Plasma autobiasing during Ohmic H‐mode in the STOR‐M tokamak

Application of a short current pulse on a nominal Ohmic discharge in the STOR‐M tokamak (Saskatchewan Torus‐Modified) [Phys. Fluids B 4, 3277 (1992)] triggers the Ohmic H‐mode characterized by reduced Hα radiation, increased electron density, and reduced edge density/magnetic fluctuations. Measurements of plasma floating potential at the plasma edge and in the scrape‐off layer indicate that the Ohmic H‐mode is accompanied by negative plasma autobiasing, which leads to a steeper radial electric field profile at the edge. Since the duration of the current pulse (≤20 kA, 100 μsec) is shorter than the resistive skin time (≂1 msec), preferential edge heating is expected, which is believed to be responsible for changes in the edge discharge condition favorable for inducing the Ohmic H‐mode. The electron density profile becomes steeper at the edge during the H‐mode, and clear formation of a density pedestal has been seen. The evolution of the density profile suggests the presence of particle pinch. An improved c...