Interaction of plasma rotation and resonant magnetic perturbation fields in tokamaks

Abstract

The interaction between plasma rotation and perturbation fields is described by the ambipolarity constraint and the parallel momentum balance, both emanating from the revisited neoclassical theory, and the electrodynamical screening of the resonant perturbation field at the singular surfaces. This screening depends mainly on the slip between the rotating plasma and the resonant field. The neoclassical theory, valid in the collision dominated regime and accounting for gyro-viscosity, includes arbitrary plasma cross-sections, anomalous viscosity, ponderomotive forces, neutral beam injection (NBI), pressure anisotropization and a momentum source due to ergodicity which has a considerable impact on the plasma rotation as demonstrated in TEXTOR.To estimate the influence of the perturbation coils on the plasma rotation, the radial magnetic field (proportional to the helical flux function) is Fourier analysed (using 'intrinsic' coordinates) and the total field is used for field line tracing thus obtaining the ponderomotive momentum input and the extension e of the ergodic layer at the edge. Both procedures account for the full plasma geometry. e is assumed to be independent of the rotational state because of the boundary condition V t = 0. In a second step the obtained velocity profiles are used to compute the screening at the singular layers and thus the reduction of the island width due to plasma rotation.The main results can be summarized as follows.Using in the case of TEXTOR shot #94092 the diffusion coefficient D M = 2 × 10 −6 m (typical for the 12/4 configuration) the observed increase in v t by v t ≈ 5 km s −1 can be reproduced. Inside the plasma the slip prevents any influence of the ponderomotive forces, thus yielding a constant increase in the v t (r)-profile by v t .Assuming in the case of the error field correction coils (n = 1) of JET the current I hel = 30 kA and using for the plasma background the data of shot #67951 in the static case, an ergodized layer ( e (n = 1) ≈ 20 cm in the vicinity of the unperturbed x-point) and large m = 2, m = 3 (n = 1) islands (W m=2,n=1 = 10 cm) are obtained. In the n = 2 configuration the analogous parameters are e (n = 2) ≈ 18 cm and W m=2,n=2 = 4 cm i.e. e stays roughly the same and the island width is strongly reduced thus indicating the superiority of this configuration. Plasma rotation (v t max = 180 km s −1 ) reduces the width W m=2,n=1 to a small value. (However, tearing mode physics which may lead to mode locking is not included in this consideration.

Aspects of three dimensional transport for ELM control experiments in ITER-similar shape plasmas at low collisionality in DIII-D

“…Exact specification of the extent of these zones in the perturbed edge layer by application of screening models and/or field amplification is strongly motivated by these experimental results and will be presented in future work. However, as this analysis was largely motivated from TEXTOR results and initial attempt was made modeling screening coefficients due to rotational screening in a visco-resistive drift-MHD model as described in references [50,51]. This allows to compare the plasmas on both experiments discussed in this paper based on the field penetration characteristics expected from this approach.…”

Modification of the edge transport barrier by resonant magnetic perturbations

“…This equation system has been invoked to interpret experiments on Tuman-3M [16]. A similar approach was used in [17] to analyse a spin up of the toroidal rotation with account of screening effects. The radial ion particle flux in the presence of RMP consists of two parts:…”

Effects of ELM mitigation coils on energetic particle confinement in ITER steady-state operation

“…For the guiding-centre equation of motion, three different types of non-axisymmetric vacuum magnetic fields, the toroidal field ripple including the effect of ferritic insertions, the error field from the test blanket modules (TBMs) and the magnetic perturbation field from the in-vessel ELM coils, are simply superposed on the axisymmetric MHD equilibrium field. Recently, some theoretical works have suggested that the vacuum ELM mitigation field, hereafter simply referred to as the EMC field, might be shielded by plasma rotation [12][13][14]. However, in the DIII-D experiments, they found a correlation between the reduction in the ELM size and the island overlap of the vacuum EMC field in the edge region [15].…”