Increasing the density in Wendelstein 7-X: benefits and limitations

Abstract

In stellarators, increasing the density is beneficial for the energy confinement. While there is no single reason for this observation, it is still very robust across different devices and this is reflected in the empirical energy confinement time scaling for stellarators, ISS04. In order to study whether this is also true for Wendelstein 7-X, the density scaling of the energy confinement time is analyzed and compared to ISS04 for the first divertor experiments. When the density is increased beyond a critical density, however, radiative collapses are frequently observed. Existing analytical models for the critical density are revisited to assess whether they can predict the accessible density range. Furthermore, since close to the collapse the radiation losses increase substantially, the impact on the global energy confinement is investigated. It is found that in plasmas with high radiation the density scaling of the energy confinement time becomes weaker, the reason for this observation is not yet clear. In the second half of the first divertor campaign, boronization was applied to W7-X for the first time. This broadened the operational window, allowing for operation at higher density and, hence, higher stored energy.

Plasma termination by excess pellet fueling and impurity injection in TJ-II, the Large Helical Device and Wendelstein 7-X

“…A broader study of plasma termination, however, is lacking for helical devices and this paper is intended to provide an initial overview of unintended plasma termination in the large helical device (LHD), Wendelstein 7-X (W7-X) and TJ-II. Radiative collapses frequently observed in discharges with highly-radiating edge and scrape-off layer (SOL) regions, however, are not covered in this study but will be reported elsewhere in the context of radiative density limits [1]. While disruptions are not expected in (currentless) stellarator/heliotron operation, thermal quenches are certainly to be illuminated for reactor scale stellarators and heliotrons as well.…”

Plasma–surface interaction in the stellarator W7-X: conclusions drawn from operation with graphite plasma-facing components

“…Most critical from perspective of radiation-induced density limits in OP1.2A [6] is the presence of O and C in the plasma edge leading to an effective charge Z eff of up to 3.5 and causing a highly radiative edge mantle. A radiative collapse of the plasma occurred before high core density operation was achieved.…”

“…Access to the stellarator-favourable high density operational regime [6] was only possible owing to boronisations in OP1.2B, thus, glow discharges in a 10%B 2 H 6 + 90%He mixture, which deposits a thin B layer on the top surfaces of the first wall and divertor PFCs and getters O. Moreover, the surface coverage also prevents further transient release of water from the graphite plasma-facing sides.…”

“…Main difference from perspective of PSI between OP1.2A and OP1.B is the application of boronisation in order to improve the wall conditions and provide access to operation at high core density around n c e = 1 × 10 20 m −3 [6]. In view of upcoming steady-state operation, these PSI processes will determine the lifetime of divertor PFCs, the fuel cycle and plasma control, as well as the C dust production.…”

Plasma radiation behavior approaching high-radiation scenarios in W7-X

“…In W7-X, the impurity radiation usually limits the maximum achievable plasma density [16,43]. In the limiter configuration, the radiation zones are ∼10 cm inside the confined plasma region and almost all of the radiated power comes from the confined plasma region (>90%).…”