Alcator C-Mod Weekly Highlights November 1, 2010 FY2011 weeks of research operations Target: 15 weeks Completed: 2.22 weeks Plasma Shots: 268 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed, with minor delays. A total of 121 plasma discharges were produced with a reliability of 87%. Experiments supported research in the Lower Hybrid Physics, Transport, and ICRF Physics topical physics areas and the ITER H-mode baseline support task. A fresh boronization was carried out over Monday night. Plasma operations are planned to continue this week. Quarterly Review ---------------- A C-Mod DoE Quarterly Review (FY2010, Q4) was held by videoconference on Wednesday, October 27. Viewgraphs for the presentations from the review can be found at www.psfc.mit.edu/research/alcator/pubs/Q-Rev_2010_10_27/Q_Rev_2010_10_27.pdf Participants included Mark Foster, Mark Koepke, Steve Eckstrand, and John Mandrekas from OFES, Randy Wilson (PPPL), Bill Rowan and Igor Bespamyatnov (U. Texas, Austin), and Earl Marmar, Miklos Porkolab, Dennis Whyte, Bob Granetz, Jerry Hughes, Brian LaBombard, Greg Wallace, Steve Wukitch, Amanda Hubbard, Bob Mumgaard, and David Terry at MIT. Operations Details ------------------ Tuesday's run in support of MP#578 continued an experimental test of EPED1.x, a class of models developed by Phil Snyder (GA) to predict the edge pedestal height and width in ELMy H-mode. A previous run from 2009 had established promising results with discharges at two values of plasma current, 0.65 and 0.9MA. We reproduced the ELMy H-mode at these currents, and added some data at 0.75 and 1.0MA. As in the previous run, Snyder participated remotely, acting as a session leader. High quality profile data were obtained in the pedestal region from both Thomson scattering and CXRS. Numerical predictions from EPED1 and EPED1.6 are being calculated and will be compared with experiment. This experiment contributes to the FY2011 OFES Joint Research Target on the testing of H-mode pedestal models. The run on Wednesday was dedicated to MP#580 "Comparison of Type I ELM access and characteristics in He4 and D plasmas in C-Mod". This experiment is designed to compare ELM characteristics in He and D majority plasmas in order to inform planning for the pre-nuclear phase of ITER operation. Following overnight ECDC in helium to remove as much deuterium from the walls as possible, a discharge was reproduced from the ELM study run on Tuesday, but using helium in place of deuterium as the fueling gas. ELMy H-modes were readily obtained, with characteristics that were quite similar to those previously produced in deuterium discharges. Data were collected at three values of plasma current, 0.75, 0.90, and 1.0MA, including scans of target density and heating power. Quantitative comparison of these results with similar deuterium discharges remains to be done, along with an accurate determination of the residual deuterium fraction in the majority helium plasmas. However, the initial results provide support for the ITER proposal to study ELM mitigation techniques in helium plasmas during the pre-nuclear operating period. The run on Thursday supported several MP's which had previously received runtime during this campaign but had not been completed. The day began with a short session devoted to MP#611 "Momentum Confinement and Lower Hybrid Source Determination". Both upper and lower null equilibria were run with LHCD, and rotation profiles were obtained with the HIREX_SR soft xray spectrometer. Upper single null discharges generally rotated in the counter-current direction, but at the lowest collisionality explored, the time scale of the rotation changes was very slow. There were also bursts of counter rotation after the LH turn off, followed by a gradual recovery toward co-current rotation. A fully non-inductive (zero loop voltage) LSN discharge at 580kA exhibited strong counter-current rotation, but with a more complicated time history. Additional shots on Thursday helped complete the dataset for MP#617 on locked mode rotations studies and calibration of the HIREX_SR spectrometer diagnostic. This portion of the experiment was aimed at testing the spectrometer in an elevated temperature environment (~30C) using new thermal pads, RTD's and temperature controllers. Reproducible locked mode shots were employed for the tests. Differences in the time history of the measured toroidal velocity profiles appear to correlate with shot-to-shot differences in the crystal temperature. Three ohmic shots similar to the target discharges from the LHCD experiment on October 19, 2010, were produced to improve the calibration of the MSE diagnostic to support the analysis of that experiment. The balance of the run on Thursday supported MP#619, "Effects of ICRF and LH power on SOL density profiles". This experiment, which employs data using the SOL reflectometer diagnostic installed next to the LH launcher, supports the research of an MIT graduate student. Data were obtained at nebar~7e19, 1.3e20, and 1.9e20/m^3, and at 800kA and 1.0MA. SOL density data were obtained during ICRF heating using each of the three antennas, and with lower hybrid power. These data are being analyzed. On Friday, we carried out the second day of experiments devoted to MP#606 "Measurements of Mode Converted Waves in D-H Plasmas with Phase Contrast Imaging", which comprises part of the research of an MIT graduate student. Friday's experiment emphasized the high hydrogen fraction portion of this MP, and required hydrogen puffing into deuterium target plasmas. Levels of nH/ne up to about 30% were obtained. Changes in the width of the signal consistent with full wave code simulations were observed. The data are being analyzed. ICRF Systems ------------ We continue to investigate the low power RF circuitry used to control the FMIT transmitter output power. The new solid state amplifier responds more quickly than the previous triode amplifier. The control system uses a gate to enable the transmitter and a separate demand to control the output power. At the gate enable, the FMIT was generating power without a demand. Delaying the gate enable to the solid state amplifier ameliorates the effect at the start of the RF pulse, but not on retries. A replacement for the modulator board to deal with this issue is begin manufactured. FMIT#1 (D-port antenna) developed an intermittent high voltage drop out that was traced to an intermittent air flow interlock, which has been repaired. Lower Hybrid System -------------------- The LH system was used to support experiments on C-Mod on Tuesday and Thursday last week. Tuesday's run involved launching LH waves into ELMy H-Mode plasmas. Production of ELMs on C-Mod requires a non-standard flux surface shape which does not conform to the shape of the LH launcher. The four-way splitter design of the launcher was able to redistribute power between the waveguide rows and performance was not adversely effected by the atypical plasma shape parameters. The LH system was used on several miniproposals on Thursday to investigate the effects of LHRF on core plasma rotation and SOL density profiles. Travel and Visitors ------------------- Following his participation in the IAEA meeting in Daejeong, Korea, Syun'ichi Shiraiwa visited several research institutes in Japan. During the last three weeks he visited TST-2 and QUEST STs, and LHD. Useful discussions were held concerning LHCD on a spherical tokamak, and a possible collaboration with LHD using their high frequency microwave sources. Paul Bonoli attended the annual workshop of the SWIM Prototype Fusion Simulation Project that was held this year at Tech-X Corporation in Boulder, CO from October 27-29, 2010. He led discussions and a series of presentations on two physics problems that are being pursued by the SWIM Project titled: (1) Time Dependent Minority Ion Tail Dynamics Study in Alcator C-Mod (2) Lower Hybrid Current Drive (LHCD) Control Studies in Alcator C-Mod The workshop was also attended remotely by John Wright and Aaron Bader who gave talks titled: Coupling TORIC and CQL3D in the Integrated Plasma Simulator (J. C. Wright) Comparisons of steady state and time-dependent AORSA-CQL3D simulations with experimentally measured fast-ion distributions on Alcator C-Mod (A. Bader) Zheng Yan, U. Wisconsin, visited MIT on Friday and gave the PSFC Seminar. She had discussions with several C-Mod scientists regarding pedestals and fluctuations, which she is studying on the D3D tokamak. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly