Patent Number: H00004073
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The invention in one embodiment converts neutron energy (E.gtorsim.1 MeV) to electric current and the concomitant development of an electrical voltage, and it produces deep ultraviolet radiation at the same time. With reference to FIG. 1, consider a swarm of 10-14 MeV neutrons (n) incident upon one or two opposing sides of a closed hollow container 11. The container sidewall or sidewalls 13a and 13b upon which the neutrons are incident is each a thin, substantially planar metallic cathode, each of thickness h, and a substantially planar screen anode 15 is positioned parallel to, and approximately equidistance from, the two cathodes as shown. Each of the cathodes contains a low atomic weight element such as Li.sup.6 as a constituent. The energetic neutrons collide with the Li.sup.6 particles in the cathodes and produce ionized helium and tritium according to reactions such as EQU n+Li.sup.6 .fwdarw.He.sup. ++ +T.sup.+ +3e.sup.-. The thickness .DELTA.h of each cathode is chosen so that the mean free path of the He.sup.++ and the T.sup.+ thus produced is greater than .DELTA.h. The mean free path of an alpha particle in a metal of effective atomic charge Z is estimated to be EQU .lambda.(He.sup.++).apprxeq.10.sup.-1 /Z.sup.2 (cm) so that the thickness of each cathode will probably be required to be .DELTA.h&lt;0.1 mm. A substantial fraction (&gt;36 percent) of the energetic He.sup.++ and T.sup.+ ions enter che interior of the container 11 where they encounter a high pressure gas (P=1-30 atm.) 17, such as He, Ne or Ar. The He.sup.++ and the T.sup.+ ions that enter the container gas deposit energy in the gas and produce additional charged particles and excited states through "Coulomb drag", and a plasma is formed. The plasma radiates deep ultraviolet photons (h.nu.=10-20 eV), many of which strike the adjacent metal cathodes and eject photo-electrons of energy determined by EQU eV=h.nu.-.PHI..sub.c, h.nu.=representative photon energy.apprxeq.10-20 eV, PA1 .PHI..sub.c =cathode work function.apprxeq.1.6-6 eV. PA1 .rho.=(.eta..sub.e1 Q-1/.eta.laser) E.sub.L .sup..nu. prf, PA1 .eta..sub.e1 =electrical energy conversion efficiency (assumed=0.4 here), PA1 Q=fusion target energy gain (assumed=80 here), PA1 .eta..sub.laser =laser energy efficiency (assumed=0.05 here), PA1 E.sub.L =laser energy delivered to target (assumed=1 MJ here), PA1 .nu..sub.prf =target fusion pulse repetition frequency. PA1 .eta.=F.sub.f I.sub.sc V.sub.oc /P.sub.in, PA1 F.sub.F (.apprxeq.0.8)=photovoltaic system fill factor, PA1 I.sub.sc =short circuit current of system, PA1 V.sub.oc =open circuit voltage of system (.apprxeq.0.85 E.sub.gap), PA1 P.sub.in =power input to system, PA1 E.sub.gap =energy gap of suitable photovoltaic semiconductor (.apprxeq.1.45-5 eV). The voltage V corresponding to the ejected photo-electrons is further reduced by the product, IR.sub.plasma of the current flow I (away from the cathode and towards the screen anode as indicated in FIG. 1) and the plasma resistance R.sub.plasma to a resultant voltage EQU V.sub.r =V-JR.sub.plasma, specific, The plasma specific resistance value is expected to be quite low here (R.ltorsim.0.3 ohm-cm.sup.2) because of the presence of the plasma in the container gas volume. The product JR.sub.plasma is no more than 3 volts for J.ltorsim.10 amps/cm.sup.2, if the container space is neutralized by the plasma; and the resultant voltage developed is then V.sub.r =5-15 volts. If one positions an impedance or other electric load 19 between. and connects it to, the screen anode and a metal wall cathode, an electric voltage will be developed across the load between cathode and anode. The action is analogous to that of a gas-filled photodiode, and the presence of the plasma avoids space charge limitations on current flow from cathode to anode in the container gas. In order to stop or substantially decelerate a neutron with initial kinetic energy E.congruent.10 MeV, one would need an equivalent thickness of cathode-anode material of at least 25-50 cm of metallic material. Thus, one can concatenate the basic cathode/anode/gas container many times, using adjacent units as indicated in FIG. 2, and repeat the above-described sequence of events several hundred times substantially simultaneously. This improves the efficiency of conversion of the energy of the neutron swarm; and since the resulting voltages V.sub.r are additive the net resulting voltage from one end of the concatenated structure to the other can be of the order of 1 kV. The presence of He.sup.+ ions in the container gas volume can promote the following reactions: EQU He.sup.+ +2He.fwdarw.He.sub.2.sup.+ +He, EQU He.sub.2.sup.+ +e.sup.-.fwdarw.He.sup.* +He, EQU He.sup.* +2He.fwdarw.He.sub.2.sup.* +He, ##STR1## FIG. 3 schematically exhibits the energy levels of the excited monomers and dimers (excimers) of interest here, He.sup.* and He.sub.2 *. The He.sub.2 * excimer will preferentially dissociate to He+He in the presence of a third particle (such as He) with emission of UV radiation at a wavelength .lambda..sub.d.apprxeq. 840 .ANG. (E=14.7 eV); this represents about 60% of the internal energy binding the two He particles. Radiation of wavelength .lambda.=640 .ANG., which is also produced by the decay shown in FIG. 3, is radiation trapped and will probably not be emitted from the container. If neon or argon gas is used rather than helium gas in the container, the UV radiation would appear at a wavelength .lambda..sub.d .congruent.1100 .ANG. (E=11.3 eV) or .lambda..sub.d .congruent.1300 .ANG. (E=9.5 eV), respectively, with about the same conversion efficiency. Thus, with approximately 60% conversion efficiency, deep UV radiation (E=9.5-14.7 eV) can be produced and emitted from the container. Much of this radiation can be used to conduct in situ experiments within the container 11, if desired. A second embodiment of the invention also provides for generation of electricity, and the concomitant development of an electrical voltage, from fast neutron reactions. with reference to FIG. 4, a closed hollow sphere 21 is provided with a sufficiently thick shell 22 to withstand internal pressures of at least p=100 atmospheres, and the sphere interior is filled with a noble or other inert gas such as He or Ne at a pressure of substantially p=100 atm. The inner diameter of the sphere should be substantially d=20 M and should have one or more dedicated sectors 23 for delivering a sequence of fusion targets T (and, separately, two or more fusion laser beams h.nu..sub.L) to the geometric center of the sphere, where laser-induced fusion occurs. Target fusion produces a plurality of high energy neutrons (n) that move through and collide with the noble gas and produce sequences of reactions such as: EQU n(fast)+He.fwdarw.He.sup.+ (fast)+n+e.sup.-, EQU n(fast)+He.fwdarw.He.sup.++ (fast)+n+2e.sup.-, EQU He.sup.++ (fast)+He.fwdarw.He.sup.++ (fast)+He.sup.+ +e.sup.-, EQU He.sup.+(fast)+He.fwdarw.He.sup.+ (fast)+He.sup.+ +e.sup.-, EQU He.sup.++ (fast)+He.fwdarw.He.sup.++ (fast)+He*, EQU He*+2He.fwdarw.He.sub.2 *+He.fwdarw.3He+h.nu..sub.d (.lambda..sub.d .apprxeq.840 .ANG.), EQU He.sup.+ +2He.fwdarw.He.sub.2.sup.+ +He, EQU He.sub.2.sup.+ +e.sup.- .fwdarw.He.sub.2 *.fwdarw.2He+h.nu..sub.d (.lambda..apprxeq.840 .ANG.). With Ne substituted for He, radiation of wavelength .lambda..apprxeq.1100 .ANG. is produced from Ne.sub.2 * decay. Assuming a reaction cross-section of .sigma.(n, He)=10.sup.-24 cm.sup.2, the mean free path between collisions of an energetic neutron with He particles is .lambda.=1N.sigma.=370 cm so that substantially all neutron energy is absorbed in (n, He) collisions within the gas before a neutron reaches the wall of the sphere. As noted above, the He or He.sup.+ or He.sup.++ or He.sub.2.sup.+ gas particles have no electronic states that may be excited by radiation of wavelength .lambda.=840 .ANG. so that the He gas is substantially transparent to such radiation. Only modest Rayleigh scattering of the radiation occurs within the gas so that most of the radiation (at photon energies up to 14.5 eV) will ultimately reach the sphere walls. With reference to FIGS. 5 and 6, these photons will substantially all pass through a first layer of thin anode plates 25, positioned adjacent to and substantially parallel to the sphere walls and spaced apart therefrom in the sphere interior. The photons then strike (and mildly heat) a thick metal cathode laye 22 (the sphere wall) that is treated in bulk with Cs, Th, Ba oxide, Sr oxide or a similar suitable atomic material to reduce the work function of the metal cathode material to .PHI..sub.c .ltorsim.1.6 eV. Preferably, the material comprising the thin anode plates 25 has a work function .PHI..sub.a &gt;&gt;1.6 eV at the temperatures of operation. The photons scatter from atoms or molecules within the cathode material; and if the photon energy satisfies EQU h.nu.&gt;E.sub.F +.PHI..sub.a, where E.sub.F is the Fermi level of the electrons in the cathode material, each such photon that scatters within the cathode material may produce one or more photo-electrons that escape from the cathode material and move to one or the other of the anodes 25, thus creating a potential difference across the cathode anode circuit (FIG. 5). With the electron work function of the anode material chosen to be &gt;&gt;1.6 eV, photo-electrons will be preferentially created in, and will preferentially exit from, the cathode material vis-a-vis the anode material so that electron flow from the sphere walls (cathode) to the anode plates will predominate. Each anode plate 25 is positioned inside the sphere so that the space 26 between each anode plate and the adjacent sphere wall 22 is also filled with the noble gas at p=100 atm pressure. The spacing d.sub.1 (=10-50 .mu.m) by insulating soacers 18 between anode plane and sphere wall in FIG. 5 is chosen small enough so that most photo-electrons emitted by the cathode will experience only a modest number of scatterings by noble gas particles in the space separating the anode and cathode and will ultimately reach and and be absorbed on one of the anode plates 25. An approximate relation for the power produced by a target fusion event is given by If one gigawatt of power is required here, the target fusion pulse repetition rate must be at least .nu..sub.prf =83 Hz, which is probably achievable with current technology. With reference to FIG. 5, the anode plates adjacent to the sphere wall can be fabricated from very thin plates of a light metal such as Al of transverse area 2-100 cm.sup.2 and with adjacent anode plates being spaced apart a small distance determined by the static voltage stand-off requirements for two such adjacent plates. The anode plates themselves may be made very thin (&lt;100 .mu.m thickness) as the plates are not required to maintain any pressure differential across themselves. With reference to FIG. 6, one may replace the cathode-anode arrangement of FIG. 5 with a photovoltaic means 25' that is adjacent to but spaced apart from the sphere wall 22, with a sequence of diodes 29 electrically connecting the photovoltaic array means 25 and the sphere wall 22. The photovoltaic means 25' should preferably have a radial thickness of at least two mean free paths for absorption of the photons of characteristic energy h.nu..sub.d. As a photocurrent is generated in 25' and moves through the diode (optional) or other electrical load, a substantial electrical voltage may be generated between the photovoltaic means 25 and the sphere wall 22. Use of the photovoltaic array of FIG. 6 offers certain advantages over use of the cathode-anode array of FIG. 5. First, the photovoltaic means 25' is capable of repairing itself in the high gamma flux and high neutron flux environment by annealing higher photon flux produces a corresponding higher current, with no direct deleterious effects. Second, the current developed acros the diodes is not space charged-limited, whereas the current flowing from cathode to anode in FIG. 5 may be so limited. For steady state operation, the power conversion efficiency for the photovoltaic process is approximately To achieve the highest conversion efficiency available (20-50 percent) for the photon energies used, one might use a semiconductor material with as high a value of E.sub.gap as possible (e.g., diamond or ZnS with E.sub.gap .apprxeq.5.4 or 3.9 eV, respectively), consistent with other physical requirements. The foregoing description of preferred embodiments of the invention is presented for purposes of illustration only and is not intended to limit the invention to the precise form disclosed; modification and variation may be made without departing from what is regarded as the scope of the invention.