Current diffusion in TEXT

A study of current diffusion in the TEXT device has been performed. The plasma current was ramped up from steady state conditions (150 kA) on a time-scale (20 ms) much faster than the classical skin time (100 ms), with ∂l p /∂t ≅ 8 MA·s −1 . A stable discharge was obtained and held at the resulting higher current (300 kA). Density (4 × 10 19 m −3 ) and position were held constant during current ramping. The profiles derived from Thomson scattering and the surface barrier diode soft-X-ray array were used in the solution of the poloidal magnetic field diffusion equation. The experimental data were also compared with the prediction of a three-zone model of radial particle and energy transport. No enhanced or anomalous resistivity is required to adequately match the data for the case where the current was ramped from a steady state condition.