A self-consistent MHD ablation model: pellet penetration depth prediction for a reactor-temperature plasma

Abstract

A time dependent quasi-two-dimensional ablation model has been developed. The ablation of a pellet injected into a magnetically confined plasma and the temporal variation of the properties of the cloud surrounding it are calculated in a self-consistent manner. The energy deposition in the shielding cloud is determined by stopping length calculations applied along the magnetic field lines with allowance for electrostatic shielding effects. The stopping length calculations are supplemented by thermal diffusion calculations, thus redistributing the energy deposited in the discrete energy group approximation. The neutral cloud is allowed to expand also in the direction perpendicular to the magnetic field. Finite rate equations are used for determining the time history of the ionization state of the ablated substance. The deceleration and full stopping of the cross-field motion is calculated by means of an MHD model (iteratively with the axial expansion dynamics), thus determining the transient variation of the lateral cloud dimension and the corresponding modifications of the field aligned density and temperature distributions used in the stopping length calculations. The expulsion of the magnetic field lines by the expanding and ionized ablatant and their rediffusion into the partial magnetic cavity thus formed are also calculated. The variation of the ablation rate along the pellet path (i.e. that of its value averaged over the residence time of a pellet of given injection velocity in a flux tube defined by the local ionization radius of the ablatant) are calculated at a sequence of flux surfaces, thus determining the particle deposition profile. Results of penetration depth calculations are compared with values stemming from experiment and from other ablation models. Good correspondence is found at moderate plasma temperatures. Predictive calculations performed for a reactor-grade plasma yield, owing to magnetic shielding effects, substantially larger penetration depths than those obtained with the standard NGS a