[Paper Review] Confinement of passing and trapped runaway electrons in the simulation of an ITER current quench

K. Särkimäki, F.J. Artola+1 more|arXiv (Cornell University)|Mar 17, 2022

Magnetic confinement fusion research40 references19 citations

TL;DR

This study investigates runaway electron (RE) confinement during the current quench phase in ITER using 3D MHD simulations from the JOREK code. By tracing RE markers in stochastic magnetic fields, it finds that field-induced transport during an 8 ms stochastic phase is sufficient to deconfine most passing and trapped REs, potentially mitigating RE beam formation before flux surfaces reform.

ABSTRACT

Runaway electrons (REs) present a high-priority issue for ITER but little is known about the extent to which RE generation is affected by the stochastic field intrinsic to disrupting plasmas. RE generation can be modelled with reduced kinetic models and there has been recent progress in involving losses due to field stochasticity, either via a loss-time parameter or radial transport coefficients which can be estimated by tracing test electrons in 3D fields. We evaluate these terms in ITER using a recent JOREK 3D MHD simulation of plasma disruption to provide the stochastic magnetic fields where RE markers are traced with the built-in particle tracing module. While the MHD simulation modelled only the current quench phase, the case is MHD unstable and exhibits similar relaxation as would be expected during the thermal quench. Therefore, the RE simulations can be considered beginning right after the thermal quench but before the MHD relaxation is complete. The plasma is found to become fully stochastic for 8 ms and the resulting transport is sufficient to overcome RE avalanche before flux surfaces are reformed. We also study transport mechanisms for trapped REs and find those to be deconfined as well during this phase. While the results presented here are not sufficient to assess the magnitude of the formed RE beam, we show that significant RE losses could be expected to arise due to field stochasticity.

Motivation & Objective

• To assess the impact of magnetic field stochasticity on runaway electron (RE) confinement during ITER's current quench.
• To evaluate whether RE losses due to stochastic transport can overcome RE avalanche growth.
• To investigate transport mechanisms for both passing and trapped REs during the stochastic phase.
• To validate and implement collision and radiation reaction operators in the JOREK particle tracer for realistic RE dynamics.

Proposed method

• Traced relativistic RE markers using JOREK's orbit-following module with guiding-center approximation.
• Incorporated a full relativistic Fokker-Planck collision operator based on Braams-Karney theory, including drag, energy diffusion, and pitch scattering.
• Applied a radiation reaction force operator to model synchrotron losses, though its impact was found negligible.
• Used time-evolving 3D magnetic fields from a JOREK MHD simulation of an ITER current quench (Case 1), which features a 50 ms current quench with vertical displacement.
• Quantified transport via radial diffusion coefficients and loss timescales, comparing cases with and without collisions.
• Analyzed trapped RE dynamics separately, identifying three loss mechanisms: collisional scattering, Ware pinch, and collisionless banana diffusion.

Experimental results

Research questions

• RQ1 Can magnetic field stochasticity during the current quench phase suppress runaway electron beam formation in ITER?
• RQ2 What is the relative importance of collisional scattering, Ware pinch, and collisionless banana diffusion in deconfining trapped runaway electrons?
• RQ3 How do transport timescales for passing and trapped REs compare with RE avalanche growth timescales?
• RQ4 To what extent do Coulomb collisions and radiation losses affect RE confinement in the stochastic phase?

Key findings

• The plasma becomes fully stochastic for 8 ms during the current quench, during which magnetic field transport is sufficient to overcome RE avalanche growth.
• Passing runaway electrons are deconfined within the stochastic phase due to radial transport, with loss timescales comparable to or shorter than avalanche growth timescales.
• Trapped runaway electrons are also deconfined via three mechanisms: collisional scattering (Ekin ≲ 200 keV), Ware pinch (200 keV ≲ Ekin ≲ 10 MeV), and collisionless banana diffusion (Ekin ≳ 10 MeV).
• The collision operator significantly affects trapped particle dynamics, while radiation reaction forces have negligible impact on RE transport.
• A small fraction of REs may survive near the magnetic axis, but the stochastic phase is generally effective in mitigating beam formation.
• The simulation suggests that RE beam magnitude is highly sensitive to losses during the early current quench, especially when stochasticity persists before flux surface reformation.