Gas-blanket studies in toroidal arcs

Magnetized arc discharges are used to gather information on transport phenomena in the partially ionized transition layer between a neutral gas blanket and a fully ionized hot plasma core. The conditions can be made similar to those that might prevail in the outer layers of a steady-state thermonuclear reactor in which fuelling and helium removal are maintained by diffusion. To study the influence of toroidicity on transport processes, discharges are made in tokamak like devices, operated at filling densities between 1020 and 1022m-3. In a first experiment the arc was struck between two disk electrodes placed 3π/2 radians apart in a toroidal magnetic field of 2 T with discharge currents up to 32 kA. Electron densities between 1021 and 1022m-3 were measured with temperatures up to 5 eV. Since the spring of 1974, the fully toroidal experiment ''Ringboog'' with the dimensions R = 0.52 m, a = 0.08 m, B = 3.2 T became operational. Preliminary results have been obtained at low power. The confinement is predominantly diamagentic, leading to βsub(p)-values up to 4. If no appropriate vertical magnetic field Bsub(v) is used for achieving toroidal equilibrium, a double-vortex flow develops which drives a vertical current limiting the flow velocity, provided that an external conductor is in contact with the plasma. Even in the case of toroidal equilibrium obtained by a proper Bsub(v), a double-vortex convection flow exists. This flow may be identical with the Pfirsch-Schlueter convection. The analysis of the conductivity temperature profile indicates losses by classical ion heat conductivity, with the toroidal correction factor (1 + 1.6 q2) included. (author)