Studies of cold-plasma/gas blankets

An impermeable cold-plasma/gas blanket may reduce plasma-wall interaction in tokamak experiments. Sufficient reciprocal shielding is provided when the well-known Lehnert criterion is fulfilled, i.e. that nsub(p)L>3x1018m-2, where nsub(p) is the local plasma density and L is a characteristic length in the transition layer. A further advantage of such a blanket is the ready possibility for fuelling and exhaust of the plasma. In this paper, experimental and numerical studies of such cold-plasma/gas blankets will be discussed. The experiments are performed in RINGBOOG II: a tokamak-like device with Bsub(T)<=3.2T, R=0.56m, a=0.08m, 5x1019m-3<=nsub(e)<=3x1021m-3. For the numerical studies a three-fluid steady-state model (electrons, ions, and atoms) is used. This model can yield either profiles in a cylindrical geometry, for plasmas with parameters comparable to those of RINGBOOG, or profiles in a slab which is sustained by heat generated in an adjacent hot plasma, thus simulating the outermost layers of such a plasma. (author)