Patent Number: 042723205
Section: description

DETAILED DESCRIPTION The high density target of this invention referred to hereinafter as a ball and shell target, is particularly advantageous for use with high power laser systems such as the above-referenced Shiva glass laser which provides sufficient energy for obtaining both high fuel densities and temperatures, not obtainable with other glass laser systems. A cryogenic DT shell with 20 TW of absorbed power can give a thermonuclear yield of several times the absorbed energy if the energy can be deposited into a Maxwellian distribution of electrons on a target with a 10-100 A surface finish. Using the laser pulses indicated in Table I, a shell of DT with an internal diameter (i.d.) of 250.mu. and 90.mu. thickness is impulsively accelerated to minimize the effects of Rayleigh-Taylor instability. TABLE I ______________________________________ TIME (SH) POWER (WATTS) PULSE ______________________________________ .0 6 .times. 10.sup.10 Constant .05 6 .times. 10.sup.10 Power .25 3 .times. 10.sup.11 Constant .27 3 .times. 10.sup.11 Power .3525 1.5 .times. 10.sup.12 Constant .3675 1.5 .times. 10.sup.12 Power .3875 5 .times. 10.sup.12 Linear Ramp .450 2 .times. 10.sup.13 Constant Power .470 2 .times. 10.sup.13 ______________________________________ Its calculated performance under various physical models is given in Table II. TABLE II ______________________________________ ENERGY ABSORBED NON-CLASSICALLY = 8.5 KJ INPUT ENERGY = 9.3 KJ .alpha. 3-T 1 2 2 4 4 ______________________________________ Inhibited Conduction NO NO NO YES NO YES Yield 23. KJ 8. KJ .53 KJ 3.28 J .131 KJ 1.55 J .rho..sup..GAMMA. MAX .91 .78 .20 .057 .046 .037 MAX AVG. FUEL .rho. 1280. 990. 135. 11.7 9.7 5.6 MAX AVG. FUEL TEMP 9.2 5.6 3.2 1.33 3.5 1.21 MAX FUEL VELOC- ITY .77 .76 .72 .38 .65 .38 (AT .rho.c) T.sub.e MAX 4.5 4.3 4.1 9.5 4.0 8.7 ______________________________________ If the energy is absorbed into a superthermal spectrum, the target performance is severely degraded, as shown. Recent experiments using the two beam Janus laser system are matched fairly well by a physical model that assumes an .alpha..about.4 and some inhibition of electron conduction, the last column in Table II. Alpha is the ratio of the effective temperature of the superthermal electrons to that of the main body electrons. Electron conduction inhibition in these calculations is based on a model for the onset of ion-acoustic turbulence, although such inhibition could come from magnetic fields as well. As shown at the top of Table II, with a pure DT target, less than 9% (8.5) of the absorbed energy was accounted for by inverse bremsstrahlung. The rest must be accounted for by non-classical processes which produce superthermal electrons. The inverse bremsstrahlung could be enhanced both by using a higher Z ablator and by frequency doubling. The combination of these two could lead to a considerable performance improvement. For the target compatible with the Shiva laser system, it is assumed that 10 TW of absorbed power is achievable and that only 1.mu. light will be available. The superthermal electrons result in a drop in the driving pressure one achieves. They also preheat the fuel which then is harder to compress. Within the constraints of this model, it is possible to pursue two lines of experiments, one which gives a larger yield but lower density than the bare DT shell, and the other which gives higher density but lower yield. DT gas-filled glass shells, generally similar in design to the exploding pusher targets imploded by the Janus laser system, give thermonuclear yields of about 1/2% of the absorbed laser energy. This is much better performance than targets previously used because the laser energy is better matched to the targets size and the density-radium (.rho.r) of the fuel is an order of magnitude higher. Mean fuel temperatures of greater than 10 keV are calculated. Such a target has the virtue of being able to tolerate large asymmetries in the absorbed energy and can be imploded without the necessity of producing a low density corona or atmosphere prior to the implosion. An embodiment of the target of this invention, referred to as a ball and shell target, shown in the drawing, is for 10 TW absorbed power and is capable of producing high densities. Referring now to the drawing, this target comprises a ball, generally indicated at 10, composed of a shell 11 of high-Z material, such as Au, which contains a quantity of gaseous DT fuel 12, shell 11 acting as a pusher and a preheat shield; surrounding ball 10 in spaced relation is an ablator-pusher shell 13 of lower-Z, lower density material, such as SiO.sub.2, defining a space containing a low-density material 14, such as CH (plastic foam), the CH serves to support the ball 10 within shell 13. By way of example, the Au pusher shell 11 has an inner radius of 0.0057 cm, an outer radius of 0.0068 cm (wall thickness of 0.0011 cm), a density of 19.3 gm/cm.sup.3, and a mass of 10.46 .mu.gm; the DT fuel has a density of 0.05 gm/cm.sup.3 and a mass of 0.0388 .mu.gm; the SiO.sub. 2 ablator-pusher shell 13 has an inner radius of 0.0400 cm, an outer radius of 0.0420 cm (wall thickness of 0.0020 cm), a density of 2.5 gm/cm.sup.3, and a mass of 105.6 .mu.gm; and the CH material 14 has a radial thickness of 0.0332 cm, a density of 0.02 gm/cm.sup.3, and a mass of 5.33 .mu.gm. It is not intended to limit the target to the specific materials and parameters exemplified above in that the pusher shell 11 could also be made of high-Z materials such as uranium (U), iron (Fe) and silver (Ag) or a mixture of selected high-Z materials, (Z of 26 and above), with an inner radius ranging from 0.005 cm to 0.01 cm and an outer radius of from 0.0055 cm to 0.012 cm. In addition the pusher could have an inner layer of lower-Z material such as SiO.sub.2 to act as a mandrel for fabrication purposes. The ablator-pusher shell 13 could be also composed Be, LiH, C, CH.sub.2, and B.sub.n H.sub.m (Z of 3 to 6) having an inner radius ranging from 0.03 cm to 0.05 cm, and an outer radius of from 0.04 cm to 0.0600 cm. The outer shell could also be a composite shell with an inner layer of higher-Z low density material such as TaCOH or SiO.sub.2. The low-density material 14 could also be composed of any low density gas or foam having a density of 10.sup.-4 to 10.sup.-1 gm/cm.sup.3. Also, the fuel 12 could vary in density from 0.01 to 0.21 gm/cm.sup.3 of DT or be composed of a solid hollow shell, or be composed of other gaseous or solid fuels such as LiD.sub..5 T.sub..5, D.sub.2, or B.sub.n D.sub.m T.sub.p. Should gaseous material be utilized as low density material 14, pusher shell 11 will be suspended within outer shell 13 by conventional support means, such as spiders, etc., well known in the art. Using the pulse shape of Table III, mean fuel densities of the above-exemplified target approach 150 g/cc with a yield of 4.times.10.sup.10 neutrons. TABLE III ______________________________________ TIME (SH) POWER (WATTS) PULSE ______________________________________ .0 10.sup.11 Constant .01 10.sup.11 Power .4 10.sup.13 Constant .42 10.sup.13 Power .43 5 .times. 10.sup.12 Linear .61 2 .times. 10.sup.13 Ramp ______________________________________ If, instead of supporting the ball 10 by the low density foam 14, as described above, it is levitated in density 10.sup.-4 g/cc gas atmosphere, the yield would be an order of magnitude higher. However, if a Maxwellian electron spectrum is generated, and a gas fill between ball 10 and shell 13 is used, then the target yields about 1/5 of breakeven. Illumination uniformity requirements are quite severe for the ball and shell target, illustrated in the drawing, in the absence of a preformed low density corona. LASNEX calculations indicate that the implosion pressures must be uniform to .+-.1% to achieve the densities indicated above. Since asymmetries of .+-.10-20% in laser intensity are expected--some corona may be necessary. This could be accomplished by surrounding the ablator shell with a 0.0001 cm thick, 0.0700 inner radius shell of SiO.sub.2 and filling the space with H.sub.2 having a density of 10.sup.-4 gm/cm.sup.3 and mass of 0.1 .mu.gm. A corona, or atmosphere, is a region of some thickness beyond the ablation surface that is penetrated supersonically by the energy deposited by the laser. This region allows electron conduction to smooth asymmetries in the absorbed energy before that energy is used to generate hydrodynamic motion of the target. To perform this function, the material in the corona must be raised to a high temperature. Theoretical calculations for a constant density corona, at a density just above the critical density for the laser being used, indicates that for a given thickness atmosphere, the required temperature is proportional to .lambda..sup.-1, where .lambda. is the laser wavelength. Similar estimates indicate that the energy required to heat the corona is proportional to .lambda..sup.-3 and that the shock generated by the prepulse used to heat the atmosphere is proportional to .lambda..sup.-7/3. It is this last effect which makes use of a high density atmosphere very difficult. Certain goals, such as the implosion of a DT shell to a density of 1000 g/cc, can only be achieved if the initial shock is limited to about one megabar. But heating a 300.mu. thick atmosphere, as exemplified above, at the critical density of 1.mu. light to the required temperature results in a 40 megabar shock. For such implosions, a 2.mu. laser will probably be required for the early part of the implosion. Because the illustrated target, modified as above to include an atmosphere, is not a low entropy implosion and because the low density foam between ablator shell 13 and ball 10 cushions the shock, it can survive the initial large shock generated by the explosion of the atmosphere forming layer about ablator shell 13. Initial 2-D calculations indicate one can probably tolerate .+-.10% asymmetries in absorbed power using an atmosphere generated in this fashion. The target illustrated in the drawing can also be modified in several other ways, which produces results intermediate those discussed above. For example, if a single thick glass shell is used, such as 13, the fuel will implode to higher density and low temperature than for the thin glass exploding pusher shell previously utilized. A similar effect is obtained by coating a high-Z material on the inside of the glass shell 13. Both of these get higher density than the exploding pusher glass shell but lower density than the atmosphere containing shell configuration described above. It has thus been shown that the present invention provides a high density laser-driven target that is compatible with high power laser systems such as the Shiva glass laser whereby both higher fuel densities and temperatures can be obtained. While particular embodiments of the invention have been illustrated and/or described, modifications will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications as come within the spirit and scope of the invention.