The development of beam-target configurations, specifically the effort to achieve increased symmetric energy deposition over the surface of the pellet, has led to what is known as indirect drive. The distinction between this approach and that of direct drive-imparting pulses of energy from lasers or ion beams directly onto the pellet as previously discussed-is as follows.

For a system utilizing indirect drive, the target consists of both a fuel pellet — similar to those discussed in Sec. ll.5-and a small cylindrical cavity, inside which the pellet is located. This cylindrical vessel, known as a "hohlraum", is a few cm long, is made of a high-Z material such as gold or other metal, and has "windows" transparent to the driver on each end, Fig.11.4. Then, instead of requiring all the driver beams to impinge symmetrically on the pellet, as is necessary for direct drive pellet compression, the beams enter both ends of the hohlraum obliquely and ablate the inner surface of the cavity. The high-Z material of the hohlraum emits soft X-rays when so irradiated, and by focusing the driver beams to the appropriate points inside the cavity, a highly symmetric irradiation of the fuel pellet results-followed by the previously discussed stages of pellet compression and heating depicted in Fig. 11.1.

As with most methods of ICF pellet compression to date, indirect drive has only been examined using laser drivers, however, the same approach is believed to be applicable to ion beams. The crucial aspect of indirect drive is establishing the optimal "pointing" of the laser beams-the positioning and focusing of the beams on the cavity’s inner surface which results in symmetric irradiation of the fuel pellet by the emitted soft X-rays. Development has shown this not only to be possible, but symmetric energy deposition on the pellet surface is achieved with fewer complications than when all the driver beams must symmetrically impinge

directly on the pellet.

The other major advantages of indirect drive as compared to direct drive are the better ablation and subsequent compression achieved with X-rays as opposed to the visible light of lasers, and reduced instabilities during pellet compression. These characteristics and the demonstration of energy deposition levels over an entire pellet surface with <1% deviation from uniformity are very appealing. However, the reduced energy coupling from the beam to the pellet, pc in Eq.( 11.17), and the increased complexities of hohlraum manufacture-in addition to the pellets alone-when scaled up to a power plant-type system are disadvantages not be overlooked. Despite these drawbacks, the inclusion of the hohlraum concept and the use of indirect drive-pellet compression does appear necessary in the continuing development of inertially confined fusion systems.

The designer of an ICF power plant faces a number of key decisions including the choice of the driver and the choice of protection scheme for the reaction chamber wall. The selection of a driver will depend on advances in the technology associated with specific types of drivers and on the beam-target coupling efficiency that can be achieved with specific beams. Protection for the reaction chamber wall from radiation and pellet debris released in a microexplosion is a unique and challenging aspect of ICF reactor design. Various possible approaches have been proposed including a large radius chamber with a "dry" wall and various "wet" wall concepts such as a falling liquid metal veil, liquid metal jets, liquid metal droplet sprays or a thin surface layer of liquid metal. The latter concepts all allow a smaller, more compact chamber but face various problems such as the difficulty in quickly pumping out vaporized material between pulses. Other unique design issues relate to pellet manufacture, pellet handling and positioning in the chamber, protection of mirrors, focusing magnets for ion beams, and other beam transport elements.