Evidence for the importance of radial transport in plasma detachment in the Nagoya University Divertor Simulator (NAGDIS-II)

Measurements of ion and electron temperatures have been performed in detach ing helium-hydrogen plasmas in a linear divertor simulator experiment using spectroscopy, a Langmuir probe, and an omegatron mass spectrometer. Detach ment in these plasmas is characterized by a significant (approximate to 20x ) reduction in the central plasma flux at the target plate as the target re gion neutral pressure is increased from 2 to 12 mTorr. The data indicate th at partially detached gas-target plasmas consist of a hot (T(e)approximate to5 eV) core region along the axis of the plasma column, surrounded by a co ld (T(e)approximate to0.1 eV) halo region of recombining plasma. At T-e=5 e V, plasma recombination is negligible compared with ionization; these exper iments therefore provide evidence that detachment is primarily caused by ra dial transport and by a gradual drop in the ionization source as the temper ature of the core region drops below 5 eV. (C) 2001 American Institute of P hysics.