Spontaneous rotation and momentum transport in tokamak plasmas

Abstract

Recently there has been widespread attention paid to rotation and momentum transport in tokamak plasmas. Of particular interest is spontaneous (intrinsic) toroidal rotation in plasmas without external momentum input. The strong co-current spontaneous rotation in enhanced confinement regimes, with ion thermal Mach numbers up to 0.3, may allow for resistive wall mode suppression in high-pressure ITER discharges, without requiring the use of neutral beam injection. Spontaneous rotation in L-mode discharges exhibits a complex dependence on plasma parameters and magnetic configuration compared to the relatively simple scaling of Alfven Mach number (MA = VΦ/CA, where CA is the Alfven speed) MA ∼ βN observed in enhanced confinement plasmas. There is currently no comprehensive, quantitative explanation of this phenomenon. An accurate prediction of the expected rotation velocity profile from whatever neutral beam injection is available on ITER requires a detailed understanding of momentum transport. There have been extensive investigations into correlations between energy and momentum diffusivities, and whether there are systematic trends of the Prandtl number with plasma parameters. Of late, there has been vigorous theoretical activity regarding a possible momentum pinch that could help enhance the rotation in the plasma interior. There has been a renewed interest in poloidal rotation, especially in ITB discharges, which is generally found to be at odds with the predictions of neo-classical theory. This calls into question the common practice of the determination of Er from toroidal rotation measurements with the assumption of neo-classical poloidal rotation.