Description
One of the potential consequences of tokamak plasma disruption is the formation of a high-energy runway electron (RE) beam that, if unmitigated, could damage plasma facing components (PFCs) and surrounding structures in future large tokamaks like ITER [1]. The primary goal of mitigating an RE beam is to effectively spread its energy and thus significantly reduce localized heat fluxes on the PFCs. A promising strategy is the so-called low-Z benign termination, where materials such as protium and deuterium are injected into a fully formed RE beam [2,3]. Experiments on various tokamaks such as JET, AUG and TCV have demonstrated the effectiveness of this technique under favourable operating regimes, for example, with neutral pressures in the range of 0.1-1 Pa on TCV [3,4]. A successful benign termination is typically attributed to the onset of a violent current-driven MHD instability that expels REs over a large wetted area on the PFCs [2-6]. More recently, efforts are being made to explore the role of the mode growth rate and amplitude in the benignness of a termination on JET [7], which calls for a multi-machine validation. In this work, we will present experimental analyses of the MHD activities during the final termination of REs in a large number of TCV discharges. Numerical simulations with the 3D non-linear hybrid fluid-kinetic code JOREK will also be discussed.
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[5] V. Bandaru et al. Phys. Plasmas 31 (2024) 082503
[6] H. Bergström et al. Plasma Phys. Control. Fusion 67 (2025) 035004
[7] C. F. B. Zimmermann et al. Nucl. Fusion 66 (2026) 056004