[Paper Review] Role of the radial electric field in the confinement of energetic ions in the Wendelstein 7-X stellarator

M. Arranz, J. L. Velasco+2 more|arXiv (Cornell University)|Feb 13, 2026

Magnetic confinement fusion research0 citations

TL;DR

The paper numerically studies fast-ion confinement in Wendelstein 7-X and shows that the radial electric field can produce an effect equivalent to increasing beta, using ASCOT5 simulations and both academic and experimentally-based scans to guide potential validation.

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 $β$ 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 $β$ 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 $β$, 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.

Motivation & Objective

• Assess how the radial electric field impacts fast-ion confinement in W7-X in comparison with beta.
• Demonstrate that E_r can mimic beta in enhancing confinement via poloidal precession.
• Identify an experimentally viable scenario to validate the optimization strategy of W7-X.
• Propose a workflow to perform both academic and experimentally-based scans of beta and E_r to validate confinement improvements.

Proposed method

• Use ASCOT5 to perform collisional guiding-center simulations of 1H+ fast ions at 50 keV in the high-mirror W7-X configuration.
• Compute energy loss fractions and loss distributions for particles born on selected flux surfaces.
• Model academic scans with predefined beta and flat E_s profiles to separate beta and E_r effects.
• Perform experimentally-based scans by inferring E_r from ambipolarity-consistent density and temperature profiles using SFINCS.
• Analyze bounce-averaged tangential drifts to relate bcvdot{nabla}s and bcvdot{nabla}alpha via theoretical expressions (I_0, I_{<beta>}<beta>, I_{E_s}E_s).
• Compare results with quasi-isodynamic (QI) assumptions and discuss implications for reactor-relevant scenarios.

Experimental results

Research questions

• RQ1 Does increasing the poloidal precession via higher beta have the same qualitative and quantitative effect on fast-ion confinement as increasing the radial electric field?
• RQ2 Can a controlled scan of the radial electric field be used for experimental validation of the W7-X fast-ion confinement optimization?
• RQ3 How do academic (idealized) and experimentally-based profiles influence the inferred relationship between beta, E_r, and fast-ion losses?
• RQ4 What flux-surface-averaged drift terms dominate fast-ion confinement in W7-X, and how do they scale with beta and E_r?

Key findings

• The study confirms that the bounce-average tangential drift 0v_d/dalphabdot is linearly sensitive to both <beta> and E_s, making beta and the radial electric field effectively interchangeable in their impact on confinement.
• An increase in <beta> produces an effect on fast-ion confinement equivalent to increasing the magnitude of the radial electric field, under conditions of the academic scan.
• Maximal fast-ion losses align with conditions where 0v_d/dalphabdot bapprox 0, and losses decrease when E_r or beta depart from the value that yields this near-zero drift.
• An experimentally-based scan using SFINCS-derived E_r profiles shows that the radial electric field variation dominates the confinement changes for the selected discharge, reinforcing the potential for experimental validation of the optimization strategy.
• The analysis suggests that an ideal validation path is to perform a controlled scan on I_Es E_s (radial electric field impact) rather than solely increasing <beta>, given experimental constraints.
• The results support the feasibility of validating W7-X fast-ion optimization by exploiting the E_r effect even at achievable beta values.