Alcator C-Mod Weekly Highlights May 15, 2006 FY2006 weeks of research operations: Planned: 14 weeks Completed: 10.9 weeks Operations ---------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 77 plasma discharges were produced with a reliability of about 80%. Experiments were carried out in support of research in the Lower Hybrid Physics, Transport, and Integrated Scenarios areas. The milestone target of 500kW of Lower Hybrid power coupled to the plasma was achieved during LHCD experiments on Thursday. One overnight boronization was performed on Monday night, and between-shot boronizations were employed during runs on Wednesday and Friday. The Monday night boronization used a smaller range for the sweep of the ECDC resonance location than previously prescribed, in an effort to minimize interaction with the ICRF antennas. The amount of boron deposited was also reduced compared to boronizations conducted earlier this year. Plasma operations are planned to continue this week. Run summaries ------------- The run on Tuesday continued our experiments in support of C-Mod MP#382 (ITPA PEP-7) on C-Mod/JET/AUG Pedestal Identity Experiments. Additional data was obtained at q95=4.1 (B=7.8T, Ip=1.3MA), but we were unable to extend the dataset to higher nu* because of limitations on the ICRF power. At higher current (Ip=1.7MA, q95=3.2) H-mode and threshold data were obtained with P_rf< 3.8MW. Stored energy above 180kJ was obtained, with pedestal nu*~0.6. Wednesday's run was devoted to MP#451 "Structure and stability of large ELMs". These experiments were carried out at B=5.6T and in the "JFT-2M shape", which features high triangularity at the lower (active) x-point and larger than standard inner and outer gaps. During this run we for the first time observed ELM filments at the inboard midplane. These propagate inward in major radius and occur before the filaments on the outboard side. We also obtained cases at low pedestal collisionality (nu*~.25, Te_ped~900eV), which exhibited strong ELMing activity. Plasmas in the same shape, but with higher density and lower collisionality, resulted in ohmic EDA H-modes. The indications are that the effect of the strong shaping of these discharges is primarily in allowing access to the low collisionality regime, rather than on the stability of the peeling-ballooning modes. The widths of the density pedestals in these discharges is observed to be wider than those in more standard shapes, implying enhanced particle transport, which favors formation of the low collisionality pedestal and therefore access to the ELMy regime. It was also noted that the global confinement in these discharges is well-fit by the ITER-98y ELMy H-mode scaling. Thursday's run continued the Lower Hybrid Coupling study (MP#418). The phase coupling study was completed using 20 different phases per shot and three positions of the grill with respect to the guard limiters. The lowest reflection coefficient obtained was ~11%, which is a significant improvement over previous results. Upon completion of the coupling scans, the LH power was increased for current drive experiments. Over 500kW of LH power was coupled to the plasma for the first time, at fields of 5.4 and 6.3T, with pulse length of 0.4 sec. LHCD data using non-compound spectra were obtained over a range of phases, densities and power. On Friday we continued experiments in support of MP#448 on small-ELM regimes in MAST, NSTX, and C-Mod. Data for the comparison were obtained at high pedestal pressure, but the small ELM regime was not accessed. ICRF power up to 4.7MW was obtained, which was an advance over the previous run day. Between-shot boronization was attempted, but subsequent plasmas experienced startup difficulties, limiting the amount of data obtained. ICRF System ------------ On Monday the E-port coaxial transmission line was opened in order to make additional electrical measurements in support of the design of new coaxial transformers to limit the voltage maximum in the phase-shifter/stub-tuner assemblies. The coax was reassembled and purged in time for operation on Tuesday. All four transmitters were operated during C-Mod experiments this week. The ICRF group tested low power rf discharges as a means of cleaning and conditioning the J antenna before runs, especially after boronization. Considerable improvement in J port output power was obtained after the Friday morning pre-run multipactoring clean-up, with up to 2.5 MW from the J-port antenna (4.7MW total) obtained by the fifth shot of the day. DNB Systems ----------- On Tuesday, the DNB was successfully operated into C-Mod plasmas, including modulated beam operation. However, late in the day, the LaB6 "washers" that comprise the cathode emitter developed cracks. A new spare cathode assembly was installed on Wednesday evening, and conditioning of the arc began. The beam was operated with all systems operational again on Thursday and Friday. Lower Hybrid System ------------------- The Lower Hybrid System operated successfully during dedicated C-Mod experiments on Thursday, achieving this year's target of over 0.5MW coupled to the plasma. The modifications to the Coupler Protection System (CPS) boards were tested during operation. Trip levels for the 48 new protection channels were checked and 14 are now being recalibrated. We also found that during operation with ICRF on, the CPS sees pickup from the ICRF, which will require addition of filters before the detector diodes. Travel and Visitors ------------------- Steve Wolfe, Josh Stillerman, and Marco Ferrara participated in the NSF Workshop on Mathematical Modeling and Control of Plasmas in Magnetic Fusion, held at General Atomics in San Diego May 11-12. Earl Marmar presented the Colloquium of the Physics Department of the University of California at San Diego on Thursday, May 11. He also had meetings with Pat Diamond, Sergei Krasheninnikov, Farrokh Najmabadi, Russ Doerner, Tom O'Neil and Fred Driscoll to discuss topics of mutual interest. Geoff Maddison (UKAEA) participated remotely from JET in the ITPA Joint Experiment PEP-7 (C-Mod MP#382) on Tuesday. David Mikkelsen spent the week at MIT discussing a planned experiment to study momentum transport in Alcator C-Mod, and examining recent data from the high frequency reflectometers. Igor Bespamyatnov, Alex Graf, Alex Ince-Cushman, Jinseok Ko, Liang Lin, Bill Rowan, and Steve Scott attended the 16th Topical Conference on High Temperature Plasma Diagnostics in Williamsburg, VA. Steve Scott also organized a "satellite" meeting on MSE at the HTPD conference. There were about 10 talks and 30 participants. The title of Steve's presentation was "Resolution of MSE Calibration Anomalies on Alcator C-Mod: The Perils of Perpendicular Injection"" _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly