Hypervelocity impact in stellar media: Frontiers in Plasma Science experiments at DIII-D.

A study of carbon ablation at high heat flux relevant to hypervelocity spacecraft entries was performed at the DIII-D as part of the Frontiers in Science campaign. Exploration missions to the Solar System's gaseous giants and hyperbolic re-entries into the Earth's atmosphere require spacecraft that can withstand high velocity (>10 km/s) and extreme heat flux (>10 MW/m2). Testing and modeling material performance in this regime is challenging due to inadequate ground testing facilities. Conditions in DIII-D L-mode edge plasma reproduce flow velocity and high heat flux comparable to those experienced during the Galileo probe's entry into the atmosphere of Jupiter. Three types of samples were used for the experiments: stationary ATJ graphite rods protruding from the floor of the vessel, 1-mm-diameter porous carbon spheres, and 700-micron-diameter glassy carbon spheres injected from the floor into the scrape-off layer and edge plasma. Experimental results for the pellet trajectories and ablation rates are compared to the UEDGE-DUSTT simulations. The resulting mass loss rates as a function of calculated heat flux to the surface of the samples are found to agree with the semi-empirical engineering models.