Threshold conditions for transitions to I-mode and H-mode with unfavourable ion grad B drift direction

Transitions from the L-mode regime to the I-mode regime, with an energy transport barrier, and to the H-mode regime with both an energy and particle transport barrier are studied on the Alcator C-Mod tokamak. Steady I-mode plasmas have been produced over a wide range of plasma field (3-6T), current (0.8-1.35MA), density and shaping in the unfavourable ion B×B configuration. The power threshold for the L-I transition is higher than scalings for the L-H transition with favourable drift, and increases with plasma current as well as density. Threshold conditions for the I-H transition are more variable. In some conditions I-mode is maintained up to the maximum available ICRF power of 5MW, nearly a factor of two above the L-I threshold, giving a robust operating window. Edge T _e at the L-I transition is in the range 250-450eV, over a range of current and density, about a factor of two higher than with favourable drift, while at the I-H transition it can be much higher (up to 1.1keV) but is again widely variable. Heat pulses due to sawteeth may play a role in transitions. Controlled I-L back transitions indicate little power hysteresis.