Gyrokinetic simulations including the centrifugal force in a rotating tokamak plasma

Abstract

Tokamak experiments operate with a rotating plasma, with toroidal velocity which can be driven externally but can also arise spontaneously. In the frame that corotates with the plasma, the effects of the centrifugal force are felt through a centrifugal drift and an enhanced mirror force [Peeters et al., Phys. Plasmas 16, 042310 (2009)]. These inertial terms become important in the case of strong rotation, as is common in spherical devices, and are also important for heavy impurity ions even at small toroidal velocities. In this work, the first gyrokinetic simulations including the centrifugal force in a strongly rotating plasma are presented. The enhanced mirror force redistributes density over a flux surface and modifies the trapping condition, destabilizing trapped electron modes. At intermediate scales this can result in promotion of the trapped electron mode over the ion temperature gradient (ITG) mode as the dominant instability, which under marginal conditions could result in an enhanced electron heat flux. The centrifugal drift acts to damp the residual zonal flow of the geoacoustic mode, while its frequency is increased. For nonlinear ITG dominated turbulence, increased trapped electron drive and reduced zonal flow lead to an increase in ion heat diffusivity if the increased rotation is not accompanied by rotational shear stabilization. An increased fraction of slow trapped electrons enhances the convective particle pinch, leading to an increase in the steady state density gradient with strong rotation. Linear ITG mode results show an increased pinch of heavy trace impurities due to their strong centrifugal trapping. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3491110

Effect of poloidal asymmetry on the impurity density profile in tokamak plasmas

“…Also, the effect of rotation, which is one of the mechanisms that causes poloidal asymmetry, has not been taken into account. Fluid and gyrokinetic modeling of rotation induced contributions to the transport fluxes, 19,20 have shown that in the ITG-dominated case, the centrifugal force leads to higher inward convection for impurities ͑increasing with impurity mass͒. This agrees with the predictions of the present work, since the strength of the asymmetry increases with Z, and accumulation at the outboard side should lead to an increased inward flux.…”

Modification of impurity transport in the presence of saturated (m,n) = (1,1) MHD activity at ASDEX Upgrade

“…The constance of the impurity density on a flux surface (condition 5 ) has recently been relaxed from a theoretical point of view since a few codes can now account for poloidal asymmetries due to centrifugal effects and poloidally varying electrostatic potential which can effect the transport [4,5,6,7]. The results from the codes have been used to model the impurity photon emission in the soft X-ray spectral region and compared to the experimental profiles as detected by the soft X-ray (SXR) diagnostic [8,9].…”

The residual zonal flow in tokamak plasmas toroidally rotating at arbitrary velocity

“…15 Recent kinetic numerical simulations for rotating plasmas indicated that the residual zonal flow level decreases with increasing rotation. 16 In a recent work, we analytically investigated the effect of sonic toroidal plasma flow on residual ZFs. The form is the same as that of the R-H expression with the parallel velocity evaluated in the rotating framework.…”

“…17 However, subsonic rotation, which has not been addressed in our previous publication, is more relevant to most experimental observations than sonic rotation. A general form of residual ZFs suitable for arbitrary rotation velocity is necessary to make a quantitative comparison with existing numerical simulations 16 and to be conveniently adopted in plasma simulations.…”