Title: Avoidance of impurity-induced current quench using lower hybrid current drive

Abstract

This work reports observations of a tokamak plasma that experienced a thermal quench due to a large, transient high-Z influx but avoided a current quench. This is argued to be caused by the presence of lower hybrid range of frequency (LHRF) waves that sustain a non-thermal, current-carrying electron population. In Alcator C-Mod L-mode plasmas at I_p = 450 kA, n¯_e = 0.5 x 10²⁰ m^–3, nearly all of the current can be sustained non-inductively by injecting ≃700 kW of LHRF power at 4.6 GHz and n_∥ = 1.9. A sudden influx of a large amount of tungsten, n_z/n_e ≃ 0.0044, triggers a cooling wave that propagates at 2–3 m s^–1 all the way into the core, dropping on-axis T _e from 3 keV to temperatures less than measurement floor of 50 eV. An off-axis reheat begins after 100 ms, but T _e profiles remain hollow for 300–350 ms after the injection. Throughout this temperature evolution, the plasma density, current and shape remain unchanged to within 10%. Following the expulsion of the tungsten, the plasma returns to its baseline conditions and the plasma ends as planned with a controlled current ramp-down. Energy balance analysis shows the LHRF power continues to bemore » absorbed in the plasma after the thermal quench, as a significant fraction of it is needed to be consistent with radiated power measurements. Examination of current relaxation time, τ_R, and fast-electron slowing down time, τ_s, indicate the LHRF must contribute to driving current, despite the low temperatures, as the current remains nominally stationary despite τ_R < 5 ms and τ_s < 50 ms for relativistic electrons. Furthermore, these measurements represent an important existence proof of a possible technique for avoidance of disruptions caused by sudden, unplanned influx of impurities in the form of dust or flakes of high-Z wall material. Implications and suggestions for future experimental and modeling and simulation work are summarized.« less

Authors:

Reinke, M. L.  ^[1];

• Search DOE PAGES for author "Reinke, M. L."
• Search DOE PAGES for ORCID "0000-0003-4413-9613"

Scott, S. ^[2]; Granetz, R. ^[3]; Hughes, J. W.  ^[3];

• Search DOE PAGES for author "Hughes, J. W."
• Search DOE PAGES for ORCID "0000-0003-4802-4944"

Baek, S. G. ^[3]; Shiraiwa, S. ^[3]; Tinguely, R. A.  ^[3];

• Search DOE PAGES for author "Tinguely, R. A."
• Search DOE PAGES for ORCID "0000-0002-3711-1834"

Wukitch, S. ^[3]

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+ Show Author Affiliations

1. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
2. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
3. MIT Plasma Science and Fusion Center, Cambridge, MA (United States)

Publication Date:

Thu Apr 25 00:00:00 EDT 2019

Research Org.:

Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)

Sponsoring Org.:

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

Contributing Org.:

The Alcator C-Mod Team

OSTI Identifier:

1558802

Grant/Contract Number:  

AC02-09CH11466; FG02-98ER54462

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 59; Journal Issue: 6; Journal ID: ISSN 0029-5515

Publisher:

IOP Science

Country of Publication:

United States

Language:

English

Subject:

70 PLASMA PHYSICS AND FUSION TECHNOLOGY; tokamak; disruption; impurity; lower hybrid; runaway electron