WO1999056521A2

WO1999056521A2 - Method and apparatus for the production of neutrons and other particles

Info

Publication number: WO1999056521A2
Application number: PCT/US1999/008941
Authority: WO
Inventors: Shui-Yin Lo
Original assignee: American Technologies Group Inc
Current assignee: American Technologies Group Inc
Priority date: 1998-05-06
Filing date: 1999-04-28
Publication date: 1999-11-11
Anticipated expiration: 2000-11-06
Also published as: JP2002514740A; EP1082727A2; WO1999056521A3

Abstract

Assemblies of multiple microdroplets made of liquid deuterium are illuminated with pulses from an ultrafast (femtosecond) laser, causing the microdroplets to turn into expanding ion clouds in which the ionized nuclei (deuterons) have kinetic energy sufficient to overcome the Coulomb barrier and cause fusion to produce free neutrons, tritium nuclei, and more kinetic energy. The droplets of liquid deuterium are first illuminated with pulses from an infrared laser in order to cause Coulomb explosion of the droplets and resulting formation of the microdroplets. Alternatively, assemblies of microdroplets of a material containing higher Z atoms such as neon or argon are illuminated by an ultrafast laser and the resulting plasma clouds collide and generate recombination x-rays.

Description

METHOD AND APPARATUS FOR THE PRODUCTION OF NEUTRONS AND OTHER PARTICLES

Background Of The Invention

Free neutrons, i.e., neutrons not bound into a nucleus, can be used for biological, nuclear, and materials research as well as power generation by inducing fission of heavy nuclei such as uranium. One important use of free neutrons is the direct transmutation of high level nuclear waste, especially the nuclei of hazardous long-lived radionuclides, such as the transuranics and certain soluble, long-lived fission products (e.g., ¹²⁹I, "Tc, and ¹³⁵Cs), into stable nuclides or shorter-lived radionuclides that will decay into stable nuclides. The destruction of nuclear waste by this means requires the separation of the radio nuclides, transmutation processes with neutrons, and eventual disposal of the low level shorter-lived radio nuclides resulting from the transmutation process.

The existing methods for neutron transmutation, such as reactors, require extremely large facilities that are costly to build and operate and present safety and environmental concerns.

Two methods of producing neutron fluxes for transmuting nuclear waste are the Advanced Liquid Metal Reactor (ALMR) concept proposed by General Electric and Argonne National Laboratory and the subcritical reactor concept proposed by Los Alamos National Laboratory that uses a proton accelerator ("accelerator transmutation of waste" or ATW). The apparatus for applying these methods will be extremely costly to build and operate.

Ditmire et al, in an article entitled "High-Energy Ions Produced in Explosions of Superheated Atomic Clusters" (386 Nature 54 (March 6, 1997)) hypothesized that a plasma formed from the explosion of deuterium and tritium gas clusters illuminated by a 150 femto-second pulse laser delivering about 100 milliJoules might exhibit a fusion yield of "roughly 10⁶-10⁷ neutrons per shot." Even if accomplished, the predicted yield is too small to be useful for applications such as transmutation. Tritium is an unstable but valuable isotope (³H) of hydrogen. Tritium primarily has been produced in specialized fission reactors that provide the necessary neutron flux. Such reactors are expensive to build and operate and also lead to the production of large quantities of undesired long-lived radio nuclides. Photons of X-ray energies are useful in many fields, from biology to electronics. A bright source of X-rays is very useful in these fields, particularly if it is relatively inexpensive and portable and can provide coherent x-rays.

A need exists, therefore, for a method and apparatus that can produce neutrons and other nuclear particles such as tritium nuclei and x-ray photons in desired quantities with relatively simple and inexpensive apparatus that does not consume much power and will not produce unwanted long-lived radio nuclides as a by-product. Summary Of The Invention

In one aspect, the invention provides methods and apparatus for generating free neutrons as well as other nuclear particles such as tritium by the fusion of light nuclei. In another aspect, the invention provides methods and apparatus for generating X-ray photons.

An embodiment of the method according to the invention involves the steps of generating a cloud or assembly of microdroplets of a material containing light nuclei capable of entering into fusion reactions that produce free neutrons or other desired nuclear particles such as tritium nuclei, illuminating the assembly of microdroplets with an ultrafast laser beam so that a plurality of the microdroplets each absorb sufficient photons from the laser beam to turn into an expanding plasma containing ions comprising the light nuclei, and colliding the ions in the plasmas of the individual microdroplets so as to generate the desired fusion reactions among the light nuclei in order to produce the free neutrons and other desired particles. The material preferably comprises deuterium. A mixture of deuterium and tritium may also be used.

In an embodiment of the apparatus according to the invention, a reservoir is provided for holding a liquid material which comprises atoms or molecules having nuclei capable of undergoing fusion. A droplet forming device, such as a capillary that emits a spray or jet of droplets, is attached to the reservoir. One or more skimmers may be provided to shape the spray or jet into a beam of droplets. A first laser may be employed to illuminate successive droplets with a pulse of light in order to cause each droplet to become a cloud or assembly of microdroplets by Coulomb explosion. A second laser illuminates the cloud of microdroplets with a brief and intense pulse of laser light that is absorbed by the electrons of a plurality of the microdroplets so that the material of each of the plurality of the microdroplets becomes an expanding plasma sphere containing ionized nuclei. The ionized nuclei in the expanding plasma spheres collide, leading to fusion of the nuclei and the ensuing production of desired particles such as free neutrons and tritium nuclei. Another embodiment of the method according to the invention involves the steps of creating an assembly of microdroplets of a material containing nuclei of sufficiently high Z number, illuminating the assembly of microdroplets with an ultrafast laser beam so that a plurality of the microdroplets each absorb sufficient photons from the laser beam to turn into an expanding plasma containing ions comprising the nuclei of sufficiently high Z number, and colliding the cooling plasmas of the individual microdroplets so as to cause recombinations of the ions with electrons in order to produce photons of X-ray energies. Currently preferred materials for practicing this method are argon (Z = 18) and neon (Z = 10).

An embodiment of an apparatus according to the invention provides a reservoir for holding a liquid material which comprises atoms or molecules having nuclei of sufficiently large Z number. A droplet forming device is attached to the reservoir. Skimmers may be provided to shape the spray or jet of droplets from the droplet forming device into a beam of droplets. A first laser may be employed to illuminate successive droplets with a pulse of light in order to cause each droplet to become a cloud or assembly of microdroplets by Coulumb explosion. A second laser illuminates the assembly of microdroplets with a brief and intense pulse of laser light that is absorbed by the electrons of a plurality of the microdroplets so that the material of each of the plurality of the microdroplets becomes an expanding plasma sphere containing ionized nuclei. The ionized nuclei in the expanding (and cooling) plasma spheres combine with free electrons, leading to the production of photons of X-ray energies.

Description Of The Figures

Further and other features of the invention will be more clear from reference to the enclosed drawings, which illustrate preferred embodiments of methods and apparatus according to the invention, and in which:

FIGURE 1 is a generalized diagram showing the steps of a method for the production of neutrons and other particles.

FIGURE 2 is a generalized diagram showing the step of illumination of an assembly of microdroplets by an ultrafast laser beam.

FIGURE 3 is a generalized schematic of an apparatus for generating neutrons and other particles according to the invention.

FIGURE 4 is an elevation view of a preferred embodiment of the apparatus shown schematically in FIG. 3.

FIGURE 5 is a combination elevation view and schematic of the source of droplets of the apparatus of FIG. 4.

FIGURE 6 is a sectional view of a portion of the source of droplets of FIG. 5 and a portion of the skimmer apparatus.

FIGURE 7 is a sectional view of a portion of the source of droplets of FIG. 5 and the lower part of an adjoining cryostat. Detailed Description Of The Invention

An aspect of this invention relies on fusion in an assembly of deuterium (or deuterium/tritium) droplets or spheres (which may be solid or liquid) of about micron sizes ("microdroplets"). An assembly of microdroplets is irradiated by an ultra fast high intensity laser in order to generate plasmas of sufficiently high temperature and density to induce nuclear fusion and produce neutrons and other nuclear particles.

FIG. 1 shows the steps in a preferred method for producing neutrons and other particles. A droplet of a liquid material containing nuclei capable of fusion is generated in the first step. The second step is to illuminate the droplet, which preferably has a charge-to-size ratio close to the Rayleigh limit, with an infrared laser pulse. In the third step, which follows the illumination of the droplet with the infrared laser, the droplet experiences Coulomb explosion into an assembly or cloud of microdroplets. In the fourth step, an ultra fast (sub-pico second) laser illuminates the assembly of microdroplets (see also FIG. 2) in order to heat the electrons in the microdroplets to kilovolt temperatures in less than the microsphere expansion time. The material of each of the microdroplets thereby becomes a plasma. In the fifth step, the atoms in the microdroplets have become ionized and the resulting ions, which consist of the nuclei capable of fusion, are accelerated away from each other. In the sixth step, ions from individual microdroplets collide with each other and cause fusion. FIG. 2 shows a "cloud" or assembly of microdroplets 1 which roughly coincide with the focal region 2 of a laser beam having a beam path 3. The microdroplets 1 preferably have diameters in the range of about 0.5 to about 5.0 microns. The focal region 2 has an approximate volume of 4/3 πL³ (where L is the characteristic length). The cloud contains many microdroplets 1 sufficiently spaced to allow laser light to propagate through the cloud (i.e., the spacing between the microdroplets 1 is at least bigger than the laser light wavelength). The cloud of microdroplets 1 preferably should be in a vacuum 4.

The fourth step involves the absorption of photons by the electrons to reach very high energy levels. The absorption of light energy by electrons normally is small. However, an anomalous process occurs in arrangements of microdroplets leading to absorption of about half of the incident laser pulse so that very high temperatures are achieved in the electron cloud. Then, in the fifth step, the ions are pulled to a high energy by the electrostatic force from the expanding electron cloud. During the expansion, which proceeds at the sound speed, the electron cloud cools down adiabatically and transfers its thermal energy to the ions, so that the ion cloud also becomes highly energetic at high temperatures, once thermal equilibrium is achieved.

The ion cloud from each microdroplet does not have many interaction collisions between its constituent ions that would result in fusion. The collision of the ion clouds originating from neighboring microdroplets leads to a significant number of fusion interactions between the ions which will generate neutrons and other particles. In order for the ultrafast laser to heat up all the microdroplets in the assembly of microdroplets, a space separation larger than the wavelength of the ultrafast laser is necessary, so that laser energy can penetrate through the assembly of microdroplets. On the other hand, as high a plasma density as possible should be maintained so that the ion collision rate (and thus the fusion rate) is consequently higher. The density of deuterium droplets is about n_D = 2.5xl0²²/cm³ and the hot ion plasma density is then typically 0.1 to 0.01 of n_D. The fusion rate N can be estimated by a well known formula to be approximately:

N F„usi .on =(10⁸-10¹⁰) 'L⁴

where the characteristic length L is equal to 10^"2 L cm and when the ion cloud temperature is 27 keV. The fusion rate is proportional to the square of the ion density and to the fourth power of the dimension L and also proportional to an exponent of the temperature. The ion density, microdroplet assembly size and the temperature should therefore be maximized in order to maximize the fusion rate. Also, if the deuterium in the microdroplet is replaced with a deuterium/tritium mixture, the neutron yield can increase by as much as 45 times.

An apparatus 10 according to a preferred embodiment of the invention is illustrated schematically in FIG. 3. A liquid droplet source 20 emits a spray or stream of droplets 5 that preferably are about 100 microns in diameter. The material forming the droplets preferably is liquid deuterium. A set 30 of skimmers, composed of first, second and third skimmers 32, 34, and 36, respectively, narrows the spray or stream of droplets 5 into a beam. The apparatus 10 is enclosed by walls (not shown in FIG. 3 but shown in FIG. 4) and various pumps (also not shown in FIG. 3, but shown in FIG. 4) are used to progressively increase the vacuum along the path of the beam 7 of the droplets 5.

A first laser 40 is aligned and focused by a mirror 42 and a lens system 44 so as to intercept and illuminate the droplets 5 in order to convert the droplets 5 into clouds or assemblies 6 of microdroplets 1 as shown in FIG. 2. Preferably, the laser 40 is an infrared laser providing a short pulse in the nanosecond range. The energy required to evaporate a deuterium droplet 5 with a 100 micron radius is about 130 microJoules. If the absorbed power is more than 130 microJoules, all of the deuterium molecules in the droplet 5 would be vaporized. If the absorbed power is much less than that, the majority of the deuterium molecules will be in liquid form and the charges on the droplet 5 will create numerous micron- sized droplets 1 by Coulomb explosion in order to provide the condition for fusion by the general process described above in connection with FIG. 1.

Preferably, the deuterium molecules in the droplets 5 will be excited by pumping the fundamental vibrational transition (υ = 0 - 1) at 2992 cm^"1 (3.3μm). Although this transition is only quadrupole-allowed in the gas phase, it is a strong absorption in the condensed phase due to the induced-dipole moment arising from intermoleculer interactions. The absorption coefficient of roughly 0.6 cm^"1 (at the Q υ = 0 band origin) is sufficiently low that the droplet is optically thin, i.e., the radiation will bathe the droplet uniformly. The absorption of light from the laser 40 will lower the surface tension of the droplets 5 sufficiently that the Rayleigh limit is exceeded and a Coulomb explosion then ensues.

Preferably, the laser 40 is a YAG-pump optical parametric oscillator with two KTP crystals as the gain medium. Such a laser is available from Coherent, Inc. of Santa Clara, California. The laser 40 preferably should be capable of fluence output on the order of 1-5 milliJoules/pulse. This output is focused to a 0.1 mm diameter spot at point 46 on the beam path (see FIG. 3) and should easily saturate the transition. The laser pulse from the laser 40 should be attenuated to a degree that will promote the Coulomb explosion of the droplet 5 without completely vaporizing the droplet.

The ensuing assemblies or clouds 6 of microdroplets 1 (see FIG. 2) produced by the illumination of the droplets 5 by the pulse from the laser 40 are subsequently illuminated by pulses of light from the second laser 50. The light pulses from the lasers 40 and 50 are aimed at essentially the same point 46 on the beam path 7 and arrive at the point 46 within about a nanosecond of each other. The light pulses from the second laser 50 are aligned and focused by a mirror 52 and the lens system 54. Both the laser 40 and the laser 50 may be triggered by a targeting system that includes a targeting or diagnostic laser (not shown), as is well known to the art. Such targeting systems have been employed, for example, in the considerable prior experimentation on laser fusion (see, e.g., U.S. Patent 3,723,703 to Ehlers, et al., the disclosure of which concerning a pellet detection and firing system is incorporated by reference). The laser 50 preferably is an ultrafast terawatt Ti:sapphire laser system. A regeneratively amplified system is commercially available from Continuum Lasers of Santa Clara, California. This system consists of a seed source, preamp/pulse cleaner, a pulse stretcher, a regenerative amplifier, a four-pass power amplifier, and a pulse compressor. Preferably, the laser 50 should be constructed to produce light having a wavelength of 790 nanometers with pulse energies of 200 milliJoules to 1 Joule at a repetition rate of 10 hertz. (Increasing the repetition rate of the laser 50 will increase the total yield of neutrons and other desired nuclear particles per second.) The pulse chirp is preferably about 130 femtoseconds. The laser intensity or power density is 2 x 10¹⁶ watts/cm² for a spot-size of approximately 0.5 millimeters. A laser system with higher power and shorter pulse duration may be used. Such systems are available, but at a greater cost.

As described above in the discussion associated with FIGS. 1 and 2, the illumination of an assembly or cloud of microdroplets of deuterium on the path of the beam 7 will cause the microdroplets 1 to become plasmas that expand rapidly and collide with each other, causing fusion of the deuterium nuclei (deuterons). The deuterium-deuterium fusion reaction has two major branches:

(1) D + D = p + t + 4.0 Mev, and

(2) D + D = n + helium-3 + 3.2 Mev.

The free neutrons (n) produced by reaction (2) can be employed to conduct experiments, to make measurements, and to transmute nuclides. In FIG. 3, the target device 60 contains equipment employing the neutrons for any of the purposes described above. Such equipment is well known and will usually include target substances for absorbing or deflecting the neutrons. The use of neutrons for transmutation of nuclear waste in particular is known and is described in Nuclear Wastes: Technologies for Separations and Transmutation (National Academy Press 1996), the disclosure of which, in connection with the transmutation of nuclear waste, is incorporated herein by reference. The device 60 thus could incorporate a moderator to slow down the neutrons to speeds more likely to cause capture events.

The protons (p), tritium nuclei (t), and helium-3 nuclei are charged particles and may be screened out by the use of metal foils of appropriate thickness in the device 60. Alternatively, the other products of the deuterium reactions may be used. For example, tritium is an essential component of thermonuclear devices and also may be used in making night sights. The tritium nuclei will become embedded in target substances (such as zirconium hydrate) in the device 60 and can be recovered by heating the target substances in a vacuum.

Although not shown in FIG. 3, the main components of the apparatus shown schematically in that drawing are mounted in an enclosure which maintains a progressively increasing vacuum. The region around the fusion location 46 on the beam path preferably should be a high grade vacuum in order to thermally insulate the expanding plasmas of the microdroplets at that point. The enclosure may contain shielding against the more penetrating particles produced by the fusion of the deuterons in the microdroplets 1, particularly the free neutrons. Preferably, the materials chosen for the enclosure and any other components in the vicinity of fusion location 46 should be selected from materials that will not form dangerous long-lived radionuclides following exposure to free neutrons. FIG. 4 shows the external configuration of an apparatus 100 according to a preferred embodiment of the invention. The apparatus 100 has a casing 110, preferably made of walls of stainless steel, that will preserve a vacuum in an internal longitudinal compartment 115. The droplet beam source 120 (not including the beam extraction elements 32-38 shown in FIG. 6) is in one end of the compartment 115 and is seen through an optical window 122. (The optical window 122, which is attached to a vacuum port in the casing 110, permits observation of the droplet source 120). A cryostat 130 cools the source to a very low temperature, preferably in the range of about fifteen to about twenty degrees Kelvin. An optical window 140 adjacent a midpoint or interaction region 117 of the compartment 115 permits entrance of light pulses from the laser 40 and the laser 50 (neither of which are shown in FIG. 4) which are mounted outside the optical window 140. The creation of assemblies or clouds of microdroplets 1 and the subsequent fusion of deuterons in the microdroplets 1 will take place in the interaction region 117. The target device 150, which is inside the compartment 115 at an end opposite to the end containing the droplet source 120, contains the testing, measuring or other devices that will employ the free neutrons that are emitted by the fusion of the deuterons in the microdroplets. The target device 150 may be retracted upwards into a side compartment 119 of the compartment 115 by rotating the extension tube 152. Tracks (not shown) in the compartment 115 and sub-compartment 119 guide the target device 150 and prevent it from rotating.

The diffusion pump 160 and the turbo molecular pumps 170 and 180, respectively, maintain a vacuum of various degrees within the compartment 115 (see the discussion in connection with FIG. 6). FIG. 5 shows the droplet beam source 120 mounted inside a portion of the compartment 115 at one end of the casing 110. Components of the apparatus 200 for maintaining vacuum or pressure in the compartments of the source 120 and the liquid helium flow cryostat 130 are shown schematically and include a second stage reservoir pump 210, a cell pump 220, a source pump 230, a differential pressure readout gauge 240, a source pressure readout gauge 250, solenoid valves 260 and 270, a differential capacitance manometer (Baratron) 280, and a source capacitance manometer (Baratron) 290. The pumps 210, 220 and 230 preferably are rotary vane pumps.

FIG. 5 also shows the input line 194 for deuterium gas, the liquid helium input line 192, and the helium exhaust line 193. Also shown are the temperature controller cable 195, the electrometer cable 196, and the high voltage power supply cable 197. The needle valve handle 191 controls the flow of liquid helium between the first and second stage reservoirs 136 and 138, respectively (see FIG. 7).

The droplet beam source 120 is shown in cross-section in FIG. 6. Briefly, the source 120 is mounted on the bottom of a liquid helium flow cryostat (see FIGS. 4 and 7) within the compartment 115.

As is best seen in FIG. 7, the cryostat 130 has first and second stage reservoirs 136 and 138, respectively, which are preferably made of copper. The second stage reservoir 138 is mounted on the source block 125 (preferably made of copper) of the source 120 and cools it. The second stage reservoir 138 is supplied with liquid helium through needle valve 137 from the first stage reservoir 136, which in turn receives liquid helium via the input line 132. Helium gas rises from the second stage reservoir 138 to the first stage reservoir via the helium exhaust line 139 and then exits the cryostat via the helium vapor pumpout line 134.

The deuterium gas is condensed in a sintered copper condenser block 199 prior to entering the source chamber 124. The source chamber 124 is a cavity formed in the base 125 that is closed by the capillary mount 121 (see FIG. 6). The sintered copper block 199 is attached to the liquid helium input line 132, as shown in FIG. 7, and is at a temperature of about 10 degrees Kelvin. The pressure in the source chamber 124 is regulated via the source chamber pump line 232 and the source chamber pressure measurement line 234, and is typically in the range 100 to 300 Torr. Liquid deuterium flows from the source chamber 124 through a small capillary 122 (60-70 μm diameter)into the droplet chamber 31 which defined by the droplet cell 127.

The capillary 122 is mounted in the source cup 121 and preferably made of glass. The droplet cell 127 is preferably made of copper and the source cup 121 is preferably made of Invar. The flow of liquid deuterium is further limited by the electrode 123, an etched tungsten wire (50 μm diameter) inserted in the capillary 122 with its tip near the capillary orifice 128. The pressure gradient across the capillary 122, which can be as low as 0.5 Torr, is regulated by controlling the pumping rate via the source chamber pump line 232 to the source chamber 124. The pressure in the droplet chamber 31 is measured and maintained, respectively, by the cell pressure measurement line 224 and the cell pump line 222.

The liquid deuterium is charged by raising the potential of the electrode 123, via the high voltage cable 197, to voltages between about two and about twelve kiloNolts. Above a threshold voltage, space-charge limited currents obeying the

Fowler-Νordheim relation are observed. Currents of nanoamperes or greater can be produced in liquid deuterium. At low pressure gradients and lower fields (but above the threshold for field ionization), a large droplet forms at the capillary orifice 128 as charging reduces the droplet surface tension. Higher electric fields lead to formation of a Taylor cone at the tip of the droplet, as seen in FIG. 6, and to subsequent droplet ejection. At higher pressure gradients, a liquid jet is observed with a typical radius of 25-50 μm even in the absence of a field. Application of an electric field reduces the jet radius and leads to Rayleigh-Plateau behavior with the field breaking up the jet into droplets at shorter jet lengths. When the Rayleigh limit is greatly exceeded, the jet undergoes a Coulomb explosion into a fine spray of small charged droplets (see FIG. 6).

Electrode wear is one of the more time-consuming problems associated with the source 120, because replacement requires removing and disassembling the source 120. The high voltage to the electrode 128 may be pulsed in order to greatly increase the lifetime of the electrode 128.

The size of the droplets 5 can be controlled by independently varying the current and the liquid flow rate. The droplet size is determined by the charge accumulated within the droplet 5. For relatively large droplets (r > 20 μm) formed by Rayleigh-Taylor instabilities at the end of a jet, the charge/length ratio q/1 of the jet is on the order of that predicted as the Rayleigh limit:

where γ is the surface tension and a is the jet radius. Fine sprays of smaller droplets are formed at lower flow rates and higher emission currents.

Quantum effects lead to large changes in the bulk thermodynamic properties of molecular deuterium. Deuterium droplets will be formed at roughly 20 degrees Kelvin, at which the vapor pressure is 220 Torr. A lower bound is a vapor pressure of 130 Torr at 18.7 degrees Kelvin, the triple point of D₂. To generate smaller droplets (radius < 20 μm), the source 120 should have very small pressure gradients across the capillary 122 and high currents. This will correspond to the mode in which a Taylor cone is formed at the tip of a stationary droplet at the capillary orifice 128, with the droplet charge-to-volume ratio determined approximately by the Rayleigh limit for a spherical droplet:

4 3 a³

3

The surface tension of deuterium is 3.56 x 10^"3 N/m. The approximate Rayleigh limit charges are approximately or less than 300 e, 10⁴ e, and 3><10⁵ e for deuterium droplets with diameters 0.1 μm, 1 μm and 10 μm, respectively. For 10 μm diameter droplets, at least 2 to 5 x 10⁵ droplets/second may be expected, depending on the actual charge state of the droplet. For 100 μm diameter droplets, at least 6000 droplets/second may be expected at the Rayleigh limit. This rate of production of droplets will be somewhat greater for droplets with lower charge density. The charged droplets are formed in the droplet chamber 31 at relatively high pressures (approximately 100 to 300 Torr for deuterium). The droplets will be extracted into the high vacuum extraction chamber 37 (which includes the beam path location 46) through two stages of differential pumping, as shown in FIG. 6. 0.5 to 1 mm apertures in the skimmers 32, 34, and 36 preferably are used between stages, but smaller apertures may be used in order to reduce pumping requirements. The skimmers 32, 34, and 36 are preferably made of copper.

The walls of the first two differential regions 33 and 35 will remain cooled to liquid hydrogen temperatures or lower, to prevent heating of the droplets 5. The first and second differential regions 33 and 35, as well as the high vacuum extraction chamber 37, are contained within the compartment 115.

The first differential region 33 will be pumped by a 130 liter per second Roots blower (not shown). The gas flow through a 1 mm aperture into the first differential region 33 will be approximately 0.12 liter per second, leading to a pressure of approximately 200 milliTorr in the first differential region 33. The skimmer 32 and the walls 38 are designed and constructed to ensure that the Mach disk that attaches to the skimmer 32 and that the shock wave at the jet boundary does not heat the central core of the beam 7. The second differential region 35 will be pumped by the diffusion pump 160

(see FIG. 4) with a pumping speed for D₂ of 4600 liter/seconds. Expansion through the skimmer 34, which has a 1 millimeter aperture, will lead to a pressure of ~ 10^"5 Torr in second differential region 35. The beam of droplets 5 will then pass through the third or final skimmer 36, which also has a 1 millimeter aperture, into the high vacuum or ion optics region 37, where the vacuum will be maintained at approximately 10^"7

Torr or lower by the turbomolecular pumps 170 and 180, which have a capacity of 500 and 1000 liters/second, respectively, and which may be backed by a dry scroll pump (not shown).

As the droplets 5 enter vacuum, they will undergo evaporation until they cool sufficiently such that their vapor pressure is in near equilibrium with the surrounding gas. The droplets 5 will reach a steady-state internal temperature limited by the rate of evaporation (and hence the droplet flight/residence time in vacuum). However, the equilibrium approximation can give a good estimate of the droplet temperature. For deuterium, a vapor pressure of 10^"6 Torr is achieved at a temperature of approximately five degrees Kelvin, indicating that the droplet will freeze in vacuum. For D₂ droplets with initial dimensions of 1-100 μm, evaporation will lead to a decrease in droplet radius of approximately 15%. Finally, the fate of the droplets 5 upon evaporation will depend upon their charge state. If the droplets 5 are at the Rayleigh limit, then evaporation will result in Coulomb explosion and subsequent fragmentation of the droplets 5. The terminal gas velocity of the jet expanding from the droplet cell 31 is about

460 meters per second. The droplets 5 will be accelerated to less than this velocity because the density of the expanding gas drops quadratically, leading to a relatively short time during which the droplets 5 are in a high pressure flow.

Gravitational fall becomes an issue for droplets 5 of larger size and thus mass. Droplets 5 with 10 μm diameter will fall only about about 250 μm in 1 millisecond, but those with a diameter of 100 μm will fall about 25 mm. The final velocity and hence flight time of the droplets will depend upon the acceleration provided by the expansion from the droplet cell 31 as well as electrostatic acceleration by ion optics (not shown) provided in the extraction vacuum chamber 37. In general, gravitational effects will be minor if smaller droplets 5 are employed. Furthermore, the gravitational force can be countered effectively with deflection fields generated by solenoids (not shown), if magnetic, or charged plates (not shown), if electric.

Once the droplets have passed through the first and second differential regions 33 and 35, they will enter the high vacuum region 37 for laser excitation, as described in connection with FIG. 3.

Droplets of liquid material containing light nuclei, such as deuterium, could be generated by means other than that described above in connection with FIGS. 4-7. For example, droplets could be generated by thermal (bubble), Piezo, or electrostatic jet apparatus and methods. A laser could be provided to send pulses into the liquid deuterium in the source chamber 124, thus creating a shock wave that propels a liquid deuterium jet from the capillary 122. Such a laser could be a Nd-YAG laser.

Microdroplets of the liquid material could be generated directly by such means and emitted in a beam of sufficient density to enable fusion to occur when illuminated by an ultrafast laser. The liquid material can comprise a mixture of hydrogen and deuterium, deuterium and tritium, or other substances containing nuclei capable of fusion.

The method and apparatus described above in connection with FIGS. 1-7 can be adapted to generate X-ray photons by changing the material of the microdroplets to include nuclei of sufficiently high atomic number (Z) that x-ray photons can be generated by recombination of highly ionized nuclei with electrons. In step five of the process illustrated schematically in FIG. 1, the atoms of the material of the microdroplets are ionized so that the microdroplets each become an expanding shell of electrons concentrically aligned with a smaller expanding shell of heavily ionized nuclei or ions. The ions and electrons originating from one microdroplet will collide with ions and electrons originating from other microdroplets.

As the ions and electrons cool, they will recombine (alternate step six in Fig. 1) and emit photons. The initial recombinations will form hydrogen-like atoms in which the transition of the electron from higher levels to lower levels (e.g., the n=3 to 2 transition) will result in the emission of photons of X-ray energies.

As shown in Valeo et al., 47 Physical Review E 1321 (Feb. 1993), which is incorporated by reference with respect to the disclosure concerning recombination X-ray generation as if fully set forth herein, the variation of variables such as the energy deposited in the material illuminated by a fast laser and the initial size of a target sphere of such material may be manipulated to increase the gain of selected transitions in such hydrogen-like ions with the possibility of achieving effective lasing if the focal region of the fast laser (the X-ray lasing region) is long and thin.

As shown by Valeo et al., suitable materials for recombination X-ray generation have Z values in the range of about 10 to about 30. Neon (Z=10) and Argon (Z=18) are noble gases that liquify at cryogenic temperatures and are preferred for employment with the apparatus 10 (FIG. 3) or 100 shown in FIGS. 3-7 in order to generate X-ray photons, although the apparatus will be at higher temperatures (in which case liquid nitrogen or other liquified gases may be used to cool the source 120). The source 120 will form the liquid neon or argon into a beam 7 of droplets 5 which are exploded into an assembly of microdroplets by pulses from the laser 40. The assembly of microdroplets is immediately illuminated by the laser 50 and converted into expanding plasma shells that cool rapidly and collide so that the ions and electrons recombine to emit X-ray photons as described. This will result in a brilliant point source of X-rays that will illuminate the target device 60 (FIG. 3) or the target device 150 (FIG. 4) for use in experiments, testing or manufacturing. As described by Valeo et al., recombination x-ray lasing can be expected if the focal region of the laser 50 is relatively long and thin so that coherent x-ray generation will be stimulated along the major axis of the focal region. This is accomplished by generating an assembly of micro spheres large enough that the laser 50 can be focused on a region of the assembly that is relatively long and narrow. Various alterations, modifications, and improvements of the invention will readily occur to those skilled in the art in view of the particular embodiments described above. Such alternations, modifications, and improvements are intended to be part of this disclosure and are intended to be within the spirit and scope of this invention. Accordingly, the foregoing descriptions are by way of example, and are not intended to be limiting. The invention is limited only as defined in the following claims and the equivalents thereof.

Claims

What is claimed is:

1. A method for generating free neutrons and other particles, comprising the steps of: generating a plurality of adjacent microdroplets of material containing light nuclei that will enter into fusion reactions that produce free neutrons and other desired particles such as tritium nuclei; illuminating the microdroplets with a laser beam so that the material of the microdroplets absorbs sufficient photons from the beam to turn into an expanding plasma with ions comprising the light nuclei; and colliding the ions in the expanding plasmas of individual ones of the microdroplets to cause fusion reactions in order to produce the free neutrons and other desired particles.

2. The method according to claim 1 in which the step of generating microdroplets comprises the steps of generating a droplet of the material containing the light nuclei and then splitting the droplet into a plurality of microdroplets.

3. The method according to claim 1 in which the microdroplets are sufficiently close to each other during the step of illumination with the laser beam so that at least some of the light nuclei fuse when the expanding plasmas of the microdroplets collide but not so close to each other as to prohibit the propagation of the light from the first laser between the microdroplets.

4. The method according to claim 1 in which the laser beam is ultra fast.

5. The method according to claim 1 in which the material comprises deuterium.

6. The method according to claim 1 in which the material comprises a mixture of deuterium and tritium.

7. The method according to claim 1 in which the microdroplets have diameters in the range of about 0.1 to about 5 microns.

8. The method according to claim 2 in which the droplets have diameters in the range of about 100 to about 1000 microns.

9. The method according to claim 2 in which the step of splitting comprises illumination of the droplet with a laser beam in order to reduce the surface tension of the droplet.

10. The method according to claim 1 further comprising the steps of exposing nuclear waste to the free neutrons in order to transmute the nuclear waste into stable or shorter-lived radio nuclides.

11. The method according to claim 1 further comprising the step of collecting the tritium nuclei produced by the fusion reactions.

12. An apparatus for generating free neutrons and other particles, comprising: a source of a liquid material containing light nuclei that will enter into fusion reactions to produce free neutrons and other particles; means for producing a plurality of adjacent microdroplets from material provided by the source; and a laser for directing pulses of light onto the microdroplets to be absorbed thereby so as to cause the material of the microdroplets to become ionized and then to accelerate and collide, thereby to fuse the light nuclei and produce free neutrons and other particles.

13. The apparatus according to claim 12 in which the laser is an ultrafast laser.

14. The apparatus according to claim 12 in which the material is deuterium.

15. The apparatus according to claim 12 in which the material is a mixture of deuterium and tritium.

16. The apparatus according to claim 12 further comprising an enclosure surrounding the means for producing microdroplets.

17. The apparatus according to claim 12 further comprising a device for holding target substances be exposed to the free neutrons.

18. The apparatus according to claim 17 in which the target substances are nuclear waste.

19. The apparatus according to claim 17 in which the target substances releaseably absorb tritium.

20. An apparatus for generating fusion products such as neutrons and tritium, comprising: a source having walls defining a reservoir for a liquid material containing light nuclei that will enter into fusion reactions resulting in products such as neutrons and tritium; a capillary in one of the walls of the source whereby the liquid material may exit with velocity from the reservoir in the form of droplets; at least one skimmer disposed adjacent to the capillary for defining a beam of the droplets, the beam having a path; a microdroplet producing laser disposed to successively illuminate droplets in order to cause the droplets to break up into assemblies of microdroplets; and a plasma-generating laser disposed to successively illuminate the assemblies of microdroplets with intense light pulses so that a plurality of microdroplets in each assembly will absorb enough photons from the light pulses in order to become plasmas having ions comprising the light nuclei that collide to produce fusion reactions resulting in the generation of free neutrons and other particles.

21. The apparatus according to claim 20 in which the source, capillary, skimmer, and first and second illumination points in the beam path are contained in a compartment enveloped by walls defining an enclosure capable of maintaining a vacuum.

22. The apparatus according to claim 21 further comprising one or more pumps for producing a vacuum in the enclosure.

23. The apparatus according to claim 21 further comprising a device mounted inside the enclosure for exposing target substances to neutrons from the fusion reactions.

24. The apparatus according to claim 21 in which the lasers are mounted outside the enclosure and the enclosure further comprises an optical window in the walls of the enclosure for allowing light pulses from the lasers to enter the compartment to illuminate the droplets and the microdroplets.

25. The apparatus according to claim 20 in which the liquid material comprises deuterium.

26. The apparatus according to claim 23 in which the target substances are nuclear waste to be transmuted into shorter lived or stable nuclides.

27. The apparatus according to claim 23 in which the target substance releaseably absorbs tritium nuclei.

28. A method for generating X-ray photons, comprising the steps of: initiating the breakup of a droplet of material containing nuclei of sufficiently high Z number in order to create an assembly of microdroplets; illuminating the assembly of microdroplets with a beam from a fast laser so that a plurality of the microdroplets each absorb sufficient photons from the laser beam to turn into an expanding and cooling plasma containing ions comprising the nuclei of sufficiently high Z number; and colliding the cooling plasmas of the individual microdroplets so as to cause recombination of the ions with electrons in order to produce photons of X-ray energies.

29. The method according to claim 28 in which the plurality of microdroplets illuminated by the laser beam is in a focal region of the laser beam.

30. The method according to claim 29 in which the focal region is sufficiently long and narrow that recombinant x-ray lasing is generated along a major axis of the focal region.

31. An apparatus for the generation of X-ray photons, comprising: a reservoir for holding a liquid material which comprises atoms or molecules having nuclei of sufficiently large Z number; a droplet forming device attached to the reservoir for emitting droplets in a jet; a first laser for illuminating successive droplets with a pulse of light in order to cause each droplet to become a cloud or assembly of microdroplets by Coulomb explosion; a second laser for illuminating the assembly of microdroplets with a brief and intense pulse of laser light that is absorbed by the electrons of a plurality of the microdroplets so that the material of each of the plurality of microdroplets becomes an expanding plasma sphere containing ionized nuclei, such that the ionized nuclei recombine with electrons to emit X-ray photons.

32. The apparatus according to claim 31 further comprising at least one skimmer for shaping the jet of droplets from the droplet forming device into a beam of droplets.