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)].

Self-consistent nonlinear kinetic simulations of the anomalous Doppler instability of suprathermal electrons in plasmas

“…The anomalous Doppler instability has enduring experimental relevance to magnetically confined fusion plasmas 5,8,11,14 , and to laboratory beam-plasma studies 17,18,27 . Hitherto theoretical interpretation has rested on linear instability analysis and quasilinear approaches.…”

“…Oscillations in the amplitude of the directional X-ray signal observed from tokamaks, reported most recently in Ref. 14, are believed to arise where the energetic electron tail distribution undergoes repeated cycles of relaxations through the isotropising effect of the ADI in its fully nonlinear regime, followed by re-extension of the tail by the current drive processes. Importantly, the ADI can occur for tail distributions that are monotonically decreasing in the v direction.…”

“…It has been found in over four decades of experiments in low density tokamak plasmas [4][5][6][7][8]11,14 with both Ohmic and lower hybrid current drive, and remains topical. For example, the ADI is believed to explain the recently observed relaxation oscillations in directional hard X-ray signals emitted by the energetic electron population in the HT-7 tokamak 14,15 . These observations reflect the underlying physics of the ADI, which involves a shift of energy from parallel to perpendicular particle motion, accompanied by the excitation of waves at frequency and wavenumber (ω, k) satisfying the anomalous Doppler resonance condition 11 ,…”

Comparison of anomalous Doppler resonance effects with molybdenum and graphite limiters on HT-7

“…The ADR effects have been studied in HT-7 from various aspects. [7][8][9][10][11][12] It was concluded from the nonlinear of runaway electrons distribution theory that the stability of the runaway electrons is mainly determined by the density critical value, g ¼ x pe =x ce , and electric field critical value, E/E 0 . 5 In this paper, the difference of the ADR effects between two different limiter conditions is studied, and the statistical results are given.…”

Simulation of the runaway electron beam formed in a discharge in air at atmospheric pressure

“…[7][8][9][10] Runaway electrons in the plasmas of tokamaks, installations intended for realization controlled thermonuclear synthesis have been extensively investigated both theoretically and experimentally. [11][12][13] In 1992, 14 the effect was predicted which was termed runaway electron breakdown (REB). For this type of breakdown, runaway electrons with energy of some hundreds kiloelectronvolts and more play the main part in the growth of the electron avalanche that gives rise to a current-carrying channel.…”