Experimental simulation of a gaseous plasma collector

Hydrogen plasma with a density of 3×1014 cm−3 and electron temperature of 20 eV is injected parallel to a 0.1 T B field, into hydrogen (and helium) neutral gas at 0.1–2 Torr. This plasma is opaque to molecular hydrogen but transparent to Franck–Condon neutrals. The axial density and temperature scale length increased with decreasing gas pressure but were insensitive to plasma density and axial magnetic field strength. The plasma decay is explained by radial ion diffusion resulting from collisions with fast neutrals while the fast neutral density is determined by the radial pressure balance between fast neutrals and the cold background gas.