Alcator C-Mod Quarterly
Progress Report – FY09 Q1
The main activity at Alcator C-Mod during the first quarter of FY09 was a continuation of the magnet and alternator inspection.
Science
Results
A C-Mod paper was on the shortlist for the 2008 Nuclear Fusion Award. The ten shortlisted papers are judged to be those of the highest scientific standard, selected from the journal volume published two years previous to the award year. Nominations are based on citation record and recommendation by the Board of Editors. The paper is: B LaBombard: (2004) Nuclear Fusion, 44 (10), pp. 1047-1066
Information about the main ion velocity in the pedestal region can be inferred by comparing estimates of the main ion diamagnetic contribution to the total radial electric field profile. This comparison is potentially useful as it lends itself to determining source information about the radial electric field and can be used to compare to theoretical predictions of main ion behavior. An estimate of the main ion diamagnetic contribution (using Thomson scattering electron profiles) demonstrates a radial electric field profile with remarkable similarity to the total radial electric field profile, but radially shifted (~4mm) and displaced in magnitude. A detailed comparison of the electron temperature profiles and the CXRS boron temperature profiles indicate that the EFIT mapping of the electron profiles is likely incorrect by at least 3 mm. This mapping error eliminates, within the error bars, the radial shift between the total electric field profile and the main ion estimate, but does not account for the magnitude displacement. To bring the main ion estimate into agreement with the calculated Er profile, the main ions must have either significant co-current toroidal rotation or ion diamagnetic poloidal rotation, or some combination of the two. The exact magnitude and shape of the main ion toroidal and poloidal velocity profiles can not be determined within the error bars, but the results are not in disagreement with neo-classical predictions of main ion velocities in the Pfirsch Schluter regime, and it is clear that the main ions cannot be stationary in the edge.
Lower hybrid waves, when applied to EDA H-mode plasmas, have been observed to promote reduced plasma density and increased temperature, through regulation of the edge pedestal. The temporal response at the edge in H-modes with applied LH power may provide clues to understand the mechanisms that are governing the pedestal modification. Edge and core phenomena are seen to occur on varying time scales, perhaps indicating an evolution in the deposition location and/or damping mechanisms of the LH waves. Prompt increases in Ly-alpha brightness and electron density are observed in the scrape-off layer when LH is applied, with the drop in both pedestal and core densities occurring on approximately 100 ms time scales. Concurrent changes in the density fluctuations measured by phase-contrast imaging are also observed, pointing to an enhancement of existing radial particle transport, which allows the relaxation of the pedestal density gradient. Previously reported core toroidal flow modification occurs following this pedestal relaxation.Jim Terry has been investigating if blob/filaments reach the divertor target. The filaments are the turbulent structures present in the far-SOL and are primarily responsible for the radial particle transport there. Previous analysis of data from Gas-Puff-Imaging (GPI) at the outboard mid-plane, and outboard of the lower x-point region, has shown that filaments map magnetically between the two regions. This result was predicted by theory. In doing so, the filaments are elongated and sheared in cross-section in the x-point regions. The new work compared the statistics of the SOL fluctuations at the mid-plane with those at the divertor target. Since the filaments dominate the density fluctuations in the main chamber and show strong intermittency there, similar statistic signatures at the divertor targets would indicate the connection of the filaments all the way to the targets. Upon examination, the statistical signatures were quite different, implying that the filaments may be "disconnected" from the targets, a phenomena described in the theoretical work on filaments by Myra and D'Ippolito. The elongation and shearing of the filaments through the X-point region plays a key role in the disconnection.The influence that magnetic shear and perpendicular plasma flow shear may have on controlling edge pressure gradients near the last-closed flux surface (LCFS) has been investigated by Brian LaBombard. In Ohmic L-mode discharges, where magnetic topology was varied systematically from double-null to inner-wall limited, scrape-off layer plasma temperature and pressure profiles are found remarkably invariant. Normalized pressure gradients, as characterized by the MHD ballooning parameter, amhd, remain persistently in the range of ~0.5 at a location 1 mm outside the LCFS, regardless of whether the LCFS is defined by the inner-wall (low magnetic shear case) or by a magnetic separatrix (high magnetic shear case). A key piece of the puzzle appears to be perpendicular plasma flow shear just outside the LCFS independent of magnetic topology; it is very strong, comparable to the ideal ballooning growth rate; the physics of flow shear may dominate over magnetic shear in these discharges. In discharges where the X-point topology was varied from lower-null to upper-null, edge pressure gradients show a systematic decrease. Coincidently, perpendicular plasma flow shear decreases, with reduced shear associated with reduced amhd values. Thus it appears that perpendicular flows play a dominant role in all these discharges, impacting the pressure gradients (amhd ) that can be attained near the LCFS, and differentiating between the physics of the near scrape-off layer region (steep gradients) and that of the far scrape-off layer region (flattened profiles with ballistic, blob-like transport).
Publications
2008 Nucl. Fusion 48 105005
TOFE Several of our engineers attended the 18th Topical Meeting on the Technology of Fusion Energy in San Francisco, CA from September 28 to October 2, and presented posters on engineering systems developed or being developed for C-Mod: Engineering Considerations in the Implementation of High Power Fast Ferrite Tuners on Alcator C-Mod, Alan S. Binus Disruption Load Calculations for Lower Hybrid Current Drive Couplers, Jeffrey Doody Transmitter Protection System Upgrade Design for Lower Hybrid Current Drive System on Alcator C-Mod, David R. Terry (presented by Pat MacGibbon) Waveguide Splitters for Lower Hybrid Current Drive, Pete Koert Conceptual Design of a New Outer Divertor for C-Mod, Peter Titus IAEA C-Mod was well represented at the IAEA Conference in Geneva, Switzerland from October 13 to 17. Attendees from the C-Mod team included Darin Ernst, Chuck Kessel (PPPL), Yijun Lin, Earl Marmar, Miklos Porkolab, John Rice, George Sips (IPP Garching), Joe Snipes (ITER IO), Dennis Whyte, Randy Wilson (PPPL), John Wright, and Steve Wukitch. There were 10 presentations which drew directly on C-Mod results, including 4 orals: Earl Marmar - Overview of the Alcator C-Mod Research Program Yijun Lin - ICRF Mode Conversion Flow Drive on Tokamak Plasmas, post-deadline George Sips - Experimental Studies of ITER Demonstration Discharges Dennis Whyte - Plasma Surface Interaction Issues of an All-metal ITER Presentations may be found at:http://www.psfc.mit.edu/research/alcator/pubs/iaea/index.htm
APS Forty-four C-Mod scientists and graduate students attended the DPP APS meeting in Orlando, Florida Nov.12-16. Four invited orals, 13 contributed orals, and 27 posters were presented on C-Mod results. Paul Bonoli gave a talk on "Lower Hybrid Current Drive Experiments on Alcator C-Mod: Comparison Between Theory and Simulation", Brian LaBombard discussed "Critical Gradients and Plasma Flows in the Edge Plasma of Alcator C-Mod", Steve Wukitch talked on "ICRF Performance with Metallic Plasma Facing Components: Revenge of the Sheath", and Valerie Izzo, University of California, San Diego, gave a talk on "MHD Simulations of Disruption Mitigation on DIII-D and Alcator C-Mod". John Rice presented a talk on "Spontaneous Rotation in Tokamak Plasmas" at the Angular Momentum Mini-Conference associated with the meeting. The presentations may be found at http://www.psfc.mit.edu/research/alcator/pubs/APS/APS2007/orlando2007index.htm
There were two mentions of C-Mod results presented at the DPP APS meeting in the Jan.2008 issue of APS News, regarding disruption mitigation and edge turbulence imaging. IEA/ITPA Workshop On December 11-13, the MIT PSFC hosted the Seventh IEA Large Tokamak Workshop (W69) on Implementation of the ITPA Coordinated Research Recommendations. This meeting is a joint workshop held under the auspices of three IEA implementing agreements related to tokamak research: the Large Tokamak I.A.; the Poloidal Divertor I.A.; and the Plasma Wall Interactions in TEXTOR I.A. Attendees included representatives from the international ITER Organization, ITPA topical group leaders, ITPA Coordinating Committee, ITER domestic agencies, the US Burning Plasma Organization, USDoE OFES and tokamak facilities from around the world. The meeting discussions included ITER high priority research needs, ITPA plans to address those needs, and related experiments and modeling. The final two half days of the meeting were devoted to detailed discussions of ITPA joint experiments proposed for calendar year 2009. Participants came to the workshop from the U.S., Europe, Japan, Korea and India. Sessions were also attended through remote teleconference by participants from the ITER Organization site in Cadarache, France.
Further analysis has been done on data obtained in experiments performed last April investigating the effects of LHCD on subsequent H-mode evolution. This effort was led by George Sips, IPP Garching, as part of an ITPA Joint Experiment in the SSO (now Integrated Operation Scenarios) group aimed at comparisons of the "hybrid" scenario. In these experiments, 800 kW of LHCD was injected into the current ramp phase of600 kA discharges, and reproducibly delayed sawtooth onset was observed until well into the flat top, indicating significantly modified j(r). Three MW of ICRH was then added, at varying times in different discharges, not only triggering H-modes, but also initiating sawteeth. Stored energy was found to be transiently higher in H-modes for which the target j(r) was modified by LHCD. Recent comparisons show that in these cases the L-H transition is significantly delayed, occurring typically 60-70 ms after ICRF turn-on, in contrast to ~ 20 ms with Ohmic targets. Rates of density rise are also slower. In all H-modes, the stored energy correlates well with edge temperature, suggesting that differences in pedestal evolution (higher Tped) are primarily responsible for the improved confinement. Modeling of LHCD in these discharges is in progress to assess the degree of j(r) modification. These results were presented by Sips at the IOS ITPA meeting in Lausanne.
Reviews Lower Hybrid Launcher Review On September 9th a successful final design review of the new lower hybrid launcher was held at MIT. Steve Knowlton, Auburn University, and Douglas Loesser, PPPL, comprised the external review panel. After a welcome by Earl Marmar, Ron Parker, Randy Wilson (PPPL), Peter Koert, Rui Vieira, Jeff Doody, Orso Meneghini, and Jim Irby gave presentations. These may be found athttp://www.psfc.mit.edu/research/alcator/data/launcher_review_.pdf
DoE Quarterly Review The FY08 4'th quarter C-Mod DoE review was held on Monday, October 27th, at MIT. Presentations were given by Earl Marmar, Jim Irby, Steve Wukitch, Bob Granetz, Rui Vieira, John Rice, and Bruce Lipschulz. Rostom Dagazian represented OFES. Randy Wilson and Chuck Kessel, PPPL, and Bill Rowan and Perry Phillips, U. Tx, participated remotely. Links to the viewgraphs from the presentations can be found at:http://www.psfc.mit.edu/research/alcator/pubs/quarterlies.html.
Steve Scott, PPPL, visited PPPL during the week of Sept. 29 for discussions with the PPPL engineers regarding the MSE upgrade.
Martin Greenwald was in Austin, TX, from October 1 to 2 for a meeting of the FESAC Alternate Concepts panel. Dennis Whyte and Amanda Hubbard visited OFES to discuss university input to their strategic planning. Jim Zaks traveled to Chicago, Ill the week of Oct. 10 to discuss production of vessel heater components with our vendor. Catherine Fiore attended the semi-annual meeting of the APS Committee on the Status of Women in Physics, held in Chicago on 10/28/2008.
Martin Greenwald, Amanda Hubbard, Ian Hutchinson, Miklos Porkolab, and Dennis Whyte traveled to Washington the week of Nov. 3 to attend a meeting of the Fusion Energy Science Advisory Committee (FESAC). Martin Greenwald was appointed this week as the new FESAC chairman. Dennis Whyte presented a talk: " The Challenges of Plasma-Surface Interactions in ITER and Beyond" Dave Terry was in California the week of Nov. 10 at CPI inspecting klystrons before shipment back to MIT, and discussing the status of the new klystrons and ICRF final amplifier tubes being fabricated.
Steve Scott, PPPL, Jinseok Ko, MIT, and Syun'ichi Shiraiwa, MIT, traveled to PPPL to discuss the engineering design of a new thermal shield and shutter for the Motional Stark Effect diagnostic at C-Mod. Scott and Shiraiwa presented information and analysis of thediagnostic. Earl Marmar participated in the NSTX Research Opportunities Forum by teleconference, and made a presentation entitled: Alcator C-Mod Research Highlights and Plans Dennis Whyte attended the International High Heat Flux Components Workshop on Readiness to Proceed from Near Term Fusion Systems to Power Plants which was held at UCSD and was jointly organized by R. Raffray of ARIES, R. Nygren of Sandia and Dennis. He presented "Physics gap issues from ITER to reactors for integrating HHFC and edge plasmas". Dennis Whyte also visited GA / DIII-D, where he discussed experimental planning of the 2009 Joule milestone on fuel retention, as well as disruption mitigation experiments for ITER. He also re-presented his FESAC talk to the DIII-D group on "The challenges of Plasma-Surface Interaction for ITER & Beyond".
Miklos Porkolab attended the Annual Fusion Power Associates Meeting and Symposium in Livermore, California, on Dec 3-4, and presented the talk "Overview of Fusion Research at MIT". John Rice, Matt Reinke, and Yuri Podpaly attended the 2008 ITER Imaging Crystal Spectrometer Workshop on Tuesday, December 16 at PPPL. Matt Reinke gave a presentation on the C-Mod experience with HiReX Sr.
Bruce Lipschultz and Dennis Whyte traveled to Nagasaki Japan for an ITPA meeting on SOL/divertor physics. The meeting was held over the period Sept 15 to 18, hosted by T. Tanabe and M. Sakamoto of Kyushu University.
From September 23 to 24, Ron Parker attended the 4th and last meeting of the European Facilities Review Panel in Brussels. The Panel report is nearly finalized, and will become public toward the end of October.
Darin Ernst, Yijun Lin, Earl Marmar, Miklos Porkolab, John Rice, Dennis Whyte, John Wright, and Steve Wukitch attended the IAEA Conference in Geneva, Switzerland from October 13 to 17. Paul Bonoli, Amanda Hubbard, Randy Wilson, Bob Granetz, and Steve Wukitch attended the ITPA meetings in Lausanne, Switzerland from Oct 20 to 22. Paul, Amanda, and Randy, attended the first meeting of the new Integrated Operational Scenarios group. Paul gave two talks: "Recent Progress on Advanced ICRF Simulations for ITER and Alcator C-Mod" and "Comparison of LHCD Predictions for Alcator C-Mod and ITER Using the LSC and CQL3D Codes". Bob Granetz attended the MHD group meeting, and Steve Wukitch attended the Energetic Particle group meeting. Jerry Hughes attended the ITPA Pedestal Topical Group meeting, and John Rice attended the Transport Topical Group meeting, both in Milan, Italy from Oct 20 to 22. The Pedestal meeting was a forum for personnel from C-Mod, DIII-D, NSTX, MAST, JET, ASDEX-Upgrade, JT-60U and TEXTOR to discuss ITER-relevant questions. Also, in a joint session with the Transport and Confinement Topical Group, they discussed efforts needed to project H-mode accessibility to ITER (i.e. L-H transition thresholds). Among the tasks needed is a careful compilation of data for L-H threshold experiments from multiple machines; an effort which will be organized by Jerry Hughes.