Title: Controlling marginally detached divertor plasmas

Abstract

A new control system at DIII-D has stabilized the inter-ELM detached divertor plasma state for H-mode in close proximity to the threshold for reattachment, thus demonstrating the ability to maintain detachment with minimal gas puffing. When the same control system was instead ordered to hold the plasma at the threshold (here defined as T_e = 5 eV near the divertor target plate), the resulting T_e profiles separated into two groups with one group consistent with marginal detachment, and the other with marginal attachment. The plasma dithers between the attached and detached states when the control system attempts to hold at the threshold. The control system is upgraded from the one described in and it handles ELMing plasmas by using real time D_α measurements to remove during-ELM slices from real time T_e measurements derived from divertor Thomson scattering. The difference between measured and requested inter-ELM T_e is passed to a PID (proportionalintegral-derivative) controller to determine gas puff commands. While some degree of detachment is essential for the health of ITER’s divertor, more deeply detached plasmas have greater radiative losses and, at the extreme, confinement degradation, making it desirable to limit detachment to the minimum level needed to protect the target plate. However, the observed bifurcation in plasma conditions at the outer strike point with the ion B × $$\nabla $$ B drift into the divertor makes this a significant challenge. If the divertor plasma were to reattach between ELMs, there would be a long (depending on delays in the gas puff system) window of high heat flux before detachment could be re-established. Thus, good understanding of detachment behavior near the threshold for re-attachment is required to properly tune an active control system to maintain ideal divertor performance without reattaching. Furthermore, the top-of-pedestal electron densities during dithering across the bifurcation and during stable marginally detached operation are the same within uncertainty, showing the need for local real-time measurements of the divertor conditions.

Authors:

Eldon, David ^[1]; Kolemen, Egemen ^[2]; Barton, Joseph L. ^[3]; Briesemeister, Alexis R. ^[4]; Humphreys, David A. ^[5]; Leonard, Anthony W. ^[5]; Maingi, Rajesh ^[6]; Makowski, Michael A. ^[7]; McLean, Adam G. ^[7]; Moser, Auna L. ^[5]; Stangeby, Peter C. ^[8]

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+ Show Author Affiliations

1. General Atomics, San Diego, CA (United States); Princeton Univ., Princeton, NJ (United States)
2. Princeton Univ., Princeton, NJ (United States)
3. Sandia National Lab. (SNL-CA), Livermore, CA (United States)
4. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
5. General Atomics, San Diego, CA (United States)
6. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
7. Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
8. Univ. of Toronto Institute for Aerospace Studies, Toronto (Canada)

Publication Date:

Thu May 04 00:00:00 EDT 2017

Research Org.:

General Atomics, San Diego, CA (United States)

Sponsoring Org.:

USDOE Office of Nuclear Energy (NE)

OSTI Identifier:

1374843

Grant/Contract Number:  

FC02-04ER54698

Resource Type:

Accepted Manuscript

Journal Name:

Nuclear Fusion

Additional Journal Information:

Journal Volume: 57; 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; divertor; tokamak; control