Estimate of pre-thermal quench non-thermal electron density profile during Ar pellet shutdowns of low-density target plasmas in DIII-D

Hollmann, E. M.; Austin, M.; Bykov, I.; Eidietis, N. W.; Embreus, O.; Herfindal, J. L.; Hoppe, M.; Lvovskiy, A.; Parks, P. B.; Paz-Soldan, C.; Shiraki, D.; Svenningsson, I.

doi:10.1063/5.0050903

Title: Estimate of pre-thermal quench non-thermal electron density profile during Ar pellet shutdowns of low-density target plasmas in DIII-D

Journal Article · Tue Jul 06 00:00:00 EDT 2021 · Physics of Plasmas

DOI:https://doi.org/10.1063/5.0050903· OSTI ID:1808713

^[1];

^[2]; Bykov, I. ^[3]; Eidietis, N. W. ^[3];

^[4]; Herfindal, J. L. ^[5];

^[4];

^[3];

^[3]; Shiraki, D. ^[5]; Svenningsson, I. ^[4]

Univ. of California San Diego, La Jolla, CA (United States)
Univ. of Texas, Austin, TX (United States)
General Atomics, San Diego, CA (United States)
Chalmers Univ. of Technology, Gothenburg (Sweden)
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

The radial density profile of pre-thermal quench (pre-TQ) early-time runaway electrons (REs) is estimated by combining electron cyclotron emission (ECE) and soft x-ray (SXR) data during rapid shutdown of low-density (n_e ≲ 10¹³ cm^-3) DIII-D target plasmas with cryogenic argon pellet injection. This technique is limited in these experiments to the pre-TQ phase and quickly loses validity during the TQ. Two different cases are studied: a high (10 keV) temperature target and a lower (4 keV) temperature target. The results indicate that early-time, low-energy (~10 keV) REs form ahead of the argon pellet as it enters the plasma, affecting the pellet ablation rate; it is hypothesized that this may be caused by rapid cross-field transport of argon ions ahead of the pellet. Fokker-Planck modeling of the two shots suggests that the RE current is quite significant during the pre-TQ phase (up to 50% of the total current). Comparison between modeled pre-TQ RE current and post-TQ RE current inferred from avalanche theory suggests that RE current increases during the high temperature target TQ but decreases during the low temperature target TQ. Here, the uncertainties in this estimate are large; but, if true, this suggests that TQ loss of REs can be larger than previously estimated in DIII-D.

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

Sponsoring Organization:: USDOE Office of Science (SC), Fusion Energy Sciences (FES); USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities Division

Grant/Contract Number:: FC02-04ER54698; FG02-07ER54917; AC05-00OR22725; FG02-04ER54744; AC05-06OR23100; AC52-07NA27344

OSTI ID:: 1808713

Alternate ID(s):: OSTI ID: 1805295; OSTI ID: 1819736

Journal Information:: Physics of Plasmas, Vol. 28, Issue 7; ISSN 1070-664X

Publisher:: American Institute of Physics (AIP)Copyright Statement

Country of Publication:: United States

Language:: English

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Title: Estimate of pre-thermal quench non-thermal electron density profile during Ar pellet shutdowns of low-density target plasmas in DIII-D

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