Alcator C-Mod Weekly Highlights November 23, 2010 FY2011 weeks of research operations Target: 15 weeks Completed: 3.76 weeks Plasma Shots: 454 Operations ----------- Plasma operations continued last week at Alcator C-Mod. Four run days were scheduled and completed. A total of 115 plasma discharges were produced with a reliability of 90%. The experiments supported research in the Lower Hybrid Physics and Transport areas, as well as Diagnostic Development. Boronization was carried out on Tuesday night. Plasma operations are planned to continue this week. Operations Details ------------------ The run on Tuesday supported MP#619 "Effects of ICRF and LH power on SOL density profiles". The objectives of this MP are to calibrate the SOL reflectometer installed next to the LH launcher with respect to other density profile diagnostics and to measure the effects of launched ICRF and LH power on the SOL density profile. Cross-calibration of the reflectometer with scanning probe measurements was performed for a wide range of densities (0.8 1.4 observed in some cases, but the ELMs were insufficient to regulate the pedestal and impurity accumulation ultimately reduced the performance. The data are being analyzed and comparisons with the code predictions will be carried out. On Thursday we carried out MP#621 "LH Wave Power Deposition Profile". The objective was to inject LH power near and above the observed density limit for efficient LH current drive, and use ECE and Thomson Scattering measurements to determine if significant power is absorbed by the plasma above the density limit, even without the usual signatures of a non-thermal population of electrons. For this purpose the LH was modulated 50 ms on, 50 ms off and the two 30 Hz Thomson lasers were synchronized to the LH pulses, with equal spaced pulses beginning at LH turn on, for a total of 7 pulses per full LH period. The target plasmas run were 800 kA, 5.4 T discharges, both USN and inner wall limited. Some difficulty with locked modes was encountered at low densities, nebar< 1e20/m^3. Analysis of the results is ongoing, but the control room observation was that negligible power is delivered to the plasma across the separatrix once the density limit is exceeded. This would seem to leave only the possibility that the SOL must absorb the LH power above the density limit. A fuller diagnosis of the SOL profiles,ionization loss, heat flowing to the divertor plates, etc. needs to be carried out under these conditions. On Friday we carried out an experiment dedicated to MP#553, "Creation of reversed sheer current profile using LH on low Ip plasmas". We increased N|| in order to move the LH power deposition (and driven current) more off-axis, and in order to compensate the reduction of total LHCD current, we operated at relatively low plasma current, 300< Ip < 450kA. We also carried out experiments using a "compound" N|| spectrum by changing the relative phasing of columns in each half of the launcher independently. Typical target densities were below 5e19/m^3. A number of successful discharges were produced with loop voltage at or close to zero. Current profile data (MSE) have not yet been fully analyzed, but we observed interesting features including bifurcation of the central electron confinement with formation of a steep electron temperature gradient near r/a=0.2, suggestive of a reversed shear electron ITB during LHCD. The return of sawtooth activity after cessation of the LH pulse was significantly extended relative to cases with lower N|| phasing, suggesting achievement of more deeply reversed q profile. We also observed substantial MHD activity at both low and high frequency during the lower hybrid pulse. In some cases the current drive efficiency was severely reduced by the appearance of a locked mode after the appearance of the ITB-like behavior. Analysis of these data is in progress. ICRF Systems ------------ The rotated 4-strap antenna design and fabrication continued last week. The first inconel strap was wire EDM'ed from a block, and a second strap is in progress. The strap radius cut and ground block design have been finalized. An inconel test braze is underway in preparation for attaching the grounding block to the current strap. In preparation for plating, the stainless steel and an inconel block have been prepared for plating. This will allow determination of the proper current density and bath size for plating the inconel straps. The TZM Faraday rod ends have been successfully threaded and will be thermally cycled to verify structural integrity. Fabricating threads on the end of the rods is now the baseline design, rather than brazing threaded inconel rod to the Faraday rod. The Faraday rod attachment baseline design is now a combination of a boron carbide resistive washer, a zirconium washer and a tube. Sample boron carbide is expected to be tested next week. The flange drawings have been reviewed and are being prepared for distribution to the vendor. The backplate has been rolled and will be inspected prior to further fabrication steps. The modification to the FMIT#2 (E-port) modulator circuit to prevent spurious demand during turn-on transients was successfully implemented. The modified system behaved nominally during plasma experiments on Tuesday and Wednesday last week. Yijun Lin and Steve Wukitch participated in a review of the heating efficiency of the ICRF heating system for the EAST Tokamak. They noted that the transmission line network needed to be adjusted to allow proper wave phasing to be launched from the antennas. Lower Hybrid System -------------------- High power calibration of the drive legs for klystrons #7 and #10 was successfully completed, and the lower hybrid system was successfully used in support of experiments on three days last week. Travel and Visitors ------------------- Steve Wukitch participated in a teleconference with our vendor regarding the design of ICRF fast ferrite tuners. A final design review is planned for the week of November 29. The present design needs to be adjusted to allow for 110deg phase shift at 80 MHz and ~100deg phase shift at 50 MHz. This change will allow full tuning capability at 80 MHz, where most C-Mod operation takes place, and allow coverage for most plasmas at 50 MHz. John Rice was in Cadarache Nov. 15-16 for a design review of the ITER pedestal x-ray crystal spectrometer system. Silvano Gnesin, a graduate student from the TCV group, CRPP, EPFL, Lausanne, visited on Thursday and Friday. He gave an Alcator seminar, "Electron Cyclotron Heating Studies on TCV" and had discussions with C-Mod students and scientists. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly