The impact of fast electrons on pellet injection in the stellarator TJ-II

Abstract

Efficient plasma core fuelling is a critical issue for achieving steady-state scenarios in magnetically confined fusion devices. The current preferred method to achieve this goal is cryogenic pellet injection (PI). PI's are now installed in most medium-and large-sized fusion devices. In this paper, recent results from PI experiments in the stellarator TJ-II are reported. Of particular interest is the influence of fast electrons. Whilst edge populations of such electrons do not affect PI in the TJ-II, fast electrons residing in its hot core have significant influence on both pellet ablation and fuelling efficiency. If present in the core, their radial location can be determined from the light emission profile produced by an ablating pellet as it traverses the plasma and confirmed from the power spectral distribution spectrograph of the total secondary beam current collected by a scanning heavy ion beam probe system. Moreover, it is found that if a pellet is subject to excess ablation due to core fast electron impacts, the resultant pellet efficiency (deposited particles/delivered particles) increases by up to 50% with respect to injections into similar, fast-electron free, plasmas. This is consistently found for microwaveheated plasmas, where pellet penetration is shallower than the magnetic axis in TJ-II, when such a population is present, as well as for neutral beam created and heated plasmas in which fast-electrons, generated during magnetic field ramp-up, persist in the core along a discharge. Similarly, when polystyrene pellets (TESPEL) are injected into the same machine sector, an analogous increase in post-injection particle deposition is seen. It is postulated that vaporization of the pellet through heating by fast electrons modifies normal neutral cloud and plasmoid development and hence affects outwards drifting that is inherent to magnetic fusion devices.

Overview of recent TJ-II stellarator results

“…However, when a fast-electron population exists in the core when an injection is made (i.e. inside ρ = 0.4), then pellet penetration is truncated and fuelling increase by up to 50% [56]. Simulations of post-injection electron density profiles made with the TJ-II HPI2 code are in good agreement with Thomson scattering density profiles for the former case.…”

“…With the expansion of the TJ-II pellet database it has been observed that pellet fuelling efficiency can benefit from the presence of a population of fast electrons in its plasma core [56]. In fact, for ECRH plasmas, it has been determined that when no fast electrons are detected (by hard x-ray emissions or by excess pellet ablation), this efficiency is typically well below 30%.…”

High-density experiments with hydrogen ice pellet injection and analysis of pellet penetration depth in Heliotron J

“…Thus, the effect of fast ions on pellet ablation is regarded as insignificant. On the other hand, over-ablation by fast electrons has been reported in the TJ-II [35], where the plasma and heating parameters are similar to those in the Heliotron J. In future studies, we should measure the fast electron energy distribution using a pulse height analyzer and utilize a new ablation code that incorporates the effect of fast electrons on pellet ablation.…”

Overview of the TJ-II stellarator research programme towards model validation in fusion plasmas

“…Indeed, tracer injections with polystyrene (-C 8 H 8 -) based TESPELs have confirmed an inverse mass dependence of the acceleration [16]. Additional experiments have also revealed that enhanced pellet ablation in the plasma core due to fast-electron impacts can lead to higher fuelling efficiencies as the outward drift acceleration is disrupted in the initial drift phase [17]. In particular, it is found that if a pellet is subject to excess ablation due to core fast electron impacts, the resultant pellet efficiency (deposited particles/delivered particles) increases by up to 50% with respect to injections into similar, fast-electron free, plasmas.…”