Alcator C-Mod Weekly Highlights August 27, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 14.6 weeks Plasma Shots: 2109 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 reliability of 92%. The runs supported research in the Pedestal Physics and H-mode Scenarios topical areas. A fresh boronization was carried out over Tuesday night in preparation for experiments. Several hours of run time were lost on Friday due to a malfunction of a cryogenic valve in the LN2 magnet cooling system, and to a power supply fault which affected the vertical stability control. Both of these problems were resolved, and the remainder of the run proceeded normally. C-Mod has now been configured for reversed field and current operation (with the ion grad B drift directed away from the vertical plate lower divertor). Plasma operations are planned to continue this week. Operation Details ----------------- Monday's operation was devoted to power system tests and initial plasma operation in the reverse field configuration. Each magnet and power supply system was first operated individually to verify proper orientation and control. We then performed a series of 1MA plasma discharges to verify full integrated operation, refine startup parameters, and provide target discharges for testing of diagnostics and other subsystems. All shots in this sequence ran successfully. The Lower Hybrid system, which has been reconfigured to operate in the reversed current configuration, was operated successfully in the new orientation. Shakedown operations for the reversed field campaign continued on Tuesday. The morning was primarily devoted to MP#605, "Lower Hybrid launcher checkout and conditioning". A series of shots were dedicated to increasing the LH power and extending the pulse length. Significant current drive was achieved at 5.4 T and 800 kA with net powers between 600 and 800 kW. The loop voltage transiently crossed 0 several times, but did not remain there as the density climbed during the LH pulse. After switching to 600 kA we sustained a loop voltage of only 0.15 V (compared to 0.8 V in the target Ohmic discharge) for the duration of the LH pulse. An upper bound on the current drive efficiency is ~2.5e19 A/W/m^2, which agrees with the efficiency found in normal field configuration. The second half of the run on Tuesday was used to focus and perform optical tests on the reconfigured HIREX_SR X-ray spectrometer diagnostic. This diagnostic has been reconfigured to provide measurements of the H-like Argon spectrum across the entire spatial profile, in preparation for anticipated high Te plasmas in planned I-Mode experiments. Previously, the diagnostic used the H-like spectrum only in the central region and provided a full spatial profile of the He-like spectrum, which is more appropriate for lower Te(r) plasmas. The new configuration also provides improved capability to diagnose H-like Ca emission, which is required for analysis of impurity transport experiments employing Laser Blow-Off (LBO) injections of CaF2. Both the H-like Ar and H-like Ca systems were successfully focused and vignetting tests were performed for the H-like argon system. Wednesday's run was dedicated to MP#694, "Transport of helium impurity in L-mode, H-mode, and I-mode plasmas". This experiment comprises part of the thesis research of a University of Texas graduate student. The dynamic evolution of trace helium injected from a piezoelectric valve is measured using the core CXRS system, with the grating changed to provide data for He emission at 4685.7 A. The intention for this run was to obtain transport data in EDA H-mode, I-Mode, and ohmic L-mode regimes. However, the H-mode and I-mode portions of the experiment were hampered by unexpectedly high influx of gas during ICRF operation, which prevented effective control of the plasma density. Difficulty in reducing the density due to wall out-gassing had been noted during the runs on Monday and Tuesday, but the effect was significantly exacerbated following Tuesday night's boronization. Steady EDA H-mode operation in the unfavorable grad-B drift orientation (LSN) was not successfully established, and while I-mode conditions were obtained, they were not sustained in steady conditions for sufficient duration to carry out the planned experiments. We were able to complete a density scan in ohmic L-mode conditions, and these data will enable analysis of the radial helium transport from the time-dependent profiles in these cases. On Friday, we began experiments dedicated to MP#707, "Pedestal and heat flux width scaling studies in I-mode plasmas". This experiment comprises part of the thesis research of an MIT graduate student. The focus of this day's experiments was on low-density I-mode operation, targeting n/nG~0.2, over a range of plasma currents. Broad coverage in I-mode across an RF power scan was achieved at Ip=1 and 1.3MA. Shots at n/nG~0.2 were also attempted at 700kA, but did not achieve I-mode. Additional experiments at n/nG~0.3 were also obtained at 700 and 850kA. ICRF Systems ------------ The ICRF was run in support of the scheduled physics runs in reverse field. Following a boronization, antenna recovery and gas evolution was significantly worse than in normal field operation. Antenna performance improved with antenna operation. Initial analysis of the thermocouples in the FA-J antenna indicate that the H side receives more energy than the K side and the overall energy to the limiters is less than 0.3% of the total input power (RF plus ohmic). Further experiments are planned to investigate the energy deposition to the RF antenna. We are nearing completion of a test setup for the new ARRA-funded FFTs to verify their electrical characteristics at low RF power. We have also received from the vendor a water flow meter and SF6 gas fittings to replace the components damaged in shipment. Lower Hybrid System ------------------- The LH phasing changeover for reverse field and current operation was completed last week. The system was successfully tested with plasma in reverse current at up to 800 kW net power. An improved thermocouple was installed on the outlet collector water cooling line of klystron 9. This thermocouple is electrically isolated from the klystron body, which results in significantly less noise on the signal. Components were ordered to install thermocouples of this design on all klystrons. Diagnostic Neutral Beam Systems ------------------------------- The DNB was operated on Wednesday in support of a physics run that focused on results from the CXRS diagnostic. The maximum beam parameters achievable at the time were 45 kV and 5 amps, which were sufficient for this physics run. On Thursday a series of tests were done which showed that both the arc and HV power systems are working properly. The source is currently being refurbished. Diagnostics ----------- The AGNOSTIC accelerator-based in situ plasma-facing surface diagnostic was run from 6pm to midnight on Wednesday 22 August, following completion of the C-Mod run. Experimental work focused on two main areas. First, the team attempted to maximize the deuteron current transmitted through the C-Mod gate valve and incident upon the inner wall PFC tiles. These tests were partially successful. Second, a toroidal field on the order of ~0.1 T was used to magnetically steer the accelerator beam. Gamma spectra were acquired from two distinct locations on the inner wall. Although analysis remains underway and increased beam current is required for more definitive tests, differences between the two spectra demonstrate that AGNOSTIC is capable of examining PFC surface conditions in different poloidal locations. Following this, it was decided to bring the RFQ beamline up to air in order to visually image the beam and perform more precise beamline alignment than is possible with the system under vacuum. Travel and Visitors -------------------- Ken Liao (Texas) was on site last week, and served as Session Leader for the run on Wednesday. Michael Brookman (Texas) completed his two week visit to MIT. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly