Resonance radiation and high excitation of neutrals in plasma–gas interactions

Experimental investigation of plasma–gas interaction has been performed in LENTA linear facility in order to model processes expected to occur in the divertor of a fusion tokamak reactor. Steady-state helium plasma with density ∼(0.2–3)×1013 cm−3 generated by beam-plasma discharge flowed into the region with high neutral pressure, interacted with neutral helium there and then reached the target plate. An intensive volume recombination and significant decrease in plasma pressure have been observed while the plasma stream interacted with gas target. Electron temperature fell below 1 eV. These processes were accompanied by an intensive emission from highly excited helium atoms and this radiation became even higher with increase in neutral pressure. Microwave emission absorption at high (Pgas=20 mTorr) neutral pressures in the gas target was detected. A model of plasma–gas transition layer was developed to provide physics understanding of these phenomena. Resonance radiation and stimulated radiative recombination play an important role in this model.