Alcator C-Mod Weekly Highlights Dec 12, 2011 FY2011 weeks of research operations Target: tbd Completed: 0.9 weeks Plasma Shots: 155 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and 3 1/2 were completed. A total of 79 plasma discharges were produced, with a startup reliability of 77%. The runs supported ICRF conditioning, diagnostic development, and Boundary Physics research. Machine cleanup and conditioning continued over the course of the week. By Friday the H/(H+D) with significant ICRF power was reduced to the 20-25% range. Ohmic H-modes, an indication of improving vessel cleanliness, were obtained during Thursday's experiment. Plasma operations are planned to continue this week, following a three-day scheduled maintenance interval. Physics -------- The paper "Search for zonal flows in the edge turbulence of Alcator C-Mod" by J Zweben, JL Terry, M Agostini, R Hager, JW Hughes, JR Myra, DC Pace and the Alcator C-Mod Group, has been accepted for publication in Plasma Physics and Controlled Fusion. The paper reports on the evaluation of time-dependent radial and poloidal velocity of edge turbulence using a 2-D time-delayed cross-correlation analysis of fast camera data from the gas puff imaging (GPI) diagnostic viewing the outer mid-plane region in Alcator C-Mod. The local poloidal velocity fluctuations are averaged over the poloidal viewing region of the GPI diagnostic and radially resolved over ±2 cm around the separatrix. The resulting poloidal velocity usually has a broadband frequency spectrum in the range ~1-20 kHz, and a radial correlation width which decreases with line-averaged density. In some cases with ICRH heating there was also a coherent poloidal velocity oscillation at 6-7 kHz which was highly correlated with a poloidal magnetic field oscillation at the same frequency. Some of these results are similar to the GAMs and/or zonal flows described in previous experiments and theory. Operations Details ------------------ Tuesday's run was devoted to MP#355c "ICRF Testing and Conditioning, with detection of mode-conversion location". The D- and E-port dipole antennas were successfully conditioned to 1.5MW each, running separately. The new ARRA-funded field-aligned four-strap J-port antenna was successfully run up to 1MW, in both 0-pi-0-pi and monopole phasing. The hydrogen fraction H/(H+D) varied in the range of 15-60% during the run day, rising with increasing RF power. The nH/ne fraction inferred from the location of the mode conversion layer observed by Phase Contrast Imaging (PCI) was found to be consistent with the spectroscopic H/(H+D) measurement. As expected, no significant heating was observed with these high hydrogen fractions. On Wednesday we dedicated a scheduled 6 hour run to MP#676 "Microproposal for Faraday Rotation Plasma Current Ramp Tests" and MP#677 "Microproposal for Faraday Rotation Vibration Tests". Two toroidal field only pulses were taken to help characterize the noise and vibration levels on the three active Faraday rotation chords. These were followed by a series of ohmic plasma shots incorporating dynamic plasma current and toroidal field ramps at several densities. TF ramps within a shot in conjunction with multiple shots at different values of Bt allow for determination of possible corruption above or below the noise floor. Finally, a pair of similar shots were taken with and without removal of a panel on the upper acoustic enclosure, indicating the sensitivity to sound has been reduced relative to last year's installation. Thursday's run was devoted to MP#670 "Statistical properties of scrape-off layer plasma fluctuations". The primary goal of the MP was to record long data records (~1s) of fluctuations in the boundary plasma as measured by mid-plane and divertor probes and Gas Puff Imaging (GPI) for a variety of densities and currents. As planned, a three-point density scan was performed at Ip=0.55MA. At Ip=0.8MA an Ohmic H-Mode developed, and because of the resulting narrowing of the heat flux, two out of four pins on the A-side Scanning Probe (ASP) were damaged. Consequently we were unable to estimate the turbulence-driven particle flux at the ASP location on subsequent shots. Fluctuation data were subsequently obtained from a three-point density scan at 0.8MA and for two densities at 1.1MA. Selected discharges were repeated with GPI diagnostics. Overall, the experiments were successful and produced many high-quality time series. Friday's run was devoted to MP#669 "Rotated antenna commissioning and initial characteristics". This MP contributes to a C-Mod "Plain English goal": Investigate ICRF sheaths and impurity generation with an advanced ICRF antenna. Standard 5.4T, 0.8MA LSN target discharges were established and tuning was quickly obtained for both D-port and J-port antennas. The power from the J-port antenna was brought up to 2MW, with some faulting, and dithering H-modes were obtained, complicating the matching. An attempt to change to upper null targets was unsuccessful because operation on the less well-conditioned upper strike point region resulted in disruptions. Conditioning shots continued in LSN with the target density reduced to avoid L-H transitions. However, more faulting began to occur on the J-port antenna and transmission line network, and measurements indicated a deleterious change in the strap decoupling. The run was ended early in order to investigate the cause. ICRF System ----------- We have successfully conditioned the D- and E-port antennas up to 1.4 MW each into L-mode discharges. By the end of the week the H/H+D was staying below 0.2 and some heating was observed. The first operation of the ARRA-funded Field Aligned (FA) four-strap antenna started out well. In 5.4 T, 0.8 MA plasmas, the FA J antenna reached 2 MW with faulting and, despite low single pass absorption, there was some indication the FA antenna had reduced radiated power per injected MW of RF. However, increased faulting and inability to maintain a match to the J-port antenna led us to stop plasma operations for the day. Subsequent investigations revealed that the electrical connection to strap#2 of the antenna had become discontinuous. Further investigations to determine the location and cause of the malfunction are underway. We are also evaluating the feasibility of continued operation of the field-aligned antenna with two of the other straps energized, pending repair of the full antenna. Lower Hybrid System ------------------- The LH launcher position limit switches were readjusted based on calibration data taken during the manned access. The launcher was inserted to within 4mm of its minimum radius during the run on Friday to test the LH Langmuir probes and X-mode reflectometer. The launcher was retracted to its maximum radius following the run. Progress on the transmitter protection system (TPS) upgrade continues. Testing and troubleshooting of timing and logic highlighted some issues that are being addressed. Beam current sensors for two klystrons on the Cart 1 TPS boards have been repaired and tested. The grounding system for the circuit commons was changed to avoid future damage. Diagnostic Neutral Beam Systems ------------------------------- The spatial profile variation of the diagnostic neutral beam has been successfully measured as a function of arc plasma current at constant voltage using a new spectroscopic diagnostic. The diagnostic will provide spatial profiles for all 4 beam components and for the emission from the background neutral gas in the duct. For the measurements, the Core CXRS spectrometer was modified to detect the spectrum in the vicinity of H-alpha. The spectrometer was then recalibrated for wavelength and intensity. A capacitance manometer was installed on the DNB tank for measurement of the pressure during a beam pulse. This is one of the measurements planned for improved measurement of beam attenuation. With the duct gauge and the pressure gauges on the tokamak vacuum vessel, a sufficiently complete measurement of the beam attenuation due to neutral gas can be made. Diagnostics ----------- The upgraded Thomson scattering lasers have been interlocked to the C-Mod access control system, in order that they may be run safely during tokamak operation. Remote operation of the lasers has been established, and successful test shots have been run. In the coming week, beam alignment will proceed, so that the lasers can be fired into the tokamak chamber. The McPherson VUV spectrometer is operational following installation of a repaired turbo-pump. The in-vessel Penning gauges are operational. All in-vessel probe arrays, as well as the A-port (ASP), F-port(FSP), and inner-wall (A-port WASP) scanning probes, are operational. A damaged probe head on the ASP will be replaced this week. Travel and Visitors -------------------- David Mikkelsen (PPPL) worked with the Alcator C-Mod group at MIT to plan new gyrokinetic turbulence simulations of rotation-reversal during density scans in ohmic plasmas, and of high-temperature I-mode plasmas.The GYRO pre-processor for experimental profile input was tested, and user instructions were placed on the C-Mod wiki. Ahmed Diallo (PPPL) made a day visit to C-Mod to discuss physics research collaboration opportunities in 2012. He had valuable discussions with MIT staff, including Jerry Hughes, Anne White and Linda Sugiyama. Prof. Alexander Wurm from Western New England University in Springfield, MA, visited last week to give a seminar at the PSFC/C-Mod Turbulence group meeting on "Introduction to Magnetic Field Line Chaos". Martin Greenwald traveled to Washington for a workshop on FES ESnet requirements. Bill Rowan and Amanda Hubbard participated, remotely, in an ITER CDR for the ECE diagnostic. They are members of the US design team. John Rice led the USBPO web seminar this week, reporting on activities of the ITPA Transport and Confinement group. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly