Pellet – Plasma Interaction: an Analysis of Pellet Injection Experiments by Means of a Multi‐Dimensional MHD Pellet Code

Abstract

This work is aimed at the analysis of pellet injection experiments by means of a magnetohydrodynamic code that computes the processes of pellet ablation, expansion, ionization, and magnetic confinement of the ablated substance. In particular, experimental pellet injection scenarios stemming from the tokamak ASDEX-Upgrade (D2 pellets) and the stellarator W7-AS (carbon pellets) are analyzed by means of this time-dependent quasithree-dimensional MHD pellet code. Pellet penetration depths measured in ASDEX-Upgrade at LFS (lowfield-side) and HFS (high-field-side) pellet injection, are compared with computational results.It was found that the penetration depths of deuterium pellets observed in about twenty randomly selected LFS and HFS injection scenarios could accurately be reproduced by a single code in which drift effects were intentionally ignored. This may be understood as an indication of the negligible effect drift exerts on the ablation process. At the same time, the distribution of the ablated and partially ionized substance may very well be affected by the drift of the ionized particles in the direction of the decreasing magnetic field.With the help of the Dα radiation patterns observed, the magnitudes of the heat fluxes affecting the pellet and the ablation cloud were estimated. The calculations show that the thermal heat fluxes (electron and ion) acting along the magnetic field lines are not sufficient to explain the size of the radiating filaments observed.The effect of extra-thermal electrons on the ablation history of pellets injected in W7-AS is demonstrated: the particle end energy fluxes causing measured pellet ablation profiles are determined.Based on a statistically well-designed computational data set, an empirical scaling of the penetration depths in terms of the background plasma parameters was attempted. It is shown that the form of the temperature profile of the background plasma cannot be ignored while deriving scaling laws; the specification of the central or the bulk temperature alone is insufficient.Quantitative data are provided for the magnitudes of various additional processes such as the anomalous thermal conduction across the field lines or the grad(B)-induced drift of the shielding cloud surrounding the pellet.The modelling of supersonic gas jets, a fuelling method that appears to be an alternative to pellet injection, is also considered. Since the massive stream of a cold gas represents a disturbance for the recipient plasma similar to that of a pellet, its dynamics can be described by the same means as the evolution of pellet clouds.

Recent results on the fuelling and control of plasmas by pellet injection, application to ITER

“…that the spherically and longitudinally expanding parts of the cloud evolve independently, the consequence of which being that most of the shielding is due to the neutral cloud, the ionized sheath acting only as a correction [7]. Practically, the penetration depths calculated with these different models are within ∼20% and reproduce reasonably well the measurements in present day machines for a wide range of pellet and plasma parameters [6,8]. Finally, 2D-time dependent MHD models are presently being developed by several authors [9,10], which do not consider a priori a total decoupling between the spherically and longitudinally expanding parts of the cloud.…”

“…Also, characterizing the smallest required perturbation is indispensable to extrapolate for future experiments. That is why specific investigations were carried out in AUG and JET on the spatio-temporal structure and dynamics of ELMs in type-I ELM H mode plasmas 8 . This was done from the analysis of the time delay between the timing when the pellet crosses the last closed flux surface and the timing of the ELM.…”

Simulations of plasma behavior during pellet injection in ITER

“…Both the 1 The article is published in the original. 0 0 P nT α ∝ oretical and experimental works on pellet ablation and penetration into the tokamak plasma, as well as other magnetic confinement devices, has been intensively explored [9][10][11][12][13][14][15][16]. An excellent review of pellet injec tion studies can be found in [11].…”

Pellet source density in toroidal plasma configurations based on a 2D Gaussian deposition model

“…More sophisticated ablation models consider the extra shielding of this cold plasmoid, electrostatic shielding due to the sheath built up at the interface of the plasmoid and the plasma, magnetic shielding due to the diamagnetism of the plasmoid [21,27]. In addition, the more complete and sophisticated ablation models also take into account one or more of following physical factors or processes, i.e., Maxwellian energy distribution of background electrons, the presence of high energy particles in plasma, the influence of inhomogeneous heat flux from plasma and inhomogeneous heating of the plasmoid, atomic physics processes in the plasmoid, the possible deformation of the pellet due to ablation asymmetry, etc [27][28][29][30][31][32].…”