The interpretation of magnetic activity associated with pellet injections into plasmas created in the stellarator TJ-II

Abstract

Pellet injection into magnetic confinement plasma is widely employed for plasma fuelling and impurity transport studies. While post-injection effects include changes in density and temperature, several short-lived effects have been identified during and immediately after pellet ablation, e.g. pre-cooling of the plasma core or a strong reduction in fluctuations. Here, transitory effects observed in magnetic signals during and immediately after the injection of fuel and tracer encapsulated solid pellets into microwave and neutral beam heated plasma in the stellarator TJ-II are reported. One effect is the excitation of a clear, short-lived, magnetic instability during the ablation phase of a cryogenic pellet. Another effect is a brief reduction in broadband magnetic fluctuations immediately after an injection while a third effect is frequency jumps of already excited Alfvén eigenmodes (AE) when neutral beam heating is employed. The interpretation of these effects is facilitated by signals from 2 recently installed toroidal arrays of magnetic coils that follow the helicity of this device along one of its quadrants. It is concluded that interactions between outward drifting pellet particles and a low-order rational surface lead to the magnetic instability, that pellet-induced plasma cooling induces a transitory reduction in broadband magnetic fluctuations and that plasma mass density changes dominate jumps in AE frequencies.

Using rational surfaces to improve pellet fuelling in stellarators

“…This is in agreement with a previous study on magnetic activity during pellet injection into TJ-II (McCarthy et al. 2021). Starting from the right, the second and third points show a relatively smaller drift.…”

“…These drift reductions could be accounted for by the effect of the rational surfaces in the vicinity of these reductions (such as the 8/5, the 11/7 and/or the 14/9), since drift damping due to the ECC mechanism is enhanced by the rational surfaces. This is in agreement with a previous study on magnetic activity during pellet injection into TJ-II (McCarthy et al 2021). Starting from the right, the second and third points show a relatively smaller drift.…”

“…Here, it is observed that, similarly to observations reported by McCarthy et al. (2021), drifting plasmoids show negligible interaction with rational surfaces when they have detached from the pellet at ρ ≥ ~0.45. However, plasmoids originated in the range of ~0.7 ≤ ρ ≤ ~0.75 are an exception, as their drift is reduced by the effect of the 8/5 rational surface.…”

“…The drift-damping produced by external currents is the dominant effect in tokamaks, whereas internal currents are dominant in helical configurations due to their shorter plasmoid-internal charge reconnection lengths (Matsuyama et al 2012a). However, a previous study in TJ-II revealed an interaction between outward-drifting plasmoids and low-order rational surfaces that can be attributed to the ECC effect (McCarthy et al 2021). Therefore, plasmoid deceleration by low-order rational surfaces cannot always be considered a minor effect in stellarators.…”

“…However, a previous study in TJ-II revealed an interaction between outward-drifting plasmoids and low-order rational surfaces that can be attributed to the ECC effect (McCarthy et al. 2021). Therefore, plasmoid deceleration by low-order rational surfaces cannot always be considered a minor effect in stellarators.…”