Abstract
Plasma terminating disruptions in tokamaks may result in relativistic runaway electron beams with potentially serious consequences for future devices with large plasma currents. In this paper, we investigate the effect of plasma elongation on the coupled dynamics of runaway generation and resistive diffusion of the electric field. We find that elongated plasmas are less likely to produce large runaway currents, partly due to the lower induced electric fields associated with larger plasmas, and partly due to direct shaping effects, which mainly lead to a reduction in the runaway avalanche gain.
Highlights
Magnetic reconnection events in tokamaks often result in a sudden cooling of the plasma associated with an increase in the plasma resistivity, which in turn induces an electric field
Apart from these differences in MHD stability, there are differences in the induced toroidal electric field and associated runaway current dynamics that depend on the plasma elongation directly, rather than indirectly via its effect on MHD stability
We focus on such effects of elongation on the coupled dynamics of runaway current generation and resistive electric field diffusion
Summary
Magnetic reconnection events in tokamaks often result in a sudden cooling of the plasma associated with an increase in the plasma resistivity, which in turn induces an electric field. Magnetohydrodynamic (MHD) simulations show that differences in the MHD activity during the thermal quench phase produce better confinement of seed runaway electrons if the plasma is limited than if it is diverted (Izzo et al 2011) Apart from these differences in MHD stability, there are differences in the induced toroidal electric field and associated runaway current dynamics that depend on the plasma elongation directly, rather than indirectly via its effect on MHD stability. We derive an equation governing the evolution of the toroidal electric field in a general magnetic geometry and consider the effect of elongation on the current dynamics in the case of an axisymmetric magnetic field with elliptical flux surfaces in the large aspect ratio limit. The Dreicer runaway production rate (Dreicer 1959; Connor & Hastie 1975), which is exponentially sensitive to the electric field, can be reduced significantly by finite elongation
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