Alpha particle driven Alfvénic instabilities in ITER post-disruption plasmas

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.

The impact of fusion-born alpha particles on runaway electron dynamics in ITER disruptions

“…CODION [57] is a numerical Fokker-Planck solver able to calculate an ion distribution under the influence of an external electric field and small-angle collisions with thermal background populations. It has been equipped with a fusion alpha source [9] and is therefore a good fit to validate our model and test the isotropy assumption. With its ability to resolve the ion distribution in pitch-angle, the influence of electric fields can be studied.…”

“…Theoretical predictions [47] show that the instability drive coming from alpha particles in an ITER plasma are on par with background damping in the hot D-T phase. The damping however originates from the bulk plasma and its efficiency drops dramatically during the TQ [9]. Together with the knowledge that alpha particles resist the thermalization for a significant amount of time (see section 2), there is an opportunity for modes to be driven unstable during the TQ of a disrupting plasma.…”

“…A plasma equilibrium can be described by magnetic flux surfaces of constant pressure and calculated via the Grad-Shafranov equation [11], requiring the input of profiles of current density and pressure. A plasma equilibrium valid for the TQ was reconstructed (using VMEC [66]) for the preceding study on unmitigated disruptions [9]. The pressure-profile used for the reconstruction consists of a thermal background pressure p 1 = 2n e0 T 1 (r, t N = 3) and an alpha particle pressure obtained from CODION.…”

“…In this paper we discuss an inherent mechanism, which could aid disruption mitigation efforts, but my also aggravate the issue. The phenomenon was originally introduced and investigated in a previous proof-of-principle study [9] and is followed up in this paper.…”

“…The destabilizing effect is counteracted by Landau damping effects coming from the bulk plasma. In the initial phase of a disruption-the thermal quench (TQ)the temperature-sensitive ion Landau damping [49] strongly decreases and was shown [9] to allow the resonant alpha particle drive to destabilize Toroidicity-induced Alfvén Eigenmodes [49] (TAEs) in the weakly damped plasma. The perturbations reach amplitudes significant for RE suppression [5,21,50] and occur during the formation of the RE beam [9].…”