Alcator C-Mod Weekly Highlights June 19, 2006 FY2006 weeks of research operations: Planned: 14 weeks Completed: 13.5 weeks Four run days were scheduled and completed last week. Approximately 3/4 of the run on Tuesday was used to bring C-Mod back into operation with fields and plasma current in the forward direction, and to bring up the J-Port antenna at 50 MHz. A full boronization of the machine was carried out on Tuesday evening. Run Reports ----------- Tuesday: Startup with forward field and 50 MHz ICRF, and MP390 -- Evaluation of D(3He) minority heating Roughly the first third of the run day was used to test power systems with the bus back in the forward direction. Much of the rest of the day was spent tuning the J-Port antenna for 50 MHz operation. An oscillation in the #4 transmitter not seen into the dummy load was eventually fixed with a matching solution change. A He3 scan of limited range was performed at power levels of up to 1.5 MW from the J-Port antenna. This scan provided a starting point for Wednesday's mode conversion current drive (MCCD) experiment. Wednesday: MP356 -- Mode conversion current drive, and MP447 MCCD experiments during ramp-up Good target plasmas for MCEH with central heating, low nebar and Zeff were rapidly obtained. Conditioning of the J-port antenna after boronization was slow, but good discharges with ~ 2 MW of rf power were eventually produced. However, significant pickup on the magnetics of the 50 MHz ICRF power limited progress on this miniproposal since it appears on the loop voltage and makes interpretation of small level changes difficult. MP447 -- MCCD experiments during ramp-up. Obtaining MCEH during ramp-up by puffing 3He into the startup plasma was easily done. The early MARFEs experienced during reversed field operation did not appear with the fields in the forward direction. An extensive scan of the puff duration was completed, which will be useful in determine optimum conditions for the next set of experiments. There is a good deal of density production associated with RF, and with both early D2 and 3He puffs, with the resulting densities probably being too large to observe Alfven cascade activity. 2 MW of power was coupled from D and E-Port antennas to these plasmas, but the J-Port antenna requires more conditioning before power levels above 1 MW can be reliably obtained. In summary, progress was made in obtaining good targets for both miniproposals. Pickup of the 50 MHz rf power on some critical diagnostics and control system signals has to be investigated and eliminated before this work can continue. Thursday: MP455 -- Determination of LH Efficiency This LHCD experiment was designed to measure the rf current drive efficiency and non-thermal ohmic component of the plasma current by performing a power scan under constant plasma conditions. The rf current is expected to be linearly proportional to power while the non-thermal ohmic current is non-linear in rf power. The lower hybrid power was scanned shot-to-shot from 100-500 kW in ~ 50 kW steps. The loop voltage was reduced from ~0.8 V to less than 0.4 V at the maximum power. The decrease was nearly linear in power. An initial fit to the data showed an efficiency of 2 to 3 A/W/10^19m-2, which is close to the expected value. Friday: MP 458 -- Type, Filament Structure, and Pacing of ELMs in High-triangularity Plasmas Primary goals for this run included determining if filaments on the inboard mid-plane continued to be present when the plasma was gradually changed to have a near DN configuration, and a determination of the power scaling of the ELM frequency. Unequivocal results for neither of the these goals were obtained. However, large ELMs in this high-triangularity/low collisionality condition were produced, and a scan of the magnetic configuration from an SSEP of < -5 cm to a near double null with SSEP=-1.5 was completed. As the SSEP got closer to 0, the ELMs disappeared and a high frequency (f~225 kHz) Quasi-Coherent Mode appeared. The high frequency of the QCM was due to the abnormally low toroidal rotation at this stored energy. Though ELMs in the near DN configuration were not produced, the inboard filaments continued to appear (as long as there were ELMs at all) even for SSEP values of -2 cm. No clear trend in ELM frequency over the power range of 2.3 to 3 MW was found. Operations ---------- A faulty relay in the alternator drive motor control system was found and replaced during the maintenance week, and the drive system performed nominally during operation last week. Lower Hybrid System ------------------- The Lower Hybrid System coupled over 500 kW of power to the plasma during the run on Thursday. All but one of the sixty new coupler protection boards operated successfully. The final board has been debugged and repaired. Travel and Visitors ------------------- Amanda Hubbard visited PPPL last week to participate in a joint ITPA pedestal comparison experiment on NSTX. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly