US 3679897 A
A beam of accelerated microparticles of substantially uniform velocity is passed transversely through an intense pulsed laser beam. In traversing the laser beam, the microparticles are thermally evaporated by the energy of the laser beam to produce either a beam or an expanding cloud of atomic particles.
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Description (OCR text may contain errors)
United States Patent Hansen et a1.
1 5] July 25, 1972 s41 LASER BOMBARDMENT 0F 58 Field of Search ..2so/41.3, 41.9 SE, 42, 84; MICROPARTICLE BEAM FOR 313/63, 230, 161, 153; 331/945; 328/227 PRODUCING ATOMIC PARTICLES IN 56] References Cited THE FORM OF A BEAM OR AN EXPANDING CLOUD UNITED STATES PATENTS  Inventors: David O. Hansen, Westminster; Saul 3,294,920 12/1966 Sewckel ..250/4L9 SE Al h l Manhattan B h; William 3,360,733 12/1967 Vali et a1 ..250/41.9 SE B i L Angeles, 11 f c m; 3,387,218 6/1968 Friichtenicht et a1.. .....250/4l.9 ME Bernard H Shaker Heights, 3,406,349 10/1968 Swain et a1. ..250/41.9 SE Ohio Primary Examiner-Anthony L. Birch  Assign: TRW inc-s Redondc Beach Cahf- Attorney-Daniel T. Anderson, Edwin A. Oser and Jerry A. 22 Filed: 7 Aug. 28, 1969 D q 1 pp 853,818 [57 ABSTRACT Related U.S. A li ati D t A beam of accelerated microparticles of substantially uniform velocity is passed transversely through an intense pulsed laser  contmuanon'm'pan of 599,163 beam. In traversing the laser beam, the microparticles are abandonedthermally evaporated by the energy of the laser beam to produce either a beam or an expanding cloud of atomic parti-  U.S. C1 ..250/42, 250/413, 250/419 SE, cles' 313/63, 313/161, 313/230, 328/227, 331/945 [51 Int. Cl. ..G0lh 21/00, 11015 3/00 16 Claims, 5 Drawing Figures Giant pulse loser Begin laser pumping Pumping Circuit 28* Fire laser 0 swnch 32 h Output 26 Circuit 34 l i -L Laser beam Velocit S l y e ector 8 D I Pulse Generator 44 z g l k v IO) Particle A 22 42 52 Emma beam 20 Particle beam J 56 Accelerator Ng gaq 4 I I6 '8 I4 36 3B 4 53- beam I Pamela J: Electric field region defectors Laser Beam Power DensitymlO Watts Sq- Cm- 8 Particle Beam I4; 50
Velocity 4 l0 CmJSec.
Fig-2 Laser Beam 35 l2 Power Density m l0 IO Watts Sq. Cm.
48 Particle Beam I4; 2
Velocity 5 x lO Cm,/ Sec.
Laser Beam 35 u l2 Power DensntywlO IO Watts Sq. Cm.
Particle Beam l4 8 VGIOCHYA I0 Cm. Sec- Fig.4
David O- Hansen, Saul Altshuler,
William Bernstein, Bernard Hamermesh,
INVENTORS AGENT LASER BOMBARDMENT F MICROPARTICLE BEAM FOR PRODUCING ATOMIC PARTICLES IN THE FORM OF A BEAM OR AN EXPANDING CLOUD CROSS REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 599,163, filed Dec. 5, 1966, entitled Neutral Atomic Beam Generating Device Employing a Giant Pulse Laser for Forming the Atomic Beam," now abandoned.
BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the generation of atomic particles, and more particularly to the interaction of accelerated microparticles with an intense laser beam for producing atomic particles in the form of a beam or a radially expanding cloud.
2. Description of the Prior Art I-Ieretofore, plasmas have been produced by irradiating with a laser beam such targets as gases, solid surfaces and thin foils. These methods have all suffered from the presence of neutral atoms either in the original gas target or being evolved from the solid surfaces. Also the presence of a solid surface will compromise any possible magnetic containment system. Another method has utilized an electrostatic suspension system for small lithium hydride pellets of about microns in diameter. This method has eliminated the effects of neutral gas and has been combined with a magnetic containment system. Still another known method involves the use of a thin quartz fiber suspension system. All of these methods have the drawback that they are limited to a single event and are not repetitive. Furthermore, they do not permit any freedom in the selection of various laser beam-target interaction regions, such as may be required in certain systems utilizing an expanding cloud of atomic particles.
SUMMARY OF THE INVENTION Means are provided for generating and accelerating a stream of microparticles along a beam path. A laser beam is provided for irradiating the microparticles transversely of the beam path. The laser beam has sufiicient intensity to convert the microparticles into atomic particles having substantially the same longitudinal velocity as the microparticles. The atomic particles can be either neutral atoms or ions, depending upon whether the laser beam has a low or a high power density, respectively. Depending upon whether the microparticles have a high or a low longitudinal velocity, the atomic particles are formed into either a beam or a radially expanding cloud, respectively.
BRIEF DESCRIPTION OF THE DRAWING FIG. I is a schematic view of apparatus for producing a neutral atomic beam according to the invention;
FIG. 2 is a diagrammatic view showing the interaction of a low velocity microparticle beam and a low power density laser beam for producing a radially expanding puff of neutral atoms;
FIG. 3 is a diagrammatic view showing the interaction of a high velocity microparticle beam and a high power density laser beam for producing a plasma jet;
FIG. 4 is a diagrammatic view showing the interaction of a low velocity microparticle beam and a high power density laser beam for producing a radially expanding plasma cloud; and
FIG. 5 is a schematic view of apparatus employing a magnetic confinement means into which laser-bombarded microparticles and high energy particles are introduced to create a high temperature plasma.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In each of the embodiments described below, a laser beam is used to irradiate a beam of discrete, electrostatically accelerated solid or liquid microparticles. The embodiments difier essentially as to the power of the laser beam and/or as to the velocity of the microparticles. As used herein, the term low velocity as applied to the microparticles means a velocity 10 centimeters per second; the term "high velocity as applied to the microparticles means a velocity 5 X 10 centimeters per second; the term low laser power density" is defined as approximately 10 watts per square centimeter; and the term high power density" is defined as approximately 10--l0 watts per square centimeter.
Depending upon the velocity of the microparticles and the laser power, the following four configurations and their uses are obtained:
1. The use of a high microparticle velocity and low laser power will result primarily in a beam of neutral atoms with a directed velocity equal to the microparticle velocityrThis neutral atom beam can be used for laboratory studies of a variety of atom-atom collision phenomena, atom-surface collisions, and chemical reactions. The method offers the major advantage of much greater instantaneous atomic fluxes in the energy region ll,000 ev than can be obtained with more conventional means.
2. The use of a low microparticle velocity together with low laser power will result in the production of an expanding cloud of neutral atoms. An application of this technique could occur in controlled fusion research where it is often desirable to introduce a controlled, localized, and wellknown puff of unionized gas into the interior of an energetic plasma for diagnostic purposes.
. The use of a high microparticle velocity with high laser power will result in the production of an isolated plasma jet or plasmoid. This technique is also relevant to the problem of plasma injection into magnetic containment systems and to laboratory studies of the interaction of such plasmoids with electric and magnetic fields.
4. The use of a low directed microparticle velocity with high laser power will result in a spherically expanding plasma with a negligible drift velocity. Because of the very small charge to mass ratio of the microparticles, this technique provides a useful method for plasma generation within magnetic confining field geometries without either any substantial modification of the field geometry or the introduction of large amounts of neutral gas or other impurities. This plasma may be used for study of the confinement properties of the containment system. This laser plasma may serve as a unique seed plasma for the subsequent generation of a plasma of fusion interest by energetic neutral injection techniques.
NEUTRAL ATOMIC BEAM There will now be described an apparatus for generating a neutral atomic beam. This apparatus employs high microparticle velocity and low laser power density as defined above. Referring to FIG. 1, an elongated vacuum chamber 10 contains at one end thereof a particle accelerator 12 capable of accelerating microscopic particles to hypervelocities up to 45 kilometers (4.5 X 10 cm) per second. The particle accelerator 12 may be one of the kind disclosed in the following publications: J. F. Friichtenicht, Two-Million-Volt Electrostatic Accelerator for I-Iypervelocity Research," Review of Scien' tific Instruments, Vol. 33, page 209 (February 1962), and H. Shelton, C. D. Hendricks, Jr., and R. F. Wuerker, Electrostatic Acceleration of Microparticles to l-lypervelocity," Journal of Applied Physics, Vol. 31, page 1243 (July 1960). The particle accelerator I2 is capable of accelerating particles ranging in size from 0.03 to 2 microns in radius. Examples of materials that have been investigated are particles of iron, carbon, and aluminum.
An alternative liquid droplet generator has been described by J. M. Schneider and C. D. Hendricks in Review of Scientific Instruments, Vol. 35, page 1349 (1964). This system generates uniform 50 to 2,000 micron diameter charged droplets of various liquids at a repetition rate up to several thousand per second. In a paper written by M. M. Hoffman entitled Formation of Uniformly Charged Conducting Droplets and Possible Propulsion Applications," issued in Los Alamos Scientific Laboratory Report LA2549, Apr. 1961, there is described a Gallium droplet generator, which is applicable to other low melting temperature metals, such as lithium.
The particles are ejected from the particle accelerator 12 in the form of a particle beam, indicated by the arrow 14, which may be 0.5 millimeters in radius, and at a rate of 10 particles per second, for example. The accelerated particles may have an electrostatic surface charge of the order of X 1016 coulombs or greater when leaving the particle accelerator 12, the surface charge having been previously induced on the particles to promote the required acceleration.
The particle beam 14 passes successively through two particle detectors l6 and 18 spaced longitudinally along the axis of the particle beam 14. Each of the detectors 16 and 18 may be one of the kind disclosed in the article by H. Shelton, C. D. Hendricks, Jr., and R. F. Wuerker, published in the Journal of Applied Physics, Vol. 31, page 1243, dated July 1960, and entitled Electrostatic Acceleration of Microparticles to Hypervelocity. The detectors 16 and 18 sense the velocity of the particles by measuring their time of flight between the detectors. When a particle of predetermined velocity passes through both detectors l6 and 18 in succession, the detectors 16 and 18 generate two electrical trigger pulses 20 and 22 which are spaced by a time interval equal to the transit time of the particle.
The trigger pulses 20 and 22 are fed to a velocity selector and pulse generator 24, which generates an output signal pulse 26 only when the trigger pulses 20 and 22 from the detectors 16 and 18 are spaced by an interval that corresponds to the predetermined particle velocity. For example, assume a predetermined particle velocity of 5 kilometers per second and a spacing of the detectors l6 and 18 of 1 centimeter. The transit time of a 5 kilometer per second particle through a distance of l centimeter is 2 microseconds. Thus, the velocity selector and pulse generator 24 produces an output signal only when the two successive trigger pulses 20 and 22 are spaced 2 microseconds apart. The velocity selector and pulse generator 24 may be one of the kind disclosed by J. F. Friichtenicht in NASA Contract Report CR-263, entitled Particle Parameter Selector System for an Electrostatic Particle Accelerator, published July 1965 and available from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia, 22151.
The output signal pulse 26 is fed to the pumping circuit 28 of a giant pulse laser 30. The laser 30 may be disposed within the chamber 10, or it may be positioned externally thereof, as shown. The giant pulse laser 30 may be one of the kind disclosed in the article by F. R. Marshall, D. L. Roberts, and R. F. Wuerker, published in the Bulletin of the American Physical Society, Series II, Vol. 7, No. 7, page 445, dated Aug. 27, 1962, and entitled Energy Storage and Radiation Emission from Kerr-Cell-Controlled Lasers. Briefly, the giant pulse laser 30 is initially pumped while a Q-switch 32 in the laser cavity is operated in a mode which inhibits regenerative action in the cavity, thereby preventing the laser 30 from generating an output light signal from the output circuit 34 thereof. The Q-switch may comprise the combination of a Kerr cell and a Glan-Thompson polarizer. At a predetermined time, the Q- switch 32 is switched to a different mode which allows regenerative action to take place in the cavity and cause stimulated emission of light therefrom. The light output signal is a high intensity light pulse of short duration, which is illustrated as a laser beam 35 exiting from the output circuit 34.
The sequence of operation of the giant pulse laser 30 must be such as to cause the laser beam 35 to irradiate the particle beam 14 for a sufficient time to vaporize the particles. Thus, taking into account the velocity of the particles, the presence of the particles of desired velocity must first be sensed, the
laser 30 must be pumped for a predetermined time, and then the Q-switch 32 must be actuated between the two modes to generate a laser beam that irradiates the moving particles.
As indicated previously, the output signal pulse 26 from the velocity selector and pulse generator 24 is fed to the pumping circuit of the giant pulse laser 30 to initiate the pumping sequence. The laser 30 must be pumped for about 60 microseconds prior to actuating the Q-switch 32. It will be understood that the pumping circuit 28 may include switching means responsive to the output signal pulse 26 for energizing the means for pumping the laser 30.
During the initial pumping period, a 5 kilometer per second particle will have travelled a distance of about 0.5 meters beyond the second detector 18. Thereafter it is desired to initiate operations for actuating the Q-switch 32 in the manner described above. The position of a particle is again sensed by means of two additional particle detectors 36 and 38. The third detector 36 is located about 2.5 meters beyond the second detector 18, and the fourth detector 28 is about l centimeter beyond the third detector 36.
The third and fourth detectors 36 and 38 are similar to the first and second detectors 16 and 18. In the example cited for illustration, the third and fourth detectors 36 and 38 sense the passage of a 5 kilometer per second particle therebetween and develop two trigger pulses 40 and 42, spaced 2 microseconds apart, which are fed to a delay generator 44.
The delay generator 44 may be one of the kind disclosed in the article by J. Bell and J. H. Green in Nuclear Instruments and Methods, Vol. 36, page 320, dated 1965, and entitled A Wide Range High Resolution Time to Amplitude Converter, or in the article by A. E. Blaugrund and Z. Vager in Nuclear Instruments and Methods, Vol. 29, page 131, dated 1964, and entitled A Time-to-Pulse-Height Converter With Simultaneous Random Coincidence Subtraction." The delay generator 44 generates a rectangular delay pulse 46 that has a delay, relative to the fourth trigger pulse 42, that is proportional to the time interval between the third and fourth trigger pulses 40 and 42. The time delay may be equal to the trigger pulse interval or it may be some multiple thereof. For example, suppose that the time delay is set to be equal to the trigger pulse interval. The delay pulse 46 will appear 2 microseconds after the fourth trigger pulse 42 occurs, and at a time when the 5 kilometer per second particle has travelled l centimeter beyond the fourth detector 38 and arrived at a location or interaction region identified in the drawing by the numeral 48.
The delay pulse 46 is fed to the Q-switch 32 of the laser 30 to switch the laser 30 to its regenerative mode. Within nanoseconds after application of the delay pulse 46, the laser beam 35 issues from the output circuit 34 and enters the chamber 10 through a window 50.
Typically, the laser beam 35 is about 2 millimeters in diameter and has a duration of 60 nanoseconds, with a total power output of about 3 joules. Thus, if the laser beam 35 is directed along an axis which coincides with the location 48 occupied by the particle when the delay pulse 46 first appears, and if the largest particle of 2 microns diameter is assumed, the particle will be within the laser beam 35, since the particle will have moved only 0.005 millimeter during the 100 nanosecond delay between switching of the laser 30 and the appearance of the laser beam 35. The particle will remain within the laser beam 35 for 60 nanoseconds, which is sufficient time to vaporize a 2 micron particle and produce a burst of atoms therefrom.
The limitation in the repetition rate of the apparatus above described is primarily determined by the power dissipation in the active material of the laser 30. A repetition rate of 1,000 per second for a period of 10-100 milliseconds can be achieved with a single laser. By using a multiple laser system, the total period can be extended to several seconds without any other substantial change in the system. Because the sensing system comprising the particle detectors 16 and 18 and their associated electronics which controls the firing of the laser 30 operates on individual particles, a high precision in the particle repetition rate is not required.
The atomic composition of the original charged particle consists essentially of neutral atoms, with a small fraction, no greater than of ions which constitute the surface charge of the solid particle. The effect of the laser beam is to break all of the atomic and ionic bonds in the solid particle. Following laser impact, a burst of free atoms and free ions results, all traveling with the same velocity as the original charged particle. Because of the high particle velocity and low laser power density, the vaporized solid particle will consist primarily of neutral atoms with a radial expansion velocity small compared to its longitudinally directed velocity--a beam of neutral atoms. Before this beam can be used, it is necessary that all possible charged particles, which can significantly influence cross-section values etc., be removed as by passing the beam through an electric field, as shown. The atoms pass undeflected through the electric field provided by a pair of spaced electrodes 52 and 54 while the positive ions are attracted to the more negative electrode 54, shown grounded. The electrodes 52 and 54 may be in the from of rectangular plates about 1 centimeter wide and 4 centimeters long in the axial direction of the particle beam and spaced 2 to 3 cenfimeters apart. The dc. voltage between the plates may be about I kilovolt. Thus a neutral atomic beam, indicated by the arrow 56, issues from the electrodes 52 and 54.
RADIALLY EXPANDING PUFF OF NEUTRAL ATOMS In this embodiment, the apparatus is the same as that shown in FIG. I, with the exception that the deflection plates 52 and 54 are eliminated. The particle beam 14 is of low directed velocity and the laser beam 35 has a low power density. As shown schematically in FIG. 2, the low velocity particle beam 14 and low power density laser beam 35 intersect in the interaction region 48 to produce a radially expanding pufi' of neutral gas, typically containing 10 atoms. The radial velocity of the neutral atoms, which is indicated by the arrows 60 is high as compared with the longitudinal velocity of the particle beam 14, and typically is 10 times as great.
The ability to inject such a puff of gas of known density and species into a specific location in a controlled thermonuclear fusion containment geometry at a selected time represents a major improvement in plasma diagnostic technique. Present techniques employ fast mechanical valves which cannot pro vide the desired localization, timing and quantitative injection.
PLASMA JET In this embodiment, the apparatus likewise is the same as that shown in FIG. 1, except that the deflection plates 52 and 54 are eliminated. The particle beam 14 is of high directed velocity and the laser beam 35 has a high power density. As shown schematically in FIG. 3, the high velocity particle beam 14 and high power density laser beam 35 intersect in the interaction region 48 to produce a drifting plasma or jet, indicated by the arrow 62 which has a high longitudinal velocity compared to its radial expansion velocity. Typically, the longitudinal velocity is ten times as great as the expansion velocity.
Plasma jets or plasmoids conventionally are produced by a variety of discharge guns such as button sources and conical theta pinch. These suffer particularly from contamination of the plasma with electrode material and the plasma composition is unknown and variable. Secondly, because they are produced in a variety of high current discharges, trapped magnetic fields of unknown magnitude and orientation can be associated with the plasmoids. Neither of these drawbacks should be associated with the laser produced plasmoid. The repetitive operation described above may also provide major advantages.
RADIALLY EXPANDING PLASMA CLOUD In this embodiment, the apparatus likewise is the same as that shown in FIG. 1, except that the deflection plates 52 and 54 are eliminated. The particle beam 14 is of low directed velocity and the laser beam 35 has a high power density. As
shown schematically in FIG. 4, the low velocity particle beam 14 and high power density laser beam 35 intersect in the interaction region 48. The injected solid or liquid particle will be vaporized and ionized producing a dense plasma. Because of the high particle temperature, the expansion velocity, which is indicated by the arrows 64, is large compared to the directed velocity and typically is ten times as great; therefore, a plasma expanding about its origin is produced. Typical temperatures may be 50-100 ev corresponding to an expansion velocity of 3 X 10 crn/sec. If p. diameter lithium droplets are used as the laser targets, then approximately 10 ions will be produced per pulse.
The beam-particle interaction can be produced within a magnetic field confinement geometry such as a mirror, torus, stellarator, etc. Because of the large mass to charge ratio of the injected particle, injection into any magnetic geometry is possible without perturbation of the trajectory.
Because the particle path and velocity are accurately known, an electromagnetic suspension system, such as that disclosed by A. F. l-Iaught and D. H. Polk, Physics of Fluids 9, 2047, 1966, or a mechanical suspension system such as disclosed by E. W. Sucov, J. L. Pack, A. V. Phelps, and A. G. Engelhardt, Physics of Fluids 10, 2035, 1967, are not required, and therefore the magnetic confinement system remains unperturbed. The systems described in the above cited references are single event configurations, as contrasted with the high repetition rate of the present system described herein.
Laser produced plasmas offer the major advantage that transient plasmas of high temperature and modest density can be produced in a confinement geometry without any perturbation of the initial vacuum and without the application of electric fields which may produce unstable conditions. In accordance with this invention, a particular application of laser produced plasmas to the problem of controlled thermonuclear fusion will be described.
One prior art approach to the problem of controlled thermonuclear fusion has been to separate the functions of containment and plasma heating to thermonuclear temperatures. This approach is represented by the PHOENIX Experiment at the Culham Laboratory, Abingdon, Berks, England, which is reported by L. G. Kuo, E. G. Murphy, M. Petravic, and D. R. Sweetman, Physics of Fluids 7, 988, 1964, and the ALICE ex periment at the Lawrence Radiation Laboratory, Livermore, California. The desired high energy protons, deuterons, and tritons are produced in a conventional 20-40 KV accelerator. In order that these particles may be trapped in a magnetic confinement geometry, they must undergo a change in either charge or momentum during their transit of the containment geometry. The usual technique is to convert the accelerated 20-40 Kev protons, deuterons and tritons to neutral atoms in a charge exchange cell. A small fraction of these beams of neutral atoms will be ionized during their transit of the magnetic confinement geometry by Lorentz ionization of those atoms in highly excited states. At 20 Kev, the trapping effi ciency is approximately 10" and decreases to 10* at approximately 1 Kev.
Collisional ionization against a seed plasma provides a useful and more efiicient alternative to Lorentz trapping. Such a plasma is produced according to this invention during each laser-particle interaction. In this case, it is desirable to use lithium as the droplet material. It ofiers the following advantages:
1. Because of its low vapor pressure, it can be cryogenically removed at the low temperature walls thus preventing a significant rise in the neutral atom pressure.
2. The low atomic number is desirable to limit possible bremsstrahlung and excitation losses.
FIG. 5 illustrates one type of arrangement which can be used to achieve trapping in the magnetic confinement region of injected energetic neutral atoms by collisional ionization against a seed plasma. For reasons given above, lithium has been chosen as the particle material. Lithium can be charged and accelerated in liquid droplet form as described in the previously cited Los Alamos Scientific Laboratory Report LA2549. The lithium droplet beam 14a issuing from a droplet source and accelerator 12a is focused by an electrostatic lens system 13 at the point 48 at which the laser-particle collision will occur. This is done in order that the laser beam may also be well focused. The lithium droplet beam 14a then passes through the four particle detectors l6, 18, 36, 38 and then into a magnetic confinement region 70 produced by a magnetic confinement means 72. The magnetic confinement region 70 may be produced in conventional manner by a pair of electromagnetic coils forming a simple magnetic mirror arrangement, for example. The outputs of the particle detectors are directed to electronics as previously described to fire the laser 30 at the correct instant to achieve the laser beam and particle beam collision. The laser beam 35 is directed into the magnetic confinement region 70 so that the laser beam 35 impacts the particle beam 14a within the region 70.
A 2-joule output laser has ten times the energy required to create a plasma of 50 electron volts temperature and l X 10 lithium ions and 1 X 10 electrons. A lithium droplet of 40 microns radius contains l X 10 lithium atoms. Therefore the laser beam radius at the collision site 48 may be 120 microns or 0.12 millimeters. These large lithium droplets will have relatively low linear velocities of the order of tens of meters per second, if an acceleration voltage of 100,000 volts is used. Because of this rather low velocity, the timing precision required is minimal compared to the previously described neutral atomic beam generation means. Because the energy absorbed by the lithium droplet is proportional to its surface area, and because the surface to volume ratio increases as the droplet size is decreased by l/radius, any smaller droplet size desired may be used.
An energetic beam 76 of one of the hydrogen isotopes such as deuterium or tritium neutral atomic particles having an energy in excess of 1000 electron volts is directed from a source 78 into the magnetic confinement region 70 at the imact point 48. The energetic neutral particles pass into the magnetic confinement region unafiected by the magnetic field. Within the region 70, the energetic neutral particles collide with the plasma created by the impact of the laser beam 35 and the lithium droplet beam 14a. Collision ionization of the energetic neutral particles occurs. Once ionized, the energetic particles are trapped within the magnetic confinement region 70 so that they occupy a restricted volume 80.
It is estimated that the lifetime of the lithium plasma will be approximately -10 aSec, with an average electron density of 2 X lO /cm The trapping efficiency L N, T, V lV 0.15 where L is the path length taken to be 5 cm N, is the electron density 2 X T. is the ionization cross section 10 cm V l V is the ratio of the electron to proton velocities approximately 4 If the energetic atomic beam 76 is operated in a continuous fashion, the trapping efiiciency will not be very high because of the low duty cycle. If the beam 76 is pulsed on simultaneously with the laser-particle interaction for only the 10 psec duration of the laser plasma, trapping efliciencies of approximately 0.15 can be achieved. Secondly, pulsing of the particle accelerator 12a may permit a substantial increase in the injected neutral beam current with a consequent increase in the instantaneous trapped plasma density. The repetition rate of l/msec is high enough to provide for accumulation of plasma over many laser-particle pulses. The initial system pressure should remain unperturbed.
A new method of energetic hydrogen injection employs atom clusters with a high mass to charge ratio; this injection technique also requires a trapping plasma of transient nature to provide breakup and ionization of the clusters.
While the foregoing specific description is based on the use of a Z-joule laser, it is apparent that with the development of higher owered lasers, much larger lithium droplets than the ones described can be employed. The consequent increase in plasma density will increase the trapping efliciency.
The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
1. in combination:
means for providing and accelerating a stream of charged microparticles along a beam path;
laser beam means for generating a pulsed laser beam directed transversely of said beam path;
particle detection means for detecting the transit of selected ones of said charged microparticles during a first time interval; and
means responsive to the output from said particle detection means for actuating said laser beam means to cause a laser beam pulse to irradiate said selected microparticles during a subsequent time interval to convert said microparticles into atomic particles;
said atomic particles forming a beam when said microparticles have relatively high longitudinal velocities in excess of 5 kilometers per second, and said microparticles forming a radially expanding cloud when said microparticles have relatively low longitudinal velocities below meters per second.
2. The invention according to claim 1, wherein said detection means includes means responding to microparticles of selected velocity.
3. The invention according to claim I, wherein said laser beam means comprises a giant pulse laser.
4. The invention according to claim 3, and further including first means for sensing the transit of selected microparticles during a first time interval and for initiating pumping of said giant pulse laser; and
second means for sensing the transit of said selected microparticles during a second time interval and for pulsing said giant pulse laser during a third time interval coincident with the passage of said selected microparticles through the path of said laser beam.
5. The invention according to claim 4, wherein said first and second sensing means comprise particle detectors spaced along the particle beam path for detecting microparticles of selected velocity.
6. In combination:
means for providing and accelerating a stream of microparticles along a beam path;
magnetic confinement means in said beam path;
means for irradiating said microparticles within said magnetic confinement means with a laser beam directed transversely of said beam path and of sufiicient intensity to convert said microparticles into charged atomic particles;
and means for injecting a stream of energetic neutral particles within said magnetic confinement means so as to collide with said atomic particles and ionize said energetic particles;
said ionized energetic particles being thereby trapped within said magnetic confinement means 7. The invention according to claim 6, wherein said microparticles have longitudinal velocities sufficiently below 100 meters per second as to form a radially expanding plasma cloud.
8. The invention according to claim 7, wherein said microparticles are lithium droplets.
9. The invention according to claim 7, wherein said energetic particles are neutral hydrogen atoms having energies in excess of 1,000 electron volts.
10. Atomic beam generating apparatus, comprising:
means for providing and accelerating a stream of charged microparticles along a beam path;
means for irradiating said microparticles with a laser beam directed transversely of said beam path and of sutficient intensity to produce a stream of atoms of substantially the same velocity as said microparticles;
and means for removing charged atoms from said stream to render them electrically neutral.
11. The invention according to claim 10, and further including means for detecting the transit of selected microparticles during a first interval of time;
and means responsive to the output from said detecting means for causing said laser beam to irradiate said selected rnicroparticles during a subsequent time interval.
12. The invention according to claim 11, wherein said detecting means includes means responding to microparticles of selected velocity.
13. The invention according to claim 10, wherein said laser beam means comprises a giant pulse laser.
14. The invention according to claim 13, and further including first means for sensing the transit of selected microparticles during a first time interval and for initiating pumping of said giant pulse laser;
and second means for sensing the transit of said selected microparticles du ring a second time interval and for pulsing said giant pulse laser during a third time interval coincident with the passage of said selected microparticles through the path of said laser beam.
15. The invention according to claim 14, wherein said first and second sensing means comprise particle detectors spaced along the particle beam path for detecting microparticles of selected velocity.
16. Atomic beam generating apparatus, comprising:
means for providing and accelerating a stream of charged microparticles along a beam path;
means for irradiating said microparticles with a pulse of coherent radiation directed normal of said beam path and of sufiicient intensity and duration to cause vaporization of microparticles in said stream and produce therefrom a stream of atoms of substantially the same velocity as said microparticles;
and means for removing charged atoms from said stream to render them electrically neutral.
* 1* l III