US3155857A - Ion beam generating system - Google Patents

Description

Nov. 3, 1964 P. FRENCH 3,155,857

ION BEAM GENERATING SYSTEM Filed June 1, 1960 masowacz 49 2a 25 24 23 22 D CsouQcE /38 g INVENTOR. z Park Fiencb ATTYS.

United States Patent 3,155,857 ION BEAM GENERATING SYSTEM Park French, Aurora, Ohio, assignor to Thompson Ramo gooldridge Inc., Cleveland, Ohio, a corporation of bio Filed June 1, 1960, Ser. No. 33,332 16 Claims. (Cl. 313-63) This invention involves an ion beam generating system in which a highly stable neutralized ion beam is produced in a simple and reliable manner.

The invention involves the use of positively charged ions which are electrically charged atoms or molecules, formed by the loss of one or more electrons. Positively charged ions are attracted to a negatively charged body in the same way that a piece of lint may be attracted to a comb. They are repelled by a positively charged body or by other positively charged ions. Positively charged ions may be emitted into outer space, or into an evacuated space, by heating certain materials to an elevated temperature. Their speed may be increased by disposing in spaced relation to the emitting surface an accelerating electrode or grid at a negative potential, so that they may be discharged into outer space or into an evacuated space. They may thus be used for propelling space vehicles and are also usable in other applications. However, certain problems are involved.

One problem is that the potential of the space vehicle or emitting body should be maintained constant. This problem can be readily solved by the separate emission of an electron beam with the total electronic current flow being equal to the ion current flow.

Another problem, not so easily solved, is that of an ion cloud, created when the ions are emitted in a broad beam, i.e. one in which the accelerating grid is spaced from the ion-emitting surface a distance which is small in relation to the transverse dimensions of the surface. In particular, the ions after passing the accelerating grid are slowed down by the electrostatic repelling forces exerted by other positively charged ions in the region. As a result, a cloud of ions starts to build up beyond the accelerating grid. As additional ions approach the ion cloud, they are slowed down even more and may be turned back toward the emitter. Ultimately, a condition is produced in which the ion cloud acts as the virtual ion source, which may be at a distance from the accelerating electrode approximately equal to the distance from the emitting to the accelerating electrode. The ions returned to the emitter are substantially equal in number to those emitted and there is substantially no net current flow. As a result, eifective propulsion is not possible.

It is possible to minimize the ion cloud problem by emitting the ions in a narrow beam, with the accelerating grid being placed at a substantial distance from the emitting surface, in relation to the transverse dimensions of the surface. Under such circumstances, the electric field in the ion beam acts mainly in a transverse direction so as to make the beam gradually expand, and not substantial ion cloud may be produced. However, to obtain sufiicient ion generation in many practical applications, it would be necessary to operate a plurality of the narrow beam sources in parallel side-by-side relation. And when so operated, it is found that the beams interact and that the total effect is substantially the same as produced by the broad beam emitter. The ion cloud problem is not solved.

To solve the ion cloud problem, it has been proposed to emit electrons directly into the ion beam to travel along with the beam and neutralize the charge thereof, and thus prevent formation of the cloud. It is found, however, that it is virtually impossible to emit Patented Nov. 3, 1964 electrons at the required speed. The reason for this is that the travel of ions is quite slow in relation to the speed of electrons from a conventional emitter, ions being much, much heavier than electrons. Sufficiently slow electrons cannot be produced by any known and practical means. In addition, even if a practical emitter could be found, it would be practically impossible to avoid violent fluctuations in potentials and instabilities. This is due to the fact that an ion cloud would almost certainly accelerate any slow electrons that might be present. For example, an ion beam by itself might generate a potential on the order of 1,600 volts in a distance equal to about of its diameter. Even if the beam were balanced 99% perfectly, it would still build up several volts in the same distance, and electrons would be accelerated by such potential differences. The beam would therefore contain electrons of energy primarily determined by the potential fluctuations within itself. Furthermore, such an arrangement has certain features of inherent instability. With an ion beam in an approximate state of balance a positive fluctuation in potential will accelerate electrons to reduce their charge density and the increased net positive space charge further increases the potential fluctuation. As a result, violent fluctuations in potential can occur even though the total number of electrons and ions is the same.

This invention was evolved with the object of providing a practical solution to the problems discussed above. By this invention, an extremely stable neutralized ion beam is produced in a simple and yet very reliable manner.

According to this invention, electrons are injected into an ion beam to travel in spiral paths with an axial velocity closely matching the ion velocity. Preferably, the spiral paths are of substantially constant radii, a condition which is produced by obtaining a balance between the force exerted by the electrostatic field of the beam, urging the electron toward the center or principal axis of the beam, and the centripetal force, proportional to the square of the tangential velocity of the electron. It is found that the injection velocity can be relatively high, far greater than the ion velocity, so as to make possible the use of practical electron emitters.

In addition, it has been discovered that by use of certain features and conditions, it is possible to obtain a total electron current flow so distributed as to obtain substantially complete charge neutralization of the beam as well as current neutralization. Furthermore, by using such features and conditions, the neutralization can be obtained with a high degree of stability.

Additional features of the invention relate to the physi cal construction and geometry of the neutralization structure. These features and other objects and advantages of the invention will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawing which illustrates a preferred embodiment and in which:

FIGURE 1 is a diagrammatic plan view of an ion beam system constructed according to the principles of this invention, with electrical connections thereto being schematically illustrated;

FIGURE 2 is a sectional view taken substantially along line II--II of FIGURE 1; and

FIGURE 3 is a sectional view taken substantially along line IlIIlI of FIGURE 1.

The ion beam system of this invention, generally designated by reference numeral 10 comprises an ion emitting surface 11, an accelerating electrode or grid structure 12 in spaced relation to the ion emitting surface 11, and a neutralizing structure 13 arranged to emit electrons into the ion beam to travel in spiral paths therein and to neua) tralize the charge of the beam, to prevent formation of an ion cloud.

The illustrated ion emitter comprises a porous tungsten wafer 14 supported on and closing the upper end of a hollow cylindrical body 15, which may preferably be of molybdenum. The body 15 has an intermediate Wall 16 dividing the space therewithin into a chamber 17 below the diaphragm 14 and a cavity 18 in which a heating coil 19 is disposed. The structure is supported on a tube 20, the upper end of which is fitted into a central opening of the wall 16. Tube 20 is preferably of molybdenum.

In operation, cesium vapor is fed upwardly through the tube 20 into the chamber 17 below the porous tungsten wafer 14. The structure is heated by the heating coil 19 and the cesium vapor diffuses through the pores of the porous tungsten wafer 14, to result in the emission of ions from the surface 11, the upper surface of the wafer 14.

The ions emitted from the surface 11 are accelerated in speed by the electric field created by the accelerating electrode or grid structure 12. The structure 12 may preferably comprise a series of concentric circular wires 21-26, disposed on suitable supports 27 of insulating material. The grid wires 21-26 may be connected together electrically and to the negative terminal 28 of a direct current source 29 having a positive terminal 30 connected to ground.

The neutralizing structure 13 comprises a series of circular concentric grid wires 31-36 disposed on radially extending supports 37 of insulating material. The grid wires 31-36 establish the general potential distribution in the neighborhood of the device. Electrons are injected from a radially extending cathode 38, supported on one of the supports 37, and having two active sides 39 and 40, from which electrons are emitted. To create an extraction field, a plurality of inverted U-shaped grid elements 41-46 are disposed over the cathode 38 and are respectively connected to the circular grid wires 31-36. The vertical leg portions of the elements 41-46 are disposed in planes in spaced relation to the active surfaces 39 and 40 of the cathode 38.

The relationship of the extracting grid elements 41-46 with respect to the cathode 38 is symmetrical, so that equal currents are emitted from both sides 39 and 46 of the cathode 38. The current from the side 3? is used for neutralization, while the current from the side 40 is ultimately collected by the circular grid structure 31-36 Where it is used in compensating circuitry to adjust the current from the side 39 to the values required for neutralization. In particular, a barrier or inactive strip 47 is disposed along the upstream edge of the side 40 to block emission. This creates a thin unneutralized ion layer between the circular grids 31-36 and the electron stream from the side 40, and causes the electron stream to be diverted in the upstream direction so as to impinge upon the circular grid structure 31-36. At the same time, the stream of electrons from the side 39 is diverted in a downstream direction, to cause the electrons to move in spiral paths at an axial velocity matching the ion velocity.

An arrangement for establishing proper operating potentials is schematically illustrated in FIGURE 1. The outer end of the cathode 38 is connected to a terminal 48 of a direct current source 49. The inner end of the cathode 38 is connected through a resistor 50 to another terminal 51 of the source 49, terminal 51 being connected to ground. Wires 31-36 are respectively connected to ground through resistors 52-57.

The form and values of the various elements and components of the system required to obtain optimum operatron are dictated by parameters of the ion emitter such as the shape of the emitter surface 11 and the potential distribution and charge densities produced by the surface 11 and the accelerating grid structure 12. In any case, the electrons should be emitted in such directions and at such velocities as to travel in generally spiral paths and should be so distributed as to effectively neutralize the ion beam through a substantial distance. With a circular ion beam source as illustrated, and with uniform ion charge and current densities, it has been found that certain conditions should prevail to obtain optimum performance.

In particular, the neutralizing structure should fulfill the following conditions:

1. The radial potential behavior of the physical structure should match that of the ion beam. At any radius r the potential should equal where a is the distance from the principal beam axis to the outer beam boundary and -V is the potential at the outer beam boundary.

2. The electrons should be injected with their average axial velocity component closely matching the ion velocity.

3. The azimuthal velocities must have the proper variation with radius. If it is assumed that the azimuthal motion carries all the injection energy, which is very nearly the case, the injecting cathode potential at all radii should equal twice the beam potential.

4. Any radial velocities should be small relative to azimuthal velocities and should average zero at any radius.

5. The injected current per unit radius should be proportional to the radius, or should equal Kr, where K is a constant.

6. The net injected electron current should equal the ion current. As a practical matter, there will be a miniminimum radius r at which neutralization can be effected, due to limitations on the minimum possible azimuthal electron velocites. The net electron current should equal To establish the proper operating potentials of the circular grids 31-36, it is found that the resistance from each grid element to ground (the resistance of each resistor 52-57) should be equal to where V is the potential at the outer beam boundary, It is the number of elements per unit radius, r is the radius of the grid element, a is the beam radius, r is the minimum radius of the beam, and I is the net injected current which would equal the iron current. This equation assumes the grid to be uniformly spaced.

It might here be noted that the grid wires may preferably be of no-sag tungsten to prevent high temperature creep.

With regard to the cathode circuit, the cathode potential should be twice that of the beam at all radii, as noted above. Thus the DC source 49 should supply a voltage equal to twice the potential at the outer beam boundary and the cathode 38 must have the proper resistance values along its length. The resistor 50 must also have the proper value. However, it should be noted that although it is thus necessary to dissipate some power along the cathode to maintain proper potentials, it is not necessary to do so for the purpose of heating, since the cathode is in close proximity to the hot ion emitter and is heated therefrom. The temperature of the emitter may be on the order of 1200 C., for example. For operation in the neighborhood of 1100 C., impregnated cathodes are satisfactory, such as sintered tungsten powder impregnated with a mixture of aluminum oxide, and barium and calcium carbonates, which become oxides upon heating. For operation around 900 C., a nickel cathode coated with the usual barium and strontium carbonate emitter mix would be preferable.

To obtain the proper potential at the inner end of the cathode 38, it is found that the resistor 50 should have a resistance equal to where V is the potential at the outer beam boundary, a is the radius at the outer beam boundary, r is the minimum beam radius and I is the total injected current, which should be equal to the ion current.

It is found that the resistance per unit length of the cathode should be equal to This cathode resistance can be obtained with a tapered slab cathode which increases in thickness linearly with radius since for ohmic resistors of slowly changing crosssection, A, the change in resistance per unit radii is equal to R /A where R is the volume resistivity.

If it is assumed that the cathode has a height it its thickness at the maximum radius, a, is equal to 4ahV 1 R I; (1

With regard to the extracting grid geometry, it may be assumed that the cathode is run space-charge limited, which is desirable for purposes of stability with respect to aging. It may be also assumed that the legs of the elements 41-46 are positioned so as to form a good approximation to a plane situated close to the emitting faces 39, 40 of the cathode 38, which is desirable to fulfill the condition of small radial electron velocities. Under such conditions, it is found that the grid-cathode spacing necessary to obtain an injected current per unit radius proportional to the radius and the proper total current is equal to where e is the electron charge, m is the electron mass and c is the velocity of light, the other parameters being as above described.

It will be observed that the grid-cathode spacing increases linearly with the radius and since the cathode faces lie along radii, the grid planes lie along radii also, as is illustrated.

The grid-cathode spacing as expressed above is the spacing in the case wherein no ions exist in the region between the cathode and the grid. Accordingly, the spacing may be increased somewhat from the value expressed or in the alternative, the values of the grid resisters may be changed somewhat.

In the operation of the device, the voltage of the acceleration grid source 29 may be adjusted to insure optimum operation. The voltage should preferably be such that the upstream current from the cathode face 49 is removed completely after one revolution, i.e. the pitch should be adjusted to be equal to the electron beam width. Under such circumstances, the direction and pitch of the downstream electron stream will automatically be established. It should be noted that the electron emission is self-regulating to a degree. If, for instance, a given region of the cathode gives insufficient emission, the circular grid at that radius will intercept less current than is normal, and its voltage will be raised because of the lowered current flowing through the associated grid resistor. Since the extracting grid loops are attached to circular grid elements at the same radius, the extracting voltage will be raised at the radius of low emission, to provide a compensating increase in extracted current and electron space charge. The operation of this compensating mechanism depends upon symmetric properties on both cathode faces, and care should be taken to insure that condition.

As noted above, radial velocities must be small rela: tive to azimuthal velocities and should average zero at any radius. Care should be taken to insure that this is the case. It is particularly important that there be a sufiicient number of extracting grids at the proper potentials and in many cases it will be desirable to provide many more than the six that are used in the device as diagrammatically illustrated. In this connection, it is noted that the cathode thickness, grid spacings, etc. of the device are exaggerated in order to more clearly show the important details of the design.

With the system of this invention as above described a highly stable and fully neutralized ion beam is produced, permitting the attainment of a large propulsive force while maintaining constant the potential of a space vehicle or other emitting body. It is found that the action of the neutralization mechanism is such as to provide an automatic compensating action with respect to expansion of the ion beam and with respect to unavoidable random variations, so as to minimize the production of unstable conditions.

It is important to note that although the principles of the invention have herein been specifically applied to a system for neutralizing a circular ion beam having uniform charge and current densities, it will be apparent that such principles can be applied as well to the neutralization of beams of other forms, by making suitable modifications in the design and proportions of the system.

It will be understood that other modifications and variations may be effected without departing from the spirit and scope of the novel concepts of this invention.

I claim as my invention:

1. In an iOn beam system, means for emitting an ion beam having a principal axis, electron-emitting means disposed in said beam for emitting electrons at points spaced from said axis to travel in generally spiral paths about said axis, said electron-emitting means having a radial potential behavior closely matching that of said beam.

2. In an ion beam system, means for emitting an ion beam having a principal axis, electron-emitting means disposed in said beam for emitting electrons at points spaced from said axis to travel in generally spiral paths about said axis, said electrons being emitted at velocities which increase with the distance from said axis.

3. In an ion beam system, means for emitting an ion beam of generally circular cross-section having a principal axis with substantially uniform charge and current densities, electron-emitting means disposed in said beam for emitting electrons at points spaced from said axis to travel in generally spiral paths, said electron-emitting means having a potential at each point thereof substantially equal to where r is the distance from said axis to said point, a is the distance from said axis to the outer beam boundary and -V is the potential at said boundary.

4. In an ion beam system, means for emitting an ion beam of generally circular cross-section having a principal axis with substantially uniform charge and current densities, electron-emitting means including a radially extending cathode disposed in said beam for emitting electrons at points spaced from said axis to travel in generally spiral paths, said cathode having an injection potential at all radii substantially equal to twice the beam potential.

5. In an ion beam system, means for emitting an ion beam of generally circular cross-section having a principal axis with substantially uniform charge and current densities, electron-emitting means including a radially extending cathode disposed in said beam for emitting electrons at points spaced from said axis to travel in generally spiral paths, the injected electron current per unit radius being proportional to the radius.

d 6. In an ion beam system, means for emitting an ion beam having a principal axis, a cathode extending radially outwardly with respect to said axis and having an eleetron-emittingsurface in a radial plane to emit electrons to travel in spiral paths about said axis.

7. In an ion beam system, means for emitting an ion beam having a principal axis, a cathode extending radially outwardly with respect to said axis and having an electronemitting surface in a radial plane to emit electrons to travel in spiral paths about said axis, and a grid structure disposed in spaced relation to said electron-emitting service for creating an extraction field.

8. In an ion beam system, means for emitting an ion beam having a principal axis, a cathode disposed radially in said beam for emitting electrons to travel in generally spiral paths about said axis, and a potential-fixing grid structure associated with said cathode and disposed in said beam in a plane transverse to said axis.

- 9. In an ion beam system, means for emitting an ion beam having a principal axis, a cathode disposed radially in said beam for emitting electrons to travel in generally spiral paths about said axis, and a potential-fixing grid structure associated with said cathode and disposed in said beam in a plane transverse to said axis, said grid structure including a plurality of generally circular elements having centers coincident with said axis.

10. In an ion beam system, means for emitting an ion beam having a principal axis, a cathode extending radially outwardly with respect to said axis and having an electronemitting surface in a radial plane to emit electrons to travel in spiral paths about said axis, a potential-fixing grid structure associated with said cathode and disposed in said beam in a plane tranverse to said axis, and an extraction grid structure connected to said potential-fixing grid structure and disposed in spaced relation to said electron-emitting surface for creating an extraction field.

11. In an ion beam system, means for emitting an ion beam having a principal axis, a cathode extending radially outwardly with respect to said axis and having an electronemitting surface in a radial plane to emit electrons to travel in spiral paths about said axis, a potential-fixing grid structure associated with said cathode and disposed in said beam in a plane transverse to said axis, said potentialfixing grid structure including a plurality of generally circular elements having centers coincident with said axis, and an extraction grid structure including a plurality of elements connected to said circular elements and disposed in spaced relation to said electron-emitting surface for creating an extraction field.

12. In an ion beam system, means for emitting an ion beam having a principal axis, a cathode extending radially outwardly with respect to said axis and having an electron-emitting surface in a radial plane to emit electrons to travel about said axis, and means for establishing a field active on the emitted electrons to cause said electrons to travel in spiral paths at a speed closely matching the ion speed.

13. In an ion beam system, means for emitting an ion beam having a principal axis, a cathode extending radially outwardly with respect to said axis and having on opposite sides thereof a pair of electron-emitting surfaces disposed in radial planes, a grid structure associated with said cathode for fixing the potential in the region thereof and to extract electrons from said surfaces to cause electron flow from said surfaces in two oppositely oriented streams about said axis, and means for causing one of said streams to spiral upstream to be collected by said grid structure and to cause the other of said streams to spiral downstream to neutralize the ion beam.

14. In an ion beam system, means for emitting an ion beam having a principal axis, a cathode extending radially outwardly with respect to said axis and having on opposite sides thereof a pair of electron-emitting surfaces disposed in radial planes, a grid structure associated With said cathode for fixing the potential in the region thereof and to extract electrons from said surfaces to cause electron flow from said surfaces in two oppositely oriented streams about said axis, and a barrier strip along the upstream side of one of said surfaces to cause one of said streams to spiral upstream to be collected by said grid structure and to cause the other of said streams to spiral downstream to neutralize the ion beam.

15. In an ion beam system, means for emitting a beam of positively charged ions having a principal axis wherein the ions travel generally rectilinearly in paths parallel to said principal axis, and means for emitting negatively charged electrons into said beam at points spaced from said principal axis and in directions such as to cause the electrons to travel spirally about said principal axis.

16. In an ion beam system, means for emitting a beam of positively charged ions having a principal axis wherein the ions travel generally rectilinearly in paths parallel to said principal axis, and means for emitting negatively charged electrons into said beam at points spaced from said principal axis and in directions such as to cause the electrons to travel spirally about said principal axis, the axial velocity of said electrons being closely matched to the velocity of said ions.

References Cited in the file of this patent UNITED STATES PATENTS Lindenblad Oct. 9, 1956 Bell et al Aug. 22, 1961 OTHER REFERENCES

Claims (1)

1. IN AN ION BEAM SYSTEM, MEANS FOR EMITTING AN ION BEAM HAVING A PRINCIPAL AXIS, ELECTRON-EMITTING MEANS DISPOSED IN SAID BEAM FOR EMITTING ELECTRONS AT POINTS SPACED FROM SAID AXIS TO TRAVEL IN GENERALLY SPIRAL PATHS ABOUT SAID AXIS, SAID ELECTRON-EMITTING MEANS HAVING A

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