Alcator C-Mod Quarterly Progress Report – FY10 Q2

The main activity at Alcator C-Mod during the second quarter of FY10 was operation of the machine. There were 31.3 research days and 7.8 research weeks.

Two Alcator C-Mod graduate students, Jason Sears, MIT Department of Electrical Engineering and Computer Science, and Brock Bose, MIT Department of Physics, successfully defended their PhD thesis work last week. Jason's thesis title is "Alfven Eigenmodes in Alcator C-Mod"; Brock's thesis title is "Plasma Turbulence following Li Pellet Injection into Alcator C-Mod Plasmas".

Physics

Several runs have been devoted to the study of I-mode:  a steady-state operational regime with H⁹⁸ ~ 1, and L-mode particle and impurity confinement. I-mode plasmas are formed with unfavorable drift using ICRF heating. Unlike H-mode, there is often no clear bifurcation. Energy confinement and intrinsic rotation are similar to those found in EDA H-mode. The pedestal collisionality can approach ~ 0.1, with no density pedestal, a long SOL density scale length and good density control. The edge particle transport may be regulated by an edge density/magnetic oscillation similar to the QCM in EDA H-mode, but at higher frequency and broader in frequency space. I-mode occupies operational space at lower q₉₅ and collisionality than EDA H-mode, as shown in Fig.1. Also, energy confinement does not degrade significantly with heating power. All of these features make I-mode an attractive operational scenario.

Meetings

The Alcator C-Mod Program Advisory Committee (PAC) met at MIT Jan.27-29. Members of the PAC include Rich Hawryluk (PPPL, Chair), Alain Becoulait (CEA), Alberto Loarte (IO), George Sips (JET), Steve Knowlton (Auburn), Craig Petty (GA), Troy Carter (UCLA), and Richard Fitzpatrick (IFS, U. Texas). Mark Foster and Nermol Podder represented the USDoE OFES. Links to the presentations to the PAC by C-Mod researchers can be found at

http://www.psfc.mit.edu/research/alcator/pubs/Cmod_PAC_2010.html

Following the conclusion of the PAC meeting, a C-Mod quarterly review was held

at MIT on Friday, Jan. 29. Participating in the review discussions were Earl Marmar, Miklos Porkolab, Randy Wilson, and Mark Foster. There were no formal presentations.

Figure 1

Collaborations

The run on Mar.17 was dedicated to MP#535a "Rotation inversion vs density and current, in limited and diverted L-mode plasmas", which falls under ITPA TC-9 and addresses toroidal rotation phenomena observed on TCV and on C-Mod in low density Ohmic discharges. Basil Duval (EPFL) served as Session Leader for this experiment. The goal of the experiment was to check the toroidal reversal with limited and diverted configurations and to attempt to move between the configurations to observe changes. Reversal of rotation as a function of density was observed in (LSN) diverted configurations, and was demonstrated to be reproducible. An example of three discharges with reversals is shown in Fig.2, achieved with density ramps. The reversals all occurred at the same electron density. The reversal was not clearly observed in the inboard limited configuration, possibly because it was not possible to access sufficiently low density. A change from counter- to co-current rotation was observed when changing between limited and diverted equilibria. A scan of q₉₅ from 3 to 4.5 confirmed the scaling with plasma current. A reversed transition was observed at q₉₅=3.6 by allowing the density to decrease after the initial part of the discharge. The data obtained support the conclusion that the toroidal velocity reversal in C-Mod is consistent with that observed on TCV. A drop in high frequency turbulence was observed with the PCI diagnostic following the reversals.

Figure 2

On Mar.19, we carried out MP#601a "Experimental Study of ITG Stiffness and Threshold (8T and 70 MHz)". This MP supports the ITPA Transport and Confinement Joint Experiment TC-13, the purpose of which is to measure ion threshold and stiffness by varying gradually the ion heat deposition profile from 100% on-axis to 100% off-axis using multi-frequency ICRH, and to compare with theory for transport model validation, with the aim of improved extrapolation to ITER. This experiment is also part of a collaboration between C-Mod and JET under IEA Large Tokamak Facility Implementing Agreement. Dr. Paola Mantica (Istituto di Fisica del Plasma CNR-EURATOM, Milano, Italy) served as co-Session Leader using remote collaboration tools from Milano. The experiment was conducted at I_p=800kA (q₉₅~6.3) in order to minimize the effect of sawteeth. On- and off-axis ICRF D(He³) heating at 80.5 and 70MHz was employed, both in the minority and mode conversion regimes. Dominant ion-heating was obtained in the minority regime, while direct electron heating and flow drive were observed in the MC regime. The MCFD was used to assess the effect of changing the velocity shear. The RF power was modulated to facilitate the analysis. Time-dependent profiles of T_i and velocity were obtained using the HIREX_SR diagnostic. We also documented one discharge with strongly ramping plasma current (0.8 to 1.2MA in 250msec) to assess the role of changing the magnetic shear on the ion profile stiffness.

Domestic Travel

John Rice attended the kickoff for the CMTFO (Center for Momentum Transport and Flow Organization) at UCSD on Jan.11-12, and presented a talk on C-Mod rotation results.

Yunxing Ma visited Gary Staebler at General Atomics from 1/11/2010 to 1/14/2010. The purpose of this visit is to install TGLF and XPTOR codes on Loki cluster at the PSFC and use them to model the transport physics in fusion plasmas. The installation has now been successfully tested on DIII-D and C-Mod cases.

Paul Bonoli and Bruce Lipschultz traveled to Princeton Plasma Physics Laboratory last week where they served on the NSTX Program Advisory Committee. The Advisory Committee Meeting was held from February 3-5, 2010.

Martin Greenwald (FESAC Chair), Amanda Hubbard (FESAC member), Miklos Porkolab, and Jeff Freidberg attended the FESAC meeting March 9-10.

Earl Marmar, Miklos Porkolab and Martin Greenwald attended the OFES Budget Planning meeting on March 11-12. Earl presented a summary of the C-Mod research plans detailed in the FY12 Work Proposal, which can be found at

www.psfc.mit.edu/research/alcator/program/Work%20Proposals/FWP_FY10-12_Draft.pdf

Miklos made a presentation on PSFC Institutional Issues.

Syunichi Shiraiwa and Orso Meneghini participated in the US-Japan Workshop on RF Physics held at General Atomics in San Diego the week of Mar.9. Syunichi presented a paper on "Development of LH full wave simulation based on FEM" and Orso presented "Coupling core and edge plasma wave simulations".

Paul Bonoli and Martin Greenwald attended a Planning Workshop for the Fusion Simulation Project that was held in Boulder, Colorado from March 15-18, 2010.

Earl Marmar traveled to Germantown to participate in the March 24 US-Japan Executive Secretary's Meeting. He made presentations on C-Mod status and plans, and on proposed collaborations.

Martin Greenwald was at PPPL to make presentations to the Fusion Simulation PAC. Earl Marmar attended as a member of the PAC, which met on March 25-26.

International Travel

Amanda Hubbard traveled to Ottawa the week of Feb.16 to participate in a review of proposals as a member of the Physics Grant Selection Committee of the Natural Sciences and Engineering Research Council of Canada (NSERC).

Robert Granetz attended an ITPA MHD meeting at the NIFS site in Japan the week of Mar.9. The two primary topics for this meeting were 3D MHD and disruptions.

John Rice was at Culham, UK to attend the ITPA Transport and Confinement group meeting Mar. 22-25.