the basic principles of the Penning discharge as `adapted for use in accelerating ions and plasmas can be found in High Current Ion Source by R. G. Meyerand, Jr., et al., in the Review of Scientific Instruments, vol. 30, No. 2, February 1959. In the Penning discharge, electrons vare emitted by a cathode and Iare accelerated electrostatically toward a hollow cylindrical anode. AThe electron drift motion in the radial direction toward the anode is constrained by a comining D.C. magnetic field generated by a solenoid which encircles the anode. Hence, the electron trajectory is a spiral along a magnetic field line contained within the anode cavity. This spiral terminates at the extremities of the potential well, and the electron thus oscllates between these extremities.

When the Penning discharge is to be used as an ion or plasma, a hollow electrode is positioned downstream of the anode and is maintained at the same potential as the cathode. During operation of the Penning device, a selfgenerated electrostatic potential gradient will be set up in the vicinity of the downstream electrode in a fashion described in Electrostatic Potenial Gradients in a Penning Discharge by F. Salz et al., Physical Review Letters, May l5, 1961. The electrons are refiected back through vthe anode cavity by this potential gradient, and are thus trapped and forced to oscillate in the potential well formed between the cathode and the downstream electrode. The expellant gas is admitted into the source and diffuses as a free molecular flow through the anode region. Ions are formed by the impact of electrons upon the expellant gas molecules thereby forming a neutral plasma.

In the above-described oscillating-electron ion engine utilizing the Penning discharge principle, it can be shown that a large portion of the thrust is transmitted to the engine through impingement of ions on the cathode. This impingement of ions on the cathode is undesirable because it represents a source of energy loss and also results in deterioration of the cathode by sputtering action.

In the present invention the efliciency of the engine is increased and sputtering damage to the cathode is reduced by generating part of the engine thrust through interaction between electrons and a magnetic mirror field whereby momentum from the oscillating electrons is transferred as thrust to the structure generating the magnetic mirror field. A nonuniform magnetic field is generated in the engine with a higher flux density in the vicinity of the engine cathode than elsewhere in the engine to form a magetic mirror in the vicinity of the cathode. The presence of the higher flux density magnetic field in the vicinity of the ice cathode results in a significant interaction between the tangential component of velocity of the electrons and the radial component of the magnetic field. Electrons are reflected away from the cathode by this interaction and their momentum is transferred to the engine as thrust on the structure generating the magnetic field.

The generation of the magnetic mirror in the vicinity of the cathode can be accomplished in several ways. The ampere turns of the engine solenoid can be increased in the vicinity of the cathode; a ferromagnetic element can be inserted in the engine behind the cathode; ferromagnetic elements can be used to provide a desired path for the magnetic lines of flux; or combinations of these alternatives can be used. Furthermore, the envelope of the engine can be contoured to conform in a general manner with the shape of the magnetic field to minimize charged particle interception losses.

Accordingly, one object of the present invention is to provide a novel and improved electrical propulsion device.

Another object of the present invention is to provide a novel electrical propulsion device having a magnetic mirror which is `utilized in the production of thrust.

Still vanother object of the present invention is to provide a novel electrical propulsion engine having greater efi'ciency and greater life expectancy than previous engmes.

Other objects and advantages will be apparent from the specification and claims, and from the accompanying drawings which illustrate an embodiment of the invention.

FIGURE l is a schematic representation of an oscillating electron ion engine with a generalized showing of a magnetic mirror in accordance with the broad concept of the present invention.

FIGURE 2 is a showing of an oscillating-electron ion engine wherein the engine envelope is contoured to conform to magnetic field lines.

FIGURE 3 is a showing of -a modified ion engine having a ferromagnetic casing to direct magnetic flux lines to form =a magnetic mirror.

FIGURE 4 is a showing of another modified ion engine having ferromagnetic rods and structure.

FIGURE 5 is a showing along line 5-5 of FIG. 4.

FIGURE 6 is a View along line 6 6 of FIG. 7 and is a sectional elevation View of a cluster of engines of the type shown in FIG. 4.

FIGURE 7 is a View along line 7 7 of FIG. 6.

In the several figures of the drawings, similar parts will receive the same numbers.

Referring now to FIG. l, the oscillating-electron ion engine 10 has an annular anode electrode 12 and an annular downstream electrode 14 separated from anode 12 by an annular insulating element 16. Downstream electrode 14 defines the open end of the ionization region chamber 17 of the ion engine. Annular anode 12 is separated by another annular insulator 18 from backplate element 20 which closes off one end of chamber 17.

A cathode element 22 is located in chamber 17 and is supplied with heating current from a current supply 24 through a transformer 26. The end terminals of the secondary of transformer 26 are connected to cathode 22 via conductors 28 and 30 which pass through backplate 2() through insulating sleeves 32 and 34, respectively. Cathode 22 is maintained at D.C. ground potential through the grounded center tap 36 of the secondary winding of transformer 26; backplate 20 is connected to ground via conductor 3S, and the downstream electrode 14 is also connected to ground via conductor 4t) connected to conductor 30 through a connection to backplate 20. The description ground implies a common potential; in a space vehicle this will generally, but not necessarily, be

the vehicle potential. A positive acceleration voltage is supplied to anode 12 through a conductor 44 Which is connected to the positive side of the power supply 45.

The expellant or propellant gas which is to be ionized in chamber 17 is introduced into chamber 17 from a supply tank (not shown) through a tube 48 which passes through backplate 20. It will be understood that the propellant gas feed can be accomplished at other locations such as radially inward through the anode as shown in succeeding figures of the drawings. Since ionization is achieved in chamber 17 through electron bombardment, any propellant gas may be used provided essentially only one charge-mass ratio species is obtained. By way of example, argon, neon, krypton, nitrogen, mercury, xenon or the alkali metals can be used as propellants.

Exteriorally of the annular envelope defining chamber 17 is a solenoid 5t) connected to a source of D.C. current (not shown) for generating a magnetic field substantially coaxial with the axis of engine 10. Solenoid 50 has an increased number of turns 52 in the vicinity of the plane of cathode 22. The increased number of ampere turns 52 serves to form a magnetic mirror by creating a magnetic field having a greater flux density in chamber 17 in the vicinity of cathode 22 than elsewhere in chamber 17. The entire magnetic eld generating structure is either supported on engine or is attached to the vehicle of engine 10. A ferromagnetic element 54 extending from backplate 20 toward cathode 22 is also shown forming part of the magnetic mirror. In accordance with the present invention, either the increased number of turns 52 or ferromagnetic element 54 can be used independently or in conjunction with the other to form a magnetic mirror in chamber 17 in the vicinity of cathode 22; furthermore, any other means of creating a magnetic mirror in the vicinity of cathode 22 can be used as equivalents within the scope of the present invention. However, it is preferred that the varying flux density magnetic field extend throughout chamber 17 and vary from a maximum flux density in the vicinity of the cathode to a minimum ux density at the discharge end of the chamber 17. That is, a magnetic mirror having a varying flux density throughout chamber 17 is preferred to one which only has a localized flux density variation in the vicinity of cathode 22.

In the operation of the ion engine shown in FIG. l, electrons are emitted from cathode 22, and these electrons are accelerated electrostatically toward anode 12. The electron drift motion in the radial direction toward anode 12 is constrained by the D.C. magnetic field generated by solenoid 50 so that the electron trajectories are spirals along axes substantially parallel to the magnetic field lines. As the electrons spiral down the: ionization chamber they collide with neutral gas molecules of the propellant introduced into the chamber via tube 48, thereby producing positive ions and forming a plasma. As explained in the above-referenced article by Salz et al., from Physical Review Letters, a self-generated electrostatic potential gradient will be set up in the plasma in the vicinity of downstream electrode 14. This potential gradient produces a potential Well which Will accelerate ions in the vicinity of downstream electrode 14 toward the open or discharge end of chamber 17 while decelerating electrons. As the ions are ejected from the engine the discharge beam potential rises slightly, and this allows a sufiicient number of cathode electrons whose mean free paths are very long to escape from chamber 17 along with the ions to form the essentially neutral plasma discharge beam necessary for propulsion.

In the standard type of oscillating-electron ion engine the thrust is transmitted to the engine through impingement of ions and electrons on the cathode. As has already been pointed out, the impingement of ions represents an energy loss which limits engine efficiency and also results in deterioration of the cathode through sputtering. The magnetic mirror construction of the present engine overcomes these shortcomings of the standard type of oscillating-electron ion engine by contributing to the generation of thrust through the interaction of electrons and the field of the magnetic mirror.

In the structure of FIG. l, the increased number of ampere turns 52 and the ferromagnetic element 54 function to bunch the magnetic flux lines in the vicinity of cathode 22 so that fiux density in chamber 17 is highest in the vicinity of cathode 16. As has already been stated, it is preferable to form a magnetic mirror so that the magnetic lines of flux diverge from cathode 22 throughout chamber 17 to downstream electrode 14 so that the magnetic mirror extends throughout chamber 17. As a result of the presence of the magnetic mirror in chamber 17 there is a significant interaction between the tangential component of electron velocity and the radial component of the magnetic field. Interaction between the ions and the field is small because of the relatively great weight of the ions with respect to the electrons. Electrons are reflected by this above-discussed interaction with the magnetic field and their momentum is transferred as thrust to the structure generating the magnetic field, i.e., on the solenoid 50 and the increased number of turns 52. The solenoids are supported by the vehicle structure and hence this thrust is transmitted to the vehicle. As a result of the application of the magnetic field mirror, engine operating efficiencies are improved because the generation of thrust is not solely dependent on the backflow of ions, and engine life is lengthened by the reduction in cathode erosion.

Referring now to FIG. 2, the annular envelope of engine 10 is contoured, i.e., the inner surfaces of anode 12, electrode 14, and

insulators

16 and 18 are shaped, along the lines of the magnetic field generated by solenoid 50. The contouring is such that magnetic field lines which pass through the cathode 22 are not allowed to intercept any other electrode surface, and as a result charged particle interception losses are minimized. The contouring of the engine surface substantially conforms to the contour of the magnetic field from the axial station of cathode 22 so that the interior surfaces of electrode 14,

insulators

16 and 18, and anode 12 are everywhere parallel to the magnetic field lines in their immediate vicinity. The configuration of the magnetic field throughout the length of chamber 17 constitutes a magnetic mirror in chamber 17 which functions as described above to transmit thrust through interaction between electrons and the magnetic mirror. The transmittal of thrust between electrons and ions is accomplished by electrostatic forces by the same principles as described -by Salz et al. in Physical Review Letters.

Referring now to FIG. 3, `a ferromagnetic sleeve 60 extends from downstream electr-ode 14 to backplate 20, both electrode 14 and back-plate 20 also being ferromagnetic. Sleeve 60 and backplate 20 `are shown in FIG. 3 as being of one piece, which would be cup-shaped in construction, but it will be apparent that sleeve 60 and backplate 20 could be separate elements. Magnetic field generating solenoid 50 is housed in the annular chamber form-ed by the envelope of the engine and sleeve 60. The ferromagnetic structure of sleeve 60, backplate 20, and element 54 provides a high permeability path for the lines of flux generated by solenoid 50. This ferromagnetic structure acts to shape the magnetic field so that it is of very high flux density in the vicinity of cathode 22 and diverges toward downstream electrode 14 so that the necessary magnetic mirror is formed.

T-he embodiment shown in FIG. 4 is quite similar to that in FIG. 3 in that ferromagnetic structure is provided exteriorally of the engine envelope to provide a magnetic ux path. However, in the FIG. 4 embodiment the external ferromagnetic structure consists of a plurality of individual ferromagnetic rods 62 placed around the engine, preferably uniformly around the engine as shown in FIG. 5 and extending between ferromagnetic electrode 14 and ferromagnetic back-plate 20. In the FIG. 4 embodiment the magnetic field generating elements are a number of separate small coils 64, each of which surrounds one ferromagnetic rod 62. The FIG. 4 embodiment functions in the same manner as the FIG. 3 embodiment, but -the FIG. 4 embodiment realizes a significant weight savings in the use of the individual rods rather than :an entire ferromagnetic sleeve, and further substantial savings are realized in the use of small individual coils rather than a massive solenoid winding.

A further significant advantage of the structure of FIG, 4 resides in the fact that it can be used in a building block concept. As can be seen in FIG. 5, by shaping t-he end plate 20 hexagonally a ready foundation is made for expansion by adding individual unit-s together. The units would be butted together along hexagonal edges, and as will be more fully described in connection with FIGS. 6 and 7, the butting units can share common rods 62 to realize :a further savings in weight and material.

FIG. 7 shows the end plate view of a cluster of seven elements of the FIG. 4 embodiment. As can readily be seen, each engine shares several rods with Ione or more neighboring engines. For example, central engine 10a has rods a, b, c, d, e and f; rods a and b are in common with engine 10b; rods bI and c are in common with engine 10c; rods c and d are in common with engine 10d; rods d and e are in common with engine 10e; rods e and f are in common with engine 10j; and rods f and a are in common with engine 10g. It can also be readily seen that engine 10b shares rod g in common with engine 10c and rod h in -Common with engine 10g, and the commonality of the `other rods will be readily apparent. The structure of FIG. 7 is shown having a single backplate 20, but it will be understood that this multi-engine structure could as well be made by joining a number of the hexagonal units shown in FIG. 5 and using common rods as described.

FIG. 6 is a view along line 6 6 Iof FIG, 7 and shows the engines 10b, 10a, `and 10e of FIG. 7. A manifold 66 surrounds each engine to supply the propellant through openings in the anode, and each manifold is fed lthrough a conduit `68 by supply tank 70. There is a valve 72 in the supply line to each engine, and any engine can be selectively turned on or off through the Valves 72 to initiate or terminate propellant flow to any engine and thereby vary the thrust of the entire cluster of engines. Although only t-he supplies to three engines are shown in FIG. 6, it will be understood that a similar supply line 68 and valve 72 is provided for each engine if desired so that any engine or combination of engines of the cluster can be manipulated to vary the total thrust of the cluster. Variations in engine thrust might yalso be realized by control of the propellant ow rate through the individual engines or 'by control of the electric potential of individual engines.

It will be understood t-hat in any of the embodiments n of this invention the ferromagnetic element 54 can be incorporated or omitted as desired. It will also be understood that all or portions of -the ferromagnetic elements may be formed of permanent magnets so that the solenoids 50 or the coils 64 may be eliminated to minimize engine weight and power requirements.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described but lmay be used in other ways without departure from its spirit as defined by the following claims.

Iclaim:

1. An electrical propulsion engine including:

a chamber having positive and negative particles therein said chamber having an open end,

a cathode in said chamber removed from said open end for generating negative particles,

means for accelerating at least part of said positive particles through the open end of said chamber and for causing at least part of said negative particles to oscillate in said chamber,

and magnetic mirror means for introducing a magnetic field of non-uniform flux density into said chamber in the vicinity of said cathode for reflecting negative particles from the vicinity of said cathode toward the open end of said chamber.

2. An electrical propulsion engine including:

a chamber having positive and negative particles thereA in said chamber having an open end,

means for generating electric field gradients in said chamber, means for generating a magnetic field in said chamber, the electric field and the magnetic field being directed with respect to each other to cause at least part of said negative particles t-o oscillate in said chamber,

and means for accelerating at least part of said positive particles through the open end of said chamber,

said magnetic field generating means including means for generating a magnetic mirror in said chamber in the vicinity of said cathode for refiecting negative particles from the vicinity of said cathode toward the open end of said chamber.

3. An electrical propulsion engine as in claim 2 wherein the field of said magnetic mirror diminishes in flux density from the vicinity of said cathode toward said open end of said chamber.

4. An electrical propulsion engine as in claim 3 wherein said magnetic field generating means includes coil means around said chamber and extending along said chamber, said coil means having a greater number of ampere turns in the vicinity of said cathode than elsewhere.

5. An electrical propulsion engine as in claim 2 wherein said magnetic mirror means includes:

a ferromagnetic element on the side of said cathode removed from the open end of said chamber.

6. An electrical propulsion engine as in claim 2 wherein said magnetic mirror means includes a ferromagnetic element on the side of said cathode removed from the open end of said chamber and wherein said magnetic field generating means includes means for generating a magnetic field of greater strength in the vicinity of said cathode than elsewhere in said chamber, said greater magnetic field generating means forming part of said magnetic mirror.

7. An electrical propulsion engine including:

an annular member forming an ionization region chamber, said annular member having first and second electrode elements, a first insulator between said electrode elements and a second insulator on the side of Said first electrode removed from said first insulator,

said annular member terminating in an open end at said second electrode and having a closure member at the other end electrically insulated from said first electrode by said second insulator,

a athode in said chamber adjacent said closure memmeans for causing said cathode to emit electrons,

means for introducing an electrically neutral gaseous propellant into said chamber,

means for imposing an electric potential on said first electr-ode to maintain said first electrode at a different potential than said cathode,

means for electrically connecting said second electrode and said cathode to maintain said second electrode and said cathode at the same potential,

and means substantially coaxial with said annular member for generating a magnetic field along at least part of the length of said chamber,

said annular member varying in diameter along at least part of the length thereof between adjacent said cathode and said second electrode,

the electric field gradients between said cathode, said first electrode and said second electrode coacting with the magnetic field to cause electrons to oscillate in said chamber to ionize said gaseous propellant whereby ionized propellant is accelerated out of the open end of said chamber to produce thrust,

and said annular member and said magnetic field coacting to form a magnetic mirror throughout the extent of said chamber diminishing in flux densty from said cathode toward said open end of said chamber for refiecting electrons from the vicinity of said cathode toward the open end of said chamber to generate thrust through interaction between electrons and said magnetic mirror.

8. An electrical propulsion engine as in claim 7 Wherein said magnetic field generating means includes means for generating a magnetic eld of greater strength adjacent said cathode than elsewhere in said chamber to form at least part of said magnetic mirror.

9. An electrical propulsion engine as in claim 7 wherein said varying diameter part of said annular member increases in diameter from adjacent said cathode toward said second electrode.

10. An electrical propulsion engine as in claim 7 including a ferromagnetic element `between said cathode and said closure member, said ferromagnetic element forming part of said magnetic mirror.

11. An electrical propulsion engine including:

an annular member forming an ionization chamber,

said annular member having rst and second electrode elements, a first insulator between said electrode elements, and a second insulator on the side of said first electrode removed from said first insulator,

a cathode in said chamber adjacent said closure memmer,

means for causing said cathode to emit electrons,

means for imposing an electric potential on said first electrode to maintain said first electrode at a different potential than said cathode,

means for electrically connecting said second electrode and said cathode to maintain said cathode and said second electrode at the same potential,

means substantially coaxial with said annular member for generating a magnetic field along at least part of the length of said chamber,

the electric field gradients between said cathode, said first electrode and said second electrode coacting with the magnetic field to cause electrons to oscillate in said chamber to ionize said gaseous propellant whereby ionized propellant is accelerated out of the open end of said chamber,

and means, including ferromagnetic means extending between said second electrode and said closure member, to provide a magnetic flux path to increase flux density in the vicinity of said cathode to form a magnetic mirror for reflection electrons from the vicinity of said cathode toward the open end of said chamber to generate thrust through interaction between electrons and the magnetic mirror.

12. An electrical propulsion engine as in claim 11 wherein said `ferromagnetic means includes an annular sleeve coaxial -with said annular member, said annular sleeve and said annular member defining an annular chamber therebetween, and wherein at least part of the magnetic field generating means is in said annular chamher.

13. An electrical propulsion engine as in claim 11 wherein said closure member is ferromagnetic, and including a second ferromagnetic element extending from said closure member into said ionization chamber toward said cathode.

14. An electrical propulsion engine as in claim 13 wherein said magnetic field generating means includes coil means around said second ferromagnetic element to generate at least part of said magnetic field.

15. An electrical propulsion engine including:

an annular member forming an ionization chamber,

said annular member having first and second electrode elements, a first insulator between said electrode elements, and a second insulator on the side of said first electrode removed from said first insulator,

a cathode in said chamber adjacent said closure member,

means for causing said cathode to emit electrons,

means for introducing a gaseous propellant into said chamber,

a plurality of ferromagnetic rods extending between said second electrode and said closure member externally of said annular member,

and means for generating a magnetic field in said chamber,

the electric field gradients between said cathode, said first electrode and said second electrode coacting with the magnetic field to cause electrons to oscillate in said chamber to ionize said gaseous propellant whereby ionized propellent is accelerated out of the open end of said chamber,

and said magnetic field generating means providing a magnetic field of increased flux density in the vicinity of said cathode to form a magnetic mirror for refiecting electrons from the vicinity of said cathode toward the open end of said chamber to generate thrust through interaction between electrons and the magnetic mirror.

16. An electrical propulsion engine as in claim 15 wherein said magnetic field generating means includes coil means around each of said rods.

17. An electrical propulsion engine as in claim 15 wherein said closure member is ferromagnetic, and including a ferromagnetic element extending from said closure element into said chamber tow-ard said cathode.

18. An electrical propulsion engine assembly includga cluster of ion engines, each ion engine of said cluster having an annular member forming an ionization charnber,

a cathode in said chamber adjacent said closure member,

means for causing said cathode to emit electrons,

means for introducing a gaseous propellant into said chamber,

and means for electrically connecting said second 9 10 electrode and said cathode to maintain said the vicinity of each cathode toward the open end of cathode and said second electrode at the same each chamber to generate thrust in each engine potential, through interaction between electrons and the maga plurality of ferromagnetic rods around the annular netic mirror of each engine.

member of each engine extending between the sec- 5 19. An electrical propulsion engine as in claim 18 ond electrode of each engine and the closure memwherein said magnetic field generating means includes ber of each engine, each engine of said cluster havcoil means around each of said rods. ing at least one of said ferromagnetic rods in com- 20. An electrical propulsion engine assembly as in mon with any adjacent engine, claim 18 wherein the closure member of each engine is and means for generating a magnetic field in the cham- 10 ferromagnetic, and wherein each engine has a ferrober of each of said engines, the electric field gradimagnetic element extending from the closure element ents lbetween the cathode, the first electrode, and the into the chamber toward the cathode. second electrode of each engine coacting with the magnetic field in the chamber of each engine to References Cited by the Examiner cause electrons to oscillate in the chamber of each 15 UNITED STATES PATENTS engine to ionize the gaseous propellant in each chammagetic field of increased flux density in the vicinity 20 of the cathode of each engine to form a magnetic MILTON O HIRSHFIELD Pnma'y Exammer mirror in each engine for reilecting electrons from D. X. SLINEY, Assistant Examiner.

Claims

1. AN ELECTRICAL PROPULSION ENGINE INCLUDING: A CHAMBER HAVING POSITIVE AND NEGATIVE PARTICLES THEREIN SAID CHAMBER HAVING AN OPEN END, A CATHODE IN SAID CHAMBER REMOVED FROM SAID OPEN END FOR GENERATING NEGATIVE PARTICLES, MEANS FOR ACCELERATING AT LEAST PART OF SAID POSITIVE PARTICLES THROUGH THE OPEN END OF SAID CHAMBER AND FOR CAUSING AT LEAST PART OF SAID NEGATIVE PARTICLES TO OSCILLATE IN SAID CHAMBER, AND MAGNETIC MIRROR MEANS FOR INTRODUCING A MAGNETIC FIELD OF NON-UNIFORM FLUX DENSITY INTO SAID CHAMBER IN THE VICINITY OF SAID CATHODE FOR REFLECTING NEGATIVE PARTICLES FROM THE VICINITY OF SAID CATHODE TOWARD THE OPEN END OF SAID CHAMBER.