Carbon migration during methane injection experiments under ITER divertor-relevant conditions

Methane was injected into a high density low temperature hydrogen plasma beam (ne ¿ 1021 m-3, Te ~ 1 eV) in Pilot-PSI to study carbon transport and provide a benchmark case for modeling. Gas injection was performed through molybdenum samples that acted as collectors of redeposits. The CH A–X band emission plumes are localized near the injection hole with a typical width of ~3 mm and a decay length of ~1.5 mm. The geometry of the emission profile is not influenced by the electron temperature. The redeposition efficiency and codeposit structure depend on the surface temperature. Surface temperatures below 500 K promote growth of a-C:H films across the surface and lead to redeposition of up to 38% of injected carbon. Above 800 K the redeposition efficiency decreases strongly and carbon microparticles are found near the injection hole.