Alcator C-Mod Weekly Highlights August 20, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 12.62 weeks Operations ---------- Following extensive testing and remedial action addressing the ground-fault which appeared on the TF magnet on August 8, research operations resumed at Alcator C-Mod on Thursday, August 16. One and a half run days were completed, in support of experiments in the H-mode and AT Integrated Scenarios thrusts. A total of 34 plasma discharges were produced with a reliability of 89%. The location of the ground-fault was determined within a fraction of a turn by a series of low voltage DC measurements. An attempt to clear the short electrically by discharging a capacitor between the magnet and ground succeeded in increasing the stand-off voltage to >2500 Volts, which is our normal maximum hi-pot value. Subsequent magnet pulses at fields up to 5.3 tesla resulted in brief recurrences of the ground, late in the ramp-down phase of the pulses. The fused ground tap was then relocated to a point on the magnet close to the identified location, in order to minimize the voltage available to drive ground current, and, following additional tests, plasma operation was resumed. Less than 2 amps of ground current was observed following the first plasma pulse. There have been no additional instances of measurable ground current in nearly 40 subsequent machine cycles, and hi-pot tests continue to be successful. We are presently monitoring the ground current on every pulse, and have instituted additional DC monitoring of the TF-ground resistance during off-hours in case the status should change. The cryopump was successfully employed for both plasma runs last week. No boronizations were carried out. Plasma operation is planned to continue at Alcator C-Mod this week. Operation Details ----------------- A half-day run on Thursday supported MP#506 "H-modes with cryopumping in upper single null topology". This run was intended to complete the evaluation of pedestal and global H-mode response to cryopumping. It was run in USN topology (unfavorable grad-B drift direction) to maximize particle flux to the pumping slots. A range of currents (Ip=0.4, 0.6, 0.8MA) and target densities (nebar from 1 to 1.6e20/m^3) were run, in an attempt to produce steady low-density H-mode regimes. Radiated power rose steadily in these H-modes, leading to pedestal cooling and plasma fueling, culminating in back transitions to L-mode. Edge fluctuation measurements revealed quasi-coherent modes, though they were apparently insufficient to control impurity accumulation. A comparison discharge without the cryopump indicated that the neutral pressure in the pump chamber was several times higher in the unpumped case, and pedestal temperatures were lower. With the pump activated, there was little long-term pump-out of the H-mode discharges following their formation. This contrasts with the result in the reversed-field USN cases run last month. The H-modes with the best confinement properties appeared to have classic grassy ELMs replacing the QCM. Detailed analysis of these results is underway. Friday's run was devoted to MP#490, "Assessing LHCD in ICRF -Heated L and H-modes". The main goal was to combine LHCD with ~ 2 MW of ICRF, for a few different physics conditions, and document both coupling and current drive. Experiments were conducted in H-mode, lower null equilibria and L-mode upper nulls. Good hard xray and loop voltage data were obtained in comparable density and LHCD conditions with ICRF off and on. The results demonstrate that ICRF and LHCD can readily be combined at substantial power levels (over 500kW of LH and up to 4MW ICRF). As expected from previous experiments, LHCD coupling during ICRF heating using the J-port antenna was better than that during ICRF at the same power using the D- and E-port antennas, which are closer to the LH grill. A strong non-linear dependence of the LH-generated non-thermal electron tail on plasma density was observed on the both hard xray and non-thermal ECE signals during this experiment. ICRF System ----------- The J-port antenna remained configured with straps 1 and 2 powered from FMIT#3 and straps 3 and 4 from FMIT#4 in (0,pi,pi,0) phasing. Steady ICRF power (from all three antennas) up to 4MW was reliably produced in support of plasma experiments last week. Lower Hybrid System ------------------- The lower hybrid system was employed during C-Mod plasma operation on Friday and performed nominally, providing reliable pulses above 500kW net power coupled into plasma as requested. Testing of a four-arm splitter, prototypical of configurations under consideration for new LH launchers was successfully carried out up to 190kW with 50 msec pulses. The results were in agreement with previous simulations. Long Pulse Diagnostic Neutral Beam System ----------------------------------------- The DNB performed nominally during plasma operation last week, supporting CXRS diagnostic measurements. A disassembly of part of the MSE polarization-analysis optics revealed damage to the mounting of the photo-elastic modulators (PEM), which may have been responsible for some of the observed irreproducibility of the MSE pitch angle measurements. The PEM unit has been returned to the vendor for expedited repairs. Travel and Visitors -------------------- Stewart Zweben (PPPL) was at C-Mod to continue the analysis of the new divertor GPI turbulence data, and to discuss ideas for the 5 year plan with Jim Terry and others. Gerrit Kramer (PPPL) visited C-Mod to discuss the cause of low signal strength in the 132 GHz reflectometer with Arturo Dominguez. The probable cause is that the waveguide system is not sufficiently straight, which leads to excessive losses particularly at these high frequencies. Discussions with the C-Mod engineering team have begun to re-design the waveguide system and its integration into the port. Paul Bonoli and John Wright hosted the annual Summer Workshop of the SciDAC Center for Simulation of Wave-Plasma Interactions that was held at MIT on August 14-16, 2007. Also attending from MIT were Jeff Freidberg, Miklos Porkolab, Steve Wukitch, Yijun Lin, and NUF students Dan Richman and Michael Hoffman. The other workshop participants were Don Batchelor, Lee Berry, and Fred Jaeger from ORNL, Dan D'Ippolito and Jim Myra from Lodestar Research, Cynthia Phillips from PPPL, David Smithe from TechX, Bob Harvey from CompX, Myunghee Choi from GA, and Yuichi Takase from the University of Tokyo. Bob Harvey also met with Paul Bonoli, Andrea Schmidt, Ron Parker, on August 17, 2007 to discuss physics issues related to the application of synthetic diagnostics for hard x-ray and electron cyclotron emission in the GENRAY-CQL3D simulation code to LHRF experiments on Alcator C-Mod. Vincent Tang (LLNL) visited the PSFC to consult with Ron Parker, Jason Sears, and other C-Mod experimenters regarding the compact neutral particle analyzer (CNPA) he developed as part of his doctoral thesis. Dr. Artem Smirnov visited with Robert Granetz and Dexter Beals during the week of 13-17 Aug to learn about our diagnostic neutral beam. Dr. Smirnov is in charge of the neutral beam heating system on a new plasma confinement device (a field-reversed configuration, or FRC) being built at UC Irvine. That NBI system will consist of a total of 6 injectors, each supplying 1 MW of power. The parameters for each of the beams will be 40 kV, 25 amps of equivalent neutral current, for 1 second. Given the neutralization efficiency at 40 kV, this implies an ion source current of 40 amps. The beams are being manufactured by the Budker Institute in Novosibirsk, Russia, which also manufactured our DNB (50 kV, 7 amps, 1.5 s). Many of the new beam components and systems will be similar to ours, and therefore Dr. Smirnov came here to observe the operation of our beam, and to learn about our experiences on various issues, such as engineering manpower requirements, operational issues, control systems, and hardware problems. Dennis Mansfield, PPPL, visited on August 7, to meet with Jim Irby to plan the PPPL/MIT polarimetry collaboration. They also discussed lithium and boron deposition techniques on both C-Mod and NSTX. Pablo Acedo, Carlos III University, Madrid, is at MIT for two weeks as part of the Carlos III/MIT collaboration on interferometry. He discussed with Jim Irby new imaging interferometry proposals for the TJ-II stellarator, and positional feedback systems for the C-Mod two-color-interferometer and polarimeter systems. From August 7-10, Sam Pierson (MIT) visited a Los Angeles vendor specializing in diffusion bonding and high temperature brazing to observe tests of a process for producing a first wall tile with a tungsten plasma facing surface diffusion bonded to a molybdenum base. The inital tests were not successful. It is believed that the nickel plating on the surfaces to be joined was too thin, such that a eutectic inter layer was not formed at temperature and thus bonding never occurred. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly