Alcator C-Mod Weekly Highlights July 19, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 10.5 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 1849 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 90 plasma discharges were produced with a startup reliability of 84%. Runs on Tuesday and Wednesday supported diagnostic development and calibration, ICRF and LH conditioning; experiments on Thursday and Friday contributed to research in Lower Hybrid and Boundary Physics. The cryopump was employed for particle control during all four runs last week, and performed nominally. Diagnostics continue to be brought on-line. All reflectometer channels except the 75GHz system are now operational. The edge Thomson Scattering system is operational and was employed during the SOL heat flux experiment on Friday; the core Thomson scattering system has both lasers operational, but requires a density calibration. Once completed, the calibration will be applied to the existing data. Plasma operations are planned to continue this week. Program Planning ----------------- The C-Mod Experimental Program Committee met on Monday, July 12. The main topic of discussion was run priorities for the rest of FY10. Three areas were agreed as due special attention: 1. High heat flux experiments in support of the 2010 joint research target 2. Lower Hybrid experiments characterizing and exploiting the new LH2 launcher 3. Supporting data for IAEA presentations including studies of confinement as a function of net and radiated power; I-mode experiments with improved Er profile data; and ITER-related ramp-up and ramp-down experiments. Two new Mini-Proposals were also considered and approved, with scheduling deferred subject to consideration of near-term priorities and operational considerations. Operations Details ------------------ Tuesday's run was devoted to a combination of MP#534 "Foil/AXUV Bolometer Calibration in Helium" with piggybacking for MP#355b "ICRF Conditioning". The purpose of the run is to calibrate the bolometry, Lyman-alpha and the HIREXSR diagnostics. Helium is used as the main-ion to reduce CX contributions on the foil bolometry and check the filter transmission of the Lyman-alpha tools. The checkout of the divertor Lyman-alpha system was completed. All three ICRF antenna systems were conditioned to ~1MW, 0.3sec pulses. Additional run time will be required to complete the other calibration tasks. On Wednesday we continued conditioning of the new Lower Hybrid launcher system (MP#605). We progressed up to ~800 kW of net LH power with reflection coefficients in the range of 30%. No arcing was seen at the launcher, even on shots at high power. Thursday's run was dedicated to MP#608 "Lower Hybrid Wave Coupling Study on LH2 Using Microwave Probe Diagnostic". Coupling was measured during scans of launcher phase (50 to 110 deg), plasma density (0.7 < nebar < 1.9e20/m^3 ), plasma radial displacement, and radial launcher location. We consistently found best coupling to be at 70 deg phasing. A reflection coefficient of ~15% was achieved transiently. The RF probe diagnostic worked well and the spectrum measured by the diagnostic agrees with the commanded one. Friday's run continued the experiments under MP#570 "Boundary layer heat transport experiments in L-mode plasmas", which contributes to the 2010 SOL Heat Flux Joint Research Target. We completed density scans at 0.54 and 0.80MA in ohmic discharges with the newly commissioned divertor heat flux diagnostics. Heat flux profile widths in attached plasmas were observed to increase with plasma density at both currents, and decrease as the current is raised at the same Greenwald fraction. These trends are qualitatively in-line with previous scaling observations for the midplane electron pressure profiles. ICRF Systems ------------ ICRF conditioning continued last week with better performance in both vacuum and plasma. The D and E dipole antennas each ran up to 1.5 MW independently; simultaneous operation was limited by spurious fault indications associated with low single pass absorption due to the high H fraction. The J-port four-strap antenna operated up to 1 MW and continues to improve. We completed a full RF analysis of the rotated antenna and found the mock up of the plasma boundary conditions needed to be adjusted. A new set of simulations will now be run to evaluate the various current strap orientations. Lower Hybrid System -------------------- The lower hybrid system performed satisfactorily in support of experiments on Wednesday and Thursday last week. One circulator developed signs of arcing during high power operation and will be replaced. Body current excursions close to the trip level on one klystron are being monitored. All LH Probe calibrations were successfully completed. These probes provided critical data for the MP#608 experiment on Thursday. Diagnostic Neutral Beam Systems ------------------------------- Last week the DNB was fired into C-Mod plasmas for the first time during this campaign. Beam parameters were 6 amps, 45 kV, and pulse lengths of 50-100 ms. No deleterious effects on plasma breakdown or performance were observed. The MSE diagnostic had measurable signals on all channels. Work will continue on correcting ground loops in the DNB system. Control System -------------- The algorithm which senses excess hard X-ray flux and automatically triggers a forced rampdown (soft abort) operated successfully on two shots last week. This routine was activated at the beginning of the summer experimental campaign in response to concern that melt damage on the outboard limiters due to runaway electrons could result in increased Mo influx during subsequent discharges. Measurements using embedded thermocouples in the outboard limiter tiles indicated that no significant energy was deposited on the limiters during the aborted shots, which were the first instances observed during this campaign of hard X-ray levels above the threshold. Travel and Visitors ------------------- Randy Wilson (PPPL) visited MIT last week to participate in the C-Mod Lower Hybrid experiments. Paul Bonoli attended the annual Scientific Discovery Through Advanced Computing Conference ("SCIDAC 2010"), held in Chattanooga, TN from July 11-15, 2010. The conference consisted of oral sessions during the day time and poster sessions in the evenings and included presentations from all of the SciDAC disciplines funded by the US Department of Energy. The conference program can be viewed at the website: http://computing.ornl.gov/workshops/scidac2010/ _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly