An advection–diffusion model for cross-field runaway electron transport in perturbed magnetic fields

Abstract

Disruption-generated runaway electrons (RE) present an outstanding issue for ITER. The predictive computational studies of RE generation rely on orbitaveraged computations and, as such, they lack the effects from the magnetic field stochasticity. Since stochasiticity is naturally present in post-disruption plasma, and externally induced stochastization offers a prominent mechanism to mitigate RE avalanche, we present an advection-diffusion model that can be used to couple an orbit-following code to an orbit-averaged tool in order to capture the crossfield transport and to overcome the latter's limitation. The transport coefficients are evaluated via a Monte Carlo method. We show that the diffusion coefficient differs significantly from the well-known Rechester-Rosenbluth result. We also demonstrate the importance of including the advection: it has a two-fold role both in modelling transport barriers created by magnetic islands and in amplifying losses in regions where the islands are not present.

Physics of runaway electrons in tokamaks

“…It is still unfeasible for the existing codes and computers to simulate the randomized field directly, because of severe resolution requirements and the need for a kinetic rather than MHD description of the emerging short scales. This situation motivates numerous sensitivity studies of the fast electron transport to magnetic fluctuations produced by the MHD codes, created by external magnetic coils or to the arbitrarily postulated ones [21,[25][26][27][28][29][30][31].…”

Runaway electron mitigation by 3D fields in the ASDEX-Upgrade experiment

“…In this sense, the combined results reported in Figure 10-11 point to two mechanisms both occurring at the same favorable coil phasing which could affect the primary generated runaway electrons, thus reducing the initial seed, or those produced in the avalanche process. Nevertheless, a deeper understanding of these issues would require a detailed analysis directly by modeling the RE trajectories in these 3D fields [56] and/or an investigation with a two-fluid approach [57], considering also non-linear effects in the plasma response to RMPs. When two-fluid terms are included in the response calculations, the ion and electron rotation velocities are no longer the same; in particular the electron velocity results to be the relevant quantity controlling the field penetration in the core of the plasma at the mode-rational surface.…”

Kinetics of relativistic runaway electrons

“…The magnetic fluctuations of interest tend to be nearly static on the electron transit time-scale, and the resulting heat diffusion coefficient should then be proportional to the electron thermal speed and scale as δ B ( ) 2 with the perturbation level. There has been an extensive work aimed at modeling and deeper understanding of the field stochastization and its consequences [9], [10], [11], [12], [13], [14]. One of the noteworthy outcomes is that the contribution of the very short wavelengths perturbations to transport is relatively small as a result of averaging along the guiding center orbit or full gyro-orbit.…”