Title: Design and operation of a high-heat flux, flush-mounted ‘rail’ Langmuir probe array on Alcator C-Mod

Abstract

A poloidal array of toroidally-extended, flush-mounted ‘rail’ Langmuir probes was recently installed on Alcator C-Mod's vertical target plate divertor. The aim was to investigate if a Langmuir probe array could be designed to survive reactor-level heat fluxes and have the ability to make measurements that could be reliably interpreted under reactor-level plasma densities, neutral densities and magnetic fields. Langmuir probes are typically built to have incident field-line angles > 10° to avoid interpretation issues associated with sheath expansion. However, at the high parallel heat fluxes experienced in reactor-relevant conditions such a probe would quickly overheat and melt. To mitigate both the issues of extreme heat flux and sheath expansion, each probe was designed to be flush with the divertor surface, toroidally-extended and field-aligned, giving it a ‘rail’ geometry. The flush mounted probes have proven to be exceptionally robust surviving the 2015–2016 campaign – a first for a C-Mod probe system. Examination of the probe current-voltage (I-V) characteristics reveals that they are immune to sheath expansion at incident field angles down to ~0.5°. Comparison of the flush probes to traditional proud probes shows that both measure the same electron pressure across the divertor plate. However, there are significant and systematic differencesmore » in the density, temperature and floating potential. This suggests that there is important physics, perhaps unique to conditions in a vertical-target plate divertor with small field-line attack angles, that affects the I-V characteristics and is not currently included in probe data analyses. Finally, the probe response is examined in the ‘death-ray’ regime, just near detachment. Previous work using proud probes has suggested that the ‘death-ray’ is an artefact of the probe bias. However, on flush mounted probes the ‘death-ray’ manifests itself under different conditions, which may provide insight into this unique phenomenon.« less

Authors:

Kuang, A. Q. ;

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• Search DOE PAGES for ORCID "0000-0002-8917-2911"

Brunner, D.; LaBombard, B.; Leccacorvi, R.; Vieira, R.

Publication Date:

Tue Aug 01 00:00:00 EDT 2017

Research Org.:

Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)

Sponsoring Org.:

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

OSTI Identifier:

1772067

Alternate Identifier(s):

OSTI ID: 1534178

Grant/Contract Number:  

FC02-99ER54512

Resource Type:

Published Article

Journal Name:

Nuclear Materials and Energy

Additional Journal Information:

Journal Name: Nuclear Materials and Energy Journal Volume: 12 Journal Issue: C; Journal ID: ISSN 2352-1791

Publisher:

Elsevier

Country of Publication:

Netherlands

Language:

English

Subject:

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY