Alcator C-Mod Quarterly
Progress Report – FY10 Q3
The main activity at Alcator C-Mod during the third quarter
of FY10 was preparing for operation of the machine.
Publications Nuclear Fusion 50 (2010) 064001 Modification of H-mode pedestal structure with lower hybrid waves on Alcator C-Mod
Nuclear Fusion 50 (2010) 064008 Rotation and transport in Alcator C-Mod ITB plasmas Plasma Physics and Controlled Fusion 52 (2010) 055010 Comparison of neoclassical predictions with measured flows and evaluation of a poloidal impurity density asymmetry Physics During the 2009-10 experimental campaign concluded in March of this year, we operated C-Mod with specially prepared boron-coated molybdenum tiles located on the top outer divertor, and plasma and RF limiters Mo tiles. These tiles were vacuum-plasma sprayed with ~100um of boron, and the coating thickness of each tile was measured prior to installation. The purpose of this experiment was to investigate and further quantify the previously reported apparent erosion rate, up to 15-20nm/sec, of boron films deposited by boronization. We anticipated that the plasma performance and RF associated impurities would be improved, requiring less frequent boronization to achieve good plasma performance. Results of this initial ~6 month operating period with B-coated Mo tiles are summarized below.
We found, consistent with previous results, that one boronization was required to reduce other metallic impurities like iron before high performance plasmas could be obtained. For ICRF heated H-modes, the core molybdenum levels have been significantly reduced and remained at low levels for increased injected RF energy. The core Mo level no longer scales with RF power in L-mode. Impurity seeding (typically nitrogen or neon) improved both the plasma and ICRF antenna performance. Surprisingly, impurity seeding did not result in increased core Mo levels and also suppressed antenna faults. Spectroscopic monitoring of the plasma limiter found that the impurity profile at the limiter was centered near the plasma mid-plane and the profile did not change shape with plasma current.
We had expected that the boron would be significantly eroded, by up to 20-30 um, and that an erosion pattern would be evident post campaign. From post campaign inspection, the B coating was not significantly eroded except in locations where melting occurred or where it peeled. Coated tiles that peeled were scattered among the good coated tiles and no discernible pattern was found. Trace material analysis also found that the B surface was contaminated with Mo, tungsten (W), and stainless steel (likely iron and chromium).
Improved performance with impurity seeding, the lack of erosion, and metallic contamination of the B coating suggest that the high boronization erosion rate is related to localized heat loads (that can lead to melting) rather than entirely a result of sputtering. Further analysis of results from the previous campaign is underway, and additional data will be obtained once C-Mod operation resumes.
We have investigated high frequency (400-900 kHz) modes observed during
ITB discharges. These modes occur during ICRF-heated EDA H-modes and ITB discharges. The features are observed both on magnetics and phase contrast imaging (PCI) diagnostics. Notable features include: (a) the presence of multiple, closely spaced modes; (b) cyclic chirping of the mode frequency over ~25 kHz range, typically at ~60 Hz; (c) persistence of the modes for 5-10msec after termination of the RF pulse. In I-mode discharges which transition into H-mode, these features are observed only during the H-mode segments. In a modulated ICRF heating experiment, a single 20-50 kHz wide fluctuation between 800 and 1000 MHz was observed. The mode appears during each RF pulse, persists for 5-10 ms after RF is turned off, and is more prevalent at higher power, consistent with fast-ion drive. In all cases, the mode frequencies are higher than the center of the gap in Alfven continuum associated with TAE's at the q=3/2 surface.
MP 602, "Core Fluctuations and Transport in Two-Frequency ICRF Heated ITB Plasmas with Jogs and Modulated ICRH" was conducted on March 24. The purpose of this experiment was to produce strongly peaked ITB plasmas, fully documented with all available fluctuation and profile measurements, and controlled with modulated on-axis heating. This work continues previous studies focused on observing TEM turbulence through direct comparison of phase contrast imaging density fluctuation spectra with gyrokinetic simulations, using a synthetic PCI diagnostic in GS2 as described at
http://www.psfc.mit.edu/research/alcator/pubs/iaea/2006/Ernst-paper.pdf
The goal of MP602 was to use modulated on-axis heating to separate core and edge fluctuations, using PCI to document core fluctuations, and PCI, reflectometry, Mirnov coils, and gas-puff imaging to document edge fluctuations. Five strong ITB's were produced with off-axis ICRH, with two successfully maintained in steady state with modulated on-axis ICRH. A full suite of profile data, including Thomson scattering, ECE, and HIREX toroidal and poloidal rotation measurements was obtained. Initial analysis of fluctuation data shows a strong increase in density fluctuations measured by PCI during on-axis heating. This is accompanied by a strong reduction in edge fluctuations associated with the quasi-coherent mode, as shown by Mirnov coils and reflectometry. On the basis of previous studies, trapped electron modes are expected to be driven unstable by the on-axis heating inside the ITB foot. Detailed analysis will be carried out, comparing nonlinear gyrokinetic simulations with fluctuation spectra.
Meetings A
quarterly review was held on Tuesday, 4/6, at MIT via video conference with DoE
(
Presentations were given on the status of the fy10 run campaign, the status of the lower hybrid launcher, plans for the current up-to-air period, and status and plans for the the Joint Facility Milestone. These presentations may be found at
http://www.psfc.mit.edu/research/alcator/pubs/Q_Reviews/C-mod_quarterly_review_FY10_Q2_draft-2.pdf
Eleven
C-Mod related presentations were given at the TTF meeting in
http://www.psfc.mit.edu/research/alcator/pubs/TTF/TTF_2010_abstracts.htm
Jerry Hughes attended the Edge Coordinating Committee meeting on April 12 to report on C-Mod's status and plans with respect to the FY11 Joint Research Milestone. Research opportunities associated with this milestone were discussed extensively throughout the week by the personnel involved.
While at the meeting, Martin Greenwald met with members of the NSTX and DIII-D teams to discuss a FY12 joint research target on core transport. There was general agreement that such a milestone would stress comparisons between codes and experimentally measured profiles, fluxes and fluctuations. Later discussions with Curt Bolton suggest that the 2012 facilities target will not be combined with the theory milestone as was done for FY11. Martin also gave a talk to the Verification and
Validation Working Group.
Several C-Mod scientists and graduate students attended the High Temperature Plasma Diagnostics Conference the week of May 18 in Wildwood, NJ. At the meeting were Syun'ichi Shiraiwa, Seung Gyou Baek, Peng Xu, Matt Reinke, Roman Ochoukov, Cornwall Lau, Keigo Arai, Zach Hartwig, Bob Mumgaard, Jim Terry, and Igor Bespamyatnov and Bill Rowan, UT-FRC.
Presentations from the conference may be found at:
http://www.psfc.mit.edu/research/alcator/pubs/HTPD/HTPD_2010.htm
Bruce Lipschultz, Brian LaBombard, Roman Ochoukov, Istvan Cziegler, Matt Reinke, Noah Smick, Zach Hartwig, Dan Brunner, and Mike Garrett (presenting a talk for Steve Wukitch), represented C-Mod at the Plasma Surface Interactions Conference in San Diego, CA, the week of May 25. Their papers and posters from the conference may be found at
http://www.psfc.mit.edu/research/alcator/pubs/PSI/psi_index.htm
Jerry
Hughes, Earl Marmar, John Rice, Yijun Lin, Orso Meneghini, and Greg Wallace
participated in the 37th European Physical Society Conference on Plasma Physics
in
http://www.psfc.mit.edu/research/alcator/pubs/eps/eps2010/eps_2010.html
Miklos
Porkolab, Paul Bonoli and John Wright attended the Anomalous Absorption Conference
2010 (AAC2010), in
Jim
Zaks visited vendors in
Paul Bonoli attended a meeting of the ITPA Integrated Operations Scenario Group this week at PPPL from April 20-23, 2010.
Dennis
Whyte, Amanda Hubbard, Bruce Lipschultz and Earl Marmar traveled to PPPL on
April 30 for collaborative discussions on research requirements for plasma-material
interactions, fusion nuclear science, and fusion pilot plants. Dennis Whyte
presented a seminar on "Taming the Plasma Interface as the Next Frontier:
An opportunity for
Martin
Greenwald was in
Steve
Wukitch was in
Jim Terry attended the ITPA Diagnostics Topical Group meeting at Oak Ridge National Laboratory, May 11-14. He presented C-Mod's recent work on and results from IR thermography. His talk was entitled "Divertor IR Thermography on C-Mod - Similarities to ITER". Jim is deputy coordinator for the US Group members.
Pat
MacGibbon was at our klystron vendor in
Arturo Dominguez was at PPPL the week of June 7 week working with Gerrit Kramer and Jay Kung building and testing the RF and I/Q circuits for the new 112 GHz reflectometer channel.
Martin Greenwald was at LLNL (6/21-24) to participate on the External Review Committee for the Physical and Life Sciences Directorate.
Yijun
Lin traveled to the Southwest Institute of Physics (SWIP) and the
Miklos
Porkolab was in
Tom Fredian and Josh Stillerman visited Cadarache the week of May 11 to discuss potential use of MDSplus for ITER. Stillerman gave a seminar on MDSplus to the ITER CODAC group. They also attended an MDSplus development workshop in Padova, meeting with Gabriele Manduchi to plan MDSplus development.
Ian
Hutchinson visited IPP Greifswald, Germany, for three weeks in May-June to collaborate
with Olaf Grulke, Thomas Klinger, and other scientists at the Institute and at
the
Ted
Golfinopolos and Geoffrey Olynyk participated in the ITER International Summer
School at the