Alcator C-Mod Quarterly Progress Report – FY11 Q1

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

The final report on the FY2010 Joint Research Target milestone was submitted to the US DoE Office of Fusion Energy Sciences on October 8. The primary goal of the milestone was to "Conduct experiments on major fusion facilities to improve understanding of the heat transport in the tokamak scrape- off layer (SOL) plasma..." Coordinated experiments were carried out on Alcator C-Mod, DIII-D, and NSTX, and a number of important results were obtained. All high level goals of the milestone were met.

C-Mod contributions to the FES Joint Facilities and Theory Research Target, FY11, Q1

The FY2011 FES Joint Research Target is focused on developing enhanced predictive capability of the pedestal in tokamaks, using a combination of experiment, theory and modeling. (The complete target statement is appended below.) Along with DIII-D and NSTX, Alcator C-Mod is committed to contributing experimental data to satisfy the research goals of the JRT, including accurate high resolution edge profiles, including radial electric field, and pedestal fluctuation characteristics. After completing a combined pedestal workshop with NSTX in September 2010, a framework for an experimental campaign in pedestal physics was developed for C-Mod, and a set of experimental sessions was developed that would meet the goals of the FY11 JRT.

Because little operation is planned for the second half of FY11, it is essential to have completed all experimental sessions needed for the JRT in the first half. In the first quarter, approximately seven run days were devoted to experiments in pedestal physics, nearly one run for each week of machine operation. In collaboration with P. Snyder (GA), we have obtained a new data set in ELMy H-mode, which are being used to test the EPED class of models for pedestal structure. A plasma current scan was completed, and the effects of elongation and triangularity on the pedestal and ELMs were evaluated. Helium H-modes, with and without ELMs, were also accessed. As of the end of the first quarter, a substantial portion of the pedestal research plan had been completed, and the priority experiments for the second quarter were determined. These included a toroidal field scan in ELMy H-mode and a session to document carefully the pedestal profiles and fluctuation characteristics in EDA H-mode as a function of plasma current, density and upper triangularity.

Collaborations with theorists and modelers are essential to successful completion of the JRT. Models for pedestal transport and structure are to be tested, as well as numerical predictions for pedestal fluctuations. Initial comparisons of C-Mod pedestal data to predictions from several versions of the EPED model (P. Snyder) have met with early success. Good agreement of the predicted pressure pedestal height with experimental values has been obtained over a wide range of plasma currents. The newest version of the model, EPED1.6, includes more accurate stability calculations, which are essential to getting good agreement with the C-Mod data. The C-Mod contribution extends the range in absolute pedestal pressure over which the EPED model has been examined, getting to within a factor of three of that expected in ITER (see figure). Continued analysis of these and follow-up data sets are planned, with particular emphasis on the entire pedestal structure (i.e. widths and gradients of both density and temperature pedestals).

Joint Research Target statement:

Experiment:

Improve the understanding of the physics mechanisms responsible for the structure of the pedestal and compare with the predictive models described in the companion theory milestone. Perform experiments to test theoretical physics models in the pedestal region on multiple devices over a broad range of plasma parameters (e.g., collisionality, beta, and aspect ratio). Detailed measurements of the height and width of the pedestal will be performed augmented by measurements of the radial electric field. The evolution of these parameters during the discharge will be studied. Initial measurements of the turbulence in the pedestal region will also be performed to improve understanding of the relationship between edge turbulent transport and pedestal structure.

Theory:

A focused analytic theory and computational effort, including large-scale simulations, will be used to identify and quantify relevant physics mechanisms controlling the structure of the pedestal. The performance of future burning plasmas is strongly correlated with the pressure at the top of the edge transport barrier (or pedestal height). Predicting the pedestal height has proved challenging due to a wide and overlapping range of relevant spatiotemporal scales, geometrical complexity, and a variety of potentially important physics mechanisms. Predictive models will be developed and key features of each model will be tested against observations, to clarify the relative importance of various physics mechanisms, and to make progress in developing a validated physics model for the pedestal height.

 
 
 
 
 
Publications
 

The paper "Inter-machine comparison of intrinsic toroidal rotation in tokamaks" by J E Rice et al., Nucl. Fusion 47 (2007) 1618, received the 2010 Nuclear Fusion journal prize.

 
 
 
 
 
Experimental study of reversed shear Alfvén eigenmodes during the current ramp in the Alcator C-Mod tokamak E M Edlund, M Porkolab, G J Kramer, L Lin, Y Lin, N Tsujii and S J Wukitch, Plasma Phys. Control. Fusion 52 (2010) 115003.
 
Flowing plasmas and absorbing objects: analytic and numerical solutions culminating 80 years of ion-collection theory I H Hutchinson and L Patacchini, Plasma Phys. Control. Fusion 52 (2010) 124005.
 
 
I-mode: an H-mode energy confinement regime with L-mode particle transport in Alcator C-Mod D.G. Whyte, A.E. Hubbard, J.W. Hughes, B. Lipschultz, J.E. Rice, E.S. Marmar, M. Greenwald, I. Cziegler, A. Dominguez, T. Golfinopoulos, N. Howard, L. Lin, R.M. McDermott, M. Porkolab, M.L. Reinke, J. Terry, N. Tsujii, S. Wolfe, S. Wukitch, Y. Lin and the Alcator C-Mod Team, Nucl. Fusion 50 (2010) 105005.
 
 
 
 
 
 
 
 
Physics
 
 

LHCD at high density on C-Mod

Significant progress has been made towards understanding the absence of LH generated fast electrons in high density (line averaged n_e > 1x10²⁰ m^-3) diverted discharges on C-Mod. Scans of plasma density in H and He discharges were conducted (Fig. 2) to determine the scaling of hard x-ray (HXR) emission during LH as a function of line averaged density. The scans show that non-thermal electron emission decreases dramatically at high density in He, although not as severely as for D. HXR emission was found to be a strong function of edge neutral pressure for each main ion species, which suggests that electron-neutral collisions in the SOL may play an important role in absorption of LH waves outside the LCFS.

Changes in the SOL density and ionization profiles during LH at high density show that the LH waves are absorbed outside the LCFS, while extremely low non-thermal emission from fast electrons inside the LCFS indicates that absorption on closed flux surfaces is weak at high density. Fig. 3 shows the increase in He line emission (and thus ionization) in the far SOL (ρ>1.5 cm) during LH. Estimates of power loss in the SOL due to the increased ionization (assuming poloidal and toroidal symmetry) are in the range of a few 100 kW.

Modeling of LH wave propagation and absorption including SOL effects shows that moving to regimes with higher single-pass absorption may help to reduce the effect of parasitic losses in the SOL. Simulations show that “prompt” losses in the SOL during the first radial transit from the launcher to the LCFS are small (< 25%). If absorption inside the LCFS is made to be significantly stronger (~100% of the power crossing the LCFS), then a majority of the LH wave power should be absorbed on closed flux surfaces. Experiments to investigate discharges using higher T_e and n_|| to increase absorption inside the LCFS have been completed and detailed analysis of these experiments is under way.

A combination of nonlinear GYRO simulations and synthetic diagnostics has been used as part of a feasibility study for a newly proposed core turbulence diagnostic. Presently at Alcator C-Mod, the only measure of core fluctuations is from Phase Contrast Imaging, which provides excellent information on the wavenumber spectrum of density fluctuations, but is a line-integrated measurement. Local turbulence measurements are desired in order better to test and validate the gyrokinetic codes. For this reason, a new Correlation Electron Cyclotron Emission (CECE) diagnostic is being designed with the aid of nonlinear GYRO simulations. A CECE diagnostic is used to measure long wavelength, turbulent electron temperature fluctuations. By applying synthetic CECE diagnostic models to a variety of experimentally realistic GYRO simulations, the feasibility of the new CECE diagnostic for C-Mod can be assessed. Specifically, the design of the quasi-optical focusing system and microwave radiometer receiver are constrained based on GYRO predictions for wavenumber spectra, power spectra, fluctuation levels, and correlation lengths of temperature fluctuations. Using nonlinear GYRO runs in the design study was required because measurements were attempted at C-mod previously, but no electron temperature fluctuations were seen and the reason for this was not understood. Because of the gyrokinetic simulation results we now have confidence that core turbulence in a typical C-Mod L-mode plasma is long-wavelength, ion-mode turbulence with a peak in the wavenumber spectrum near . Synthetic diagnostic modeling shows that the normalized fluctuation amplitude, , at can be clearly measured as long as the sample volume size, , is reduced to less than 1 cm. GYRO therefore explains why the past C-Mod attempts at CECE did not work - indicating that it was because the spot-size of the old system was too large at = 3 cm. The new CECE diagnostic at C-Mod will be built taking account of the GYRO results and attempts to measure turbulence will be made in the type of L-mode plasmas used in the design simulations. When the CECE diagnostic comes online and makes (or fails to make) core turbulence measurements, we will already know a great deal about the accuracy of GYRO predictions for core fluctuations in C-Mod. If successful, this test will provide increased confidence in applying the GYRO code to predict turbulence and transport in other tokamaks and burning plasma experiments where local, core fluctuation measurements have not yet been made.

 
 
 
Meetings
 

C-Mod was well-represented at the 23rd IAEA Fusion Energy Conference in Daejeon, Korea. Participants affiliated with C-Mod included Earl Marmar, John Rice, Yijun Lin, Greg Wallace, Syunichi Shiraiwa, Dennis Whyte, Chuck Kessel (PPPL) , Randy Wilson (PPPL), Val Izzo (UCSD), Miklos Porkolab, Bob Granetz, Peter Catto, Wendell Horton (U.Tx.), Orso Meneghini, and Alberto Loarte (ITER). Presentations and papers may be found on the web at

www.psfc.mit.edu/research/alcator/pubs/iaea

Miklos Porkolab served as chair of the International Program Committee for the Meeting.

A C-Mod DoE Quarterly Review (FY2010, Q4) was held by videoconference on Wednesday, October 27. Viewgraphs for the presentations from the review can be found at

www.psfc.mit.edu/research/alcator/pubs/Q Rev_2010_10_27/Q_Rev_2010_10_27.pdf

Participants included Mark Foster, Mark Koepke, Steve Eckstrand, and John Mandrekas from OFES, Randy Wilson (PPPL), Bill Rowan and Igor Bespamyatnov (U. Texas, Austin), and Earl Marmar, Miklos Porkolab, Dennis Whyte, Bob Granetz, Jerry Hughes, Brian LaBombard, Greg Wallace, Steve Wukitch, Amanda Hubbard, Bob Mumgaard, and David Terry at MIT.

Most of the scientific staff participated in the APS Division of Plasma Physics conference in Chicago. Copies of the C-Mod presentations are being posted on the web at

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

which included 4 invited talks, 15 contributed orals and 33 contributed posters.

Miklos Porkolab attended the Fusion Power Associates (FPA) meeting in Washington, DC, where he presented a talk on "Continued Research through the Next Decade on Existing Tokamaks is Critical to Make Magnetic Fusion a Viable Energy Source"; the vugraphs can be found at

http://www.psfc.mit.edu/research/alcator/pubs/FPA.Porkolab.Final.2010.pdf

At the meeting, Miklos also received a Distinguished Career Award from the FPA. The citation reads: In selecting Prof. Porkolab, the FPA Board recognizes his decades of career contributions to fusion research and development, including his pioneering contributions to wave-particle interactions, plasma heating and diagnostics development and the leadership he has provided to the MIT and world fusion programs.

Domestic Travel

Paul Bonoli attended the annual workshop of the SWIM Prototype Fusion Simulation Project that was held this year at Tech-X Corporation in Boulder, CO from October 27-29, 2010. He led discussions and a series of presentations on two physics problems that are being pursued by the SWIM Project.

Dennis Whyte presented "Implications of Plasma-Material Interactions" to the annual meeting of the Fusion Power Associates in Washington D.C. during the week of Dec. 1.

He also attended the first meeting on materials requirements for Fusion Nuclear Science Pathways in Gaithersburg, MD.

Miklos Porkolab also attended the Fusion Power Associates (FPA) meeting in Washington, DC the week of Dec. 1

Martin Greenwald was at PPPL Dec. 13-16 for a meeting of the FSP management team.

Dennis Whyte traveled to Washington DC for the first meeting of the NRC Committee on Inertial Fusion Energy.

International Travel

Earl Marmar, John Rice, Yijun Lin, Greg Wallace, Syun’ichi Shiraiwa, Dennis Whyte, Miklos Porkolab, Bob Granetz, Peter Catto and Orso Meneghini attended the IAEA FEC in Daejeon, Korea Oct. 10-16.
 

Amanda Hubbard attended the meeting of the Integrated Operational Scenarios ITPA TG in Seoul, Korea, Oct.18-21 and made presentations on C-Mod contributions to IOS, on the progress of IOS JE5.3 (on LHCD at high ne), on Pedestal Issues for ITER Integrated scenarios (to a joint session) and on C-Mod I-mode results (to the Transport and Confinement Group).

John Rice attended the ITPA TC group meeting in Seoul, Korea Oct. 18-20.

Jerry Hughes attended the ITPA Pedestal Topical Group meeting from 18--20 Oct in Seoul, as well as a session of the Transport & Confinement group meeting. He presented the status of several ITPA joint experiments in which C-Mod is participating (TC-3, PEP-6, PEP-26, PEP-27, PEP-28) and reviewed the status and plans for the ITPA H-mode threshold databases.

Bruce Lipschultz attended the ITPA SOL/Div group meeting in Seoul. Bruce chaired the committee meetings, which were held on October 18-20. The first day consisted of joint meetings of the SOL/Div group with the Pedestal group (on ELMs), with the MHD and EP groups on Disruptions, and with the Pedestal, IOS and Transport groups on fueling.

Following his participation in the IAEA meeting in Daejeong, Korea, Syun'ichi Shiraiwa visited several research institutes in Japan, including the TST-2 and QUEST STs, and LHD. Useful discussions were held concerning LHCD on a spherical tokamak, and a possible collaboration with LHD using their high frequency microwave sources.

Bruce Lipschultz visited JET Oct 28 and Nov 1 for discussions on CXRS diagnostics with representatives from JET and MAST. Plans for upgraded spectroscopy diagnostics for JET were discussed. Bruce also met with representatives of ADAS for discussions of what could become available for low-energy CX cross-sections. In-depth discussions were also held regarding the engineering of the JET ITER-like wall which may have applicability to the new C-Mod divertor. Bruce then attended the EU task force meeting on Plasma Wall Interactions in Vienna Nov 3-5, where he gave a talk on ITPA work in the same area and participated in discussions about the direction of the EU program.

John Rice was in Cadarache Nov. 15-16 for a design review of the ITER pedestal x-ray crystal spectrometer system.

Steve Wukitch visited our vendor's site in Backnung, Germany the week of Dec. 1for the final design review for the new fast ferrite tuners. He also participated in ITER ICRF antenna feedthrough design activities.

Greg Wallace attended the IAEA Steady State Operations meeting in Vienna Dec.6-8 where he gave a presentation on "Lower Hybrid Current Drive Studies on Alcator C-Mod."

Bruce Lipschultz traveled to Cadarache to attend two international meetings. The first, held Monday, Dec. 13, was the ITPA coordinating committee meeting, where he represented the ITPA SOL/Divertor group in discussions of ITER high priorities. On the second and third day he attended the IEA/ITPA joint planning meeting, both as the representative of the SOL/Div group and also as the facility representative from the C-Mod group.