Alcator C-Mod Weekly Highlights October 3, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 14.5 weeks Plasma Shots: 1706 Research -------- One of the key questions regarding the I-mode regime, which features a steady thermal barrier without a particle barrier, is understanding the conditions under which it is obtained (L-I transition) and maintained without transitioning to H-mode (I-H transition). A large database of discharges and parameters has been assembled from past campaigns, which is being used for threshold analysis, focusing on the most common condition with unfavorable drift, and B 5-6 T (over 170 I-mode time slices, about 40 L-I and I-H transitions). The L-I thresholds have relatively well defined trends. Power thresholds increase with both current and density, which are partially correlated in our database. Edge temperatures at psi=0.95 are in a range of 250-400 eV, independent of current and density, suggesting a possible threshold condition. This is about twice that of previously published L-H thresholds with favorable drift. Conditions for I-H transitions are less clear; there is considerable overlap between powers and edge parameters before transitions and in plasmas which remained in I-mode. A power window up to 1.8x the L-I threshold scaling has been achieved. It appears that the power window may be largest at lower density. These results will be presented by Amanda Hubbard at the upcoming International Workshop on H-mode Physics and Transport Barriers, Oxford, UK. Results motivate further experiments to clarify the I-mode density dependence and extend the power window (planned for the FY12 campaign), and the need for comparisons with other tokamaks through ITPA joint experiments. Operations ---------- We continue to prepare magnet power supplies, ICRF systems, and the alternator for operation. Spare fuses and other components needed to support the run campaign are on-order or are now in-house. In-vessel work last week included precise positioning of the lower hybrid limiter molybdenum protection tiles 0.2 mm forward of the coupler waveguides. The Fast Ion Loss detector installation on the outer wall is complete, and its fiber optic cable has been run to a lower vertical port vacuum flange. A transformer expected to be used in the EF2 upgrade has arrived at the vendor's site and is being prepared for testing. ICRF Systems ------------ Modifications to the back radius of the ARRA funded rotated antenna backplates have been machined successfully by our vendor. Four servo amplifiers needed for the ICRF FFT system arrived last week. The Ferrite Tuner Test Fixture is being brought back into operation so that tests of the new components can begin. FMIT#1 and FMIT#2 have been brought up to high voltage, and they have both passed the crowbar wire test. FMIT#1 and FMIT#2 screen crowbars also tested successfully. Our vendor working on the fast opening switch replacement for the FMIT high voltage crowbars visited last week. They expect to provide a demonstration of the new switch technology soon. Lower Hybrid Systems -------------------- The first set of six transmitter protection system (TPS) control boards arrived from the SBIR vendor. The remaining four sets of six boards are expected to arrive early next week. Each set of six boards controls a cart of four klystrons. A decision was made to proceed with the TPS upgrade for the upcoming run campaign based on this development. Communication with the TPS boards was established via telnet. A stress test of 500 read/write iterations with the TPS boards completed successfully. Fiber optic communications hardware was installed in the controls rack and testing is under way. A script was written for down-loading user defined parameters from the TPS GUI to the control boards. TPS upgrade fiber mapping for all four carts was completed. The copper contamination on the vacuum side of window 7D was successfully removed by grit blasting. RF tests show that transmission through the window has returned to normal levels. The optimum length of the sliding short on column 16 was determined. The sliding short was removed from the air side of column 16 and will be replaced by a fixed length short. Remote Participation, Travel, and Visitors ------------------------------------------ Miklos Porkolab attended the NRC Plasma Science Committee meeting at the Beckman Center of the National Academy of UC Irvine, CA on Friday and Saturday, September 23 and 24. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly