US3182220A - Multiply neutralized ion source - Google Patents

Description

May 4, 1965 D. GABoR 3,132,220

' MULTIPLY REUTRALIZED ION SOURCE Filed March 18. 1960 s Sheets- Sheet 1 DEN/V15 GABOR B/y yon-0w, Mm. T

Inveni'm' May 4, 1965 Filed Marh 18. 1960 D. GABOR 3,182,220

MULTIPLY NEUTRALIZED ION SOURCE 5 Sheets-Sheet 2 DEA/N16 6? 0R 3), KW W? or Cys May 4, 1965 p. GABOR MULTIPLY NEUTRALIZED ION SOURCE 5 Sheets-Sheet 3 Filed March 18, 19 60 Ins/e 76:- DE NNIS C ABOR 7 I rncys y 4, 1965 D. GABOR 3,182,220

MULTIPLY NEUTRALIZED ION SOURCE Filed March 18. 1960 5 Sheets-Sheet 4 In ven'far QfN/VIS .519 R y 1965 D GABOR 3,182,220

MULTIPLY NEUTRALIZED ION SOURCE Fil ed March 18. 1960 5 Sheets-Sheet 5 I'M/8015! 'DENNIS 6950K a; flaw, 422%.... 7

0"? eyS United States Patent 19 Claims cl. 313-63) This invention relates to large power ion beam devices. An object of the invention is to provide a device capable of functioning as a source of an ion beam of high energy.

A further object of the invention is to provide a device capable of delivering a high energy ion beam in which space charge is neutralized by electrons.

It is a still further object of the invention to provide a device in which a high energy ion beam may be set up and converted into an ionic flywheel.

More particularly the invention provides an ion beam source of a novel kind in which a large ion current which may be of the order of hundreds or even thousands of amperes is produced and accelerated to energies of the order of 5-100 kev. or more. A further optional element of the invention consists in means associated with the said ion beam source, which shapes the ion beam into the form of a hollow cylinder, in which the ion rotate at high energies around in axis, while their space charge is neutralized by electrons injected into the beam. This cylinder will for brevity be referred to as an ionic flywheel, as it contains, like a flywheel, a large kinetic energy in the form of rotational motion.

According to this invention in one aspect an ion beam source comprises means for setting up an ion stream and means for injecting electrons transversely into said stream at a plurality of locations along said stream.

In another aspect the invention provides an ion beam source comprising means for introducing gas molecules into an ionising location, means for agitating electrons released in said ionising location to ionise said gas molecules, means for accelerating the ions so formed away from said ionising location and means for injecting further electrons transversely into the accelerating ion steam so as to neutralise the space change in the accelerating region.

According to a feature of the invention the ion beam source may be associated with means for diverting the ion beam into an ionic flywheel, the means for effecting this diversion comprising means for setting up a magnetic field in the form of a resultant of three componet fields namely, a first azimuthal component, a second component derived from at least one current loop surrounding the axis of symmetry of the ion beam source and a third uniform axial component produced by at least one coil surrounding said current loop and energized in opposite sense to said loop.

The ion beam sources described hereinafter comprise two principal parts, an ion source proper, that is to say a location in which gas molecules are ionized, and an accelerating region in which ion from the source are accelerated to a higher energy level and formed into a beam. A third region may be included in which the ion beam is converted into a ionic flywheel. In the ion source a high frequency field is used to agitate the electrons and so produce ionization by collisions with the gas molecules and fields are set up to cause the ions so generated to move towards the accelerating region while the electrons are caused to move towards the walls of the ionizing location where they are drawn off by suitably placed electrodes. In the accelerating region an accelerating field is set up to provide acceleration of the ions away from the source and the space charge in this region is neutralized by means of electrons injected transversely into the beam "ice at various points along it, acceleration of the electrons out of the beam being avoided by means of an arrangement of electric and magnetic fields.

Ion beam devices according to the invention have rotational symmetry around an axis and for clarity the various directions which are to be referred to are defined as follows. Directions parallel to the axis will be referred to as axial, directions at right angles to this in planes containing the axis as radial, directions at right angles to V the axial and radial directions as azimuthal. Planes containing the axis will be referred to as meridian planes, and directions other than axial or radial contained in these planes will be referred to as meridional directions. The general arrangement comprises means nearest the axis of symmetry for introducing gas molecules into an annular ionisation location or source and surrounding this is the acceleration region, which is also annular. The ion beam is accelerated radially outwardly and given at the same time a rotational motion so that it leaves this region substantially tangentially and is then diverted into an axial direction to form a cylindrical circulating ionic flywheel. A gas such as hydrogen or deuterium is admitted, continuously or in short bursts, to the ion source. A strong high frequency azimuthal electromagnetic field is set up in this space, which will accelerate electrons to such mean energies, preferably of the order 50-200 ev., that they not only ionise the gas, but break up. at least part of the molecules, converting at least a part of the neutral gas into protons or deuterons before the gas has pene trated beyond the ion source. The motion of the electrons is an energetic oscillation in the azimuthal direction with the frequency of the HR field, with a superposed slow axial drift towards one or both of the walls. The motion of the ions is mainly radial, that is to say parallel to the walls of the ion source. This type of operation is made possible by the provision of a strong azimuthal magnetic field which may be produced by a very strong current in the conductor located in the central axis. This strong azimuthal field slows down the sideways drift of the electrons to such an extent that the mid-plane of the plasma between the two walls assume a potential negative in relation to that of the walls, so that a field is produced which tends to drive the electrons to the walls, while keeping the ions away from them. In the ideal operation of the device the drift of the ions is radial, with a certain amount of rotation around the axis, but without axial components. In order to achieve this kind of operation, which produces maximum ion economy, electric fields are set up in the ion source, which have a radial component which accelerates the ions outwards, and an axial component which just compensates the magnetic force produced by the radial motion of the ions in the azimuthal magetic field. This field may be set up by electrodes located on both walls of the ion source, and arranged so as not to prevent the penetration of high frequency electrical energy into it. They are therefore not annular electrodes, but a plurality of point-electrodes. They may be connected to the accelerating potentials, which are substantially steady during operation, by means of transmission lines which are a quarter wave-length long at the high frequency so that they appear as substantially an open circuit for the high frequent energy in the ionisation location, while presenting low impedances for the steady currents, that is to say for the electron currents which flow to them from the plasma.

Preferably the ion source is arranged in a region in which the meridional magnetic field is very small. This has the advantage that all ions are produced in annular zones which are linked with very nearly the same magnetic flux. Consequently, by the law of conservation of total momentum, all ions, wherever they have originated in the source, will aquire the same axial momentum in the magnetic fields to which they are subjected later. In order to make the beam homogeneous as regards total energy also, the accelerating voltage drop in the ion source is kept as low as possible, preferably within a few hundred volts. The result is that all ions will have nearly the same dynamic data, as if they had originated at one point.

The ions produced in the ion source now proceed into the accelerating space, which can also be called the ion gun. While in the ion source the space charge of the ions is neutralized by the secondary electrons, which are produced in equal numbers. In conventional ion sources the acceleration is effected in vacuo, with a very strong ion space charge developing, which reduces the avaliable ion currents to small fractions of an ampere. In the present invention neutralization is affected by employing for the acceleration electrodes capable of emitting electrons. In known designs of ion sources this would lead to a breakdown of the high accelerating voltage by arching bet-ween the electrodes. This is avoided in the accelerator according to the invention by two co-operating means. One is a meridian magnetic field, which steadily increases in strength from the ion source outwardly attaining large value of the order of several thousand gauss in the outer zones of the accelerator, where the accelerating electric gradient is large. The other is the arrangement of annular accelerating electrodes in pairs in the opposite walls of the accelerating space, in such a way that the electrical equipotential lines joining them coincide with the magnetic field lines, that is to say the electrodes of one pair connected by a magnetic field line have the same potential. In a strong magnetic field electrons can move easily only along the magnetic field lines, and as these coincide with the equipotential lines, electrons will never be greatly accelerated, but will oscillate in the ion cloud, accumulating in it to the extent which is necessary to compensate its space charge. The emission of electrons can be effected by hot cathodes, but also by cold metal electrodes, using the phenomenon of the unipolar arc. This effect, which has constituted a frustrating phenomenon in many thermonuclear devices, consists in the formation of an arc discharge whenever a plasma approaches a metal electrode whose potential is negative with respect to the plasma. The result is that the plasma will be charged up by electrons emitted by the electrode to within a score of volts of its potential. This phenomenon is used here to good purpose, because in the strong magnetic field the unipolar arc will charge up to a constant potential only one narrow zone of the ion beam, without danger of destroying the gradient between this zone and the next, owing to the fact that in the strong meridian field the plasma is a bad conductor at right angles to the said field, in the radial direction.

The high frequency electric field does not penetrate into the accelerating space, as it is screened off by the annular electrodes, but the azimuthal magnetic field extends into it. As this would bend the ion beam, in order to maintain the ion motion in the required plane the equipotential lines are skewed relative to the direction of ion motion, to such an extent as to compensate the magnetic force component at right angles to the said motion.

The invention as so far described, that is to say the ion beam source itself, has practical applications such as the production of neutrons in strong bursts, or the production of tritium by deuteron bombardment of lithium. Further extensions will now be described which are of importance in the application of the invention of the production of very high temperatures and of thermonuclear power.

The ions leave the ion beam source with a radial and an azimuth component of velocity, the azimuth i.e. rotational velocity being preferably large compared with the radial. In order to store up large kinetic energies the ions are now directed, by a combination of electric and magnetic fields into a hollow cylindrical space, preferably having a small radial thickness compared with the radius and length of the said space, and they are trapped in this space, so that a high ion density is built up in it. The space charge of the ions is neutralized by an equal number of electrons, injected by special hot or cold cathodes. This neutral assembly of rotating ions and electrons will be for brevity called a flywheel.

In the flywheel the ions rotate around the axis, and are alternately reflected at the inner and outer boundaries. At these boundaries the ion motion is either purely azimuthal, or at most has a small axial component. The major part of the flywheel is situated in a substantially uniform, axial magnetic field. The sign of this field is opposed to that produced by a circular loop or loops situated near to the ion source at the axis. In other words, the magnetic flux through a circle drawn coaXially through the ion starts with a, say, negative value, and increases steadily as the ion moves outwards, attaining a positive value at or before the exit of the ion gun. By the law of constant total angular momentum, which is the sum of the mechanical angular momentum of the ion and of the magnetic flux traversed by the ion, the angular momentum of the ion will steadily and steeply increase in absolute value as the ion circles outwards. As it reaches a certain radius, all its kinetic energy imparted by the accelerating voltage drop will be used up as rotational energy. Hence this is a maximum radius, at which the ion must turn back. This determines the outer boundary of the flywheel. But there will also be an inner boundary, because an ion launched under the conditions as described can never reach the axis in the uniform field. At the axis the magnetic flux is zero, but as the ion has started at a radius with a negative flux inside it, the flux traversed by it is still positive, hence at the axis it would still have to possess a finite positive angular momentum which is impossible. Consequently, if the conditions as previously specified are observed, the flywheel will be limited inside and outside by two radii, which can be brought close together by suitable design. Limitation of the flywheel in the axial direction may be achieved by the simple device of increasing the magnetic field intensity at the required height, for instance by adding at this level an extra coil to the coil system which produces the uniform axial magnetic field.

Only very small energies could be stored in a flywheel without space charge neutralization because of the enormous space charge in ion beams. Space charge neutralization is achieved in the flywheel by electron injection from a system of hot or cold cathodes arranged in coaxial circles. In order to avoid ion losses at these electrodes, they are so positioned that owing to the magnetic limitation of the boundary of the flywheel explained above, the ions pass close to them, but do not make contact with them. When excess positive space charge starts accumulating in the flywheel, the repulsion of the ions will make them approach the cathodes, until a neutralizing stream of electrons flows into it from the cathode, by hot emission or by a unipolar are. It is known that this is possible only at the expense of losing some ions to the cathode, but the ratio of ion current to electron current is about equal to the square root of the ratio of electron mass to ion mass. In the case of deuterons for instance this means that one ion is lost to the cathode for about 60 electrons which flow into the flywheel, hence the loss is not heavy.

The electrons injected into the flywheel will move freely along the magnetic field lines only, hence surfaces of revolution drawing through the field lines will automatically become equipotentials, with potentials practically equal to that of the cathode which injected the electrons. By impressing suitable potentials on the array of annular cathodes it is thus possible to regulate the electric potential distribution in the flywheel. This is an essential advantage of the invention. It is well known that, even in discharge devices with exactly rotationally symmetrical electrodes, systems of ions and/ or electrons have a tendency to depart from the rotationally symmetrical shape, and break up into uncontrollable strands. This danger is greatly reduced in devices according to this invention by the local control of potentials, which are made forcibly rotationally symmetrical.

Various equilibrium configurations of the flywheel can be produced in impressing suitable potentials on the said injection-cathodes. An equipotential flywheel is suitable for only small accumulations of charges. In such a flywheel the ions would describe very nearly circles, alternately contacting the inner and outer boundary. In a preferred form of the invention a radial electric field is impressed on the flywheel by an anray of cathodes, of such intensity that the ion trajectorie in the bulk of the flywheel become straight lines, which are fairly abruptly reflected at the outer boundary. In this preferred operation the ions and the electrons rotate. together in the bulk of the flywheel, the electrons on coaxial circles, the ions along polygonal paths. Hence in this operation not only the space charge but also the ion current is neutralized in the bulk of the flywheel. This has the advantage that the position of the flywheel remains unchanged during the build-up period, while the large equal positive and negative charges accumulate in it, hence the injection conditions remain constant. But it is not possible to maintain current-neutrality in the whole of the flywheel. The ions must be reflected at the outer boundary 'by a rather abruptly increasing magnetic field, which is screened off by a circulating current. In the preferred operation according to the invention this current is concentrated in a rather thin surface layer at the outside of the flywheel. It can be seen that this current must have the sign of an ion current, but it is preferable to produce it not by ions but mainly by electrons circulating opposite to the ions in the said outer surface layer or skin. Consequently the radial electric field, which in the bulk of the flywheel was of such sign as to accelerate ions outwards, must be reversed in the said skin.

The magnetic field at the outside of the flywheel must be stronger than the field at the inner boundary, because the magnetic pressure difference must be such as to balance the centrifugal force in the rotating flywheel. Consequently the magnetic field at the outside must be increased as mass and charge accumulate in the flywheel. This can be achieved by allowing the flywheel to expand a little and to compress the magnetic field between it and the conducting vacuum envelope. It is, however, preferable to keep the flywheel in its original position during the build-up process, and to increase the magnetic field produced by the outer coils, by increasing their current according to a certain schedule.

In a preferred operation of the invention the flywheel is built up rapidly, for instance in one millisecond, until it contains e.g. ione coulomb charge. This requires an ion current of 1000 amperes. With an accelerating voltage of e.g. 100 kilovolts the energy stored in the flywheel, almost entirely in the form of kinetic energy of the ions, will be 100,000 joules. Rapid build-up has the advantage that the ion motion remains essentially regular for short times. In longer times the ions will collide with one another and develop a random radial motion superimposed on their regular rotation. In even longer times the ions will share their random energy with the electrons, and these will radiate it away. Once the ions have shared their energy with the electrons the flywheel changes into a rotating plasma, with a certain temperature, and it is known that plasmas will diffuse through magnetic barriers. Hence precise confinement of the flywheel is facilitated by rapid build-up. On the other hand a loss of a small percentage of the energy of the ions to the electrons is beneficial, because ions which have lost energy in the flywheel will not be able to retrace their steps and flood back to the ion gun. The time 6 of build-up is therefore a matter of advantageous compromise between the beneficial and deleterious effects.

A flywheel containing e.g. 1 coulomb of charge of either sign, i.e. 6.10 ions in a volume of, for instance 10 cm. with an energy of e.g. kev. offers an advantageous starting point for the production of thermonuclear power. One way of achieving this is to start with a flywheel of deuterons and to admit to the vacuum envelope containing the flywheel an equal molecular quantity of tritium gas, that is to say one tritium atom for two deuterons. After the collision of the rotating deuteron flywheel with the practically stationary tritium, the total rotational energy will drop to about 40% of the original, and 60% of the energy becomes available for sharing between deuterons, tritons and an equal number of electrons. This is rapidly randomized, and is converted into a temperature of several hundred million degrees, suflicient for starting the D-T fusion reaction. In order to make this economical it is necessary, however, to compress the mixture about a hundred times by rapid increase of the magnetic fields. The flywheel will then be converted into a very hot toroidal plasma in which the fusion reaction will go on so long as the magnetic fields can contain it. The device according to the invention has an important advantage over the known so-called mirror machines in that the plasma is torodial, and will remain so owing to its rotation which is not randomized. Consequently the plasma will not leak off through the axis, as in conventional mirror machines. Moreover, it will carry a certain large current, and thereby provide by itself a'certain fraction of the magnetic fields required for its containment. A further advantage is that an azimuthal stabilizing magnetic field can be impressed on this toroidal plasma through the central conductor.

The invention will be better understood from the following description of two embodiments thereof given with reference to the accompanying drawings in which:

FIG. 1 is a schematic illustration of the two magnetic fields which together compose the meridonal magnetic field.

'FIG. 2 illustrates the result of the superposition of these two fields.

FIG. 3 is a section through a first form of the ion beam source, with a schematic illustration of the limitations of the flywheel.

FIG. 4 is a side view, partially broken away, of the whole device of FIG. 3.

FIG. 5 is a horizontal sectional view of the device of FIG. 3 taken substantially on the line 5-5 in the latter figure, but omitting certain of the elements in the interest of clarity.

FIGS. 6 and 7 are fragmentary sectional views of two alternative forms of the injection cathode of FIG. 3.

FIG. 8 is an axial section through an improved form of the ion beam source, accelerator and reflector with cathodes for injecting electrons into the flywheel.

FIG. 9 is a fragmentary horizontal sectional view of the device of FIG. 8 taken substantially on the line 99 in the latter figure, but omitting certain of the elements in the interest of clarity.

FIGS. 1 and 2 illustrate the design of the meridional magnetic field for the ion beam source. 1 is a conductor forming a circular loop around the axis Y. The magnetic field is represented not by field lines but by the lines of constant vector potential A, which is defined as A =/21rr, where r is the radial distance from the axis, and 4) the magnetic flux passing through the circle of radius r. The lines A=a constant are somewhat different from the field lines which are the lines of constant flux, i.e., =a constant. They have the advantage over a representation by field lines that a uniform axial magnetic field is represented by equally spaced lines A=a constant, it A increases in equal steps. FIG. 1 shows the two components of the meridional magnetic field separately. The A=a constant lines of the first component field, produced by the current in the loop 1, form closed lines around it. The second component field is produced by coils outside the limits of FIGS. 1 and 2, but illustrated as coils 36 in FIGS. 4, 5 and 9. This second field is uniform near the level of 1, and for some distance above it, up to the level to which one wishes the flywheel to extend. The field intensity increases immediately below the level of 1 and above the previously mentioned top level.

In FIG. 2 the field resulting from the superposition of the two component fields is shown. As the currents in the loop 1 and in the above-mentioned outer coils 36 have opposite signs, the result is that they weaken one another inside the loop 1, and reinforce one another outside it. At any point of a certain line A=0, shown in dashed lines, the flux between the point and the axis is zero. This line is therefore also a field line, 1 5:0.

Elsewhere field lines and lines A=a constant differ from .one another, as shown by the field line F, shown in dotdashed lines passing through the point A=-3 at r=r In the present example it is assumed that the ions originate at this point A r The figure shows also, in dashed lines, certain contours of constant rotational energy. These are determined, apart from a constant factor, by the formula m= o e 2 Any one of these lines is the boundary of the accessible space for ions which have been accelerated by the electric voltage drop to a total energy equal to the figures marked on these lines, because at these lines the whole energy is used up in rotation. It is seen that all ions of given energy are thereby confined to closed regions.

FIG. 3 is a schematic axial section, FIG. 4 is a side view and FIG. 5 is a cross-section through one design of the ion beam source, FIG. 3 also illustrating the shaping of the flywheel by the magnetic fields previously described. An axial tubular conductor 2 which carries the axial current I runs through the whole structure and produces a strong azimuthal magnetic field. In this design a mechanically controlled gas inlet is provided. A movable circular plate 3 preferably of ceramic material is arranged between two dishes 4 and 5, and can be raised by means of a solenoid 6. A diaphragm 7 of silicone rubber or the like fits snugly to the bottom plate and closes the gas inlet 8. On raising 3 a certain small amount of gas is admitted through an annular gap between the two dishes 4 and 5 into the ion source. In this drawing the circular loop 1 is placed immediately below the entrance 9 of the ion source space 10, and this loop carries, in addition to the steady current which generates a component of the meridional magnetic field, a high frequency current for energizing the source, that is to say for ionizing the gas stream on its way through it. The source is schematically indicated as bounded by a double array of circular cathodes 11, which must not be full loops, or else the high frequency power could not penetrate into the space between them. The ion source space 10, that is to say the space in which ionization of the gas stream takes place, extends as far as two outer dishes, 12 and 13, preferably of ceramic material, while the accelerating space 14 is outside them. This space is flanked by cathodes 15, connected in opposite pairs, which carry voltages gradually decreasing towards the outer periphery. As previously explained, in spite of the high potential differences between neighbouring electrode pairs 15, no discharge will take place between these, by reason of the strong meridional magnetic field produced by the loop 1 and the outer coils which prevents electrons from moving radially.

FIG. 3 illustrates also how the ion beam is diverted into an ionic flywheel, in the simple example of an equipotential flywheel. The lines of constant rotational ion energy, shown in dashed lines, are transferred into this drawing from FIG. 2. As an example, the numbering of these lines may correspond to kilo-electron-volts; for instance the line 56 means that this is the outer boundary of the region accessible for ions with 50 kev. energy. At the lower end of this region an injection cathode 16 is arranged the contour of which follows this line along a short are. Ions accelerated to 50 kev. will be just able to reach this surface. As the ions accumulate, the ion cloud takes on a more positive potential, and a discharge occurs between 16 and the cloud, injecting electrons into it. As previously explained, this can be either thermal emission or a unipolar arc discharge produced by any suitable form of emissive cathode. A known form of thermal emissive cathode is illustrated in FIG. 6 and comprises an electron surface of contoured shape provided with heater wires in thermal contact therewith. A known form of unipolar arc discharge cathode is illustrated in FIG. 7 and comprises a row of sharp metallic teeth forming a contoured surface. The result is that the ion cloud becomes neutralised, but only in the region which electrons can reach. This region crosshatched in the drawing is limited by two of the dotdashed lines starting from the cathode, which represent field lines, and which, by reason of the large mobility of the electrons in the direction of magnetic field lines,

ecome equipotential lines. It may be noted that in the presence of a strong azimuthal field the field lines are not in meridian planes, but form helices around the axis, but this does not affect the representation in the drawing, because the generatrix of the surfaces containing the field lines is still determined by the meridional magnetic field alone.

The flywheel will thus occupy the cross-hatched area, which has a long cylindrical extension in the region in which the magnetic field is uniform and axial, a region which has been indicated by a gap in the drawing. At the top, the flywheel is limited by the increased magnetic intensity at this level.

The vacuum boundary 17, which is preferably in the form of a metallic envelope evacuated by a suitable pump through a pipe 37 (see FIG. 4), is preferably maintained at a potential equal to that of the injection cathode 16, ie 50 kv. below that of the ion source in the above example, or somewhat lower. Coils 36 outside vacuum boundary 17 providing a magnetic field which, as previously described with reference to FIGS. 1 and 2, forms the second component of the meridional magnetic field.

An improved design of the ion beam source is shown in FIG. 8 in axial half section and in cross-section in FIG. 9. Starting from the axis, the central conductor 2 which carries the axial current I producing the azimuthal magnetic field is a tube, provided with perforations 18 at the level of the ion source for the admission of dosed quantities of gas. Dosing is achieved by injecting, by a syringe or the like, a certain small volume of gas at the bottom of a column of mercury 19 covered by a layer of silicone oil 20 supported in a tube 21 housed within the tube 2. The gas will rise in the tube 21 in the form of a bubble. In the tube 21 there is an insulating ring 22 which insulates the end portion 23 from the main part of the tube 21. When a gas bubble breaks the column of mercury at this region the end portion 23 is isolated and a signal is thus provided which is fed through lead 24. This signal is used to start the electrical supplies.

The gas proceeds from the holes 18 in tube 2 outwards between two ceramic plates 4 and 5 as before into an annular space which has a constriction at 9, preferably provided with tangential grooves, so as to slow down the speed with which gas molecules travel through the ion source space 10.

The means for generating the meridional magnetic field differ from those shown in FIG. 3 in that the loop 1 has been split into three parts, 1', 1", and 1". 1 has two turns, and all four turns in 1', 1" and 1 carry the same current, in like sense, in series. This has the advantage that the centre of the ion source space 10 is now at a point of zero meridional field, and the field density is very small throughout this space, thus ensuring uniformity of angular momentum of the ion beam, as previously explained. A further difference is that the high frequency current which induces the HR field in the source and thereby effects ionization therein, has been separated from 1 and is now a separate coil 25.

As explained in the introduction, the design of the ion source is such that the ions formed in the space 10 by HE. bombardment proceed radially outwards, while the secondary electrons produced in the ionization process proceed to one or both of the walls. In this design of the ion source the meridional magnetic field is weak while the azimuthal field, produced by the central current I is strong. The effect of this is to bend the ion trajectories in the meridian plane. In FIG. 8 it is assumed that the sign of this field is such as to bend the trajectories downwards. In order to compensate this effect the electric equipotentials are skewed in such a way that there is an axial component of the electric field which tends to bend the trajectories upwards, just compensating the effect of the magnetic field. This skew system of equipotentials is produced by two radially offset arrays of electrodes 26 and 27 corresponding ones being at the same steady potential.

The electric supply to the ion source must be such that it offers a small impedance to the strong electron current, equal to the ion current, which flows out through the said electrodes, while offering a high impedance to oscillatory currents of the frequency of the H.F. power supply. This is achieved in the present design by making these electrodes 26 and 27 in the form of a double array of wires embedded in a suitable dielectric 28, 29 so that the length between the inner ends in contact with the ion source and the outer ends at which they are connected together by annular metal bands 30 and 31 and thereby azimuthally short-circuited is equal to a quarter of the vacuum wavelength corresponding to the said high frequency. Convenient frequencies for the operation of this device are in the range 10-30 meters vacuum wavelength. By embedding these conductors in a certain titanateceramic, known under the registered trade name of Faradex H, which at the said frequencies has a dielectric constant of about 2500, the quarter-wave wires embedded in the said material can be made to have lengths of only 15 cm. The embedding must be very perfect, as any small air gap between the wires and the ceramic will substantially increase the length of the quarter-wave lines; hence it is preferred to fire the ceramic after the wires are embedded in a powder or paste of the said material.

By breaking up the electrodes which supply the ion source with steady potentials into a double array of quarter-wave wires, connected into annuli at the outer ends only, two purposes are achieved. One is that the high frequency power can penetrate almost freely between the wires into the ion source. The other is that the HR impedance of the said electrodes is very large, hence no H.F. oscillating current can develop in the discharge space, because it cannot continue in the outer circuit. This is of particular value if one desires the ions in the said space to oscillate as well as the electrons, and to contribute to breaking up the gas molecules. In this case instead of a completely azimuthal magnetic field one operates the device with a slightly helical field, preferably with an inclination angle of the order (m/M) m/M being the mass ratio of electrons to ions. This is about 1/60 for deuterons. The effect is that the ions too will oscillate, with an energy about equal to the electrons, but because of their higher mass they will describe orbits of much smaller dimensions. By making the HF. impedance of the outer circuit large by the means described the plasma will automatically keep at a certain small distance from the conductors, because no H.F. current can flow through these. Such a slightly helical field can be achieved by making the meridional magnetic field in the beam sources not exactly zero, but giving it a slight axial component.

The steady potential supplies to the ion source through the metal bands 30 and 31 have a low potential, and can be led through the vacuum, as shown at the top and bottom of the figure. On the other hand the high voltage supplies to the hot or cold cathodes 15 in the accelerating part 14 of the ion beam source must be well insulated throughout, by ceramic conduits 32 and 33.

FIG. 8 differs from FIG. 3 also in the way in which the flywheel is controlled. While in FIG. 3 the flywheel branches out upwards and downwards, in FIG. 8 it has only a single upward arm. This is produced by a reflector 34, preferably of ceramic material, in which is embedded an array of annular injection cathodes 35. By giving these cathodes appropriate potentials, only a little less than the maximum potential drop, the ion beams leaving the ion beam source can be deflected by degrees. The potential gradient between these cathodes regulates the radial potential distribution in the flywheel. In a preferred operation, as previously explained, the electric gradient is directed outwards and has such a value that it just compensates the inwardly directed magnetic force, so that inside the bulk of the flywheel, the ions move in straight lines. They are reflected in a thin outer layer or skin of the flywheel, which alone carries a nonzero circulating current. In this layer the electric gradient is reversed. This gradient too can be impressed by the cathodes 35, but the said layer can also be left to itself. By the high conductance of the plasma, due chiefly to electrons, a current will automatically develop in the outer layer of such intensity as to screen off the outer, strong magnetic field because, by the means described, the circulating currrent is suppressed in the inner layers of the flywheel. As previously described, in order to keep the flywheel in a fixed position during the build-up period, the outer magnetic field must be run up during this time from a small Value to a larger one, sufficient to compensate by its magnetic pressure the centrifugal pressure of the rotating ions.

The means for the electrical operation of the ion beam source as described are well known in the art of electrical engineering and need only be briefly listed. The large central current I, which may be of the order of e.g. 200 kiloamperes, can be advantageously produced by a half-wave of mains frequency, 50 or 60 cycles/sec, in a step-down transformer, with synchronous switching at the zero points of the current on the primary side. The same technique is also advantageous for the currents circulating in the loops 1', etc. which are of a somewhat smaller order, and for the coils outside of the tube which produce the uniform axial field. It is preferable to have the operating period near the maximum of the said currents, at which moment they are almost steady. In order to ensure a safe balance of the fields at the position of the ion source it is advantageous to have these loops and coils all in series. These and all other supplies are preferably timed by the arrival of the gas bubble at or near the top of the liquid column, but the start of this bubble must be timed in relation to the mains, so that the bubble arrives near the time of a current maximum. At the moment when the gas arrives in or near the top of the tube 21, the high frequency power supply is switched on. The steady potential supplies to the ion source and accelerator and to the injection cathodes can be previously energized, because these will discharge no current until the ion beam has started.

Instead of building up the rotating ion mass into a high energy flywheel the invention may also be used for producing an axial ion beam. This may be done by locating the ion source at any radius on the surface marked A=0 in the diagram of FIG. 2, that is to say on the surface which encloses zero magnetic flux. In addition the axial magnetic field which is uniform in the region of the source is allowed to fall to zero at a level above the source. This is in contrast with the previous flywheel containing field which is made stronger at the higher level as shown in FIG. 2. With such a field, and with source located as described, the ions will start rotating around the axis but they will gradually be aligned with it as they enter the region of decreasing field and finally leave in an exactly axial direction.

Such an arrangement may be used for ionic bombardment of targets such as lithium or could be employed, for example, for propulsion of a space vehicle.

It will be appreciated that such a beam will still be neutralized along its whole length by the electrons injected as previously described.

I claim:

1. Ion beam source comprising means for setting up an ion stream, means positioned at a plurality of locations along said stream for injecting electrons transversely into said stream so as to neutralise the space charge of the ions in said stream, and means for preventing flow of injected electrons between difierent electron injecting means.

2. Ion beam source comprising means for introducing gas molecules into an ionising location, means for agitating electrons released in said ionising location to ionise said gas molecules, means for accelerating the ions so formed away from said ionising location, means for injecting further electrons transversely into the accelerating ion stream so as to neutralise the space charge in the accelerating region, and means for so constraining injected electrons as to prevent acceleration thereof by said accelerating means.

3. Ion beam source comprising a vacuum envelope, means within said vacuum envelope defining an ionisation location, means for accelerating ions away from said ionising location, means for injecting electrons transversely into the stream of accelerating ions, said electron injecting means comprising at least one array of electrodes in close proximity to the stream of ions, and

means for providing easy flow paths for the injected electrons transverse to the ion stream.

4. Ion beam source comprising a vacuum envelope, means within said vacuum envelope for setting up an ion beam, and means for accelerating the said beam comprising at least one array of electrodes in suificiently close proximity to said beam that electrons will be emitted therefrom transversely to said beam, means for applying potentials to the electrodes of said array in descending gradation to provide an accelerating field, and means for setting up a magnetic field in a direction transverse to said beam and perpendicular to the electric field set up by said electrode array.

5. Ion beam source comprising a vacuum envelope, means within said vacuum envelope for setting up an ion beam, and means for accelerating the said beam comprising an array of electrodes on each side of the beam in sufiiciently close proximity thereto that electrons will be emitted therefrom, means for applying potentials t9 the electrodes of each array in descending gradation with the electrodes of one array forming pairs of equal potential with the electrodes of the other array, and means for setting up a magnetite field transverse to the beam and the field lines of which are such as to pass through both electrodes forming an equipotential pair.

6. Ion beam source comprising in a vacuum envelope means defining an ionisation location rotationally symmetrical about an axis, means for introducing gas molecules into said location, means for agitating electrons released in said location by ionisation of gas gas molecules, means for drawing of said electrons in an axial direction, and means for setting up an azimuth magnetic field within said location.

7. Ion beam source comprising in a vacuum envelope means defining an annular ionisation location centered on an axis, means for setting up within said location a high frequency electromagnetic field, means for impressing a radial electric gradient across said location, and means for setting up an azimuthal magnetic field within said location.

8. Ion beam source comprising in a vacuum envelope means defining an annular ionisation location centred on an axis, means for introducing gas molecules into said location, means for setting up in said location a high frequency electromagnetic field, means for setting up across said location a radial electric gradient, means for setting up within said location an azimuthal magnetic field and, surrounding said location an annular accelerating egion, means for setting up a radial potential gradient across said region, means for introducing electrons into said region, and means for setting up in said region a magnetic field transverse to said beam and to the electric gradient in said accelerating region.

9. Ion beam source as claimed in claim 8 wherein said means for setting up within said ionisation location a radial electric gradient comprises a plurality of concentric arrays of conductors extending away from the boundaries of said location on either side of the beam, the conductors of each array being connected together at their outer ends, the length of said conductors being electrically equivalent to a quarter wavelength at the frequency of the high frequency excitation in the ionisation location.

10. Ion beam source as claimed in claim 9 wherein said conductors are embedded in dielectric material.

11. Ion beam source as claimed in claim 8 wherein the means for setting up the azimuthal magnetic field in said ionisation location comprises a conductor passing axially through said vacuum envelope.

l2. Ion beam source as claimed in claim 11 wherein the axial conductor is in the form of a tube, and wherein gas is introduced to the ionisation location through this tube.

13. Ion beam source as claimed in claim 8 wherein the means for setting up within said accelerating region a radial potential gradient comprises a plurality of electrodes located on each side of the beam, each of said electrodes encircling the axis at a different radial distance without forming a completely closed loop, the electrodes on one side of the beam being paired with the electrodes on the other side of the beam to form equipotential pairs defining equipotential surfaces skewed to the axial direction.

14. Ion beam source as claimed in claim 13 wherein at least some of said electrodes are electron emissive.

l5. Ion beam source as claimed in claim 8 including means for diverting the ion beam into an ionic flywheel, said means comprising means for setting up a magnetic field in the form of the resultant of three component fields, namely, an azimuthal component field, a meridional component field derived from at least one current loop surrounding said axis, and a uniform axial component field produced by at least one coil surrounding said loop, and energised in opposite sense to the said loop.

16. Ion beam source as claimed in claim 15 wherein said meridional component field is produced by at least two current loops encircling the axis.

17. Ion beam device as claimed in claim 15 including means for deflecting the beam into the axial direction.

18. Ion beam device as claimed in claim 17 wherein said deflecting means comprises an array of annular electrodes surrounding said accelerating region, and means for applying potentials thereto.

19. Ion beam source comprising means for setting up an ion stream, means positioned at a plurality of locations along said stream for injecting electrons transversely into said stream so as to neutralise the space charge of the ions in said stream, and means for setting up a magnetic field in a direction transverse to said stream so as 13 14 to cause the injected electrons to move along the mag- 2,873,400 2/59 Cook 313-231 netic field lines. 2,880,337 3/59 Langmuir et a1 313-2315 References Cited by the Examiner 2,392,114 6/ 59 p 3113-53 UNITED STATES PATENTS 5 GEORGE N. WESTBY, Primary Examiner.

2,576,601 11/51 Hays 250-4191 ARTHUR GAUSS, RALPH G. NILSON, Examiners.

Claims (1)

1. ION BEAM SOURCE COMPRISING MEANS FOR SETTING UP AN ION STREAM, MEANS POSITIONED AT A PLURALITY OF LOCATIONS ALONG SAID STREAM FOR INJECTING ELECTRONS TRANSVERSELY INTO SAID STREAM SO AS TO NEUTRALISE THE SPACE CHARGE OF THE IONS IN SAID STREAM, AND MEANS FOR PREVENT-

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