Edge ion parameters at the L–H transition on JET

Abstract

L–H transition experiments have recently been performed on JET to characterize the local edge ion parameters. Evidence of a threshold in the edge ion temperature to access the H-mode is presented and comparisons are made with edge electron temperature. These results provide the first scaling of pedestal ion temperature on JET. The threshold edge ion temperature scales approximately linearly with toroidal magnetic field and shows no dependence on electron density in the range explored. Reference shots have been repeated with the ion ∇ B drift away from the X-point and counter neutral beam injection. The L–H transition is also considered in terms of dimensionless parameter space. The effective edge ion collisionality does not appear to play a significant role in the L–H transition, providing no clear distinction between the L- and H-modes. In contrast, the normalized poloidal ion gyroradius provides a sharp H-mode boundary at any given field and current, consistent with a threshold ion temperature for H-mode access being dependent on toroidal field but not on density. However, comparison of ρ* and with forward and reversed fields across a range of fields and currents demonstrates that these dimensionless parameters simply describe an operation trajectory for the JET L–H transition.