Alcator C-Mod Weekly Highlights November 16, 2009 FY2010 weeks of research operations Base Target: tbd ARRA Target: 5 weeks Completed: 3.1 weeks Operations ----------- Plasma operations resumed at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 120 plasma discharges were produced with a startup reliability of 90%. All four runs supported experiments from the ICRF Physics topical area. An overnight boronization was carried out following the conclusion of Tuesday's experiment. Plasma operations are plannned to continue this week. Operation Details ----------------- Monday's run was devoted to initial evaluation of ICRF heating using the J-port antenna at 50MHz, employing He^3 minority and mode conversion heating. The He^3 concentration was scanned and monitored in the mode conversion regime by break-in-slope analysis of the electron cyclotron emission, as well as spectroscopically. Effective minority heating was obtained in USN L-mode discharges at low He^3 concentration. Further optimization of the He^3 concentration was continued on Tuesday. The first part of the run on Tuesday was devoted to MP#581 "High density target plasma antenna operation". The NINJA inner wall capilary gas system was used to raise the target density while operating the cryopump for density and gas control. The J-port antenna was operated at 50MHz for heating of D(He^3) plasmas. It was found that the neutral pressure limit of the J-port antenna, referred to the midplane pressure gauge, was lower with the inner-wall puff than with fueling from the outboard piezo-valve, but the accessible density was about 15% higher, up to nebar~2.5e20/m^3. Thursday's experiment was devoted to MP#508 "Evaluation of D(He^3) heating", comparing heating at 50MHz (J-port antenna) with 80MHz D(H) heating using the D- and E-port antennas, all at Bt~5.3T. He^3 minority heating was found to be less effective than H minority heating. Even in L-mode discharges, the plasma heating effectiveness for H minority absorption was significantly higher than He^3 minority. The initial low single pass absorption makes the operational space more limited. More effective He^3 minority heating was obtained by pre-heating the plasma with the D/E antenna; impurity seeding with neon was also found to be advantageous for the D(He^3) minority heating. On Friday we carried out MP#583 "Optimization of mode conversion flow drive". This experiment is aimed at further documenting the rotation (flow) drive previously observed during mode conversion heating on C-Mod and confirmed during experiments on JET. Scans of several relevant parameters, including He^3 concentration, toroidal field, plasma current, and antenna phasing, were carried out. The existence of an optimum He^3 concentration for flow drive efficiency was confirmed. The field scan indicated that the most efficient flow drive appears when the Mode Conversion and Ion Cyclotron layer are close to the axis. Dipole and +90 degree antenna phasing resulted in similar rotations, while -90 degree phasing generated slower rotation, but still in the co-current direction. Prelimimnary analysis of the current scan data indicate more efficient flow drive at the higher current, but more data are required to confirm this result. ICRF Systems ------------ Several operational issues were identified and resolved during the week. On Monday, the FMIT#1 fast breaker failed to close properly and the problem was investigated. A failed relay contactor was found and the system was tested repeatedly prior to placing it back in service on Thursday. FMIT#3 FPA bias regulator and stub tuner controller were both repaired. The FFT computer used for control of the Fast Ferrite Tuner (FFT) had difficulty on reboot but was recovered after re-seating memory cards; a replacement computer has been identified in case this problem recurs. Lower Hybrid System ------------------- Seven of sixteen waveguide assemblies have been successfully welded to the four-way splitters. Fit-ups of the waveguide assemblies to the vacuum flange have begun. ARRA Upgrade activities ----------------------- Purchase orders for fabrication and testing of four additional high power microwave sources (klystrons) for the Lower Hybrid system were placed on Nov 5, 2009. This completes an ARRA FY10 milestone, "Order additional high power microwave sources", almost two months ahead of schedule. Travel and Visitors ------------------- Manfred Bitter and Ken Hill from PPPL visited MIT on Nov.11-13 to assemble and install the x-ray fluctuation spectrometer. Thomas Putterich from IPP-Garching visited MIT on Monday and Tuesday last week. He discussed impurity and momentum transport with C-Mod scientists and graduate students. He also presented a talk on Tungsten operation in ASDEX-Upgrade at the weekly C-Mod Science Meeting. Olaf Grulke, a long-time collaborator from IPP-Greifswald, has returned for a two week visit. He will continue his analyses of edge turbulence in C-Mod, as well as propose new experiments investigating the parallel structure and dynamics of the blobs/filaments. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly