Alcator C-Mod Weekly Highlights June 7, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 8 weeks Operations ---------- Three run days were planned and completed last week. On Wednesday only three full-length plasma were produced as a problem with the OH1 power supply was diagnosed. On Thursday work continued very successfully on SOL Phase Space Studies, MP#409. On Friday very good results were obtained using the new high pressure disruption mitigation gas jet for the first time. After the OH1 supply was fixed, startup reliability was 92% for the remainder of the week. Plasma operation will continue this week. Physics ------- Thursday's run of MP#409 "SOL Phase space studies" was very productive. A careful scan of line averaged plasma density over the range of 0.25 to 1e20 /m^3 in matched upper and lower null topologies (Ip = 0.4 MA, Bt = 4.0 Tesla) was completed. A density scan at Bt=2.7 Tesla was started, setting up lower and upper null discharges at a line averaged density 0.83e19 /m^3. Based on our experience during this run, we should be able to produce the rest of the discharges in this density scan quickly. Boron Doppler spectroscopy on the inner wall was attempted, but the low density plus low intrinsic boron levels in the machine made the signal levels too low to be useful. Data collected from the A-Port scanning probe will be used to determine how the SOL profiles evolve as n/nG is raised at low absolute densities. Does a similar 'shoulder' appear in the far SOL at these densities compared to discharges with twice the current? This question will be addressed by loading the new data into the edge database and comparing profiles at otherwise similar n/nG values. Stewart Zweben, PPPL, was at C-Mod during this run and took GPI data on edge turbulence. Two ultra-fast CCD cameras were used to take the data. The PSI-5 camera viewed the GPI gas puff and the PSI-4 camera viewed radially in from the outer mid-plane. The edge turbulence signal levels were low due to the low density of these discharges, but useful data was obtained. Stewart has calculated the correlation lengths of turbulence in the BOUT turbulence modeling run of Umansky to be 1.3 cm in the poloidal direction and 0.5 cm in the radial direction, which are within a factor of two of the experimentally measured correlation lengths. MP #424, Initial Tests of Disruption Mitigation Using a Gas Jet, was run on Friday. This run was very successful with the newly installed gas jet hardware working as soon as it was enabled. Two gases (He and Ne), a range of pulse lengths (1-5 ms), and ohmic (0.38 Atm) and RF heated L- and H-Mode plasmas ( 0.55 to 0.75 Atm) were tried during the run. The gas jet reliably disrupted the plasma with very good reproducibility. Jitter was only about 0.5 ms, which facilitated optimization of the timing of the Thomson system and the ultra-fast camera imaging. The Thomson scattering data gave good temporal information on electron temperature and density profiles which will be very useful for the NIMROD modeling. The Thomson system uses two lasers, each operating at 30 pulses per second. Up until now, these have been operated with equal spacing between the pulses, giving a 60 Hertz measurement rate. During this run, the relative delay between the two sets of pulses was, for the first time, varied from shot to shot, so that the evolution just before and during the disruptions could be obtained. The camera captured good sequences in He0, He+, and Ne0 light. Helium gas jets did not change the usual disruption time scales, nor did they reduce halo currents. Neon (at only 49 bar) did speed up the quench, and reduced the halo currents. Detailed bolometry and AXUV profile data was taken and will be analyzed. Future runs on this mini-proposal will explore higher pressure (up to 70 bar) neon and argon puffs and H-mode plasmas with higher plasma pressure. Finally, it has been shown unequivocally that gas jet mitigation does not have a negative impact on disruption recovery as startup remained robust throughout the day. There were 24 consecutive successful startups following the 1 MA disruptions. Lower Hybrid ------------ Stefano Bernabei, Bob Ellis, Joel Hosea, Gerd Schilling, and Randy Wilson, all at PPPL, attended the weekly Lower Hybrid meeting via teleconference with MIT on 6/2. Several options are being considered by the combined team for a reworked coupler design. The MIT group presented analysis of the initial braze tests of alumina microwave windows brazed directly into a stainless steel 3-window coupler prototype. FEA results simulating the full 24-window stainless steel coupler which indicated acceptable stresses in the windows were also presented. PPPL presented progress on an alternate design using alumina windows brazed into a copper sleeve. MIT is taking the lead on evaluation of a third design in which extruded stainless steel waveguides with alumina windows brazed into each channel are welded together into a coupler. Long Pulse Diagnostic Neutral Beam ---------------------------------- Beam conditioning continued last week with beam currents now in the 5A range, and full energy fraction ~63%. ICRF Systems ------------ ICRF antenna recovery from the vent has continued. The E-Port antenna has reached ~1.5 MW in upper single null plasmas and 1 MW in lower single null. The J-Port antenna has reached 3 MW with some faulting and 2.7 MW fault free in both upper and lower single null discharges. The D-Port antenna continues to improve slowly and is now at the 0.3 MW level. Conditioning in helium discharges will begin this week which should improve the recovery rate. A meeting was held last week at MIT to discuss operational issues that could impact the lifetime of our 4CM2500KG 2 MW ICRF final amplifier tubes. In attendance were Marshal Loring (High Power Electronic Systems and Components consultant who participated in the development of the 4CM2500KG tube), Chris Brunkhorst (PPPL RF engineer), Dave Terry, Andy Pfeiffer, J. Fred Riley, Steve Wukitch, and Peter Koert. The primary recommendations included improvements in the cooling water system, improved monitoring of cutoff voltages and idle currents, and changes to the grid power supply crowbar. Tube lifetimes are currently approximately 2500 hours at MIT. Travel and Visitors -------------------- David Mikkelsen, PPPL, visited MIT to discuss details of a planned experiment to explore the conditions needed to form ITBs in C-Mod. A number of plasmas were found that are suitable for testing a proposed method of detecting small changes in electron temperature gradient scale length. Stewart Zweben was at MIT taking data with the ultra-fast cameras. Marshal Loring, High Power Electronic Systems and Components, was at MIT to participate in the meeting on FPA tube lifetime. Joseph Snipes visited T&C Power Conversion in Rochester, NY to test a spare C-Mod active MHD antenna with their amplifiers to determine if they will be suitable driving TAEs in C-Mod and JET. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly