Radial Profiles of the Neutral Hydrogen Density in the F-1 Cold Gas-Blanket Experiment

The neutral density profiles in the F-1 device have been calculated using the measured electron density and temperature profiles. The results from the fluid description have been compared with the numerical solution of the Boltzmann equation. It is found that the fluid model slightly underestimates the neutral density in the center of the discharge. The profiles indicate that a minimum input power of 0.3 MW is needed to keep the central plasma temperature above the hydrogen ionization limit. For higher input powers the plasma becomes impermeable to the neutrals and thus a true cold gas-blanket state is attained. The size of the ionization region of this blanket decreases with increasing input power and this confirms qualitatively the earlier obtained results.