Title: Runaway electron seed formation at reactor-relevant temperature

Abstract

Systematic variation of the pre-disruption core electron temperature (T_e) from 1 to 12 keV using an internal transport barrier scenario reveals a dramatic increase in the production of ‘seed’ runaway electrons (REs), ultimately accessing near-complete conversion of the pre-disruption current into sub-MeV RE current. Injected Ar pellets are observed to ablate more intensely and promptly as T_e rises. At high T_e, the observed ablation exceeds predictions from published thermal ablation models. Simultaneously, the thermal quench (TQ) is observed to significantly shorten with increasing T_e—a surprising result. While the reason for the shorter TQ is not yet understood, candidate mechanisms include: insufficiently accurate thermal ablation models, enhanced ablation driven by the seed RE population, or significant parallel heat transport along stochastic fields. Kinetic modeling that self-consistently treats the plasma cooling via radiation, the induced electric field, and the formation of the seed RE is performed. Including the combined effect of the inherent dependence of hot-tail RE seeding on T_e together with the shortened TQ, modeling recovers the progression towards near-complete conversion of the pre-disruption current to RE current as T_e rises. Measurement of the HXR spectrum during the early current quench (CQ) reveals a trend of decreasing energy with pre-disruption T_e.more » At the very highest T_e (≈ 12 keV), ≈ 100% conversion of the thermal current to runaway current is found. The energy of this peculiar RE beam is inferred to be sub-MeV as it emits vanishingly few MeV hard x-rays (HXRs). These measurements demonstrate novel TQ dynamics as Te is varied and illustrate the limitations of treating the RE seed formation problem without considering the inter-related dependencies of the pellet ablation, radiative energy loss, and resultant variations of the TQ duration. Finally, if the observed shortening of the TQ with increasing T_e extends to fusion-grade plasmas, than their propensity to form large quantities of RE seeds at high T_e may be far worse than previously thought. Positively, the high T_e scenario in DIII-D produces REs so prodigiously that it can serve as a meaningful new platform for demonstrating RE avoidance techniques.« less

Authors:

Paz-Soldan, C.  ^[1];

• Search DOE PAGES for author "Paz-Soldan, C."
• Search DOE PAGES for ORCID "0000-0001-5069-4934"

Aleynikov, P. ^[2]; Hollmann, E. M. ^[3]; Lvovskiy, A.  ^[1];

• Search DOE PAGES for author "Lvovskiy, A."
• Search DOE PAGES for ORCID "0000-0002-3649-1169"

Bykov, I. ^[3]; Du, X.  ^[1];

• Search DOE PAGES for author "Du, X."
• Search DOE PAGES for ORCID "0000-0001-6127-2825"

Eidietis, N. W.  ^[1];

• Search DOE PAGES for author "Eidietis, N. W."
• Search DOE PAGES for ORCID "0000-0003-0167-5053"

Shiraki, D. ^[4]

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+ Show Author Affiliations

1. General Atomics, San Diego, CA (United States)
2. Max-Planck Inst. for Plasma Physics, Greifswald (Germany)
3. Univ. of California, San Diego, CA (United States)
4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)

Publication Date:

Thu Apr 16 00:00:00 EDT 2020

Research Org.:

General Atomics, San Diego, CA (United States); Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)

Sponsoring Org.:

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

OSTI Identifier:

1615333

Alternate Identifier(s):

OSTI ID: 1615125; OSTI ID: 1657915

Grant/Contract Number:  

FC02-04ER54698; AC05-00OR22725; FG02-07ER54917

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 60; Journal Issue: 5; Journal ID: ISSN 0029-5515

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY