Experimental test of the neoclassical theory of impurity poloidal rotation in tokamaks

Abstract

Despite the importance of rotation in fusion plasmas, our present understanding of momentum transport is inadequate. The lack of understanding is in part related to the difficulty of performing accurate rotation measurements, especially for poloidal rotation. Recently, measurements of poloidal rotation for impurity ions (Z>1) have been obtained in the core of DIII-D [J. L. Luxon, Nucl. Fusion 42, 6114 (2002)] plasmas using charge exchange recombination spectroscopy. The inferred poloidal rotation is based on careful consideration of the effective energy-dependent cross section and of the gyromotion of the ions. The rotation measurements are found to be consistent with the radial electric field determined independently from multiple impurity species as well as from motional Stark effect spectroscopic measurements. The poloidal rotation measurements have been compared with predictions based on the neoclassical theory of poloidal rotation from the code NCLASS [W. A. Houlberg et al., Phys. Plasmas 4, 3230 (1997)]. The comparison shows that the neoclassically predicted poloidal rotation is in general significantly smaller than the actual measurements.

Poloidal rotation and its relation to the potential vorticity flux

“…[5][6][7] Furthermore, recent experiments at DIII-D indicate that deviations in the magnitude as well as direction of the poloidal rotation of impurity ions from that predicted by neoclassical theory can be present throughout the plasma volume. 8 Within the above studies, measured poloidal flows have been observed to deviate from neoclassical predictions by as much as an order of magnitude. These substantial deviations of poloidal flows from neoclassical predictions are suggestive of the critical role in which turbulent stresses are likely playing in the generation of mean poloidal flows.…”

Residual parallel Reynolds stress due to turbulence intensity gradient in tokamak plasmas

“…While we use this basic picture to justify using the parallel Reynolds stress to represent the toroidal one, it should be noted that there are recent indications that poloidal rotation may also be anomalous 40,41 or at least radially varying deviations from an average neoclassical poloidal rotation should be expected. 42 The remainder of this paper is organized as follows.…”

Collisionality scaling of main-ion toroidal and poloidal rotation in low torque DIII-D plasmas

“…In many cases, aside from strong core transport barriers, the neoclassical prediction of impurity poloidal rotation is quite small and near the resolution level of the diagnostic designed for the measurement. Early accounts of poloidal rotation contained zero within the error bars of the measurements [21], but through sophisticated atomic calculations [22,23] and novel diagnostic viewing configurations [24,25,26] the measurements uncertainties have been reduced to such levels that the poloidal flow theory may be tested.…”