Patent Number: H00005088
Section: description

DETAILED DESCRIPTION OF THE INVENTION For any ICF target system to have a high gain, the dimensions of the ICF target must drastically reduce in size as the target is driven to implode. For both laser and ion beam drivers, the degree to which this may be accomplished depends upon energy deposition efficiency and uniformity, which together particularly limit the implosion symmetry that may be practically achieved. Since, generally speaking, in presently existing driver systems energy deposition efficiency and uniformity are optimized only in the neighborhood of a single target radius, as the target implodes these parameters quickly deteriorate. The implosion of an ICF target generally occurs in two phases: an initial compression phase, during which the target rapidly and very significantly diminishes in size, and a final peak power phase, during which the reduced dimensions of the target remain relatively static. The initial compression phase has especially high energy deposition uniformity requirements, and the final peak power phase is especially sensitive with respect to energy deposition efficiency, even though the two parameters are always of critical concern during the full course of the implosion. The present invention comprises the realization that a vast improvement in gain, at fixed total driver energy, may be achieved by separately and optimally driving the two phases of ICF target implosion. The invention has six potential embodiments: laser-ion beam; ion beam-laser; laser-laser; ion beam-ion beam; single laser; and single ion beam. The present invention is discussed by the inventor, James Wai-Kee Mark, in Lawrence Livermore National Laboratory document UCRL-97110, dated July 24, 1987. As a preliminary consideration, reference is first made to FIG. 1, which is a cross-sectional, schematic view of an ICF target 10. The target 10 is comprised of a hollow spherical ablator 12, that is surroundingly disposed around a quantity of fusion fuel 14. It is emphasized that the figure is very schematic. ICF targets are very well known in the prior art, and they can be of very great complexity, comprising many additional features and components than those shown in FIG. 1. For example, known ICF targets often have multiple internal regions for containing fuel or performing various hydrodynamic functions. Fusion fuel 14 may comprise deuterium, tritium, any other fusionable isotopes, in solid, liquid, and/or gaseous form. Ablator 12 may comprise any structural material such as aluminum or beryllium. Ablator 12 is so called because, in operation, material is ablated from the surface of ablator 12 to drive the inward implosion of the remaining portions of ICF target 10. The diameter of typical ICF targets, such as target 10, is usually on the order of one millimeter. Reference is now made to the present preferred embodiment of the invention, which is illustrated in FIG. 2. This figure is a perspective, schematic view of a hybrid-drive implosion system 20 for an ICF target 22, made in accordance with the invention. ICF target 22 is exactly similar to target 10 of FIG. 1. In particular, target 22 comprises a hollow spherical ablator surroundingly disposed around a quantity of fusion fuel, which are not shown because of their very small size. Hybrid-drive implosion system 20 comprises a laser system 24, that provides laser beams 26. Beams 26 are shown reflected, by mirrors 28, to ICF target 22. As very well known in the prior art, laser beams 26 focus on ICF target 22 and compress the ablator of target 22 to higher density. As shown, laser system 24 and laser beams 26 schematically represent an axially symmetric illumination scheme as disclosed by Mark in Physics Letters 114 A, 458 (1986). In this Gaussian-quadrature illumination strategy, which is equally applicable to laser or ion beam drivers, the laser or ion beams are situated on the rims of cones whose angles are the zeroes of Legendre polynomials. The scheme achieves symmetry comparable to that achievable with the same number of beams placed uniformly over the surface of a sphere. The number of beams required is minimized if the beams have unequal powers, as appropriate. Thus, the ablator of ICF target 22 may very efficiently be compressed to higher density by laser beams 26 of laser system 24 by techniques that are well known and well established in the prior art. It is emphasized that this invention is in no way limited to the Gaussian-quadrature illumination strategy. As further shown in FIG. 2, hybrid-drive implosion system 20 additionally comprises an ion beam system 30, that provides ion beams 32. Ion beam system 30 may be similar to any of the many presently known ion beam drivers for ICF targets. Ion beams 32 are focused by known magnetic or electrostatic focusing mechanisms, not shown, and caused to impinge upon ICF target 22 where they deliver energy into the ablator of target 22, after it has been compressed by laser beams 26 of laser system 24. It is particularly noticed that ion beam system 30 may be fashioned in an axially symmetric, Gaussian-quadrature illumination configuration as discussed above. The relative orientation of ion beam system 30 to laser system 24 need not be co-axial. Rather, the orientation may have any convenient configuration. The illumination geometry of ion beam system 30 and ion beams 32 is optimized to a target radius within the compressed ablator of target 22 so that energy is optimally delivered from ion beam system 30 to the ablator of target 22. Since most (approximately 80% in many situations) driver energy is delivered to the ICF target 22 during the second, ion beam driven, phase of operation, the optimization of energy delivery during this phase greatly increases ICF target gain. The energy delivered to the ablator of target 22 by ion beams 32 causes the ablator to implode and compress the quantity of fusion fuel within ICF target 22 to conditions wherein fusion reactions occur. Reference is now made to FIG. 3 which shows a second embodiment of the invention. The figure shows a hybrid-drive implosion system 40 for an ICF target 42, that is very similar to system 20 of FIG. 2, with the single exception that the first phase of ICF drive is performed by ion beams rather than laser beams. Once again, ICF target 42 is exactly similar to ICF target 10 of FIG. 1, and comprises a hollow spherical ablator surroundingly disposed around a quantity of fusion fuel, neither of which are shown in FIG. 3 because of their extremely small relative size. A first ion beam system 44 provides ion beams 46 that compress the ablator of ICF target 42 to higher density. Ion beam system 44 may be configured with the axially symmetric, Gaussian-quadrature illumination strategy discussed above, and energy delivery is optimized for compressing the ablator of ICF target 42 to higher density. The means of doing this are very well known in the prior art. After the ablator of ICF target 42 is thus compressed to higher density, a second ion beam system 48 provides ion beams 50 that directly deliver energy into the compressed ablator of target 42. Ion beam system 48 is described exactly as ion beam system 30 of FIG. 2. That is, ion beam system 48 may be configured with the axially symmetric, Gaussian-quadrative scheme as discussed above, and is optimized to a target radius within the compressed ablator of ICF target 42 so that energy is optimally delivered from ion beam system 48 to the ablator of ICF target 42. Because of this, the ablator of ICF target 42 implodes and compresses the quantity of fusion fuel within ICF target 42 to conditions wherein fusion reactions occur. Ion beam systems 44 and 48 may, but need not be co-axial, but rather may have any convenient relative relationship. It is thus appreciated that the ICF hybrid-drive implosion systems 20 and 40, shown in FIGS. 2 and 3, respectively, illustrate methods and apparatus for increasing ICF target gain while keeping the total amount of available driver energy fixed, and will thus make the goals of ICF potentially more attainable. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. For example, as mentioned above, four additional embodiments of this invention are potentially possible. They are, in addition to the two embodiments described in detail, a third embodiment wherein the first phase of ICF drive is performed with ion beams and the second phase with laser beams; a fourth embodiment wherein each of the two phases of ICF drive are performed by a separate laser system; a fifth embodiment wherein each of the two phases of ICF drive are performed by ion beams supplied by a single ion beam driver; and, a sixth embodiment wherein each of the two phases of ICF drive are performed by laser beams supplied by a single laser beam driver. In the last two embodiments, five and six, a single driver produces two multiplicities of driver beams which must be time delayed from one another and separately focused on the ICF target during each of the two separate phases of ICF target drive. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.