Dependence of the H-mode power threshold on toroidal plasma rotation in the DIII-D tokamak

Abstract

The power required to induce the transition from L-mode to H-mode plasmas (L-H transition) is dependent on the applied beam torque and the edge toroidal plasma rotation. For upper single null discharges in which the ion grad-B drift is away from the X-point, the L-H transition power threshold is reduced by nearly a factor of 3 by changing from predominantly co-injection (∼6 MW) to predominantly counter-injection (∼2.2 MW). A similar, but less prominent dependence is observed in lower single null discharges, in which the ion grad-B drift is towards the X-point, where the power threshold is reduced by nearly a factor of 2. Over a similar torque range of 2.0 Nm to -0.5 Nm, the USN discharges exhibit a reduction in threshold power of over 50% and the LSN cases show a power reduction of about 30%. The threshold power decreases with decreasing edge toroidal rotation and, at low edge toroidal rotation, large changes in the shear of the poloidal velocity of the edge turbulent eddies are observed prior to the L-H transition that may be strong enough to induce the transition. For the first time, the L-H transition has been induced at constant input power below the nominal threshold power by reducing the input torque from all co-beams to balanced beams. The variation in threshold power with applied torque is not found to be dependent on the ion orbit losses in the plasma. © 2008 IOP Publishing Ltd.