Observation of thermal quench induced by runaway electrons in magnetic perturbation

Abstract

Experimental observations in Korea Superconducting Tokamak Advanced Research (KSTAR) plasmas show that a loss of pre-disruptive runaway electrons can induce a rapid radiative cooling of the plasma, by generating impurity clouds from the first wall. The synchrotron radiation image shows that the loss of runaway electrons occurs from the edge region when the resonant magnetic perturbation is applied on the plasma. When the impact of the runaway electrons on the wall is strong enough, a sudden drop of the electron cyclotron emission (ECE) signal occurs with the characteristic plasma behaviors such as the positive spike and following decay of the plasma current, Dα spike, big magnetic fluctuation, etc. The visible images at this runaway loss show an evidence of the generation of impurity cloud and the following radiative cooling. When the runaway beam is located on the plasma edge, thermal quenches are expected to occur without global destruction of the magnetic structure up to the core.

Study of runaway electron dynamics at the ASDEX Upgrade tokamak during impurity injection using fast hard x-ray spectrometry

“…Also, in the presence of electrons with energies of several tens of MeV, the use of methods based on measuring synchrotron radiation in the visible (VIS) and infrared (IR) ranges can help. These methods have been implemented in the RE studies at the TEXTOR [18], DIII-D [19,20], HT-7 [8], EAST [21], J-TEXT [22], KSTAR [23], Alcator C-Mod [24], and AUG [25] tokamaks.…”

Observation of trapped and passing runaway electrons by infrared camera in the EAST tokamak*

“…Synchrotron radiation images of passing runaway electrons have been observed in many tokamaks, such as EAST, [15] KSTAR, [16][17][18] C-Mod, [2] and DIII-D. [19][20][21][22] The position and orientation of the infrared camera on EAST are described in Ref. [9].…”

“…Since the reduced radial transport happens near the low-order rational surfaces, the runaway electron beams tend to gather here to form a ring structure. [17,24] Only in very few discharge cases, may the runaway electrons be uniform distribution. So, six types of synchrotron radiation images can be observed by infrared cameras as shown in Table 1.…”