Behavior of Plasma Rotation and Radial Electric-Field with Density Ramp Rate in an Ohmically Heated Tokamak
Measurements of toroidal and poloidal rotation of the TCA plasma with Alfven wave heating and different levels of gas feed are reported. The temporal evolution of the rotation was inferred from intrinsic spectral lines of C V and C III, and, using injected helium gas, from He II. The light collection optics and line intensity permitted the evolution of the plasma rotation to be measured with a time resolution of 2 ms. The rotation velocities were used to deduce the radial electric field. With Alfven wave heating there was no observable change of the electric field that could have been responsible for the density rise which was characteristic of the RF experiments on TCA. The behaviour of the plasma rotation with different plasma density ramp rates was investigated. The toroidal rotation was observed to decrease with increasing plasma density. The poloidal rotation was observed to follow the value of the plasma density. With strong gas puffing, changes in the deduced radial electric field were found to coincide with changes in the peaking of the plasma density profile. Finally, with frozen pellet injection, the expected increase in the radial electric field due to the increased plasma density was not observed, which may explain the poorer confinement of the injected particles. Even in an ohmically heated tokamak, the measured plasma rotation and radial electric field are shown to be strongly related with the confinement. A thorough statistical analysis of systematic errors is presented and a new and significant source of uncertainty in the experimental technique is identified.