Gyrokinetic simulations including the centrifugal force in a rotating tokamak plasma

F.J. Casson, A.G. Peeters, C. Angioni, Y. Camenen, W.A. Hornsby, A.P. Snodin, and G. Szepesi

Physics of Plasmas, 17, 102305 (2010)

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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 A.G. Peeters et al. Physics of Plasmas 16, 042310, (2009). These inertial terms become important in the case of strong rotation, as is commonin 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, destabilising 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, whilst 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 stabilisation. 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. ©2010 American Institute of Physics

The figure shows the magnitude of perturbed electron distribution in velocity space for the GKW-TEM case at the outboard midplane with increasing rotation. Arbitrary units and contours are equally spaced at same intervals for each plot. The trapping boundary is overplotted as a dashed line.