Runaway electron generation and loss in EAST disruptions

Abstract

Runaway currents have been detected during unintended disruptions in the circular plasma with the limiter configuration in the Experimental Advanced Superconducting Tokamak (EAST). The runaway electron (RE) plateau can carry up to 80% of the pre-disruption plasma current. The highest runaway currents correspond to the lowest loop voltage, which is contrary to the observations made in most tokamaks. This anomalous behavior is attributed to the acceleration of the pre-existing wave resonant suprathermal electrons by lower hybrid waves during the disruption decay phase. Two distinct types of RE-related relaxation phenomena, distinguished on the basis of the amplitude of the magnetic fluctuations, have been found during the disruptions. Large-amplitude magnetohydrodynamic activity with indications of RE loss is observed during the RE plateau when the edge safety factor decreases to less than 3, and the external kink mode is discussed to resolve this anomaly. Burst-like relaxations with small-amplitude magnetic fluctuations and ∼0.6 kHz frequency are confirmed from the spikes in the hard x-ray array signals under a negative loop voltage. Measurement of the RE energy distribution suggests that the instability is driven by medium-energy REs.

Runaway electron dynamics in Experimental Advanced Superconducting Tokamak helium plasmas

“…Here, n e is the electron density and lnΛ is the Coulomb logarithm. Several mechanisms can cause electrons to run away, including the Dreicer generation, [9,10] hot tail RE generation, [11][12][13][14] RE avalanching, [15] tritium decay, [16] radio-frequency wave heating [15,17] and Compton scattering of γ-rays from the activated wall. During the flattop, the Dreicer generation is the dominant primary RE mechanism and creates adequate RE seed population which is further amplified by the secondary RE generation named avalanche mechanism.…”

“…During the flattop, the Dreicer generation is the dominant primary RE mechanism and creates adequate RE seed population which is further amplified by the secondary RE generation named avalanche mechanism. On the other hand, there are some limits on the maximum energy the REs can reach, which are set by, amongst others, bremsstrahlung radiation, [15,16] synchrotron radiation, [18] drift orbits, [19] magnetic field ripples [20] loop voltage and magnetic turbulence. [21,22] These phenomena can limit the energy of REs, remove the REs from the confined plasma and damp the RE current.…”

Observation of electrostatic fluctuations driven by runaway electrons in EAST disruptions

“…The highenergy tail of the RE distribution function can drive kinetic instabilities [17] in a massive injection of argon experiments, and the instabilities eventually contribute to the RE loss during the current quench (CQ), which finally suppresses the RE plateau formation. Moreover, abnormal RE loss due to kinetic instabilities has been recently observed during the RE plateau phase in the Experimental Advanced Superconducting Tokamak (EAST) with unintended disruptions [18].…”

Calculation of collisionless pitch-angle scattering of runaway electrons with synchrotron radiation via high-order guiding-centre equation

“…Plasma disruption (Lehnen, Aleynikova & Aleynikov 2015; Tang, Zeng & Chen 2021) is almost inevitable in the operation of the tokamak. One of the most dangerous hazards of plasma disruption is the generation of large amounts of runaway electrons (REs) (Paz-Soldan, Cooper & Aleynikov 2017; Breizman et al.…”