Role of the radial electric field in the confinement of energetic ions in the Wendelstein 7-X stellarator

TL;DR

This study uses numerical simulations to analyze how the radial electric field influences energetic ion confinement in the Wendelstein 7-X stellarator, confirming its effect is similar to that of plasma beta and identifying a scenario for experimental validation.

Contribution

It provides the first numerical analysis of the radial electric field's impact on fast-ion confinement in W7-X, aiding experimental validation of magnetic configuration optimization.

Findings

Radial electric field effect on fast-ion losses is similar to plasma beta.

Simulations confirm the impact of electric field on confinement.

A viable scenario for experimental validation is proposed.

Abstract

Good fast-ion confinement is an essential requirement for a fusion reactor. The magnetic configuration of the Wendelstein 7-X (W7-X) stellarator is partially optimized in this regard in a reactor-relevant scenario: it is expected to show improved fast-ion confinement when $\beta$ is high and the effect of the radial electric field is negligible. The experimental validation of this optimization is difficult since, with the available power, achieving high $\beta$ under appropriate conditions for the validation is challenging and the effect of the radial electric field is inevitable. In this work, the confinement of fast ions in W7-X has been studied numerically for a variety of scenarios via the ASCOT5 code. The effect of the radial electric field on fast-ion losses is confirmed to be equivalent to the one produced by $\beta$, and this is characterized by means of scans on both parameters. Through a preliminary study with experimentally-based profiles, a viable scenario is identified that takes advantage of this effect for the experimental validation of the optimization strategy of W7-X.