The hohlraum hard X-ray flux, like the M-band radiation of gold, may preheat the ICF target and alter the implosion dynamics significantly. Besides its the widely discussed influence on the fuel/ablator interface Atwood number, numerical studies have suggested that higher M-band fraction would also lead to higher implosion velocity (v_imp) of the cryogenic DT fuel (by a few percent), accompanied by a decrease in the remaining ablator mass. Very often, this phenomenon is simply attributed to an increase of ablative rocket efficiency.

However, we found with 1-D simulations that even if the radiation module is completely shut down for the final stage of implosion (after shell radius ≈ 1/2 initial radius), v_imp of the DT fuel is basically not affected – it will continue rising for about 30%, and shows the difference caused by M-band radiation. Especially, it is in this final stage of acceleration, rather than the earlier times with ablation on, that the difference in v_imp emerges.

By a careful investigation of the 1-D radiation hydrodynamic simulation results, we think the increase in DT velocity due to M-band preheating is primarily a hydrodynamic effect, caused by the radial expansion of the remaining ablator material – with entropy set mainly by the M-band energy deposited earlier in these matters. In this manner, the difference in remaining mass is more likely to be the result of this hydrodynamic process, rather than its cause.

Based on these understandings, we propose that the difference in DT v_imp caused by M-band radiation may be largely suppressed by increasing the dopant level – if the final chunks of ablator material were well protected from M-band preheating for most of the implosion time, their entropy as well as hydrodynamic behavior would be similar in the final stage, insensitive to the M-band flux screened by the outer layers. i.e., higher ablator opacity may not only alleviate the high-mode hydrodynamic instability at the fuel/ablator interface, but also help to reduces the overall low-mode asymmetry introduced by asymmetric M-band flux.
 

inertial confinement fusion,implosion,preheating effect,1-D Simulation