Alcator C-Mod Weekly Highlights August 23, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 13.3 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 2311 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 100 plasma discharges were produced with a startup reliability of 92%. Runs supported research in the Boundary Science, Transport, and Lower Hybrid Physics topical areas. A fresh boronization was carried out on Wednesday night. Plasma operations are planned to continue this week. Program Planning ---------------- A Conceptual Design Review (CDR) for the outer divertor upgrade was held at MIT on August 13th with remote participation from PPPL and GA. At GA were Peter Peterson, Arnie Kellman, Paul Anderson, and Mark Foster. At PPPL were Timothy Stevenson, Bob Ellis, Phil Heitzenroeder, Peter Titus, Han Zhang, Rich Hawryluk, and Charles Kessel. At MIT were Soren Harrison, Rui Vieira, Bruce Lipschultz, Jeff Doody, Lihua Zhou, Brian LaBombard, Jim Irby, Miklos Porkolab, Bob Mumgaard, and Mike Garrett. The objectives of the new divertor were discussed, as well as the preliminary design, and thermal and mechanical stresses. The presentations may be found at: http://www.psfc.mit.edu/research/alcator/data/eng_reports.html On August 18, Earl Marmar and Jerry Hughes participated in a USBPO-organized conference call on ELM control research, along with many other scientists from the US and from the ITER Organization. During the call, important upcoming decisions regarding ELM control tools on ITER were highlighted, and plans for US research with the potential to inform these decisions were discussed. Operations Details ------------------ Tuesday's run supported MP#579 "Characterization and optimization of I-mode". The goal of this experiment was to obtain new profile and fluctuation data from CXRS, using both the DNB and local gas puffing, in USN I-mode discharges at 1.1MA and 0.8MA, with Bt=5.7T. Neon puffing was employed to reduce the heat flux to the strike points at high RF power (up to 4MW). These data are being analyzed. Wednesday's run supported two experiments in the Lower Hybrid Physics area, MP#608 "Lower Hybrid Wave Coupling Study on LH2 Using Microwave Probe Diagnostic" and MP#463 "Reaching Zero loop Voltage with LHCD". The former experiment aimed to measure curves of reflection coefficient versus density at the coupler for different phasings and launcher positions, and to investigate the effect of localized gas puffing using capillaries next to the launcher. We obtained measurements of reflection coefficient vs density at the grill in the range 4e17 to 4e18/m^3 for three relative phase settings at two Launcher positions. The reflection coefficient improved as we pushed the launcher in. The same discharges that had good coupling at low power resulted in faulting at high power. Localized gas puffing improved coupling at high power, and with it we were able to reach the same level of reflection coefficient as at low power. The results support the conjecture that the reason for high reflectivity is very steep density gradients in front of the LH grill. Simulations to address this effect are in progress. The effect of localized gas puffing is promising, but further work to optimize the effect is required. The second part of this run was devoted to MP463 "Reaching zero loop voltage with LHCD". The object of this experiment was to optimize conditions for LHCD (coupling, power and density) with the aim to reduce the loop voltage to zero (fully non-inductive current). At a power of 700kW the loop voltage was transiently reduced to 0.2V, although it increased later in the pulse. About 500eV of electron heating was observed at this power level. The best results were obtained with coupled LH power at or below 700kW and nebar < 6e19/m^3 and plasma current at or below 700kA. The complex interplay between current, gap, configuration, and LH power needs to be better mapped out to find the best path for nulling the loop voltage. Thursday's run was devoted to MP#591 "Boundary Layer Heat Transport Experiments in H-mode Plasmas", which contributes to the 2010 SOL Heatflux Joint Research Target. The goals of this experiment were to measure heat flux footprints in LSN EDA H-mode discharges with improved ramped tile diagnostics and with the strike point on the vertical target face. Experiments were carried out over a range of P_sol and at different plasma currents between 0.5 and 1.0MA. Friday's run continued the series of experiments devoted to MP#608 "Expanded exploration of LHCD Density Limit". The main goal of this run was to obtain scanning probe and MSE data for discharges below and above the LHCD density limit, in limited and diverted configurations. Good data were obtained in several configurations: lower null low density, double null low density, inner wall limited high density, and lower null high density. In double null equilibria at high density, the experiment was complicated by H-mode transitions during the Lower Hybrid pulse. The data from these experiments are being analyzed. ICRF Systems ------------ The J-port antenna was vacuum conditioned to ~38kV prior to plasma operations last week. The antenna conditioned quickly, and required only 3 pulses to reach 30kV, compared to 20 pulses the previous week. During plasma operation, the antennas still exhibited faulting at high current, but their performance was improved through impurity puffing, suggesting excessive heat flux to the antenna may be responsible for some fault indications. As is typical after boronization, the antenna performance was transiently degraded but recovered with plasma conditioning. ARRA Activities ---------------- The remaining feedthrus and the raw molybdenum alloy (TZM) rod for the ICRF antenna have been received (ARRA funded). We have completed the detailed drawings for the outer conductor tube assembly and will next detail the center plate. We have completed the wire EDM programming for the inner two straps and are preparing to do a test cut. Lower Hybrid System -------------------- The Lower Hybrid Current Drive system supported runs on Wednesday and Friday. Wednesday's experiments focused on optimizing reflection coefficients and also increasing LH power at low density. Friday's experiments were aimed at better understanding the topology dependence of the LHCD density limit on C-Mod. Measurements of current profiles by MSE and SOL density/temperature profiles by scanning probes were made in lower and double null and inner wall limited configurations at a range of densities. The LH-SOL reflectometer has been fully assembled in preparation to collect first data. Installation plans for instrumentation and data acquisition equipment for the reflectometer are being developed. DNB Systems ----------- The diagnostic neutral beam operated on all run days this week. Typical ion beam parameters are now 50 kV and 7 amps (full performance) throughout the plasma flattop (modulated). Diagnostics ----------- The optical table for the WideView CXRS system was changed to one of larger size so that the new BES system can be installed in the existing lab area. The spectrometer was realigned and refocused with essentially no change in spectral resolution in the range required for Ti measurements. Narrower slits than normally used were tested with calibration lamps and with plasma emission to find the best trade-off between spectral resolution and spectrometer throughput. This is the final test needed before purchasing several new sets of slits. Travel and Visitors ------------------- Prof. Hyeon Park, Pohang University of Science and Technology, Korea, visited on Thursday, 8/19. He presented a PSFC seminar, "ECE Imaging Systems on TEXTOR, AUG, DIII-D, and KSTAR", had discussions with John Wright, Paul Bonoli, Anne White, Amanda Hubbard, Earl Marmar, Miklos Porkolab, and Ron Parker, and also met with several graduate students. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly