Alcator C-Mod Quarterly Progress Report – FY08 Q2

 
The main activity at Alcator C-Mod during the second quarter of FY08 was operation of the device. 
 
 

Science Results

 
 
Following application of lower hybrid current drive (LHCD) power, the core toroidal rotation in Alcator C-Mod L- and H-mode plasmas is found to increment in the counter-current direction, in conjunction with a decrease in the plasma internal inductance, li. Shown in Fig.1 is a comparison of two discharges, with and without LHCD. Along with the drops in li and the core rotation velocity, there is peaking of the electron and impurity density profiles, as well as the ion and electron temperature profile, as shown in Fig.2. The mechanism generating the counter-current rotation is unknown, but it is consistent with an inward shift of energetic electron orbits, giving rise to a negative core radial electric field. The peaking in the density, toroidal rotation (in the counter-current direction) and temperature profiles occurs over a time scale similar to the current relaxation time but slow compared to the energy and momentum confinement times. Most of these discharges exhibit sawtooth oscillations throughout, with the inversion radius shifting inward during the LHCD and profile evolution. The magnitudes of the changes in the internal inductance and the central rotation velocity are correlated and found to increase with increasing LHCD power and decreasing electron density. The maximum effect is found with a waveguide phasing of 60o (a launched n|| ~ 1.5), with a significantly smaller magnitude at 120o (n|| ~ 3.1), and with no effect for negative or heating (180o) phasing. These results are consistent with the current drive efficiency scaling as PLH/nen||2). Regardless of the plasma parameters and launched index of refraction n||, there is a strong correlation between the rotation velocity and li changes, possibly providing a clue for the underlying mechanism.
 

 
 
 
 
 

Fig.1. Electron density, LHCD power, internal inductance and core rotation velocity for discharges with (red) and without (green dashed) LHCD.
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Fig.2. The electron density, rotation velocity and temperature profiles before (green) and during (red) LHCD.
 
 
 
 

Meetings

 
 
The 2008 meeting of the C-Mod Program Advisory Committee was held at MIT Feb 6-8. The focus of this year's meeting was the proposal for the next five year research plan on C-Mod, covering the period November 2008 through October 2013. Presentations on the plan from the C-Mod team were made on Feb 6 and 7; links to pdf versions of the viewgraphs can be found at
 
http://www.psfc.mit.edu/research/alcator/pubs/Cmod_PAC_2008.html. 
 
The PAC members are: Chuck Kessel, PPPL (Chair), Brian Lloyd, UKAEA, Alberto Loarte, ITER Organization, Craig Petty, General Atomics, George Sips, IPP Garching, Paul Terry, U. Wisconsin, Madison, Jim Van Dam, U. Texas, Austin, and Rostom Dagazian, DoE-OFES (ex-officio). The panel provided the C-Mod team with an initial debriefing, which included recommendations concerning the plan and associated proposal. 
 
 
A quarterly review (FY08 Q1) was held at MIT on Thursday, February 7. In attendance were Rostom Dagazian, Earl Marmar, Miklos Porkolab and Jim Irby. Current operational status and near term plans for C-Mod were discussed.
 
 
 
 

Collaborations

 
 
Run time was devoted to MP#522, "Lower Ip Long Pulse L-Mode and H-Mode Advanced Scenarios". Charles Kessel (PPPL) served as Session Leader using remote collaboration tools. The experiment continued development of target discharges for AT scenarios at 450kA (q95~7) with slow current ramps, using ICRF heating injected well before the start of the current flattop at 0.5sec. H-modes were obtained during the ramp using 2-3MW of ICRF. Control of the density to levels above the low-density threshold around 8x1019/m3 was necessary for entry to H-mode, with lower density cases remaining in L-mode. H-modes produced around 200ms were short-lived (dithery), with the earliest sustained examples having L-H transitions around 0.25sec. The H-modes were ELM-free, and often suffered radiative back transition to L-mode, although some had reduced radiation and lasted for over 250 ms. The earliest H-mode attempts at 200 ms did appear to delay sawteeth and/or change their character. A cursory look at the temperature profiles showed that the longer H-modes had good pedestal temperatures and were steady, in spite of being ELM-free. More detailed examination will focus on systematic changes in internal inductance, loop voltage, stored energy, and other plasma parameters.
 
 
 
Run time was devoted to MP#524 "Development of ITER relevant current rise phase", and was led by George Sips (ASDEX-Upgrade). The purpose of this experiment, which is part of a coordinated effort including ASDEX Upgrade, JET and DIII-D, is to obtain a dataset to be used in validating the proposed ITER Scenario 2 ramp-up phase. The ITER reference case calls for a ramp up to 15MA in 80 sec with 0.7< li < 1.0 throughout the ramp. The corresponding ramps on C-Mod have a flattop current of 1.35MA and ramp-up lasting between 0.4 and 0.6 seconds. Successful examples maintaining li<1 were obtained in the C-Mod experiments. The time of X-point formation was also brought as early as 0.23sec, when the plasma current is at just over half its flattop value. In addition to Ohmic ramp-ups, we also employed up to 3 MW of ICRF heating to increase the temperature during the current rise. The principal influence on li under all circumstances was dI/dt.
 
 
 
Half of a run day was devoted to MP#478 "Joint Experiments with JET on non-resonant n=2 magnetic braking", which supports ITPA Joint Experiment MDC-12. This experiment was led by David Howell (UKAEA), from the JET site using remote videoconferencing tools. This run continued C-Mod experiments begun in June 2007, looking for evidence of magnetic braking from non-resonant n=2 perturbations applied using the non-axisymmetric coilset (A-coil). In the earlier experiments we observed no braking of the intrinsic rotation in EDA H-modes with ICRH power of 2MW and plasma current of 800kA (q95~5). This contrasted with results from JET which had observed braking in NBI discharges, which was interpreted as being consistent with theoretical expectations based on neoclassical toroidal viscosity (NTV). Since the theory predicts stronger braking at higher ion temperature, these experiments were carried out with twice the auxiliary power (4MW). We obtained data at 800kA, for comparison with the previous experiment, and at 1MA (q95~4), which was closer to the q95 value in the JET experiments. No magnetic braking was apparent at either plasma current, implying that a straightforward extrapolation of the JET result based on the NTV theory is not consistent with the results. If the NTV theory is valid, the discrepancy might be due to differences in the plasma regimes (e.g. collisionality) or related to different response of the intrinsic rotation in C-Mod compared to the NBI-driven rotation in JET.

Two run days were devoted to MP#448a, "Comparison of Small ELM regimes on C-Mod, MAST and NSTX", which supports the C-Mod component of ITPA Joint Experiment PEP-6. In the first experiment, Rajesh Mainji (PPPL) served as session leader, and Hendrik Meyer (UKAEA) participated by remote video-conferencing from Culham. An RF power/plasma beta scan was accomplished for a series of discharges with small ELMs. The ICRF power threshold for small ELMs was ~ 3.1 MW, and the ELMs were still observed at the maximum ICRF power of 4.5 MW. The signatures of these small ELMs on soft X-rays, fast magnetics, and GPI were clearly observed. These ELMy discharges exhibited excellent confinement relative to the H98 scaling, and this will be an attractive regime for other experiments. ELMy H-mode access appeared to be remarkably sensitive to details of equilibrium shape, as was the pedestal temperature and the character of the quasi-coherent mode.
In the second run day, Rajesh Maingi (NSTX) and Hendrik Meyer (MAST) were at MIT to participate as co-Session Leaders. The first part of this run consisted of systematic scans of lower X-point position and Ip, with the aim of determining the ELM operational window. We achieved a fine scan of X-point positions, decreasing RXL in steps of 2 to 4mm for a total of -1.2cm shift. Contrary to our previous impression, ELMs proved accessible across this range (0.54 < dl < 0.61). Variation in the characteristics of the ELMs as a function of triangularity over this range is being analyzed. Based on initial impressions, the most reliable figure of merit for small ELM access in this range of triangularity is simply the elevation of the pedestal Te above ~600eV. A current scan from 700kA to 850kA revealed a fairly narrow window in Ip (q95~4.7 +/- 0.3) for obtaining these ELMs. Large ELMs were also revisited in this run, by running an increased lower triangularity of 0.75 at low elongation. A broad dataset of small ELMs in near-DN shape was obtained, including useful comparative discharges with varied shaping and collisionality, which will help advance our H-mode Baseline Integrated
Scenario studies.

Domestic Travel

Jim Terry was at General Atomics in San Diego the week of Jan.14 to participate in the DIII-D Program Advisory Committee meeting.

Bruce Lipschultz traveled to PPPL for the NSTX PAC meeting from Jan.22-24. NSTX plans for the periods through 2010 and 2013 were discussed.

Miklos Porkolab attended the first meeting of the newly formed multi-institutional group "Gyrokintic Simulation of Energetic Particle Turbulence and Transport", under the leadership of Prof. Zhihong Lin, UC Irvine. Porkolab is on the PAC of this group and the first meeting was organized on Jan 25, at UC Irvine, to discuss the goals of this program, including a strong element of validation by periodic comparisons with experiments from C-Mod, DIII-D and NSTX.

Martin Greenwald traveled to Washington the week of Feb.20 to participate in a meeting of the Fusion Energy Sciences Advisory Committee (FESAC). Ian Hutchinson and Miklos Porkolab also attended this meeting.

Amanda Hubbard and Jerry Hughes visited PPPL Feb. 28--29 in order to collaborate on two NSTX experiments, both of which contribute to ITPA joint experiments (PEP-6 and PEP-16), and in which C-Mod is a joint participant. The first experiment examined the dependence of ELM characteristics on magnetic balance, in particular the sensitivity of ELM type to proximity to double null. The following run constituted an NSTX contribution to a comparison study of small ELM and EDA regimes in C-Mod, MAST and NSTX. 
 
S. Wukitch, Randy Wilson, Paul Bonoli, John Wright and Steve Richardson (MIT PSFC Theory Division) presented papers at the US-Japan workshop held at PPPL Feb 27-28.
 
Dennis Whyte traveled to Princeton to present the PPPL colloquium "The Challenges of Plasma-Surface Interactions in ITER & Beyond"
 
Bruce Lipschultz attended the one day IAEA paper selection committee meeting in Gaithersburg MD on February 28. 
 
C-Mod engineers Jon Savage, Atma Kanojia, and Bill Burke attended a class on CPLD programming in Texas the week of Feb.28.
 
Earl Marmar attended and participated in a Fusion Facilities Coordinating Committee meeting on Monday, March 10 at OFES in Germantown, MD. Earl and Miklos Porkolab attended and made presentations at the OFES FY2010 Budget Planning meeting, March 11-12, in Gaithersburg, MD. The C-Mod presentation can be seen at URL
 
http://www.psfc.mit.edu/research/alcator/program/work_proposals.html
 
 
Martin Greenwald traveled to Washington, D.C. for the OFES budget meetings and to attend a two-day workshop on FES network requirements.
 
Martin Greenwald, Igor Bespamyatnov, Istvan Cziegler, Eric Edlund, Darin Ernst, Catherine Fiore, Jerry Hughes, Alex Ince-Cushman, Brian LaBombard, Liang Lin, Ken Marr, Rachael McDermott, Miklos Porkolab, John Rice, Bill Rowan, Steve Scott, Noah Smick, and Joe Snipes participated in the 21st US Transport Task Force Workshop in Boulder, CO the last week in March.
 

International Travel

Bruce Lipschultz presided at the 10th ITPA SOL/divertor meeting in Avila Spain January 7-10. He also presented 4 talks at the meeting - 'Divertor characteristics puzzles', 'Enhanced sheaths during ICRF', 'Projection of ITER wall fluxes', and 'A new surface ion beam diagnostic' (the last on behalf of D. Whyte). Following the ITPA meeting he traveled to Garching and on January 11 presented a joint ASDEX-Upgrade and materials science division seminar on 'D retention in a high-Z tokamak, Alcator C-Mod'. Following the talk he spent the day in discussions with the materials science group headed by J. Roth on the C-Mod and ion-beam results. Lastly, Bruce Lipschultz traveled to the FOM Institute at Rijnhuizen to be part of their scientific advisory committee meeting January 14-15.

Brian Labombard attended the ITPA meeting on SOL/divertor physics and a meeting of the international program committee for the upcoming Plasma-Surface Interaction Conference (May 2008). Both meetings were held in Avila, Spain.

Yijun Lin was invited to visit the Institute of Plasma Physics of Chinese Academy of Sciences (ASIPP) in Hefei, China, from 1/10 to 1/12 and to give a series of lectures on ICRF physics and applications on tokamaks at a plasma winter school sponsored by the Chinese Academy of Sciences. On 1/12, he discussed potential collaborations between ASIPP (EAST and HT-7) and C-Mod on ICRF, LH, divertor tungsten tile fabrication, long pulse MDSplus and other issues.

During the last week of February, Robert Granetz attended an ITPA MHD meeting
in Naka. The meeting was combined with the IEA Large Tokamak Workshop (concentrating on resistive wall mode physics) and the US/Japan MHD workshop.

Near Term Plans

Schedule

A draft schedule for the FY08 run period can be found at
 

http://www.psfc.mit.edu/research/alcator/facility/Operations/operation_schedule.pdf