US 3321919 A
Description (OCR text may contain errors)
. 1. MAROLDA 3,321,919
Y PLASMA May 30, 1967 A PARATUS FOR GENERA TIN@ HIGH DENSIT 4 Sheets-Sheet Filed July a, 1964 53! DIC VAE/0451.6
DC. SUPP/ Y SEQUENT/L SWITCH/NG' MEA/VS May 30, 1967 A. J. MAROLDA 3,321,919
APPARATUS FOR GENRATING HIGH DENSITY PLASMA Filed July 1: 1964 4 Sheets-Sheet :f
5oz/RCE GAS SUPPLY DC SOURCE DC SOURCE paz 5E GIA/[PA VAR/ABLE SUPPLY 30 Fig. 4
APPARATUS For@ c-ENLRATIN@ HIGH DENSITY PLASMA Filed July 1964 4 Sheets-Sheet 1 D@ J I .SOURCE 51 GAS 55 ,5i/6 l l SUPPLY yd 4s Dig IT f2 m 38 4'/ May 30, 1967 A. J. MAROLDA 3,321
ENSITY PLASMA 'LENERATING HIGH D AFPARATUS FOR 4 Sheets-Sheet a Filed July United States Patent Ofice 3,321,919 Patented May 30, 1967 This invention relates to the generation and stable confinement of high density, high temperature plasma. In particular, there is comprehended a novel combination of plasma generating, heating and confining apparatuses which cooperate to provide an improved plasma device. Although the principles taught herein can be applied to various devices without departing from the spirit and scope of the invention it is currently believed that the high current ion source, the thermonuclear react-or, and the plasma rocket engine represent particularly suitable embodiments thereof.
There currently exists the need for an improved high density plasma generator. In the eld of physics research such a device would provide a desirable source for the type of very high current ion beam that might be required for use in conjunction with a particle accelerator. Another use for the plasma generator comprehended herein would be to facilitate the controlled fusion of ions. One approach to controlled fusion involves injecting low energy deuterium ions into a containment field wherein they undergo dissociating collisions and are trapped for long periods of time. Density-containment product considerations require that the injected beam be as large as possible to enable the particle density to build up to a high value in a time less than the minimum containment time. The plasma generator of the present invention is well adapted to provide this very large injected beam. Still another example of the utility of the invention is its use as a plasma rocket engine. The present invention. being directed toward generating a copi-ous supply of ions, is particularly effective in meeting this requirement.
Inasmuch as these and other applications of the plasma generator exist and since known prior art devices are in certain respects unsatisfactory, it is the principal object of this invention to provide a new and improved high density plasma generator.
lt is another object of this invention to provide a plasma generating device employing a unique combination of plasma creating, heating and confining apparatuses which, when actuated in accordance with a particular sequence of operation, will effectively provide an improved source of high density, high temperature plasma.
Another object of this invention is to provide a new and improved high current ion source.
Another object of this invention is to provide a new and improved thermonuclear reactor.
Yet another object of this invention is to provide a new and improved plasma rocket engine.
These, together with other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings in which like elements are given like reference numerals throughout and wherein:
FIGURE l illustrates, in section, the ion source embodiment of the invention including schematically represented control means;
FIGURE 2 is a sectional view of FIGURE l taken at 2 2;
FIGURE 3 is an isometric view of the ion source illustrating certain plasma confinement and heating apparatus;
FIGURE 4 illustrates, in section, the thermonuclear reactor embodiment of the invention including schematically represented control means;
FIGURE 5 illustrates, in section, the plasma rocket engine embodiment of the invention including schematically represented control means;
FIGURE 6 illustrates the angle H which a charged particle with a velocity vector makes with the z axis in a slowly increasing magnetic field FIGURES 7a and 7b illustrate instability characteristics of plasma under certain conditions; and
FIGURES 8a through Se illustrate the plasma envelope during one cycle of the traveling strong mirror field operation.
Basically the present invention is a so-called hydromagnetic shock plasma generator. In such a generator the gas in an elongated container is ionized by creating an electrical discharge at one end of the container. The electrical discharge ionizes the gas between electrodes at one end of the container and creates a hydromagnetic shock wave which travels the length of the container ionizing the remaining gas as it goes. In order to achieve the various objects of the invention, the plasma thus created must be stably confined. The plasma is also preferably heated by ion cyclotron resonance heating techniques. Furthermore, a particular pulsing sequence that is synchronized with cyclic operation of the plasma confining magnets is required to effect an operable device. The essence of the invention therefore comprises the novel combination of ionizing means, plasma confining means, and plasma heating means, and also a novel mode of operation whereby such means cooperate in particular synchronism to overcome various problems inherent in prior art devices. Ion sources, thermonuclear reactors, and plasma rocket engines, the preferred embodiments of the invention, are hereinafter described in detail so that the novel combinations and the mode of operation of the invention will be more clearly understood.
Ion sources adapted to provide output currents on the order of amperes, require an ion density of about 1015 ions/cc. over a large area aperture, for example in the order of one eighth to one half inch in diameter. The so-called duoplasmatron ion source achieves such high ion densities, but it is limited in aperture size to the physical distribution of the plasma in the arc discharge (usually less than fifty thousandths of an inch in diameter).
An ion source which effectively produces output currents in the order of amperes is comprehended by the present invention and it is intended that such a source utilize hydromagnetic shocks to create the high density plasma. A hydromagnetic shock can be created by the radial electrical discharge between a centrally disposed electrode and the walls of a conducting tube. Such a shock travels the length of the tube causing almost ionization of the gas present. A tube having a gas density of 100 microns would thus provide a plasma having an ion density of greater than 5X1()15 ions/cc. along the tube axis. However, in order to apply this hydromagnetic shock principle to an ion source, the plasma generated thereby must be kept away from the source walls in order to prevent recombination and cooling. This is accomplished with a properly shaped magnetic field such as the confinement field developed by the magnetic mirror configuration used in controlled fusion research. Such a field, when operated with one weak mirror to allow plasma leakage into the extraction region, completely confines the plasma from all the material walls. Although 7) the magnetic mirror exerts the desired confining infiuence upon the plasma, such a configuration in and of itself is not wholly effective since it is unstable, and, if a small perturbation is allowed on the plasma surface, it will grow rapidly in time with the result that the `plasma will soon be entirely lost to the walls. Theoretical hydromagnetic considerations show, however, that a plas-ma confined in a region of radially increasing magnetic field will be stably confined. Although the mirror field increases along the axis, it decreases in the radial direction. ln accordance with the teachings of the present invention, the field is altered so that it increases in the radial direction. This is done by placing current-carrying conductor rods parallel to the tube axis such that alternate conductors have current owing in opposite directions. The resulting field imposed on the mirror field thus creates a stable confinement configuration.
The measured temperature of the shock created plasma is on the order of 3 104 K. and corresponds to an energy of a few electron volts. lf the energy of the plasma can be raised to an energy of approximately 1 keV., the extracted current density will be increased by a factor of 1.5 due to the reduction in space-charge limitation. To take advantage of this fact, an ion-cyclotron resonance heating unit is included in the ion source of the present invention. This method of heating involves alternately compressing and expanding the plasma in a special way at a frequency close to the socalled cyclotron frequency of the ions. In this way, the plasma rapidly absorbs the applied energy and becomes very hot in a short time. In addition to the beneficial decrease in space-charge limitation obtained by heating the plasma, further advantages include insurance of 100% efficiency of ionization and a more copious proton yield resulting from increased molecular dissociation.
Measured values of plasma decay times in shock tube experiments indicate that volume-recombination occurs in the order of several hundred microseconds. Much longer decay times are obtained by using the stabilized containment configuration and resonance heating to keep the plasma hot. In order to maintain the plasma density at the required level, then it is required merely to pulse the central electrode in a time less than the time necessary for the plasma density to reach a value of 1015 ions/cc. ln this way, a D.C, source output can be maintained with economical operation of the input power supply.
In addition to the technique of creating plasma by hydromagnetic shock and the methods of heating and confining such plasma outlined above, it is also a feature of the invention to provide means for maintaining the plasma thus created at a constant density. This is accomplished by providing a traveling strong mirror field which has the effect of decreasing the volume of the plasma as ions are withdrawn from the source. Such a traveling strong mirror field and its relationship to the Ipresent ion source will be hereinafter described in detail.
A synopsis of the theory relating to the various concepts referred to above is here presented 'by way of more completely describing the invention.
In order to experimentally generate hydromagnetic waves, a cylindrical tube of conductive material having non-conducting end plates and containing a gas at about 100 microns pressure is used. Means are also provided to establish a uniform axial magnetic field therein. When a high radial electric field is applied to one end of the tube by discharging a bank of condensers to a centrally disposed coaxial electrode, electrical breakdown of the gas occurs resulting in a radial fiow of current. Due to the imposed axial magnetic field, an azimuthal force is applied to the plasma according to causing the plasma to rotate. The ionization front thus developed, proceeds down the tube at a velocity dependent upon the plasma and magnetic field characteristics. If the wave is allowed to strike the non-conducting plate at the end of the tube while the ionizing current continues to fiow, impurities are introduced into the plasma. For this reason it is necessary to stop the ionizing current before the wavefront strikes the plate, This is accomplished by crowbarring the ionizing current with an ignitron switch which effectively provides a short circuit between electrode and the tube. The effect of the shunt is to impose on the plasma the condition where E, (r) is the radial electric field and r1 and r2 are the radii of the coaxial electrode and the cylindrical tube respectively. Since the angular velocity of the plasma, v0(r) is dependent on E, (r), (2) implies f1 d j; mfr) r-O (3) However, due to the l/r dependence of both the radial current and crowbar action in addition `to the viscous drag forces on the plasma, swirls develop in the plasma resulting from the varying radial velocity distributions. The swirling motion `of the plasma induces local electric fields according to E=U B (4i However, if these fields are made to equal zero by placing a conducting grid in electrical contact with the plasma, the swirling motion can be strongly inhibited.
A magnetic field imbedded in a conducting fiuid is frozen in and constrained to move with the fiuid. Thus such fields may propagate hydromagnetic shocks. The velocity of propagation for such shock waves is given by 1'0=2E/fz (6) where E is the ionization energy of the plasma per unit mass.
Using the generalized ohms law,
'Vlfn=VrBz (7) where v; is the resistivity perpendicular to Bz we find for the radial velocity The time required, then, for the plasma to drift from the central electrode to the walls is given by where u and b are radii of the cylinder and electrode respectively. If l is the length of the tube and U is the frontal velocity, then to avoid radial drift It is therefore seen that to avoid radial drift a high axial magnetic field must be used.
The magnetic moment of a charged particle under the inliuence of a magnetic field sin (u) The adiabatic invariance of ,a then implies that as B increases, vr2 increases proportionally to maintain the constancy of p.. Therefore,
sin2 0= sin2 60 where the sub-naught components indicate initial values. It is clear, then, that when B/Bo reaches a value of l/sin2 00 that sin2 6 will equal l. This implies that there is no velocity component parallel to the z axis and that all the translational energy has been turned into rotational energy. Thus the particle turns around, still spiralling in the same sense, and returns in the negative z direction. It is seen that any charged particle which enters the field with its velocity vector making some angle 0 with the z axis will be reflected by the increasing B field.
However, there exist several loss mechanisms which allow the plasma to leak out of the containing field. These losses may be intolerable for a fusion reactor but some can be used to advantage for an ion source. If all the charged particles were refiected and completely contained between the mirrors, then no plasma would leak into the extraction region and therefore no beam would be produced. It is possible, however, by introducing certain non-adiabatic effects into one of the mirror fields, to allow the desired amount of plasma to leak through one of the mirror fields into the extraction region.
For a particle which has its turning point at BM, the maximum value of B along the tube where Wt is the total energy of the particle.
At Bm, the minimum value of B, it has been demonstrated that Bm (l) Combining 14 and l5 sin2 @czBm/BM (16) which indicates that a particle which is reflected at BM makes an angle 0c at Bm. Thus, only particles which make an angle greater than or equal to Hc at Bm can be reflected by the increasing field. Equation i6 then defines a loss cone in velocity space at the midplane or point of minimum B. Any particle which crosses this plane with its velocity vector oriented within this cone will not be fully retarde-d at BM and thus will be lost from the system. Further, it has been shown that if the adiabatic assumptions given previously are not met, then this loss cone will be effectively enlarged and the plasma loss even greater. Since the particles are continually undergoing collisions in the plasma which tend to knock them from one field line to another, it is reasonable to assume that all the particles, except possibly the last, will eventually be knocked into the loss cone and be lost from the system.
When the two mirror fields are not of equal strength, and if the above loss mechanism is the dominant one, then most of the trapped components will escape through the weaker mirror field. It is therefore possible to use this loss mechanism as a method of `leaking the plasma into the extraction region. By making the mirror field closest to the extraction aperture weak, relative to its counterpart and insuring that the adiabatic assumptions are violated, it is possible to leak enough plasma therethrough so that a large, high ion density plasma boundary will form from which the desired beam could be extracted.
Such a mirror field will also tend to reduce any axial variation in the shock created plasma since the particles all tend to return to the midplane between the two mirror fields.
Plasma, confined by a magnetic eld, will be stable if all types of infinite small perturbations lead to damped oscillations about the equilibrium state and unstable if one or more types of perturbation grow exponentially. It will now be shown that the mirror configuration is unstable by finding at least one perturbation that will grow in time.
Let the radius r and the angie 0 be the cylindrical coordinates of a point on the plasma surface. If the surface is perturbed by allowing it to move radially inward or outward according to r=er sin NH h Bnl! (crcstJ--L lid! (through) (18) where d is the element of length along each path. Since the distance from a to b is greater along the crest than along the trough, the value of B must be less along the crest than along the trough. Therefore, the magnetic pressure in the trough must be greater than the magnetic pressure on the crest and since the plasma pressure is everywhere the same, an inbalance exists tending to make the troughs deeper and the crests higher. Thus the perturbation can only grow. This instability, if allowed to exist, would soon cause all the plasma to be lost to the walls. It is therefore necessary, in the interest of source efficiency, to stabilize the conned plasma and to contain it for as long as possible. A method for accomplishing this stability is hereinafter described.
A plasma can be stably confined by a field which increases everywhere toward the periphery. Therefore, there exists some point or in some cases a closed surface, which is a minimum of B2. There also can exist then, a group of closed surfaces defined by Bzzconstant where surfaces of larger B2 enclose surfaces of lower B2. Since the magnetic pressure" exerted on the plasma surface is given by the pressure on the plasma increases in all directions. It is in such a region that stable plasma confinement may be achieved.
The fieldstrength of a pair of mirrors increases along the z axis away from the midplane but decreases in a radial direction away from the axis. A method, to have B2 increase in all directions toward the periphery is to impose on the mirror field, a second field which increases in a direction radially away from but is constant along the axis. Such a field is the multipole field provided by 2l straight rods parallel to the z axis with adjacent rods carrying current in opposite directions. Thus the plasma could be enclosed by surfaces of increasing B2 and stably confined so that almost all the losses will be through the weak mirror as desired.
There are many advantages to be obtained by heating the source plasma to high energies. In order to partially overcome space-charge limitations in the extraction of ions emitted from the plasma boundary, for instance, it is desirable to increase the initial energy of the ions before they are extracted. In this way, it is possible to increase the ratio of the current density i that can be Withdrawn when the particles have an initial energy eV to the current density when eV@ is small according to where Vex, is the extraction voltage. Since, as mentioned above, the temperature of the shock formed plasma is only a few electron-volts, the plasma could be heated by another method such as ion-cyclotron resonance heating and therefore benefit `by the above relationship, Other benefits that might be expected to be gained from a high energy plasma include long life for the contained plasma, insurance of complete ionization and the possibility that more dissociating collisions will occur.
In its simplest form, considering only a single particle, ion cyclotron resonance heating involves pumping energy into the motion of an ion in a magnetic field by perturbing the steady induction `by a small oscillation at the ion cyclo tron frequency given by where e/M is the charge-to-mass ratio of the ion and B is the induction. Ions gyrating out of phase are decelerated; however, they can only lose whatever energy they have and then they, too, start in phase. The heating process is very rapid, so that an ion can be heated to n high energy in a short time.
In a dense plasma, however, heating by the above method is ineffective due to the self-shielding effects set up in the plasma by a radial oscillating space-charge electric field that is created `by charge separation. To overcome this difficulty, it is proposed that two units be `used operating in different phase. The effect is to create two waves in the plasma traveling in opposite directions down the tube.
In general, the dissipative attenuation of a wave in a plasma is very small and such Waves can be transmitted through or reliected from a plasma with little loss of power. There are, however, some combinations of frequency, ion density and magnetic field strength for which the wave number, K, of certain plasma waves become infinite. At these singularities, the phase velocity of the wave, given by V pIf/ K (22) where f is the frequency and K the wave number, becomes zero and the wave would dissipate its energy before traveling a definite distance. Such singularities occur at the ion and electron cyclotron frequencies. Therefore, waves propagated in the plasma meeting above condition would dissipate their energy in the plasma thereby heating it.
Plasmas contained in the mirror-cusp type field have heretofore been stably confined for perioids on the order of lF2 seconds. If we duplicate this containment time with the ion source of the present invention, it is only necessary to pulse the source every -2 seconds to have the plasma regenerated. However, during the time of confinement the number of particles is steadily decreasing CII due to the beam being drawn off. This is equivalent to about 3 1019 particles/sec. for a 5 A. beam. This of course causes a decrease in ion density, and, since plasma boundary formation is a function of iron density, the boundary will tend to recede. Thus in order to maintain a non-fiuctuating D.C. output, it is necessary to maintain a constant ion density.
In accordance with the present invention, this is accomplished by decreasing the volume of the contained plasma in `proportion to the loss of ions. Referring to FIGURES 8A-8E, there is illustrated thereby such means for decreasing the volume of contained plasma in proportion to the loss of ions through the weak mirror field. Initially the shock created plasma is contained for the length of the source between the two end mirrors as illustrated by FIG. 8A. As the ions are extracted through the weak mirror, a series of strong mirror coils are energized progressively down the source, confining the plasma to a smaller and smaller volume in the manner illustrated in FIGURES 8B and 8C. At the end of the cycle when it is necessary to pulse again, the neutral gas is let into the container, and the central electrode is pulsed in time to overtake the diffusing gas just as it reaches the far end. As the ionizing front progresses down the tube, the last strong mirror coil is turned off and the first mirror coil is again energized as illustrated by FIGURE SD and 8E, and the cycle begins again.
Apparatus employing the principle of the present invention can be readily adapted as a thermonuclear reactor. In order to obtain controlled fusion, there are certain requirements such a device must meet. If fusion reactions are to occur, the particles must have enough energy to overcome the mutual Coulomb barriers so that the close range nuclear forces can take over. For deuterium, this sets a minimum plasma temperature of about 40 kev. or 4.7 108 K. In the present invention, this high ternperature is achieved by the use of an ion-cyclotron resonance heating unit.
The total energy generated per cubic centimeter per bremsstrahlung radiation which is given by,
where N is the density of plasma nuclei, a is the cross section, v the particle velocity and En is the energy released in each fusion reaction. lf the reactions are to be self-sustaining it is obvious that the energy released must be greater than the energy losses or the temperature `will not be maintained. The greatest energy loss is through bremsstrahlung radiation which is given by,
P,=l.42 10-2'1Z2N2 (24) where Z is the atomic number. Thus it is required that Pg Pr for a useful device. It also can be seen from (2) that P,.=Pr(z2) so that if P, is to be minimized, low Z nuclei must be used. For this reason, isotopes of hydrogen are preferably used in the fusion reactor of the present invention. The Pr dependence on Z2 also points out the necessity of having a highly pure plasma. This is accomplished in the manner described above by not allowing the Wave front to reach the far end of the device while the ionizing current is still owing.
Most of the collisions that occur in the plasma will be elastic with no event of interest taking place. Thus to obtain useful power from such a device it is necessary to confine the particles in a small volume so that they may have many collisions and will eventually undergo fusion. The requirement that Pg P, as defined above sets a minimum density-containment time product of l()16 see/cc. for the D-D reaction and 1014 sec./cc. for the D-T reaction. However, for various reasons hereinafter discussed, too large a density is not permissible and the problem is thus reduced to that of providing a sufiicient containment time.
The plasma pressure is given by Rp=NKT (2S) where N is the ion density, K the boltzman constant and T the temperature.
The magnetic pressure is given by,
where B is the field strength. The ratio :Rl,/RB is a useful quantity and is often referred to in the literature. Clearly, if a plasma is to be confined at all, the maximum value of is one which sets a limit on the particle density determined by the magnitudes of the magnetic field and the particle temperature.
Stable plasma connement has heretofore been the main obstacle to successful fusion efforts.
Although the confinement fields provided by the present invention provide a relatively long stable containment period, the mirror systems employed have other natal-al" losses due to scattering of the particles into the loss cones and diffusion across the field lines. If ion-ion scattering is the dominant loss mechanism, for instance, then the time required for the plasma density to drop l/e of its value would be lafrvvalntan/e2) 27) where is the scattering angle in radians, E is the ion temperature in ergs, N is the ion density, v the ion velocity, e the electronic charge and a the nearest neighbor distance. The problem, then, is to determine the initial ion density required in order that at the end of the stable confinement period the equation is satisfied, where N7 is now the ion density at time T. it is seen that A' Kr e (29) where T is the time required for the initial plasma density, Ni, to drop to l/e of its value by any mechanism and K is an experimentally determined dimensionless constant. Equation 28 then becomes and defines the initial ion density required to get useful energy from the device for given T. Assuming that T=^r for O T t when 0 Lr t where z is the time required for N1 to drop to l/e of its value by the dominating scattering loss mechanism, then it is apparent that 2.72 K 1. If, however, rt, then T=t and D K 1.
Very high magnetic fields are necessary in order to contain the very large densities at thermonuclear ternperatures required by such a device. If an axial field of 200 kg. is applied, by a superconducting magnet for instance, then a density as high as 2.4 1016 ions/cc. can be utilized. The containment time needed would then be 42x10*1 seconds. This density must be present at the beginning of a stable period in order to meet the densitycontainment requirement. The initial density obtainable with the device of the present invention is on the order of 5 l015 ions/cc. It is therefore necessary to have a method of building the initial ion density so that after a number of containment periods, s, the initial ion density required by Equation 30 for the T. obtained would satisfy the equation. The stable confinement provided by the present invention permits the density to increase until 11:1 is satisfied. When this value of the density satisfies Equation 30, useful controlled fusion results. In the event that stable confinement is not possible for very long periods of time, then another process is necessary to build up the initial density so that at the beginning of some later stable period Equation 3() will be satisfied. Such a method will now be described.
to scattering losses, so that at the end of the period the ion density will be N,=PTN,-L (32) where N1 is the ion density resulting from one shot and Pe- KT L--H -N. (33) in accordance with Equation 29. At the end of the containment period, some instability will develop which will result in most of the plasma being lost to the material walls. A certain fraction, )=N'/N of the plasma will be contained after this time, however. The ion density immediately after the containment period is then given by N'zfrPTM-L) (34) Accordingly it is possible to satisfy Equation 30 for the 1- obtained. The particle density at the end of s periods therefore is s=l (3U.) This approach to meeting density containment requirements, then comprehends starting with as high an initial density as possible by running with a large B field and, allowing the density to build up from shot to shot until the critical Ni is reached.
The principles of the present invention may also be applied to a plasma accelerator to propel space craft. By removing the end plate and adding a magnetic nozzle coil an engine is provided that will be capable of about l lb. of thrust depending on the propellent and dimensions chosen, at a megawatt input power.
The plasma is created in the same manner as in the above-described ion source and thermonuclear device and is then expelled through the magnetic nozzle by the effect ot' the sequentially energized strong mirror coils. A weak mirror coil and nozzle coil have the etect of increasing the velocity of the propellent which determines the specitic impulse of the engine by IspzVe/g (36) where Ve is the exhaust velocity and g is the acceleration of gravity at the earths surface. Isp has the units of seconds and physically means the number of seconds an engine will deliver a pound of thrust on a pound of fuel.
Referring now to FIGURES 1 and 2, there is illusti-ated an ion source which employs the principles of the present invention. An elongated cyiindrical container 11 is provided to confine the plasma. Container 11 is of nonmagnetic, non-conducting material, preferably ceramic, and includes end plates 12 and 13 which are fabricated of like material. A rst electrode 2l, which may comprise an annular copper insert, is located at one end of container 11 and a second electrode 20 of electron emissive material is coaxialiy disposed within electrode 2l and insulated therefrom by means of end plate 12. Container ll is initially filled with a gas to be ionized such as hydrogen from gas supply 34 through conduit 35. The pressure of such gas may be, for example, one hundred microns, and valve 36 provides pulsed operation of the gas supply in accordance with the mode of operation hereinafter described. End plate 13 has a large aperture 14 therein wherethrough ions are extracted by means of extraction electrode 17. Extraction electrode 17 is operated at conventional extraction potentials and is removed from container 11 by tube extension 15 and insulator 16. Conducting grid 49 may be placed in transverse relationship to container 1l for the purpose of inhibiting swirling motion of the plasma. A plurality of straight rods 18 are disposed on the outer surface of container 11 parallel to the major axis thereof. Rods 18 are electrically connected in series in the manner illustrated by FIGURE 3 such that the current in adjacent rods travels in opposite directions. Direct current source 33 causes sufficient current to flow through rods 18 to establish a plasma confining field in the ol'der of three to fifteen percent of the total field of the device, Since it is undesirable that such a field appear in the ion extraction region, mu metal shields 19 are used to cover rods 18 and their electrical connections in this area. Coil 26 of electrically conductive material is wound around the outer surface of container 1l as illustrated in FIGURE 3. An R.F. field is es tablished on coil 26 by means of R.F. source 32 and conductor 22. Such an R.F. field is effective to cause ion cyclotron resonance heating of the plasma generated within container 11. A solenoid 23 is provided which coaxially surrounds container 11 and coil 26. Solenoid 23 is supplied from direct current source 29 and should be capable of providing an axial magnetic field in the order of twenty kilogauss. An electrical coil 25 is positioned over solenoid 23 at the extraction end of container 11. When supplied with a variable direct current from variable D.C. source 30, coil 25 establishes a controllable mirror field that can be operated to permit the extraction of ions in any desired quantity. The field thus established is herein referred to as the weak mirror field since it permits the escape of some ions from the system at the aperture end of the device and may be distinguished from the strong mirror field (hereinafter identified) which permits the escape of no ions from the other end of the device. A plurality of annular coils 24 are arranged in juxtaposition with coil 25 and in combination with direct current source 28 they comprise means for establishing said strong mirror field. Sequential switching means 27 is provided between direct current source 28 and coils 24 and effects selective energization of such coils in accordance with the hereinafter described mode of operation of the invention. Inasmuch as switching means 27 energizes coils 24 in such a manner as to cause the strong mirror field created thereby to move from the electrode end of container 11 to the aperture end, the field is herein referred to as a traveling strong mirror field. Sequential switching means 27 may, of course, be any switching means such as a drum type controller or a bank of flip op switches that will switch coils 24 on and off in the sequence called for by the invention.
The initial hydromagnetic shock is created in the abovedescribed device by discharging condenser bank 4l between electrode 20 and electrode 21. Condenser bank 41 comprises a combination of condensers 39 and inductances 40 arranged as shown and has sufiicient capacity to deliver l thousand ampere current pulses. Direct current supply 38 is effective to charge the condenser bank to a potential of approximately fifty thousand volts.
Each cycle of operation of the present device is initiated by a pulse delivered from trigger pulse generator 45. Pulse generator 45 may be any conventional means such as a stable multivibrator that is suitable to generate a train of control pulses. Such a trigger pulse first biases the grid of triode 44a such that valve actuator 37 is activated by direct current source 31. This has the effect of opening valve 36 for a sufiicient length of time to fill container 11 with gas from gas supply 34 to one hundred microns pressure. Following an appropriate delay provided by delay line 46 the trigger pulse then biases the grid of triode 44h permitting direct current source 31 to fire ignitron 42. The firing 0f ignitron 42 thus causes condenser bank 41 to discharge between electrodes 20 and 21 thereby creating the hydromagnetic shock required to ionize the gas in container l1. After further delay provided by delay line 47 the trigger pulse biases the grid of triode 44C thereby permitting direct current source 31 to fire ignitron 43. Firing ignitron 43 has the effect of short circuiting electrodes 20 and 21. The time delay of delay line 47 is such that these electrodes are short circuited just before the hydromagnetic shock wave reaches end plate 13 thereby avoiding unwanted contamination of the plasma.
The container 11 at this stage of the cycle of ion source operation has a confined plasma of a given density that is substantially one hundred percent ionized. However, since ions are being extracted through the weak mirror field something further must be done to maintain constant density of the plasma. This is accomplished in accordance with the present invention by the traveling strong mirror field hereinafter described with reference to FIGURE 1 and FIGURES 8a through 8e. When the hydromagnetic shock wave reaches the end of container 11, the plasma has a given volume and its envelope has approximately the geometric shape illustrated by FIGURE 8a. The volume and geometric shape are determined by the combined effects of the various confining elds. If the volume were to remain constant during the time ions are drawn off through the weak mirror field the plasma would become less dense. In order to maintain a constant density therefore it is intended to effect a concomitant reduction in volume to compensate for the loss of ions. The traveling strong mirror field of the present invention does this by compressing the plasma envelope from the electrode end to the aperture end of container 11. For the initial condition (designated Time=0 seconds in FIGURE 8a) the strong mirror coil 24 at the electrode end of container 11 is energized by direct current source 28 through sequential switching means 27. By way of illustrating the sequence of operation more clearly, energized mirror coils in FIGURES 8a through 8e are designated by heavy cross hatching. Following the operation of` ignitron 43 the trigger pulse is delayed a brief interval to assure complete ionization of the gas in container 11, after which said trigger pulse initiates operation of sequential switching means 27. Sequential switching means 27 successively energizes strong mirror field coils 24 in the sequence of MC1, MC2 MCN. Although an operable device will be provided by energizing only individual coils it is preferred that a plurality of coils be energized at a time. For instance coils MC1, MC2 and MC3 may be energized at time Time=0. Switching means 27 would then cause the strong mirror field to travel toward the aperture end of container 11 by energizing MC4 and de-energizing MC1; then energizing MCS and de-energizing MC2 and so forth thus providing a smooth continuously moving field.
Trigger pulse generator 45 must provide a control pulse for each cycle of operation of the ion source. It is therefore designed to generate a train of pulses that are timed to occur each time the above-recited cycle of operation is completed. In the present example this would occur every 10-2 seconds but would depend on the stable confinement period.
Another embodiment of the present invention is represented by the thermonuclear device of FIGURE 4. In this embodiment the ion extraction means of the above-described ion source are eliminated and the apertured end plate 13 is replaced by solid end plate 52. Also, since the problem in obtaining a controlled fusion of ions is chiefly one of stable plasma confinement. the traveling strong mirror field is not required in such a device. In its place a fixed mirror field is provided by means of annular coil 56 and variable direct current source 53. The device is operated as hereinbefore described with trigger pulse generator 45 and de'lay lines 46 and 47 effecting the desired operating sequence.
Having reference now to FIGURE 5, there is represented thereby a plasma rocket engine that is fabricated and functions in accordance with the principles of the present invention. The plasma rocket engine is essentially the same device as the above-described ion source with the exception that the apertured end plate has been removed and the ion extraction means have been replaced by magnetic nozzle coil 51. This device is operated in the same manner as the ion source and the weak mirror coil in combination with nozzle coil 51 have the effect of increasing the velocity of the ions as they are expelled from the open end of container 11 by the traveling strong mirror field.
Although only several particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects and therefore it is intended that the appended claims cover all such changes and modifications that fall Within the true spirit and scope of this invention.
What is claimed is:
1. A high density plasma generator comprising the combination of an elongated non-magnetic container, means for `injecting a quantity of gas therein, means for creating a hydromagnetic wave for the purpose of ionizing said gas to produce hydro-magnetic shocks, ion cyclotron resonance heating means in operable relationship with said container and adapted to heat said ionized gas, a plurality of Substantially parallel conductors longitudinally disposed adjacent the outer surface of said container, said conductors being electrically connected such that current in adjacent conductors fiows in opposite directions means for causing a current to flow through said conductors, a solenoid longitudinally surrounding said container, a direct current source connected thereto and in combination therewith being adapted to establish a magnetic field in the order of twenty kilogauss, and means for superimposing a plurality of magnetic mirror fields thereon.
2. A high density plasma generator including in combination, hydromagnetic shock ionizing means, ion cyclotron resonance heating means, and means for providing a stable plasma confining field, said last mentioned means comprising the combination of a first magnetic field established by a plurality of current carrying conductors, said conductors being disposed in parallel relationship proximately surrounding said generator and being electrically interconnected such that the current in adjacent conductors flows in opposite directions, a second magnetic eld established by a current carrying s-olenoid, said solenoid being overwound in proximate relationship to and extending over the length of said generator with its major axis in parallel relationship to said current carrying conductors, and third and fourth magnetic fields established by magnetic mirror coils, asid magnetic mirror coils being coaxially positioned over said solenoid.
3. An ion source comprising an elongated ceramic container, means for injecting gas under pressure into said container, ion cyclotron resonance heating means, first and second electrodes disposed within one end of said container, an apertured end plate adapted to effect closure of the other end of said container, an extraction electrode proximate to said end plate adapted to extract ions from said container through said aperture, means for periodically discharging high potential electrical pulses across said first and second electrodes whereby a serie-s of hydromagnetic shock waves effective to create a plasma are initiated within said container, means for short circuiting said first and second electrodes during part of each interval between successive high potential electrical pulses, first plasma confinement field producing means comprising a plurality of substantially parallel current carrying conductors longitudinally disposed adjacent the outer surface of said container, said conductors being electrically :onnected such that current in adjacent conductors flows ln opposite directions, second plasma confinement field producing means comprising a current carrying solenoid longitudinally surrounding said container, third plasma confinement field producing means comprising a magnetic mirror coil coaxially positioned over said solenoid at the extraction end -of said container, said magnetic mirror coil including a variable direct current supply, fourth plasma confinement field producing means comprising a plurality of magnetic mirror coils coaxially positioned over said solenoid, and means for sequentially energizing said last mentioned magnetic mirror coils such that plasma confined within said container is maintained at constant density.
4. A thermonuclear reactor comprising an elongated ceramic container, means for injecting gas under pressure into said container, ion cyclotron resonance heating means, first and second electrodes disposed within one end of said container means for periodically discharging high potential electrical pulses across said first and second electrodes whereby a series of hydromagnetic shock waves effective to create a plasma are initiated Within said container, means for short circuiting said first and second electrodes during part of each interval between successive high potential electrical pulses, first plasma confinement field producing means comprising a plurality of substantially parallel current carrying conductors longitudinally disposed adjacent the outer surface of said container, said conductors being electrically connected such that current in adjacent conductors flows in opposite directions, second plasma confinement field producing means cornprsing a current carrying solenoid longitudinally surrounding said container, third plasma confinement field producing means comprising a magnetic mirror coil coaxially positioned over said solenoid at each end of said container, said magnetic mirror coils including a variable direct current supply.
S. A plasma rocket engine comprising an elongated ceramic container, having an open end and a closed end, means for injecting gas under pressure into said container, ion cyclotron resonance heating means, rst and second electrodes disposed within the closed end of said container, a nozzle coil proximate to the open end of said container, means for periodically discharging high potential electrical pulses across said first and second electrodes whereby a series of hydromagnetic shock waves effective to create a plasma are initiated within said container, means for short circuiting said first and second electrodes during part of each interval between successive high potential electrical pulses, first plasma confinement field producing means comprising a plurality of substantially parallel current carrying conductors longitudinally disposed adjacent to the outer surface of said container, said conductors being electrically connected such that current in adjacent conductors flows in opposite directions, second plasma confinement field producing means comprising a current carrying solenoid longitudinally surrounding said container, third plasma confinement field producing means comprising a magnetic mirror coil coaxially positioned over said solenoid at the open end of said container, said magnetic mirror coil including a variable direct current supply, fourth plasma confinement field producing means comprising a plurality of magnetic mirror coils coaxially positioned over said solenoid, and means for sequentially energizing said last mentioned magnetic mirror coils such that plasma generated within said container is forced out through the open end thereof.
References Cited UNITED STATES PATENTS 3,088,894 5/1963 Koenig 176-1 REUBEN EPSTEIN, Primary Examiner.