Alcator C-Mod Weekly Highlights July 2, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 9.3 weeks Operations ---------- Plasma operations continued at Alcator C-Mod last week. 3.75 run days were scheduled and completed. A total of 103 plasma discharges were produced with a startup reliability of 94%. This week's experiments supported miniproposals in the Divertor/Edge, Transport, ICRF Physics, Diagnostic Development, and MHD areas. The cryopump was employed to aid density control during the run on Tuesday. An overnight boronization was performed prior to the run on Thursday. We continue to work with our power utility to reduce the voltage drop on our 13.8 kV line. A meeting with utility engineers last Thursday clarified why the problem had arisen, and a plan was developed to reduce the impedance of the line to an acceptable value. This week is a scheduled maintenance week, with no plasma operations planned. Operation Details ------------------ Tuesday's experiment was a scheduled six hour run devoted to MP#489 "Damping rate of TAE modes at the threshold of stability in the presence of ICRF". The goal of this experiment, which comprises part of the thesis work of an MIT graduate student, is to observe the stability of moderate n (n=6-14) TAEs relative to the temperature of the ICRF-generated ion tail using Active MHD Spectroscopy. The target plasmas were inner-wall limited L-mode discharges with q~5. Unstable TAE's were observed at high ICRF power, indicating the tail temperatures were sufficient to reduce the total damping to zero. Data were obtained at ICRF powers between 2 and 3 MW, completing the dataset begun during a previous run day devoted to this experiment. Additional data were obtained in a lower q discharge, for which the q=3/2 surface should be closer to the active MHD antennas and magnetic probes. Analysis of these data is ongoing. Wednesday's run supported MP#480 "Topology effect on SOL total flow vector" from the Divertor/Edge Physics Group. This MP also contributes to the thesis research of an MIT graduate student. Flow data using all three scanning probes, including the inner-wall "WASP" probe, was complemented by data from additional diagnostics including CXRS measurements based on the DNB and on NINJA gas puffing, as well as X-ray spectroscopy. Plasma density was scanned between 0.8< nebar < 1.6e20/m^3 in upper and lower null discharges. Fluctuation data were also obtained in both topologies at nebar~1e20/m^3. Flow data were also obtained in near-balanced double null discharges slightly biased toward the upper X-point, to complement data obtained previously in near-double lower null discharges. Thursday's run was devoted to obtaining profile and fluctuation data in ohmic Internal Transport Barrier (ITB) discharges (MP#477), exploiting improved diagnostic capability developed since these plasmas were studied on C-Mod five years ago. The ohmic ITB's were formed from ohmic H-modes obtained by dynamically reducing the toroidal field until an L-H transition occurs, typically around Bt~3.5, q95~2.7, then raising Bt again to increase q95>4, after which the ITB develops out of an EDA H-mode. During this run we observed numerous cases with short-lived ITB's, terminating after a hundred milliseconds with H-L back transitions apparently resulting from radiative collapse as the impurity profile continue to peak along with the density. We did obtain some discharges in which the ITB was sustained for over 0.5 seconds. Central densities up to ne0~7e20/m^3 were measured by Thomson scattering. Fluctuation data were obtained from reflectometry, phase contrast imaging, and magnetics diagnostics. Ion temperature and rotation profile data were obtained from xray spectroscopy of hydrogen- and helium-like argon (HIREX_SR); it was found necessary to restrict the amount of argon injected to reduce the radiated power and extend the length of the ITB's. Unfortunately, the DNB-based CXRS diagnostics were unavailable because of a problem with the beam cryogenics. Two experiments were carried out on Friday. The first, MP#486 from the ICRF group, was aimed at investigating the origin of impurity generation during ICRF heating and associated localized erosion of the boron layer. Special tiles with niobium coating over the molybdenum substrate had been installed in C-Mod prior to the current campaign, in the location outboard of the lower divertor which had previously been identified as mapping to field lines passing in front of the J-port ICRF antenna. This area was also implicated as a likely source of Mo impurities after degradation of between-shot boronizations, although more recent experiments have called this interpretation into question. Experiments on Friday were carried out to determine whether Nb impurities were in fact generated and observed spectroscopically in the core of the discharge during J-port antenna operation. Up to 1.3MW was coupled through the J-port antenna, and up to 3MW total ICRF was delivered to the plasma during this experiment. No Nb was observed using normal LSN discharges for which the antenna was expected to be magnetically connected to the region with the niobium-coated tiles. However, when the strike point was moved close to the marker tiles on the divertor shelf region we did observe Nb lines from the core of the discharge, indicating that the coating was still present and the diagnostics were sufficiently senstitive to detect the tracer. The implication of this experiment is that the marker tiles are probably not in fact at the locations of the dominant ICRF-induced impurity sources for typical diverted discharges. The second half of the run on Friday was used for calibration of the MSE diagnostic using a new method - radial sweeps of the plasma edge past the MSE viewing chords (MP#491). Low elongation inner-wall limited discharges were employed with dynamically varying major and minor radius, such that the outer edge of the plasma swept across the MSE view. The plasma current was also varied to keep the edge q constant. The field angle measured by MSE can therefore be compared with the angle inferred from magnetic equilibrium reconstruction, which is expected to be most accurate at the plasma boundary. The minor radius was varied from the nominal 22cm down to 15.5cm in these experiments. Physics Program --------------- The C-Mod Experimental Program Committee met on Monday, June 25, to review miniproposals for upcoming experiments. A total of ten MP's were approved, and several others are under revision and will be reconsidered at a future meeting. ICRF Systems ------------- The coax for FMIT Transmitters #3 and 4 were swapped back to their normal configuration. These had been reversed during the previous week following observation of HV faults in the FMIT#3. These faults were later determined to be associated with sagging of the AC line power due to the utility reconfiguration. Lower Hybrid System -------------------- The Lower Hybrid system was not involved in experiments last week. Maintenance activities were performed, including partial disassembly of the "jungle gym" waveguide system for inspection and cleaning and removal and inspection of some water loads. Maintenance on the LH system will continue this week. Low power testing of the 4-way splitter prototype continued last week. Adjustment of the matching transformer has resulted in extremely well balanced splitting of the power and very good phase control from port to port. These data very closely match simulation results. Long Pulse Diagnostic Neutral Beam System ------------------------------------------ The DNB cryopump system developed a heat leak on Wednesday, probably due to formation of an ice dam. As a result the LHe transfer on Wednesday evening was unsuccessful and the pump regenerated overnight and had to be warmed up to room temperature on Thursday. A successful LHe transfer was performed on Friday morning, and the beam was returned to service in time for the MSE calibration run on Friday afternoon. Travel and Visitors -------------------- David Howell (UKAEA/Culham) completed his visit to C-Mod, during which he served as Session Leader for the experiment devoted to MP#478 "Joint experiments with JET on non-resonant n=2 magnetic braking". Analysis of the results of this experiment are continuing. Paul Bonoli attended the annual SciDAC 2007 Conference in Boston, MA from June 25-29, 2007. He gave an invited talk at that conference entitled "Evolution of nonthermal particle distributions in radio frequency heating of fusion plasmas". Paul's talk was one of only two invited orals at this meeting in the area of fusion plasmas. Paul also hosted a visit by Professor John Cary (University of Colorado and TechX Corporation) at the Plasma Science and Fusion Center on June 27, 2007. Bill Burke, Pete Koert and Dave Terry attended the SOFE 22 conference in Albuquerque, NM. and presented posters and submitted papers on Lower Hybrid related projects. Martin Greenwald was in San Diego for a meeting of the FESAC Planning Panel, which he chairs. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly