Thermal electron transport in direct-drive laser fusion

Thermal electron transport plays a key role in direct-drive laser fusion (1). In efficient high-gain targets illuminated by submicrometre laser light, the laser light is absorbed by inverse bremsstrahlung in the low-density corona up to the critical density where the light is fully reflected. The energy absorbed is then conducted by electron thermal transport to the ablation surface where the cold, solid target material is ablated producing the "rocket" action that drives the target inwards. Figure 1 describes schematically the processes and the conditions in the target that pertain to the thermal electron transport. The absorbed laser energy creates a heat front that progresses through the cold target material; the corona, the region outside the critical surface, is almost isothermal because of the good thermal conductivity and the low density of the blowoff material. The primary effect of the thermal electron transport is on the efficiency of the drive; any reduction in the heat flux between the corona and the ablation surface directly results in diminished drive efficiency. When the thermal flux is reduced, the energy not transported into the ablation region accelerates the coronal material. A secondary effect of the thermal heat transport is on the absorption fraction. When the thermal flux is reduced, the excess