Alcator C-Mod Weekly Highlights June 11, 2007 FY2007 weeks of research operations: tentative target: 15 weeks Completed: 6.57 weeks Operations ---------- Four run days were scheduled and three were successfully completed last week with a plasma startup reliability of 86%. On Tuesday we investigated operation of the J-Port antenna in the dipole configuration and also studied TAE damping rates. On Wednesday a fault in the PLC server in the morning and other computer related problems later in the day limited us to only two discharges. On Thursday, the interaction of lower hybrid and ICRF operation was studied in both L- and H-mode plasmas. On Friday, wall pumping was studied at several densities in both helium and deuterium majority plasmas. Operation Details ------------------ On Tuesday, in an effort to understand the operational neutral pressure limits of our ICRF antennas, the J-Port antenna was operated in a 2-strap dipole configuration, and plasmas were produced with a variety of densities and plasma currents. The dipole configuration ran over a much broader neutral pressure range in comparison to the 4-strap configuration. In addition, we found that higher plasma currents resulted in more reliable operation of all the antennas. We plan to investigate these limits further by producing plasmas over a larger current and density range. Also on Tuesday, we ran MP 489 "Damping rate of TAEs near stability threshold with ICRF fast ions." This run observed the stability of moderate n (~6-14) TAEs relative to the ICRF generated ion tail temperature as a function of ICRF power. This topic was investigated in earlier campaigns, but has been resumed now with an augmented set of magnetic field probes for improved toroidal mode identification. A toroidal field of 5.9 T was established as a good compromise to move the heating towards the TAE resonant surface while maintaining good ICRF antenna performance. TAE resonances were observed in most shots during ICRF and analysis is ongoing. Thursday's run was devoted to MP 490 "Assessing LHCD in ICRF Heated L- and H-modes." The purpose of the first day of this experiment was to measure LHCD coupling in the presence of ICRF power, in various plasma conditions, particularly q95 which affects the magnetic mapping between the ICRF antennas and the lower hybrid couplers, in both L and H-mode. For this purpose we used relatively short, low power pulses of LH, with phasing varied within a shot. The first half of the day was devoted to L-mode plasmas, using USN geometry to raise the L-H threshold. Somewhat surprisingly, based on past results, adding ~ 1 MW of ICRF from either J-port or E-port antennas did not cause a large increase in LH reflection coefficients, over a range of current from 0.65-1.0 MA. However, adding power from D-port, which is immediately adjacent to the LH antenna, did lead to increased reflections. These plasmas were quite promising as LHCD targets, since they have low densities and high Te (up to 5 keV); confinement enhancement for these L-Mode edge plasmas was typically H_89~1.3. Some test were done with the cryopump on, and puffing with the NINJA system. This scenario did raise edge densities at the grill, with a core density rise much smaller than without the cryopump, and this technique may be a useful tool for future experiments. Ip was also scanned in LSN geometry, and in H-mode plasmas with 1 MW each from E- and J-port ICRH. Reflections were generally higher but, again, coupling did not change a great deal due to ICRF. Because it had been some time since a boronization, the quality of these H-modes was low, and it would be useful to revisit these conditions with stronger edge pedestals. Data from this experiment will contribute to the C-Mod non-Joule Milestone on LHCD into L and H-modes, and combining with ICRF. A planned second day will use increased LH power for selected conditions, to assess current drive. On Friday, MP 419 "Fiducial Wall Pumping Measurements for Comparing Operating Conditions" was begun with two goals in mind to help understand gas retention in the walls. The working hypothesis is that the deuterium retention observed in Mo is not due to co-deposition but rather to ion fluxes implanting deuterium into the near surface of the tiles at a rate faster than recombination of deuterium atoms into molecules can occur at the front surface. This leads to localized high deuterium pressures within the Mo lattice followed by damage, or displacements in the lattice. Such displacements add locations where D2 can reside over and above that expected in 'normal' Mo. Such D2 traps can then diffuse through the tile leading to much higher deuterium retention than some laboratory experiments would have implied. To test this hypothesis two experiments were performed: 1) Discharges were made with helium as the main gas and recycling species. Helium does not need to recombine at the front surface since it is a noble gas, so recombination is not a limiting process and helium ion fluxes should not lead to enhanced pressures and damage within the lattice; 2) By running D2 gas limited plasmas the ion fluxes to tiles will be lowered at least a factor of 10 compared to diverted discharges where the high recyling condition occurs. Thus the ion flux is smaller compared to the limiting surface recombination rate for D into D2 and again there is less likelihood of pressure-drive damage and trap creation. The torus gatevalves were toggled closed before each discharge, and kept closed for several minutes after, so that the gas inventory could be measured following the shot. Initial analysis during the run day indicated that the logic described above is correct: The retention during helium and inner wall limited discharges appeared to be essentially zero. ICRF Systems ------------- New solid state driver amplifier electronics for all the transmitters are being developed. These amplifiers will be able to supply more drive power when needed, particularly for transmitters #3 and #4. Phase demodulator calibration procedures are being updated. Lower Hybrid System -------------------- Work continued on simulations of a new 4-way splitter that may be used in the 2nd lower hybrid launcher, and work also continued on design options for the launcher. Qualifications of twelve new water loads were completed last week. Travel and Visits ----------------- Dennis Whyte presented two talks "Update on PSI diagnostics and research on Alcator C-Mod", and "DIONISOS result on D retention in Molybdenum" at the Plasma Facing Component Annual meeting held at Argonne National Laboratory June 4-6. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly