Observation of rapid frequency chirping instabilities driven by runaway electrons in a tokamak

Lvovskiy, A.; Heidbrink, W. W.; Paz-Soldan, C.; Spong, D. A.; Dal Molin, A.; Eidietis, N. W.; Nocente, M.; Shiraki, D.; Thome, K. E.

doi:10.1088/1741-4326/ab4405

Title: Observation of rapid frequency chirping instabilities driven by runaway electrons in a tokamak

Journal Article · 12 September 2019 · Nuclear Fusion

DOI: https://doi.org/10.1088/1741-4326/ab4405 · OSTI ID:1569036

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Oak Ridge Associated Univ., Oak Ridge, TN (United States); General Atomics, Energy & Advanced Concepts, DIII-D
Univ. of California, Irvine, CA (United States)
General Atomics, San Diego, CA (United States)
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Università di Milano-Bicocca, Milan (Italy)

In this work, we report the first observation of chirping instabilities driven by runaway electrons (REs) in a tokamak. The instabilities are accessed during the post-disruption RE beam stage in a low density background plasma (n_e < 10¹⁹ m^-3) on DIII-D. The chirping instabilities are observed when a decelerating loop voltage is applied to the RE beam. The frequency chirping is detected in two distinct frequency bands: 0.1–10 MHz and 30–80 MHz. The mode frequency increases linearly when the toroidal magnetic field sensed by the RE beam increases. The frequency chirps by 0.3–2.4 MHz on a timescale of 1 ms. Modification of the RE distribution function is directly measured during the chirping in the low-frequency band consistent with the hole–clump model for frequency chirping. The low-frequency instabilities also correlate with an increase of intermittent RE loss from the plasma. Lastly, these observations provide a novel experimental platform for fundamental studies of nonlinear chirping. They also support continued investigation of opportunities to utilize kinetic instabilities for RE mitigation in a tokamak reactor.

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Research Organization:: General Atomics, San Diego, CA (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES)

Grant/Contract Number:: FC02-04ER54698; AC05-00OR22725

OSTI ID:: 1569036

Journal Information:: Nuclear Fusion, Journal Name: Nuclear Fusion Journal Issue: 12 Vol. 59; ISSN 0029-5515

Publisher:: IOP ScienceCopyright Statement

Country of Publication:: United States

Language:: English

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