Alcator C-Mod Weekly Highlights April 12, 2004 Research operations continued at Alcator C-Mod last week. Five run days were scheduled and completed. The total number of research days accomplished during the FY04 campaign is now 61; the operational milestone for the fiscal year is 72 days (18 weeks). Experiments were carried out in support of Transport, MHD, RF Physics and Burning Plasma Support. Progress also continued on the Lower Hybrid Project. Physics operation is planned to continue next week. Operations ---------- Physics operations were carried out all five days last week. A total of 99 plasma discharges were produced, with a startup reliability of over 85%. Twenty of the plasma discharges were at high field (B>7.7 tesla) as the research program continues to exploit this unique capability of the C-Mod facility. Monday's run continued high field operation in support of MP#371, aimed at developing high current operation. Several TF power supply faults limited the number of useful shots; these were determined to be related to insufficient gate drive signals from a number of driver boards; these boards were identified and replaced. Additional instrumentation was implemented in the supply cabinets to determine the source of spurious fault signals that occurred during the run. Tuesday's run was in support of MP#391 investigating Alfven cascades during the current rise with high power ICRF heating. This experiment made use of a newly installed magnetic probe head on the A-side scanning probe to provide additional information on the toroidal mode numbers of the observed magnetic modes, as well as the PCI diagnostic. ICRF power up to 5MW was coupled during the current rise phase of the discharges, and the toroidal field was scanned between 4.8 and 6 tesla to determine the role of the resonance location. Moving the cyclotron minority resonance to the high field side eliminated the cascades, whereas moving the cyclotron minority resonance to the low field side maintained the cascades. At least qualititively, moving the resonance slightly to the low field side (5.7T) resulted in the richest cascade spectrum, on both the PCI and magnetic loop probes. Phasing the J-port antenna to co or counter current drive did not have an obvious effect (at 5.7T) although these data will require a closer examination. On Wednesday, we resumed high field operation with MP#203 on optimization of D(He3) minority and mode conversion heating at B~8T. The He3 concentration was varied by changing the gas puff duration and the level monitored spectroscopically. The experiment was complicated by a tendancy for the He3 to accumulate from shot to shot, impacting reproducibility. In L-mode, central temperatures up to 6 keV were obtained for ~4.4 MW of ICRF power. The H-mode threshold was found to be ~4 MW, which is higher than observed last summer. From the RF operation point of view, too little He3 appears to result in faulting in D and E antennas. Thursday's run was devoted to MP#361, an ITPA Joint Experiment on error field effects and locked modes using non-dimensionally identical parameters on C-Mod, JET and DIII-D. The JET experiments were completed in January and the DIII-D experiment is scheduled for next month. Tim Hender (UKAEA) is visiting MIT and led the C-Mod experiment. We obtained locked mode threshold data at different densities at a field of 6.3T and current of 1.3MA (q95=3.2), which match dimensionless parameters of JET discharges previously obtained. The C-Mod A-coil was operated with a mode spectrum closely matching that of the JET EFCC coils. Additional data, at different densities and using different A-coil configurations, is required to complete the C-Mod dataset for this experiment, and more run-time has been scheduled. Friday's run completed MP#375 on the role of magnetic balance between the upper and lower x-points on rotation effects and the L-H threshold. Threshold and rotation data were obtained for a fine-scale scan of SSEP, the measure of balance between the two separatrices, for slightly upward-biased (i.e. active X-point opposite to direction of ion grad-B drift) equilibria, completing the dataset for this experiment. The results are consistent with previous observations: both rotation and the L-H threshold power exhibit strong dependence on the value of SSEP, at the scale of 1-2mm separation between the two separatrices (measured at the outboard midplane). Diagnostics ----------- Repairs for the 16 low field side ECE Radiometer channels continue. The repair for the local oscillator (LO) for these channels is in progress at the vendor and is expected to be completed next week. This schedule will allow reinstallation and testing for a reasonable length of time before the end of the campaign. ICRF System ----------- All four transmitters operated very reliably last week. Up to 5MW was delivered to the plasma as required in support of research operations.It was noted that the loading variation during current rampup discharges is quite large and these plasmas would make good targets for real time matching techniques using ferrite tuners. A pair of 4CM2500KG tubes formerly in FMIT#1 and #4 have arrived at EIMAC for evaluation. These tubes expired after ~2500 filament hours (about twice the lifetime of the previous tubes) and we would like to investigate the failure mechanism. EIMAC is manufacturing the first tube of the three ordered at this time with an expected completion on 05/13/04. This tube will be sent to PPPL and we will pick up their production slot. This will not pose a problem for C-Mod since the delivery date is after the planned close of the present experimental campaign, and further tubes should be delivered before we start operating again. The first new 4CM2500KG will replace the low power tube in FMIT #1. A prototype DC break similar to the PPPL outer conductor design was built and capacitance characterized. With 0.003" kapton without Cu cladding, the capacitance was 13 nF (~125 mOhm) at 50 MHz. Further tests with different materials will be performed this week to investigate whether the capacitance can be increased further. Lower Hybrid System ------------------- Alumina windows needed for fabrication of the lower hybrid couplers were copper plated last week. This process is required before the windows can be brazed into the couplers. On Friday, following a 500C bake needed to verify the quality of the plating, a significant number of the windows showed blistering of the copper surface. We are in the process of determining the cause of the blistering, and planning for a solution. We continued work on the low power microwave systems last week. The vector modulator, I/Q detectors, SFOL links, and other components related to this systems are being tested as a full feedback loop. Eight of 12 ciculators are now fully tested and ready for operation. Possible problems with 2 of the remaining are being evaluated, and may require return to the vendor for rework under warranty. Travel and Visitors ------------------- Tim Hender (UKAEA/JET) is visiting C-Mod to participate in Joint Experiments on error fields and locked modes; he served as Session Leader for Thursday's experiment. Matt Sampsell (UT-FRC) visited C-Mod, where he worked with Bill Rowan on CXRS calibration and measurements. The setup work included establishing a reference source for regular intra-shot CXRS spectrometer calibration. The experimental work included CXRS measurements at various B's, various RF powers, and during locked modes. Analysis of this data will focus on v-poloidal profiles in the outer third of the plasma for the various conditions. Doug Loesser and Dave Miller (PPPL) visited this week to discuss the LH waveguide heater status.