Abstract
Fusion-born alpha particles in ITER disruption simulations are investigated as a possible drive of Alfvénic instabilities. The ability of these waves to expel runaway electron (RE) seed particles is explored in the pursuit of a passive, inherent RE mitigation scenario. The spatiotemporal evolution of the alpha particle distribution during the disruption is calculated using the linearized Fokker–Planck solver CODION coupled to a fluid disruption simulation. These simulations are done in the limit of no alpha particle transport during the thermal quench, which can be seen as a most pessimistic situation where there is also no RE seed transport. Under these assumptions, the radial anisotropy of the resulting alpha population provides free energy to drive Alfvénic modes during the quench phase of the disruption. We use the linear gyrokinetic magnetohydrodynamic code LIGKA to calculate the Alfvén spectrum and find that the equilibrium is capable of sustaining a wide range of modes. The self-consistent evolution of the mode amplitudes and the alpha distribution is calculated utilizing the wave-particle interaction tool HAGIS. Intermediate mode number (n = 7–15, 22–26) toroidal Alfvén eigenmodes are shown to saturate at an amplitude of up to δB/B ≈ 0.1% in the spatial regimes crucial for RE seed formation. We find that the mode amplitudes are predicted to be sufficiently large to permit the possibility of significant radial transport of REs.
Highlights
The subject of runaway electron (RE) mitigation is of crucial importance to the success of reactor-relevant tokamaks such as ITER [1,2,3,4,5]
We show that the alpha particle distribution remains sufficiently energetic during the disruptions considered to drive TAEs in the current quench, where the plasma temperature ( Landau damping) has dropped significantly
The onset and saturation of the modes occurs before the rise of the electric field induced in the current quench, before the start of significant runaway avalanche
Summary
The subject of runaway electron (RE) mitigation is of crucial importance to the success of reactor-relevant tokamaks such as ITER [1,2,3,4,5]. Energetic beam experiments at the AUG tokamak revealed the importance of the heavily temperature dependent Landau damping and its role in allowing strongly unstable modes to exist in a cold plasma [43,44,45]. In this paper we consider the active phase of ITER, where suprathermal alpha particles born through the fusion process in the burning plasma exist at significantly higher energies than present day experiments. Good alpha particle confinement in the thermal quench stage is a necessary condition for the scenario described in the paper This may not be universally true in all disruptions. We show that the alpha particle distribution remains sufficiently energetic during the disruptions considered to drive TAEs in the current quench, where the plasma temperature ( Landau damping) has dropped significantly.
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