Alcator C-Mod Weekly Highlights January 18, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 8.7 weeks Plasma Shots: 1049 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Two run days were scheduled and completed. The experiments supported miniproposals in the Transport area. A total of 64 plasma discharges were produced with a reliability of 94%. Prior to these runs, the J-port four strap antenna and associated transmitters were reconfigured from 78 MHz to 70 MHz operation. Plasma operations are planned to continue this week. Run Planning ------------ The 2011 C-Mod Ideas Forum was held on Monday and Tuesday last week. 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 meeting of the C-Mod Experimental Program Committee was held on Friday. Follow-up activities after the Ideas Forum were discussed. Each topical area or task leader will post a brief summary of the presentations in their areas on the web, and each topical group will convene to discuss organization and priorities for run time. In addition we reviewed four new or revised MiniProposals for runtime in the current experimental campaign. Operation Details ----------------- Thursday's run was devoted to MP#609a, "Dependence of momentum and particle pinch on collisionality, R/Ln and q". This MP supports an ITPA Joint Experiment TC-15 to clarify the parametric dependencies of momentum pinch on several plasma parameters that are known to play a key role. The separation of diffusive and convective (pinch) terms is to be accomplished by introducing a periodic perturbation on the toroidal rotation while performing shot-to-shot scans of plasma parameters, including collisionality, profile peakedness (R/L_n), and safety factor. Thursday's session explored three techniques to create a periodic rotation perturbation in C-Mod discharges: LHCD modulation, DNB modulation, and SSEP modulation. LHCD modulation was found to create a good v_tor modulation but the question remains whether the concomitant perturbations of density and electron temperature were too large for satisfactory analysis of the dependencies of the momentum transport. Modulation of the near-perpendicular diagnostic neutral beam did not result in sufficient rotation modulation for momentum transport studies. SSEP modulation, repetitively transitioning the equilibrium from lower to upper null, appears to be a satisfactory technique, as it results in a clear rotational perturbation and does not create any other significant perturbation in plasma parameters. Further run time is needed to optimize its use and to carry out the parametric scans required for the Joint Experiment. On Friday we carried out MP#631, "H-mode power threshold scaling with plasma density using off-axis ICRF heating". This MP comprises part of the research of an MIT graduate student. The goal of this run was to investigate the effect of heating deposition profile on the H-mode power threshold and L-H transition. Additional goals of the run were exploration of the Bt dependence of Pth, and low density H-mode behavior. In all cases, the pedestal parameters at the L-H transition were documented in order to elucidate the underlying physics of any variation in the threshold power. The first portion of the run was conducted with intermediate target density, nebar~1.5e20/m^3, at three values of toroidal field, 5.2, 4.5, and 3.9 tesla. On and off-axis heating employed fundamental proton minority heating at frequencies of 80 and 70 MHz. It was found that the threshold was essentially unchanged (P_th~2 MW) except in the case where the resonance was placed far off-axis to the high-field side (B=3.9T, f=80MHz), in which case the threshold power doubled to P_th~4MW. The difference in this case may be related to RF physics. Further analysis of these results is in progress. The second part of the run was carried out at lower density, nebar~0.9e10/m^3 at Bt~5.2T and nebar~0.7e20/m^3 at Bt=4.5T. Again, no significant difference was found between the 70MHz and 80 MHz threshold powers for these conditions. The comparison with the resonance to the high field side (Bt=3.9T) was not completed, but the threshold for f=70MHz appeared to be elevated. It was also noted that at low target density (nebar~0.7e20/m^3) we observed transitions to ELMy H-modes at 3.9 and 4.5 tesla. The H-mode phase of these discharges also had a rather low density, with nebar~1e20/m^3. ICRF Systems ------------ FMIT#3 and #4 and J antenna transmission network was tuned to 70 MHz last week, and then successfully operated with up to 3MW of coupled power late in the week. However, one of the demodulator channels used in the arc detection protection circuit developed a significant noise problem. We will calibrate a replacement for this demodulator this week. We have begun manufacturing the short strip line feeds for straps #2 and #3 for the ARRA funded rotated antenna through conventional machining. Our EDM machine has been serviced and qualification tests will be done to verify operation. The resumption of EDM machining will allow us to finish the current strap machining in preparation for brazing the bridge and strip lines to straps #2 and #3. The port flange surface preparation is nearing completion and we have sent the manufacturing drawing for the top/side/bottom plates out for quote. The TZM side tiles design has been completed and will be sent out for quotes next week. We have received the anode from a failed GA FPA tetrode tube, and plan to cut samples for pull tests. The leak location observed on this tube is similar to those found on our tubes which experienced similar failures over the years. Our efforts are aimed at clarifying the failure mechanism in these devices with a goal of improving the design to extend tube lifetime and reliability. Lower Hybrid System -------------------- The LH system was used in support of Thursday's run to investigate the effects of LH on toroidal plasma rotation. This required operation of the LH system with constant and square wave modulated power waveforms. The LH system performed nominally. Faulty power supply cables connecting to the #7 klystron PIN switch were replaced. 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. Travel and Visitors ------------------- 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 including divertor modeling, heat load footprints, CXRS measurements, impurity source rates, and impurity seeding. Tuomas Tala from VTT in Finland visited between Jan.10-18 and served as Session Leader for the run on Thursday, devoted to MP609a, the study of momentum transport coefficients from velocity perturbations. David Brower and Wiexing Ding, UCLA, visited MIT last week to attend the Ideas Forum and to work with Peng Xu, Will Bergerson, and Jim Irby on the C-Mod Polarimeter. Good progress was made on identifying sources of phase noise in the system. Randy Wilson (PPPL) visited MIT on Thursday and Friday to work with the Lower Hybrid group. Mark Foster (OFES) visited MIT last week and participated in the C-Mod Ideas Forum and the C-Mod Quarterly Review. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly