Patent Number: 045253234
Section: description

DESCRIPTION OF THE INVENTION The present invention is an ion-beam fusion target, described in greater detail hereinafter, but basically consisting of a spherical shell of frozen DT surrounded by a low-density, low-Z pusher seeded with high-Z material, and a high-density, high-Z tamper (see FIG. 1). As pointed out above, this target satisfies many of the requirements or criteria for inertial confinement type commercial power production applications, although it can be also effectively utilized in the other various applications described above. Also, it is again noted that the principal feature of this target is the low-density, low-Z pusher instead of the prior known high-density, high-Z pusher. In addition, this target is composed of inexpensive material and can be readily fabricated by existing technology. The high-density tamper, such as Pb in the illustrated embodiment, serves as a confinement shell to increase the efficiency of the implosion. The pusher material, such as TaCOH, shown in the illustrated embodiment, is CH.sub.2 that has been seeded with a high-Z material, such as tantalum oxide, and is thus a a low-density, low-Z material. The tantalum in the illustrated embodiment constitutes only about 1 atomic percent (at %) of the pusher, but this is sufficient to inhibit energy transport into the fuel, preventing preheat. Because of its low-density, the pusher can be relatively thick to decrease the fluid-instability problem and yet contain little mass. In addition, the fluid instabilities causing pusher-fuel mixing during the final stages of the implosion are ameliorated as a result of the small density difference between fuel and pusher. Calculations typically give Atwood numbers much less than 1 across the pusher-fuel interface during the final stages of the implosion. In some cases, the fuel actually becomes denser than the pusher, resulting in a stable condition. Even where some mixing does occur low-Z materials cause less burn degradation than high-Z materials. In calculations involving high-Z pushers, utilized in previously known targets which are composed of a shell of fuel surrounded by a high-Z pusher material, such as gold (Au), the density times radial thickness (.rho.r) of the pusher in targets comparable to the embodiment illustrated in FIG. 1 is roughly 10 g/cm.sup.2 during thermonuclear burn. In the target of this invention, the bulk of the high-Z material remains uncompressed at a large radius. The total .rho.r of the high-Z material in both the pusher and tamper is less than 1 g/cm.sup.2. Thus, the low-Z pusher target produces less than 10% as much high-Z radioactive debris as a target with a high-Z pusher. Low-Z materials stop ions more effectively than do high-Z materials. Thus the ion-beam energy is preferentially deposited in the pusher region where it is most effective (see FIG. 2) the curve in FIG. 2 was calculated for 6.5 MeV protons at temperatures and densities occurring 18 ns into the implosion, a typical temperature in the deposition media being in excess of 200 eV. To achieve high gain, fuel must be efficiently compressed and only the central portion ignited. A radially propagating burn ignites the remainder of the fuel. These two conditions are achieved by using the pulse shape shown in FIG. 3. FIG. 4 is a plot of pressure as a function of density at the innermost portion of the fuel and at radial distance containing roughly one-half of the fuel. The zero temperature Fermi-degenerate pressure is shown for comparison. It is evident that most of the fuel has been compressed relatively efficiently while the central part of the fuel has been placed on a high adiabat and driven to ignition. Plots of radius as a function of time are shown in FIG. 5. The maximum velocity of the pusher-fuel interface is 33 cm/.mu. sec. Referring now to FIG. 1, the illustrated embodiment of the target comprises a central hollow shell of fuel 10 defining therein a void 11, a pusher shell 12 surrounding fuel shell 10, and a tamper shell 13 surrounding pusher shell 12. As shown in FIG. 1, the fuel 10 is composed of deuterium-tritium (DT) having a density of 0.21 gm/cm.sup.3 mass of 1.00 mg, an inner radius of 0.19004 cm, an outer radius of 0.20000 cm, forming a wall thickness of 0.00996 cm; pusher 12 is composed of TaCOH having a density of 1.26 gm/cm.sup.3, mass of 16.8 mg, inner radius of 0.20000 cm, outer radius of 0.22360 cm, forming a wall thickness of 0.02360 cm; with tamper 13 composed of lead (Pb) having a density of 11.3 gm/cm.sup.3, mass of 72.1 mg., inner radius of 0.22360 cm, outer radius of 0.23333 cm, and wall thickness of 0.009973 cm. While specific parameters and materials of the FIG. 1 embodiment of the target have been described and/or illustrated, the fuel 10 can also be composed of any other thermonuclear fuel such as DD, LiD, etc., but such fuels impose more difficult requirements on ion beam power and energy. The quantity of fuel as well as other parameters such as tamper thickness can range over large limits depending on such things as ion beam voltage, power and energy and desired target yield. The tamper can be composed of any dense material (density of 0.5 to 25 gm/cm.sup.3), for example, 0.5 gm/cm.sup.3 is dense compared to DT. In addition to the above-detailed embodiment the tamper could be composed of materials selected from the group consisting of Pb, Fe, Cu, W, Ag, Ta, Au and any other high Z material. The pusher and intermediate layer can be any low Z material. Seeding material can be nearly anything also. In addition to the above-detailed embodiment the pusher could be composed of materials selected from the group consisting of Ta, COH, Li, Be, B, C or any other low Z material seeded with Ta, W, Pb, Au, Fe, Cu or any high-Z material. The densities of these layers range from 0.07 gm/cm.sup.3 to 10 gm/cm.sup.3. Z of the pusher ranges from 1 to 30 and of the seeding material from 2 to 92, or greater if one wants to use man-made elements. One can also use mixtures of elements. The basic idea is that anything works as long as the tamper is denser than the pusher. The overall gain of the FIG. 1 target is 88. The energy output is 113 MJ; the energy input is 1.28 MJ at a peak power of 2.4.times.10.sup.14 Watts. It is understood that the illustrated target can be readily scaled to different sizes and ion-beam voltages. By way of comparison, the performance of the FIG. 1 target has been compared with a high-Z pusher target composed of a hollow shell of DT having a density of 0.21 gm/cm.sup.3, inner radius of 0.190 cm, and outer radius of 0.200 cm; and a high-Z pusher composed of gold (Au) having a density of 19.3 gm/cm.sup.3, an inner radius of 0.200 cm, and an outer radius of 0.221-0.223 cm; imploded by an unshaped ion beam pulse of 600 TW. A detailed comparison of the FIG. 1 target and the high-Z pusher target is given in Table I. TABLE I ______________________________________ Input Energy Output Energy Peak Input Target Target (MJ) (MJ) Power (TW) Gain ______________________________________ Low-Z 1.28 113 240 88 Pusher High-Z 7.2 247 600 34 Pusher ______________________________________ Two dimensional LASNEX code calculations show the stability of the low-Z pusher to be superior to that of the high Z target. The accuracy of the LASNEX code has been verified by actual implosion experiments (see above-referenced papers UCRL-77056, UCRL-77094, and UCRL-77725), thus unquestionably establishing the code as an effective tool for target design, and thus each target configuration or modification thereof need not be actually imploded to establish the energy produced thereby or energy required to implode same. It has thus been shown that the target of the present invention satisfies many of the requirements for commercial power production by ion-beam fusion, by positioning a layer or shell of low-density, low-Z pusher material seeded with high-Z material between a hollow shell of fuel and a high density tamper. This low-density, low-Z pusher target produces results substantially greater than the targets using a high-Z pusher. While FIG. 1 illustrates a specific embodiment of the invention, it is not intended to limit the invention to the specific materials and parameters illustrated in that embodiment, since as pointed out above, other materials and parameters can be used. Also, if desired for certain applications, the abovedescribed target can be modified by placing a low-density absorber layer between the pusher and the tamper. It has thus been shown that the target of the present invention substantially advances the state of the ion-beam targets for the inertial fusion applications exemplified above. While a particular embodiment of the invention has been described, modifications and changes will become apparent to those skilled in the art, and it is intended to cover in the appended claims all such modifications and changes as come within the spirit and scope of this invention.