Behaviour of Runaway Electrons during Injection of High Z Impurities/Gas Puffing in HT-7 S.Sajjad INSTITUTE OF PLASMA PHYSICS,HEFEI CHINA

Outlines 1.Motivation 2.Experimental Background 3.Density Ramp up 4.Background Results 5. Parameters for Experiment

Motivation 1. The avoidance and mitigation of disruptions are critical issues in advancing the tokamak concept as a viable energy source using magnetic confinement fusion. 2. The highly resistive plasma causes the confining poloidal magnetic field to decay during the toroidal current quench (CQ). 3. The disruption causes damage by these means: (i) Plasma-conducted thermal loading of wall surfaces during the thermal quench, (ii) J X B forces from vessel poloidal halo currents induced by the current quench. (iii) The conversion of the toroidal plasma current into relativistic runaway electrons (RE) that eventually are stopped by the wall.

1. It has been found that a massive injection of a high-Z gas such as neon, argon and xenon can terminate a disruption- generated runaway currents before the runaway electrons hit the walls. 2. High-pressure gas-jet injection of neon and argon is shown to be a simple and robust method to mitigate the deleterious effects of disruptions on the DIII-D. 3. The low pressure noble gas puffing and killer pellet (neon ice pellet) injection in the JT-60U has been used for avoidance and mitigation of disruptions. 4.It has been also shown that runaway electrons cannot gain high energies in the presence of high-Z noble gases, and the runaway electrons are also cooled down to thermal velocity. References: 1. D. G. Whyte et al, Physical review letters, (2002) 89, M. Bakhtiari et al, Physical review letters, (2005) 94, Experimental Background

Density Ramp up

Results from DIII-D

Parameters for Experiment Ohmic shots: Ip=100kA ne= , , ,0.7-2 IT= 4000A Puffing gas= Neon, Argon, Xenon Gas Pulse Width: 5ms, 10ms, 15ms 20ms LHCD Shots: Ip=120 kA ne= , , ,0.7-2 PLH=200kW

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