Alcator C-Mod Weekly Highlights November 23, 2009 FY2010 weeks of research operations Base Target: tbd ARRA Target: 5 weeks Completed: 4.0 weeks, 483 shots Operations ----------- Plasma operations resumed at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 110 plasma discharges were produced with a startup reliability of 90%. The runs supported experiments from the ICRF Physics and Transport topical areas and the Advanced Scenarios task. The start of Friday's run was delayed in order to complete repairs of the FMIT#1 breaker, which was required for the day's experiment. Plasma operations are plannned to continue this week. Operation Details ----------------- Tuesday's run was devoted to MP#585 "Low Bt Discharges to Support AT Development Using 50 MHz ICRF". Plasma startup at B~3.4T with early x-point formation (100msec) was optimized, and current ramp-up with ICRH-induced H-modes during the ramp was explored. The D(H) minority heating scenario was used with f=50MHz from the J-port antenna. Volt-second savings and reduction in li associated with the ramp-up H-modes were documented. Flattop targets at Ip=450 and 600kA (q95=4.6 and 4.0, respectively) were obtained, with ELM-free and EDA H-modes. On Wednesday, we continued experiments under MP#583 "Optimization of mode conversion flow drive". L-mode experiments were carried out in Upper Single Null discharges at B=5.1T. A current scan from 0.6< Ip < 1.35MA (2.8 < q_95 <5.6) confirmed that the rotation efficiency is roughly linear with current. Moreover, the contrast between antenna phasing (+90 vs -90 degrees) is more pronounced at the higher currents, with up to a factor of two difference in drive efficiency at the higher current and negligible difference at low current; the direction of the driven flow is always co-current. The flow drive with dipole antenna phasing is always similar to, but slightly weaker than for +90 degrees. The dependence on Ip may be due to the 1/q dependence of mode conversion to the ion cyclotron wave branch, and possibly momentum confinement time difference. A density scan (0.8 < nebar < 2.1e20/m^3) at Ip=1MA indicated that the flow velocity is approximately inversely proportional to density, implying constant momentum drive over this range. Experiments in L-mode with lower single null equilibria showed similar flow drive, indicating no apparent dependence on plasma topology. H-mode results were inconclusive due to inadequate wall conditions to maintain steady EDA targets. Modulation of the ICRF resulted in clear modulation of the rotation profiles; these data are being analyzed. Thursday's run was devoted to MP#536 "Evaluation of FWEH, FWCD and MCCD". The goal was to optimize fast wave electron heating at 5.2T with 50MHz (J-port), using D(H) minority heating at 80MHz (D- and E-port) to pre-heat the target plasma. The experiments were conducted in upper single null equilibria at low density (nebar < 0.8e20/m^3) to avoid H-mode and maximize Te in the target plasma. The cryopump was employed for particle control. Fast wave electron heating was clearly observed on the initial discharges, but optimization was hampered by inadequate impurity control and ineffective impurity seeding in this configuration. On Friday we investigated the Improved L-mode (I-mode) characteristics at lower field than our previous experiments, employing D(H) minority heating at 50MHz (J-port) at B=3.4T. Some experiments were also conducted adding off-axis D(H) second harmonic minority heating at 80MHz from D- and E-port antennas, at B=3.0 T. A scan of q95, varying both B and Ip was performed, down to q95~2.4. All experiments were carried out in USN equilibria. As in previous experiments, the H-mode transition threshold was well-correlated with q95, decreasing monotonically with q95 from 2.4 to 3.3. Fluctuation signatures of the I-mode, disappearance of the broad-band L-mode turbulence and spin up of a "quasi-quasi coherent" oscillation, were observed by reflectometry. The latter mode is strongly reduced upon entry into the H-mode. The enhancement to the Te pedestal was not as pronounced as for I-modes at higher field, but the global energy confinement did increase. Program Planning ---------------- The C-Mod Experimental Program Committee met on Monday, Nov 16. Three new Miniproposals were considered and approved. The operation schedule for the remainder of this month and the early part of December was discussed, including the timetable for frequency changes for the J-port antenna and installation of the Lower Hybrid launcher. A tentative operations plan was determined. ICRF Systems ------------ The D-port antenna was out of service on Wednesday while its primary breaker was repaired. The repairs were completed on Thursday, and all four transmitters performed nominally for the remainder of the week. . The rotated ICRF antenna design continues to progress. After RF modeling with CST, we have frozen the strap design, and a 1/2 scale plastic model will be built. In parallel, one strap is being fabricated from stainless steel to investigate manufacturing issues. Lower Hybrid System ------------------- All sixteen in-vacuum waveguide assemblies have been welded, successfully leak-checked, and fit-up to the vacuum flange. Final cleaning of the assemblies and installation of the rf probes are underway. Installation of the waveguide runs from the klystrons has begun. Components needed to couple the waveguide runs to launcher and measure the forward and reflected power are also being assembled. Travel and Visitors ------------------- Dr. Olaf Grulke of IPP Greifswald completed the second week of his visit to C-Mod. A mini-proposal to study the parallel mapping and dynamics of turbulence structures in the C-Mod SOL was written and accepted. Olaf finished writing the codes needed to predict the mappings and do the cross-correlations. He will interact with the C-Mod team remotely when the MP is scheduled. Dr. Matteo Agostini, a collaborator from the RFX device in Padua, Italy, has begun a 6 week on-site visit. This is a continuation of his work on edge turbulence in C-Mod that began with an on-site visit in 2008. He will be working with J. Terry and S. Zweben, analyzing data from the Gas-Puff-Imaging fast cameras and the inboard-midplane GPI diode array. He will be applying wavelet transform analyses that have been developed by the RFX group. Stewart Zweben (PPPL) was at C-Mod last week. He bench tested and installed one of the new Phantom 710 fast cameras on C-Mod, and began to acquire data with it on Friday. He also continued analysis of the convective SOL transport associated with the turbulence seen in GPI, and discussed turbulence data analysis with C-Mod visitors Olaf Grulke from Greifswald and Matteo Agostini from Padova. Manfred Bitter and Ken Hill from PPPL visited from Nov.18-20 to install and successfully operate the x-ray fluctuation spectrometer. Monday November 16th Catherine Fiore spoke at the Women in Science and Engineering Workshop that was held at Jefferson Laboratory in Newport News, Virginia. Her talk was titled "Report from the CSWP Gender Equity Conference." _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly