Runaway electron dynamics during impurity gas puffing on HT-7 tokamak

Abstract

Runaway electrons have been studied by injection/gas puffing of argon and neon gases during low-density, current ramp-down deuterium discharges in the HT-7 tokamak [X. Gao, Plasma Sci. Technol. 1, 25 (1999)]. The argon and neon gases have been injected/puffed separately during runaway Ohmic discharges, and the effects of these gases on runaway electron dynamics have been observed with different gas pulse widths and varied injection times. It has been found that the injection of a noble gas such as argon or neon can terminate the already generated runaway currents in the discharge before the runaway electrons hit the limiter or vessel structures. It has also been observed that runaway electrons cannot gain high energies in the presence of noble gas, and thereby are cooled down to their thermal velocities. During puffing of argon gas, a time delay has been observed before triggering the secondary emission of runaway electrons with a gas pulse width of 10–20 ms, whereas the same effect has also been observed with neon gas with a gas pulse width of 40–50 ms. For a 60–100 ms gas pulse width the runaway electrons have been partially suppressed, and earlier termination of the discharges was observed due to sawtooth collapse of heat energy in the center of the discharge, characterized by hard x-ray and γ ray emissions. The maximum energy of runaway electrons (12 MeV) inferred from gamma spectra is in agreement with the runaway limiting energy predicted by a test particle description of the runaway dynamics [J. R. Martín-Solís et al., Phys. Plasmas 5, 2370 (1998)].