The scaling laws of a cold gas-blanket experiment

Experimental investigations of a cold gas-blanket surrounding a fully ionized plasma core have been performed. The gas blanket is produced in the F-1 rotating plasma device having a purely poloidal magnetic field, B <= 0.5 T, major radius R = 0.3 m, electron density in the range of 1021 m-3, electron temperature T = 5 eV and rotation velocity upsilonphi = 105 m/s. It is found that the transverse particle diffusion is classical and the scaling of the plasma density, nsub(b), with the neutral hydrogen density at the wall, n1sub(w), follows the law nsub(b) proportional to (n1sub(w)B2)sup(1/3) agreeing with theory based on the momentum and particle balances. The scaling of the temperature with the power input cannot be accounted for on the basis of the transverse heat-balance alone. The thickness of the boundary layer decreases with increasing power input, but the maximum power input compatible with a gas-blanket could not be determined due to the Alfven critical-velocity effect, which limited the rotation velocity at the insulator surface. The minimum power input needed to maintain an impermeable plasma core was found to be 0.25 MW. (orig.)