Effect of non-axisymmetric perturbations on the ambipolar E _r and neoclassical particle flux inside the ITER pedestal region

Abstract

The transport dynamics of impurities in the pedestal region of ITER plasmas is of crucial interest since this regulates the penetration of impurities from the edge into the core plasma, where an excessive accumulation of impurities can degrade their fusion performance. In the pedestal region of H-mode tokamak plasmas, anomalous transport is highly reduced and impurity transport is found to be well described by the neoclassical theory. Under these conditions, perturbations to the axisymmetric tokamak geometry can strongly affect both the radial electric field and particle transport. In this work, we describe the results of numerical studies performed to quantify the effects on the pedestal ambipolar electric field and radial particle fluxes of the non-axisymmetric fields, associated with both the intrinsic toroidal field ripple and extrinsic fields applied for ELM control, for ITER Q = 10 plasma conditions with emphasis on high Z impurity transport. It is found that the effect of the ITER toroidal field ripple on high Z impurity transport is negligible. On the contrary, extrinsic three-dimensional fields applied for ELM control cause a strong modification of the pedestal ambipolar electric (to less negative values) and the appearance of multi-valued solutions for the pedestal electric field, analogue to core stellarator transport significantly increasing the outward character of neoclassical pedestal transport for both the main plasma ions (D and T in ITER) and high Z impurities, suggesting a strong modification of the background plasma profiles. Finally, it is found that for the Z impurity, its quantitative evaluation has uncertainties (with important implications for the radial flow direction) associated with the high poloidal Mach number ~ 1, due to the high pedestal electric field and large ion mass, which renders the first order neoclassical theory questionable. Detailed ITER MHD 3D equilibria and a benchmark of SFINCS against NEO and NCLASS codes has also been obtained.

FLIPEC, an ideal MHD free-boundary axisymmetric equilibrium solver in the presence of macroscopic flows

“…In order to validate the implementation of curvilinear coordinates in FLIPEC, we first reproduce the same ITER equilibria that were obtained with the first version of the code using a circular control contour [17], but using an ellipse instead. The case investigated corresponds to the ITER baseline configuration, with a plasma toroidal current I p = 15 MA and toroidal field B ϕ ≃ 5.3 T (for more details see [25]). The ITER magnetic field is created by 18 toroidal coils, 6 poloidal coils and 6 central solenoid modules.…”

“…The contribution from the coils does not change once the coil currents have been fixed, so it is evaluated only at the start of the calculation. The MAKEGRID code, part of the STELLOPT stellarator optimization suite [19], is used for this task [25]. It provides the vacuum magnetic field from which the associated poloidal magnetic streamfunction is obtained.…”

Statistical description of collisionlessα-particle transport in cases of broken symmetry: from ITER to quasi-toroidally symmetric stellarators

“…For the toroidally symmetric configuration the natural decision was to consider a ripple-less ITER [19] tokamak with the following parameters: B ~5.3 T, a = 2.67 m, R = 6.2 m and V ~900 m 3 . The four quasi-toroidally symmetric configurations are loosely based on the NCSX [20,21] stellarator scaled up to have the same ITER nominal magnetic field and volume, which results in a minor and major radius of a = 2.15 m and R = 9.8 m respectively.…”