A study of the physics of pellet injection in magnetically confined plasmas in stellarators

Abstract

Plasma core fuelling is a key issue for the development of steady-state scenarios in large magnetically-confined fusion devices. This is of particular importance for helical-type machines, due to hollow density profiles predicted by neoclassical theory for on-axis microwave-heated plasmas. At present, cryogenic pellet injection is the most promising technique for efficient fuelling. However, further experimental and theoretical studies are necessary to fully understand all the mechanisms involved in pellet ablation and in the subsequent particle deposition, since a complete understanding of experimental results from non-axisymmetric devices remains outstanding. In this work, pellet ablation and fuelling efficiency experiments, using a pipe-gun type cryogenic pellet injector, are carried out in electron cyclotron resonance (ECRH) and neutral beam injection (NBI) heated hydrogen plasmas of the stellarator TJ-II. Here, all injections are made from the outer plasma side (inner plasma side injections are not possible for technical reasons). Ablation profiles are reconstructed from light emitted by the cloud that surrounds an ablating solid hydrogen pellet and collected by silicon photodiodes and a fast-frame camera system, under the assumptions that such emissions are loosely related to the ablation rate and that pellet radial acceleration in the plasma is negligible. Light emissions are also used to study the pellet penetration dependence on pellet and plasma parameters, such as pellet velocity, pellet mass, and plasma density for pellet injections from the outer plasma side of TJ-II. Pellet penetration in TJ-II, as in other magnetically confined plasma devices, increases with increasing pellet mass and velocity as well as for high plasma density and low temperature. However, if suprathermal electrons are present in the plasma core, they can limit pellet penetration due a sudden excess of ablation. In addition, pellet dynamics inside the plasma are analysed employing fast-camera images. Pellet radial acceleration is found to be zero or negligible. In addition, it is found that pellet injected into unbalance NBI-heated plasmas are deflected toroidally and poloidally. Furthermore, the drift direction and magnitude of the ionized fraction of the cloud, or plasmoid, is investigated using this fast-camera system. Plasmoids drifting, at between 0.5 and 20 km/s, towards the outer and lower plasma edge are observed. However, when pellets penetrate beyond the magnetic axis, plasmoids seem to drift towards the plasma centre. A dependence between plasmoid drift and plasmoid detachment position, related to rational surfaces, is observed. Also, pellet particle deposition profiles and fuelling efficiency are determined using pre- and post-injection density profiles provided by a Thomson Scattering (TS) system. Moreover, the influence of plasma heating methods on pellet ablation and material deposition is considered. Efficiency is found to depend significantly on pellet penetration depth. This is especially noted for NBI plasmas, since pellets penetrate beyond the plasma axis. In order to attain a deeper understanding of pellet injection physics in the TJ-II, experimental results are compared with theoretical predictions. In first instance, a neutral gas shielding-based code is adapted for TJ-II to compare experimental ablation rates for pellets injected into both ECRH and NBI-heated plasmas with simulated rates. Although penetration depths are well predicted by this model, ablation profiles only agree with experimental results for injections into ECRH plasmas. In addition, the Hydrogen Pellet Injection (HPI2) code, in its stellarator version, is used to simulate pellet injections into ECRH plasmas in TJ-II. With this code, using TS electron density and temperature profiles as input, ablation and material deposition predictions are compared with experimental measurements. Good agreement between experiment and simulations for pellet injections in TJ-II (ECRH) is obtained, except when suprathermal electrons are present in the plasma core. This agreement gives confidence in codes for stellarators, allowing predictions to be made with some sureness for the large W7-X device. The HPI2 code is then used to predict ablation and deposition profiles for pellets injected into relevant ECRH plasma scenarios in the stellarator W7-X, in particular corresponding to the second part of its initial operational phase, OP 1.2. Furthermore, comparisons with preliminary experimental results from OP 1.2 are presented. Predicted density profiles cannot reproduced experimental results, this being mainly attributed to the presence of suprathermal electrons. Finally, the HPI2 code is also used to simulate ablation and deposition profiles for pellets of different sizes and velocities injected into future relevant W7-X plasma scenarios, while estimating the plasmoid drift and the fuelling efficiency of injections made from two W7-X ports. These simulations allow identifying an advantageous port for efficient pellet injections into W7-X. The thesis presented here is divided into five chapters; of these, experimental and simulated results reported in Chapters 4 and 5 partially coincide with the main published contributions derived from this work [1–4].