Yasuhiro Kagei

Drift resonance effect on stochastic runaway electron orbit in the presence of low-order magnetic perturbations

During major disruptions, an induced loop voltage accelerates runaway electrons (REs) towards high energy, being in the order of 1–100 MeV in present tokamaks and ITER. The stochastization mechanisms of such high-energy RE drift orbits are investigated by three-dimensional (3D) orbit following in tokamak plasmas. Drift resonance is shown to play an important role in determining the onset of stochastic drift orbits for different electron energies, particularly in cases with low-order perturbations that have radially global eigenfunctions of the scale of the plasma minor radius. The drift resonance due to the coupling between the cross-field drift motion with radially global modes yields a secondary island structure in the RE drift orbit, where the width of the secondary drift islands shows a square-root dependence on the relativistic gamma factor γ. Only for highly relativistic REs (γ 1), the widths of secondary drift islands are comparable with those of magnetic islands due to the primary resonance, thus the stochastic threshold becoming sensitive to the RE energy. Because of poloidal asymmetry due to toroidicity, the threshold becomes sensitive not only to the relative amplitude but also to the phase difference between the modes. In this paper, some examples of 3D orbit-following calculations are presented for analytic models of magnetic perturbations with multiple toroidal mode numbers, for both possibilities that the drift resonance enhances and suppresses the stochastization being illustrated.

MHD simulation of relaxation to a flipped ST configuration

The dynamics of spherical torus (ST) plasmas, when the external toroidal magnetic field is decreased to zero and then increased in the opposite direction, has been investigated using three-dimensional magnetohydrodynamic (MHD) numerical simulations. It has been found that the flipped ST configuration is self-organized after the ST configuration collapses because of the growth of the n = 1 mode in the open flux region and a following magnetic reconnection event. During the transition between these configurations, not only the paramagnetic toroidal field but also the poloidal field reverses polarity spontaneously.

Stochastic Transport of Runaway Electrons due to Low-order Perturbations in Tokamak Disruption

During disruption events in tokamak devices, runaway electrons are often generated due to the toroidal loop voltage induced with increasing the resistivity. Experimentally, avoidance of runaway generation has been demonstrated by means of magnetic perturbations applied externally or induced spontaneously [1]. If runaways are escaped from the core region in the timescale much shorter than that of avalanche multiplication, the runaway current observed after the plasma current quench is expected to be suppressed significantly. However, necessary levels of magnetic perturbations were not yet fully understood [2], which requires detailed 3-D simulations of runaway electrons in mixed magnetic topologies including nested flux surfaces, island chains and stochastic volumes.