Fast Plasma Shutdown in JT-60U Tokamak with Increased Critical Electric Field below which Runaway electrons are avoided
Mohammad Bakhtiaria, Ryuji Yoshinob, and Yasushi Nishidaa
a) Energy and Environmental Science, Graduate School of Engineering, Utsunomiya University 7-1-2 Youtoh, Utsunomiya, Tochigi 321-8585, Japan
b) Japan Atomic Energy Research Institute Naka Fusion Research Establishment 801-1 Mukouyama, Naka-Machi, Naka-Gun, Ibaraki-Ken 311-0193 Japan
Abstract.
Disruptions triggered by killer pellets injected into the JT-60U plasma are investigated. A one dimensional computer model has been developed to simulate the bahavior of disruptions. The code was first validated with previous experimental results in JT-60U. Using this code a new technique to shutdown the JT-60U tokamak with avoiding runaway electron generation was found, whereby a certain amount of deuterium mixed with impurities is injected into the tokamak plasma. The injected deuterium increases the electron density and radiation power to induce a density limit disruption. Injection of a small concentrate of impurities speed up the disruption sequence. The runaway electrons can not be generated because of the high density produced by deuterium injection. The higher the density the higher critical electric field for runaway electron generation. However the values of deuterium which should be injected are limited. If the deuterium and impurity densities exceed a critical value, which depends on the tokamak size and paremeters, the temperature suddenly drops and runaway electrons are likely to be generated.
From calculations it is shown that deuterium pellets mixed with small amount of krypton make a fast shutdown and deuterium pellets mixed with small amount of neon induce a slow shutdown. This phenomena depends on the radiative properties of the impurity ions with different temperatures.