Alcator C-Mod Weekly Highlights Dec 5, 2011 FY2011 weeks of research operations Target: tbd Completed: 0.46 weeks Plasma Shots: 76 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. Power systems and other tokamak subsystems operated reliably. A total of 76 plasma discharges were produced, with a startup reliability of 60%. Reliability and plasma performance increased markedly over the course of the week as cleanup and conditioning proceeded. Full length (~2sec) plasmas with current up to 1MA were obtained. Following two runs devoted to startup and conditioning, the runs on Thursday and Friday supported research in the Pedestal Transport (diagnostic development) and Boundary Physics topical areas. Operations Details ------------------ Runs on Tuesday and Wednesday were devoted to plasma startup and conditioning, with the goal of obtaining reliable discharge operation and beginning the conditioning process to reduce the impurity and hydrogen concentration to levels suitable for beginning the research program. These experiments were successfully concluded on Wednesday, culminating in a sequence of ten full length ohmic lower single null diverted discharges with flattop current of 700kA and nebar~1e20/m^3. The H/D ratio was approximately 1, with Zeff between 2 and 2.5. These discharges were also used to begin checkout and commissioning of numerous diagnostic systems. All plasma operation was at Bt=5.4 tesla. We also produced one long flattop TF-only pulse for use in calibrating the toroidal tilt of the magnetic diagnostic loops and for evaluation of pickup in various diagnostic systems. Thursday's run was primarily devoted to MP#678 "Edge CXRS Related Diagnostic Development", from the Pedestal Transport Group. The primary purpose of these experiments is to debug and characterize newly installed diagnostics and gas systems used in conjunction with the edge Charge eXchange Recombination System (CXRS). These include a 2nd diagnostic gas puffing (NINJA) system and diodes and Photomultiplier tubes (PMT's) used for measuring Dalpha. These experiments support the thesis research of an MIT graduate student. Wednesday's experiment provided initial comparison of the new NINJA system manifold at F-port with the previously-installed C-port NINJA, and tested the D-alpha detectors at the outer wall mid-plane view at G-port. The capillaries at G-Mid-Out and B-Mid-In from both NINJA's functioned correctly. Data are being analyzed to provide gas transit times of each of four capillaries tested; estimated gain settings for the diode amplifiers; quantitative effect of lowering plenum pressure and increasing puff duration, based on results of a gas pressure scan; and comparison of the new D_alpha detectors (slow diodes) with the GPI High Field Side fast diodes, which will allow benchmarking of radiance calibrations. Friday's run was dedicated to MP#674 'Death-Ray' Physics and Sheath Heat Transmission Coefficients in Helium Plasmas. Measurements of heat flux with surface thermocouples during last year's MP#570 Boundary Layer Heat Transport Experiments in L-Mode Plasmas (all shots run with deuterium) indicated that the 'death-ray' divertor overpressure, a factor of ~2 enhancement of the divertor plasma electron pressure relative to the upstream value, was localized to the Langmuir probes. The 'death-ray' was previously thought to be a region of slightly elevated electron temperature near the strike point where neutrals, flowing at about the plasma sound speed due to charge-exchange collisions, were ionized and collected as an overpressure. We now believe that the negative bias of the Langmuir probe is the source of the local (to only the probe) elevated electron temperature. MP#674 aimed to test this theory by running helium plasmas. Helium has lower electron impact ionization and charge exchange cross sections than deuterium. Fine density scans with both helium and deuterium were performed around the region in edge collisionality where the 'death-ray' is typically seen--just before detachment. Initial results indicate that the 'death-ray' exists at the same electron temperature but is smaller in helium than deuterium, consistent with helium's lower electron impact ionization cross section. ICRF System ----------- First ICRF operation for conditioning purposes was performed using D and E antennas. The conditioning had a good start with each antenna reaching 1.5 MW without overlap. The H/H+D climbed from 0.3 to greater than 0.5 and the density increase was significant with RF. The resonant and decoupling loops for the new antenna have been assembled and characterized. There is significant coupling from the field aligned J antenna to the QCM antenna as measured without plasma. We will also make measurements of this coupling with plasma. The next step is to connect the resonant loops to the 9" transmission lines. Before we can fill the lines with SF6 and do significant power tests, the relative humidity in the lines needs to be reduced to < 2% by purging with dry nitrogen. Lower Hybrid System ------------------- Progress on the transmitter protection system (TPS) upgrade continues. TPS directional coupler detector diodes were calibrated. All of the cart TPS boards were checked on the bench for correct gains and trip level settings. There are a few remaining issues with the TPS firmware, but testing will continue to see if there are more before the next firmware update. Implementation of the new water cooling instrumentation was tested and bugs resolved. Redundant phase shifters have been removed from the low power microwave rack (LPMR). LPMR output power levels were measured and adjusted. Fixed attenuators were added in series for those channels which had their variable attenuators previously set near maximum attenuation. Diagnostic Neutral Beam System ------------------------------ The DNB continues to operate reliably at nominal parameters (50kV, 7-8 amps). With the DNB running at full parameters, the newly refurbished thermocouple instrumentation on the DNB calorimeter was used to do a perveance scan. The beam current was varied from 5 amps to 8.2 amps, with the beam voltage fixed at 50 kV. The smallest beam radius, 4 cm half-width at 1/e, was found at the maximum beam current. Dr. Weiguo Que, a PPPL electrical engineer, arrived last week to begin his long term assignment at MIT. He is currently working on an upgraded control system for the DNB. Diagnostics ----------- The HIREX_SR spatially resolving high resolution xray system is aligned and taking data. Final focusing operations are pending. The LoWEUS (Long Wavelength Extended Ultraviolet Spectrometer) is now operating in conjunction with the C-Mod shot cycle, acquiring Mo line-emission previously obtained from the McPherson spectrometer. The McPherson is presently off-line, awaiting repair of its turbo-pump. The XEUS (X-ray Extended Ultraviolet Spectrometer) is operating in conjunction with the shot cycle, viewing low-Z emission. The MKS baratron pressure gauges are on-line and operational. The PCI (Phase Contrast Imaging) fluctuation measurement diagnostic is operational. The SOL reflectometer system located adjacent to the Lower Hybrid launcher at C-Port is operational. The first data of the campaign from the FRCECE Electron Cyclotron Emission radiometer diagnostic was obtained on Thursday. The data from last week will be calibrated at the first of this week, and the calibration will be maintained thereafter. The first CXRS data from the core Charge Exchange Recombination System was acquired during startup plasmas operation on Wednesday. Cables and filtering for nine differential interferometer channels has been installed. Work is in progress on the interface to the MDSPlus tree, preparatory to shake-down and calibration of the system. Travel and Visitors -------------------- Timothy Stoltzfus-Dueck from IPP-Greifswald, visited MIT last week. He presented a talk on models for intrinsic rotation in the SOL and edge to the PSFC turbulence group meeting, and had conversations with several C-Mod scientists and students. U.S. Congressman Chris Murphy of Connecticut spent an hour at the MIT Plasma Science and Fusion Center (PSFC) on Monday 11/18, learning about fusion research at MIT and touring the Alcator C-Mod facility. Miklos Porkolab, Martin Greenwald, and Earl Marmar guided Congressman Murphy through the C-Mod control room and cell, explaining how MIT is contributing to the international quest for fusion power. On Thursday, Rachel Stephen-Smith, Minister-Counselor for innovation, industry, science and research at the Australian embassy in Washington, DC, visited the PSFC, where she met with Miklos Porkolab, Ian Hutchinson, Martin Greenwald, and Earl Marmar. She also toured the C-Mod facility and had discussions based on her general interest in fostering collaborations between Australia and the US. Syunichi Shiraiwa attended the 21th International Toki Conference held in Japan from Nov. 27 to Dec. 1st, and gave an invited talk on "Progress in LHCD modelling and experiments towards AT regime on Alcator C-Mod". He also visited Kyushu University on Friday, to learn about their RF heating technology. U.S. Congressman Martin Heinrich of New Mexico spent an hour at the PSFC on Saturday 12/3, learning about fusion research at MIT and touring the Alcator C-Mod facility. Martin Greenwald guided Congressman Heinrich through the C-Mod control room and cell, explaining how MIT is contributing to the international quest for fusion power. Rep. Heinrich, who was trained as a mechanical engineer, represents the 1st congressional district of New Mexico which includes Sandia National Laboratory. He has a strong interest in clean energy technology. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly