Effect of magnetic fluctuations on the confinement and dynamics of runaway electrons in the HT-7 tokamak

Abstract

The nature of runaway electrons is such that the confinement and dynamics of the electrons can be strongly affected by magnetic fluctuations in plasma. Experimental results in the HT-7 tokamak indicated significant losses of runaway electrons due to magnetic fluctuations, but the loss processes did not only rely on the fluctuation amplitude. Efficient radial runaway transport required that there were no more than small regions of the plasma volume in which there was very low transport of runaways. A radial runaway diffusion coefficient of Dr≈10 m2s-1 was derived for the loss processes, and diffusion coefficient near the resonant magnetic surfaces and shielding factor ϒ=0.8 were deduced. Test particle equations were used to analyze the effect of magnetic fluctuations on runaway dynamics. It was found that the maximum energy that runaways can gain is very sensitive to the value of αs (i.e., the fraction of plasma volume with reduced transport). αs=(0.28−0.33) was found for the loss processes in the experiment, and maximum runaway energy could be controlled in the range of E=(4 MeV-6 MeV) in this case. Additionally, to control the maximum runaway energy below 5 MeV, the normalized electric field needed to be under a critical value Dα=6.8, and the amplitude normalized magnetic fluctuations b̃ needed to be at least of the order of b̃≈3×10−5.