Alcator C-Mod Quarterly Progress Report – FY11 Q2

The main activity at Alcator C-Mod during the second quarter of FY11 was operation of the tokamak.

 
Publications
 
Investigation of the validity of quasilinear theory for electron Landau damping in a tokamak using a broad-band wave effect 
Jungpyo Lee, Paul Bonoli, and John Wright 
Phys. Plasmas 18, 012503 (2011); doi:10.1063/1.3541836 
 
 
 
The effect of the radial electric field on neoclassical flows in a tokamak pedestal 
Grigory Kagan, Kenneth D Marr, Peter J Catto, Matt Landreman, Bruce Lipschultz and Rachael McDermott
2011 Plasma Phys. Control. Fusion 53 025008 doi: 10.1088/0741-3335/53/2/025008
 
 
 
Characterization of impurity confinement on Alcator C-Mod using a multi-pulse laser blow-off system 
N. T. Howard, M. Greenwald, and J. E. Rice 
Rev. Sci. Instrum. 82, 033512 (2011); doi:10.1063/1.3565448
 
 
 
Physics
 
Recently, Alcator C-Mod has been engaged in active research regarding lower hybrid current drive (LHCD) induced rotation. It had previously been observed that Alcator C-Mod exhibited counter-current LHCD induced rotation while Tore Supra and JT-60U exhibited co-current induced rotation. The results from all of the machines appear to be robust, but it seemed impossible for one mechanism to drive both counter- and co-current rotation. This work performed at Alcator C-Mod has made great progress towards resolving this conundrum by identifying the parameters that appear to control LHCD rotation direction and magnitude. It has been observed that the rotation change induced by LHCD has a magnitude that scales with plasma current and can change direction based on the current as well (see figure 1). This is the first observation of rotation changes in both the co- and counter-current direction on one machine. It is further observed that the orientation of the plasma, lower single null (LSN) or upper single null (USN), has an effect on the point of the LHCD zero rotation crossing.
 

Figure 1. Changes in plasma rotation as a function of plasma current. Changes are taken solely during the LHCD phase of the C-Mod discharge.
 
The mechanism driving this rotation is currently being explored. An interesting observation is that the rotation appears to be localized mostly to the core of the plasma (see figure 2), lending credence to the idea that this is primarily a LHCD deposition effect rather than being due to modification of the edge pressure gradient. It is being explored if the effect of direct momentum input from the wave on the plasma can explain the counter current rotation. Another approach that has been proposed is that LHCD is modifying the turbulence in the core, possibly destabilizing ITG turbulence and thus changing the rotation behavior. This is currently being tested with gyrokinetic simulations and seems to be supported by the observation of turbulence behavior during the counter-current rotation phase of the LHCD plasma.

Figure 2. Characteristic rotation profiles of the USN LHCD case. Lower hybrid turns on at 0.8 seconds and lasts until 1.3 seconds. 
 
Given this work’s new nature and possible impacts of controlling the rotation profile and better understanding of current drive, it is being actively pursued. More detailed analyses are also ongoing including turbulence simulations, density and temperature profile changes, and transport simulations to calculate the q and shear profiles.
 
 
 
 
 
 
 The formation of nano-filaments, or “fuzz”, on the surface of high temperature refractory metals exposed to low-energy helium plasmas has been well documented in linear plasma devices around the world. However, these nano-filaments have never been grown inside a tokamak. The high power density and refractory metal wall of Alcator C-Mod makes it well suited to achieve the surface conditions necessary to grow this fuzz (Tsurface > 900 K, ΓHe+>1022 m-2s-1, 10 eV < EHe+< 150 eV). In the C-Mod lower divertor there are a set of tiles ramped ~2o into the parallel plasma flux for thermal load analysis with a nearby tungsten Langmuir probe ramped ~11o (see figure 3a). On these ramped tiles we were able to achieve the necessary surface temperatures for fuzz growth (figure 3b). On the last day of the campaign, shots were dedicated to accruing growth time at these conditions, which resulted in ~12 s of total growth time. Examination of the surfaces after removal from C-Mod revealed the growth of fuzz on the tungsten Langmuir probe as indicated by an optically dark surface (figure 4). The nano-filaments look quite developed for such a short amount of growth time, which may extrapolate to very thick layers of fuzz growth in a long-pulse device such as ITER. The consequences of these fuzz layers on reactor performance and lifetime remain unclear. There was no observable fuzz growth on the ramped molybdenum tiles or calorimeters. Possible reasons for this lack of fuzz are the higher sputtering rates of molybdenum as compared to tungsten, interference of boron plasma impurities and deposits, or the electrically/thermally isolated and more steeply ramped tungsten probe reaching and remaining at the correct temperature range for fuzz growth for a longer time than the molybdenum tiles. Follow-up experiments on the DIONISOS linear plasma experiment are planned to clarify the lack of fuzz growth on the molybdenum surfaces.
 

Figure 3: a) Photograph of the W Langmuir probe (blue) and Mo calorimeters (red and green) after removal from Alcator C-Mod. The black surface on the Langmuir probe is indicative of the presence of “fuzz”. b) An IR image of the ramped tiles during a plasma discharge in Alcator C-Mod. Tmax ~1300 K for these discharges.
 

Figure 4: An SEM image of the surface of the W Langmuir probe shown in figure 1. W nano-tendrils are fully formed on the surface.
 
 
 
 
 
 
 
Meetings

The 2011 C-Mod Ideas Forum was held Jan.10-11. The Forum aids in the planning of the FY11/12 C-Mod Operating Campaigns by providing an opportunity for presentation of specific ideas appropriate for experiments during the upcoming runs. The entire forum was available for remote participation using a variety of conferencing tools. 113 presentations were made by participants from multiple institutions around the world, including 11 from the ITER organization. Copies of the presentations are available on the web at:

http://www.psfc.mit.edu/research/alcator/program/ideas2011/Ideas_forum_agenda-2011.htm

A breakdown of ideas by topic is shown here:

Quarterly Review

The C-Mod Quarterly Review for FY2011, Q1, was held on Monday, Jan. 10. In attendance were Mark Foster (OFES), Earl Marmar, Miklos Porkolab and Jim Irby. Discussion topics included updates on ARRA projects, C-Mod facility status, near term operations plans, the FY2013 Work Proposal schedule, the March, 2011 PAC meeting, and the mid-term facility review expected in June.

PAC Meeting

The Alcator C-Mod Program Advisory Committee met on Mar. 2-4 at MIT. PAC members attending included Rich Hawryluk (PPPL, Committee Chair), Troy Carter

(UCLA), Steve Knowlton (Auburn), George McKee (U. Wisc.), Richard Fitzpatrick (IFS, U. Texas), Phil Snyder (GA), and Richard Pitts (ITER Organization). Mark Foster (OFES) also participated in the meeting. C-Mod personnel provided presentations on recent C-Mod results and plans; slides are available on the web at

http://www.psfc.mit.edu/research/alcator/pubs/Cmod_PAC_2011.html

The PAC was asked to provide advice on our research and upgrade plans through

FY2013. Rich Hawryluk presented a preliminary panel report at the end of the meeting. DoE-OFES representatives participated in the debriefing session by videoconference.

ITPA Pedestal Group

The PSFC hosted the 20th meeting of the ITPA Pedestal Topical Group from March 30 through April 1. Thirty ITPA members and guest experts attended the meeting in person while several more connected remotely, presenting an array of recent results of experiment and modeling in the pedestal research area. Over 20 institutions from the US, EU, China and Korea and Japan were represented. The scientific program included reports of progress in the areas of pedestal structure, ELM mitigation and the L-H transition. Amanda Hubbard, Jerry Hughes and Yunxing Ma presented recent results and plans on I-mode physics, pedestal control and L-H transition studies on C-Mod. Several meeting participants took advantage of their visit to discuss ongoing and proposed collaborations with C-Mod staff and students.

Tom Osborne (GA), Marc Beurskens (CCFE) and Lorenzo Frassinetti (KTH) completed their visits to collaborate on analysis of pedestal data in ELMy H-mode. While on-site, Tom completed an installation of his latest pedestal profile fitting tools and demonstrated their ability to produce meaningful profile analysis and kinetic EFITs using C-Mod data. The incorporation of these tools directly assists our efforts to complete the FES FY11 Joint Research Target (JRT) in pedestal physics.

A mid-term meeting of the JRT contact persons was held in conjunction with the ITPA meeting. This meeting was chaired remotely by Rich Groebner (GA), and had participation from several involved scientists, including the main contact persons from C-Mod, DIII-D and NSTX, and the GA, LLNL and PPPL theory groups.

Domestic Travel

Dennis Whyte and Paul Bonoli attended the NSTX Program Advisory Committee Meeting at PPPL on January 26-28, 2011. They are both currently members of the NSTX PAC.

Dennis Whyte attended the Fusion Nuclear Science Pathways Assessment meeting at UCLA Jan 24-25.

Dennis Whyte attended the NRC committee meeting on Inertial Fusion Energy in

San Ramon, CA.

Paul Bonoli and Martin Greenwald attended a Planning Workshop for the Fusion

Simulation Project that was held at General Atomics on February 8-11, 2011.

Martin Greenwald was at General Atomics in San Diego for a meeting of the

DIII-D PAC; Martin serves as chair of this committee.

Martin Greenwald and Amanda Hubbard traveled to Washington D.C. to participate in a meeting of the Fusion Energy Sciences Advisory Committee. Martin chairs the committee, and Amanda is a member. Miklos Porkolab also attended the meeting.

Martin Greenwald traveled to Washington to participate in a Fusion Simulation Program briefing for Ed Synakowski and OFES staff.

Martin Greenwald traveled to PPPL for a meeting of the Fusion Simulation Project (FSP) planning team, then to Washington DC where he presented a briefing to the NRC Plasma Science Committee.

Amanda Hubbard visited Univ. Wisconsin at Madison on Mar.14, and gave a seminar on "Separation of energy and particle transport barriers in the I-Mode regime in Alcator C-Mod". She saw and discussed research on several U.W. plasma experiments in different departments, and had discussions on future collaborations regarding I-mode models.

Catherine Fiore gave an invited talk at the American Chemical Society meeting on March 28th in Anaheim, California in the Francis P. Garvin-John M. Olin Symposium in Honor of Sherry Yennello. Her talk, 'Gender Equity Conversations: A CSWP program,' was given on behalf of the APS Committee on the Status of Women in Physics.

Mar.28-30 Earl Marmar was at PPPL for a meeting of the FSP Program Advisory Committee, on which he is a member. Martin Greenwald participated in the meeting as a member of the FSP planning team.

International Travel

Bruce Lipschultz attended the review of the Max Planck Institute of Plasma Physics program January 12-14, 2011. That program includes both the Wendelstein and ASDEX experiments. Bruce also spent time on January 11 and part of January 14 meeting with IPP-Garching staff on a range of topics

Bob Granetz participated in the ITPA MHD topical group meeting held in Ahmedabad, India. While in India he gave two presentations to the C-Mod PAC via remote conferencing tools, including divertor modeling, heat load footprints, CXRS measurements, impurity source rates, and impurity seeding.

Robert Granetz attended the 1st Indo-US workshop on collaboration in fusion, held at the Institute for Plasma Research (IPR) in Bhat, Gandhinegar. The major US fusion facilities presented summaries of current research and near-term plans. During extended discussion periods, the most fruitful areas for collaborations were discussed.

Paul Bonoli attended the US-Japan Workshop on Integrated Modeling and Simulation in Toroidal Plasmas that was held at Kyoto University from March 9-11, 2011. The meeting was jointly hosted by Professor Atsushi Fukuyama from Kyoto University and by Paul Bonoli from MIT. Paul Bonoli gave two talks at the meeting titled "Integrated Modeling Activities in the US" and "Recent Advances in Integrated Modeling in the SciDAC Center for Simulation of Wave-Plasma Interactions".

Bruce Lipschultz traveled to the ITER site to chair a conceptual design review for the pressure gauges to be used in the divertor and main chamber of ITER. The meeting was held March 15-16.