Direct drive target survival during injection in an inertial fusion energy power plant

Abstract

In inertial fusion energy (IFE) power plant designs, the fuel is a spherical layer of frozen DT contained in a target that is injected at high velocity into the reaction chamber. For direct drive, typically laser beams converge at the centre of the chamber (CC) to compress and heat the target to fusion conditions. To obtain the maximum energy yield from the fusion reaction, the frozen DT layer must be at about 18.5 K and the target must maintain a high degree of spherical symmetry and surface smoothness when it reaches the CC. During its transit in the chamber the cryogenic target is heated by radiation from the hot chamber wall. The target is also heated by convection as it passes through the rarefied fill-gas used to control chamber wall damage by x-rays and debris from the target explosion. This article addresses the temperature limits at the target surface beyond which target uniformity may be damaged. It concentrates on direct drive targets because fuel warm up during injection is not currently thought to be an issue for present indirect drive designs and chamber concepts. Detailed results of parametric radiative and convective heating calculations are presented for direct-drive targets during injection into a dry-wall reaction chamber. The baseline approach to target survival utilizes highly reflective targets along with a substantially lower chamber wall temperature and fill-gas pressure than previously assumed. Recently developed high-Z material coatings with high heat reflectivity are discussed and characterized. The article also presents alternate target protection methods that could be developed if targets with inherent survival features cannot be obtained within a reasonable time span.

Review on high repetition rate and mass production of the cryogenic targets for laser IFE

“…The main steps of this production line have been investigated over the period of 1995-2016. They are as follows: mass production of shells [34,[36][37][38][39][40][41] , shell coating with protective layers [42][43][44][45] , fuel filling [30,31,44,46,47] , cryogenic fuel layering [45,46,48,49] , target acceleration and injection [47,[50][51][52][53][54][55][56][57] , flying target tracking and shooting [50,[57][58][59][60][61][62][63] . It is supposed that the first power plants will work with radioactive D-T fuel, which is a major limitation to IFE.…”

Lifetime of silica final lenses subject to HiPER irradiation conditions

“…Contrary to the case of indirect drive targets, with direct drive targets gas protection is a bad option. This is due to the fact that (i) the presence of a residual gas pressure (typically 100 Pa of Xe at standard temperature) makes difficult to keep the target temperature below its triple point 22 ; (ii) direct target tracking is difficult with the presence of a residual gas; (iii) for the particular case of fast ignition a 100 kJ, 7 PW, 15 ps laser pulse is required that is incompatible with the background pressure necessary for ion mitigation. The only realistic ion mitigation strategy to protect the final lenses relies on ion diversion by magnetic or electric fields.…”

Thermal Behavior and Operating Requirements of IFE Direct-Drive Target

“…A very reflective target surface is required to minimize the total absorbed heat flux. For the very thin (275-375 Å) coating of gold on the target, a reflectivity of ~96% is anticipated [1]. An effort is underway to estimate more accurately the radiated energy absorption and reflection based on a multi-layer wave model to provide a stronger basis for this assumed reflectivity.…”

“…The resulting target thermal behavior is a particular concern because the target can deform due to melting and evaporation of the DT. Although the maximum temperature limit to prevent unacceptable target outer layer deformation is not well known, the previous assumption was to maintain the target DT temperature (TDT) below the triple point (TTP,DT = 19.79 K) [1]. This paper presents an updated and more detailed assessment of the target thermal behavior during injection.…”