Special design issues. Laser driver-reaction chamber interfaces

The interface systems between the laser driver beam and the reaction chamber in IFE (inertial confinement energy) plants are described. The main purposes of these systems are to prevent material in the reactor from escaping and contaminating the driver beamlines and to protect the optical elements of the laser driver from the effects of the fusion pulses. After discussing options for a final optics system (to provide a seal against the escape of materials from the reactor and shield the delicate upstream laser components) the material properties are mentioned that make amorphous fused silica the most desirable material currently known for use at the laser-reactor interface to support the wedge deflector concept. Next, the need for gas protection from fast ions of the final laser optics is discussed. Such damage occurs through two mechanisms: softening of the material when the specific energy deposited is above the melting threshold, and front surface cracking when the peak Grueneisen pressure exceeds the yield stress for a sufficiently long time. As the X ray absorption cross section falls off cubically with X ray energy both effects diminish rapidly with increasing photon energy. Consequently, direct drive systems even have a regime in which gas protection from X rays is not required, although gas protection from debris ions still is likely to be necessary. In indirect drive systems, on the other hand, the hohlraum can capture a greater fraction of the fusion energy and reradiate it as cold X rays. Thus, without gas protection the fluences at the final optics systems may exceed damage thresholds for typical IFE reactor conditions. However, fast ions are more of a problem for direct rather than for indirect drive. As a main conclusion: gas protection is expected to be necessary for direct and indirect laser drive targets. For the threats of high speed vapours, condensates and droplets the most acceptable solution is a counterflow of noble gas, requiring a noticeable fraction of the plant power. Alternatively, a sufficient amount of ambient gas may be allowed to remain in the chamber, but this presents a challenge to the cryogenic target. 16 refs, 4 figs, 2 tab