Role of the radial electric field in the transition from L (low) mode to H (high) mode to VH (very high) mode in the DIII-D tokamak*

Abstract

The hypothesis of stabilization of turbulence by shear in the E×B drift speed successfully predicts the observed turbulence reduction and confinement improvement seen at the L (low)–H (high) transition; in addition, the observed levels of E×B shear significantly exceed the value theoretically required to stabilize turbulence. Furthermore, this same hypothesis is the best explanation to date for the further confinement improvement seen in the plasma core when the plasma goes from the H mode to the VH (very high) mode. Consequently, the most fundamental question for H-mode studies now is: How is the electric field Er formed? The radial force balance equation relates Er to the main ion pressure gradient ∇Pi, poloidal rotation vθi, and toroidal rotation vφi. In the plasma edge, observations show ∇Pi and vθi are the important terms at the L–H transition, with ∇Pi being the dominant, negative term throughout most of the H mode. In the plasma core, Er is primarily related to vφi. There is a clear temporal and spatial correlation between the change in E×B shear and the region of local confinement improvement when the plasma goes from the H mode to the VH mode. Direct manipulation of the vφi and E×B shear using the drag produced by a nonaxisymmetric magnetic perturbation has produced clear changes in local transport, consistent with the E×B shear stabilization hypothesis. The implications of these results for theories of the L–H and H–VH transitions will be discussed.

Transition physics and scaling overview

“…In this section, several aspects of the terminology and experimental technique are reviewed in order to facilitate commUncation. Techniques for studying the formation of E, and its role in H-mode physics have been recently discussed ( Burrell et al 1994a, Moyer et al 1995 and will not be repeated here.…”

Fully electromagnetic nonlinear gyrokinetic equations for tokamak edge turbulence

“…[19][20][21][22] ii) After an H-mode transition, E r well is formed just inside the LCFS. The pressure gradient scale length and the radial electric field scale length in the edge E r well is on the order of the ion poloidal gyroradius, 23 …”

Neoclassical physics in full distribution function gyrokinetics

“…2,[28][29][30] 4. An increasing number of experimental observations [31][32][33][34][35] insists on the central role of plasma rotation (toroidal and poloidal) for the onset and control of enhanced confinement regimes and often report strong departures of poloidal rotation from its oft-expected neoclassical predictionpossibly due to the turbulence 2,36,37 -either before, at, or enduring past the transition to an enhanced confinement state.…”