Tomographic Analysis of the Evolution of Plasma Cross-Sections in Hbt.

A diagnostic has been developed for measurements of the evolution of plasma density distributions in 2D using the principles of computed emission tomography. The system measures the near infrared emission along several chords through the plasma cross-section and the emission profile is then reconstructed using a maximum entropy based algorithm. Measurements have been made on a poloidal cross -section of the Columbia University high beta tokamak HBT. It was possible to determine the complete time history of the plasma density distribution during the tokamak phase of the discharge. Variations of the plasma location, size, and elongation were measured with a spatial resolution of about 2 cm. The reconstructed profiles were observed to be correlated with magnetic structure of the plasma. Correlation experiments were performed to show that the density profile can be inferred from the square root of the infrared emission profile. The data obtained indicate that the discharges examined had beta's from about 15% to more than 24%. The values of (epsilon)<(beta)(,p)> were in the range 0.38 to 0.62 which indicates that the discharges were either at or above the boundary for stable low beta operation. The values of (epsilon)<(beta)(,p)> were not high enough to put the plasma in the second stable regime. No evidence of MHD instability was seen, possibly because the 20 (mu)S lifetime of the discharges was not long enough for any instabilities to grow large enough to be detected.