Alcator C-Mod Weekly Highlights July 6, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 9.9 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 1702 Operations ----------- Plasma operations resumed at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 72 plasma discharges were produced with a startup reliability of 60%. The runs supported startup and conditioning activities under Miniproposals from the Operations, ICRF Physics, and Lower Hybrid Physics tasks, and began the physics characterization of the new lower hybrid launcher. Plasma currents up to 800kA in the lower single null diverted configuration were produced, at toroidal fields between 5.4 and 6.0 tesla. Plasma operations are planned to continue this week. Operations Details ------------------ Plasma operation was successfully re-established on Tuesday, and clean-up and conditioning discharges were continued through Wednesday. Initial discharges were short and resistive, with low startup reliability. Continued operation produced notable improvements in cleanliness, and full length diverted discharges with Zeff~2 were produced with high reliability on Wednesday. Once satisfactory full length ohmic discharges were available, we began characterization of the new LH antenna under MP#605 "LH2 Lower hybrid Launcher and Conditioning". On Wednesday, the upgraded coupler protection system (CPS) trip levels were adjusted for 60, 90, and 120 degree phases (n|| = 1.3, 1.9, and 2.6), using low-power (~100kW), short (< 20msec) pulses. After verifying proper operation of the CPS at a range of phases, the LH pulse length and power were increased on Thursday, as we continued MP#605. By the end of the day, pulse lengths of 0.5 s and coupled power levels of 675 kW had been obtained. Non-thermal ECE and hard x-ray signatures were seen during LH pulses at net power levels greater than 300 kW, although small changes in loop voltage do not indicate significant driven current. Little driven current was expected due to the high line averaged densities, which were near the previously observed LHCD density limit for C-Mod of ~1e20 m-3. Friday's run was dedicated to MP#355b, "ICRF Testing and Conditioning, with Detection of Mode-Conversion Location". The principal goal of this MP is conditioning of the ICRF antennas to obtain reliable operation at nominal power levels for support of the experimental program. The secondary goal of this MP is to use the observation of the mode converted (MC) wave by the phase contrast imaging (PCI) diagnostic to characterize the hydrogen fraction in the plasma core. For the initial series of conditioning discharges, each antenna/transmitter was exercised sequentially in time to limit the instantaneous power delivered to the divertor and limiter surfaces. The D- and E-port antennas were successfully conditioned up to 1MW power levels, with some faulting still present. The two paired straps of the J-port antenna were run independently in dipole phasing, and low-faulting operation was not obtained above the 500kW per dipole level. We were successful in measuring the MC waves with PCI during discharges at B=6T. The H/D values inferred from the MC location appear to differ significantly from those measured spectroscopically looking at balmer-alpha emission from the plasma edge. Further investigation and analysis is required to obtain quantitative conclusions from the mode conversion measurements. ICRF Systems ------------ Conditioning of all three antenna systems into plasma was begun, as described in the Operations section. The solid state, 10 kW amplifiers, designed to replace the triode based Intermediate Power Amplifiers (IPA) for FMIT#1 and #2, have been delivered from the vendor. A new sample holder for the RF breakdown device has been manufactured. This new design should allow for smaller, better controlled samples to be tested. Space constraints make attachment of the Faraday screen for the new rotated antenna difficult. We are considering modification of the antenna geometry and a reduction in the size of the non-inductive resistors used to limit disruption currents as possible solutions to this issue. Two more feedthrough assemblies have been completed by our vendor. Lower Hybrid System -------------------- Last week marked the first operation of the new "LH2" lower hybrid current drive antenna on C-Mod. Two days were dedicated to miniproposal #605, "LH2 Lower Hybrid Launcher Checkout and Conditioning", as described in the Operations Details section above. Calibration of the LH probes continued last week. Adjustments were made to the probe attenuator settings. Calibration of two probe channels with the modified attenuator value was completed. DNB Systems ------------ The two visiting DNB experts from the Budker Institute continued working with us on our arc source. They rebuilt the cathode heater, and had a new outlet nozzle manufactured for the hollow cathode. Late in the week they began testing the arc source with the new cathode components. Travel and Visitors ------------------- Last week Stewart Zweben (PPPL) was at MIT to help start up the GPI diagnostic and to discuss plans for the run. Excellent GPI images were obtained with the outer midplane camera at 400,000 frames/sec. Images were also obtained with the X-point region camera at 400,000 frames/sec, but with much lower signal to background level. Plans were made to use the GPI cameras during the upcoming runs for the FY2010 SOL heat flux joint facility research target, and to compare these GPI results with SOLT simulations of SOL turbulence by Lodestar. Randy Wilson (PPPL) visited MIT last week to participate in the initial experiments using the new LH launcher. He served as Session Leader for the experiment on Thursday. Martin Greenwald was at LLNL (6/21-24) to participate on the External Review Committee for the Physical and Life Sciences Directorate. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly