Cluster formation and hydrogen co-deposition by colliding ablation plasma plumes of lithium and lead

Summary form only given. In high-repetition inertial confinement fusion (ICF) reactors, the interior of target chamber is exposed repeatedly to intense pulses of fusion neutrons, X-rays, unburned DT-fuel particles, He-ash and pellet debris, the total deposited energy of which could amount to a few tens of Joules/cm²/implosion. As a result, wall materials are subject to ablation, emitting particles in the state of plasma. Ablated plasma particles will either be re-condensed elsewhere on the wall or collide with each other in the center-of-symmetry region of the target chamber. Colliding ablation plasma particles may possibly form clusters which can grow into aerosol, which may affect subsequent implosion performance. Despite its critical importance, the chamber clearing issue has not widely been recognized in the ICF research community. In our previous studies, the dynamics and recondensation behavior of colliding plasma plums of selected materials for solid wall ICF reactors, including W and C, was investigated using an experimental setup referred to as LEAF CAP [1] (for the Laboratory Experiments on Aerosol Formation by Colliding Ablation Plumes). The present work focuses on Li and Pb, materials envisaged for ICF reactors with a liquid first wall. In the LEAF-CAP setup two arc shaped targets are irradiated in vacuum by 6ns pulses of 3ω-YAG laser at 10Hz, the deposited energy of which ranges from 1 to 10 Joules/cm²/pulse. Ablation plasma plumes thus generated are to collide with each other in the center-of-arc region which is diagnosed by a CCD/ICCD camera, qudrapole mass analyzer, Langmuir probe, visible spectrometer, etc. From ICCD camera observations, Li plumes collide to merge with each other, traveling to slow down in the compound velocity direction, suggesting an inelastic process. Consistently, cluster ions of Li²⁺ have been identified in mass spectra. These findings are similar to those on colliding C plumes, forming- C_n clusters and nano-scale aerosol [2]. In contrast, colliding Pb/Pb and Li/Pb plumes appear to penetrate each other, similarly to W/W plumes [3].