Re-acceleration of Energetic Ions via Small-Scale Reconnection in Magnetic Fusion Plasmas

TL;DR

This paper reports the first observation of energetic ion re-acceleration via small-scale magnetic reconnection in a spherical torus, demonstrating a new auxiliary heating mechanism without large-scale MHD disruptions.

Contribution

It provides experimental evidence and simulations showing small-scale reconnection can efficiently energize fast ions in magnetic fusion plasmas, a novel auxiliary heating process.

Findings

Energetic ions can reach up to 2.5 times injection energy without large MHD bursts.

Small-scale reconnection energizes seed fast ions but not thermal ions.

This mechanism does not degrade core confinement and is ubiquitous in magnetic devices.

Abstract

We report the first observation on the EXL-50U spherical torus that energetic particles injected by neutral beam injection (NBI) can be stably accelerated to significantly higher energies - reaching up to 2.5 times the injection energy, occurring without significant large-scale magnetohydrodynamic (MHD) bursts. Simulations based on EXL-50U parameters indicate that small-scale magnetic reconnection, mediated by multiple magnetic islands, fails to accelerate bulk thermal ions but efficiently energizes seed fast ions. Unlike global MHD events, such small-scale reconnection is ubiquitous in magnetic confinement devices and does not degrade core confinement. This mechanism offers a novel and potentially universal channel for auxiliary ion heating in future fusion reactors.