Ion Thermal Diffusion in the Texas Experimental Tokamak.

Profiles of the ion thermal diffusion coefficients have been determined experimentally for several TEXT plasma discharges using the steady state power balance method. The ion temperature profiles have been measured by the active charge exchange method where high energy hydrogen atoms are injected into the plasma as artificial targets for plasma ions to charge exchange with. A scanning multichannel neutral particle analyzer has been used to detect the emerging neutral particle flux from which the ion temperature is deduced. The modulation of the diagnostic neutral beam allowed the determination of the ion temperature profiles before and after injection of hydrogen pellets into the plasma. For most TEXT plasma discharges studied, the experimentally inferred ion thermal diffusion coefficients were found to be larger than the values predicted by neoclassical theory by a factor of two to six. Furthermore, no evidence of a reduction of thermal losses through the ion channel was observed in pellet-fueled discharges. The anomaly in the ion thermal diffusion coefficients was compared with transport coefficients predicted by different models that describe the eta_ i-instability. The effect of convective ripple transport on the ion distribution function has been investigated. A series of experiments dedicated to the study of the up-down asymmetries observed in some measured ion temperature profiles, have established the evidence of a strong influence of the magnetic field ripple on the tail of the ion distribution function. These asymmetries have been modified by changing the distribution of the magnetic field ripple through vertical displacements of the magnetic axis. These results are important for a correct determination of the ion temperature from active charge exchange data and suggest that the convective ion transport due to the magnetic field ripple can be an important energy loss channel for plasma ions and the primary loss mechanism for beam ions.