PPPS-2013: Aerosol formation and buffer gas effects in a high-repetition rate ICF reactor

Summary form only given. With the recent 192-beam experiments at the National Ignition Facility, attention has been directed towards the next step to build a DEMO reactor with a reasonably high-repetition rate (~10Hz) of implosion. Along with repeated implosion, however, the interior of target chamber is exposed to intense pulses of DT-fusion neutrons, X-rays, unburned fuel particles, He-ash and pellet debris. As a result, wall materials will be subject to ablation, emitting particles in the plasma state. Ablated particles will either be re-deposited elsewhere on the wall or collide with each other in the center-of-symmetry region, if any, of the target chamber. Colliding ablation plasma particles may lead to the formation of clusters which can grow into aerosol, possibly floating thereafter. This would affect the subsequent implosion. It is also predictable that tritium be incorporated into re-deposited materials. Despite their importance, the chamber clearing and tritium codeposition issues have not yet been addressed in the inertial fusion community. To investigate the behavior of colliding ablation plasma plumes, a series of systematic experiments have been carried out over the past several years [1, 2, 3]. Used for these experiments is a well-diagnosed laboratory setup in which two arc-shaped targets are set in such a way that ablation plasma plumes, generated simultaneously by 10Hz, 6ns, YAG laser beams, collide with each other in the center-of-arc region. The laser power density ranges from 1 to 30 J/cm2/pulse and the repetition rate is set at 10Hz. These laser ablation experiments have been done both in high vacuum, in hydrogenic atmosphere, and additionally in the present work, in glow discharge plasmas of inert gases. Employed as the target materials are: Li, C, W and Pb, all candidate materials for ICF reactor applications. Data indicate that metallic targets tend to form aerosol in the form of droplet, the diameter of which is of the orders of 100nano~1- icron meters, whereas colliding carbon plumes form carbon nano/micro-tubes and fullerene onions, etc. Contrary to our prediction, hydrogen co-deposition has been found to be more significant with carbon than with lithium, the latter of which forms a hydride, though. Possible mechanisms are proposed to understand the aerosol formation and hydrogen co-deposition behavior and effects of background inert gas plasma on them.