Turbulent Resistivity, Diffusion and Heating

Current flow with density j > necs (cs ≡ √Te/mi = ion acoustic velocity) should encounter resistance due to ion acoustic turbulence when Te >> Ti. This has been observed in experiments with a streaming caesium plasma (β = 0), in which electron current, potential rise due to turbulent resistivity, spectrum of unstable ion acoustic waves, and associated electron heating were all measured directly. Kinetic theory calculations for an expanding, unstable plasma, give results in agreement with experiment. The effective electron collision frequency is of older 5 to 10 ωpi. In a strong magnetic field, with Te/Ti ∼ 1 and current densities less than neae (ae = √Te/me), typical for present Tokomaks. the plasma is stable to ion acoustic but unstable to current driven electrostatic ion cyclotron waves. Relevant characteristics of these waves are calculated and it is shown that for ion β > me/mi, the electromagnetic ion cyclotron wave has a lower instability threshold than the electrostatic one. However, when ion acoustic turbulence is present (Te/Ti > 5) experiments with Double Plasma (DP) devices show rapid anomalous heating of an ion beam streaming through a plasma. These DP-devices also produce large-amplitude, collisionless ion acoustic shocks, both laminar and turbulent. At low Mach numbers, a beam of reflected ions is observed. Turbulence, either natural or triggered by fluctuations of injected ion waves, scatters the reflected ions and heats the shocked ions (observed broadening of distribution function). Shock structure is very sensitive to Te/Ti and especially to the presence of light ions: a few per cent He in an A plasma can significantly change both the shock amplitude and the laminar/turbulent transition. At higher density ratios (obtained with photoionization and microwave, upper hybrid, resonant heating) the Mach number increases (maximum observed ∼ 1.8) and the structure washes out. (author)