H-mode access and the role of spectral shift with electrode biasing in the TCABR tokamak

The transition from the ohmic heating L-mode to the H-mode in the tokamak TCABR induced by an external electrode biasing is addressed in this work. A gradual improvement in the plasma confinement with the biasing voltage was observed as a result of the enhancement of the E × B shear flow at the edge. This result was supported by the measurement of plasma parameters in different radial locations by two electrostatic probes. Radial profiles of the electron temperature (from the standard sweeping voltage technique), electron density, and radial electric field, as well as the local turbulence features, were measured. We observed that the electron density profile became steeper as the voltage on the electrode increased, while no significant change in the temperature profile was noticed. In addition, the data were compared with the spectral shift model [G. M. Staebler et al., Phys. Rev. Lett. 110, 055003 (2013)] at different bias voltages, i.e., different E × B shear flows, showing a good agreement. Particularly, the gradual improvement was held: the higher the biasing voltage, the larger the radial wavenumber shift and so the turbulence suppression. Moreover, by studying the edge radial profile of the mean radial wavenumber, we show that the large shift occurs in the same position where the transport barrier is created.