Alcator C-Mod Weekly Highlights December 7, 2009 FY2010 weeks of research operations Base Target: tbd Base Completed: 0.16 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 615 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled, and two and a half were completed. A total of 67 plasma discharges were produced with a startup reliability of about 80%. The runs supported experiments from the Transport Physics and Boundary topical areas and the Diagnostic development effort. Plasma operations are plannned to continue this week. Operation Details ----------------- Two experiments were carried out on Wednesday. The morning was devoted to diagnostic development and calibration activities, in support of a proposal to carry out trace helium transport experiments using the Wide View CXRS diagnostic. Varying amounts of helium were introduced into a series of discharges to determine the sensitivity and signal to noise ratio (SNR) that could be achieved for HeII radiation using a newly-installed spectrometer. We found that sufficiently high SNR was easily obtained at levels of ambient helium that did not significantly perturb the edge electron temperature. Two different gratings were evaluated, and it was determined that the higher resolution grating would be more appropriate for the proposed experiments. The second half of Wednesday's run and the full day on Friday were devoted to MP#586a, "Impurity Transport Studies in L-mode Plasmas". These experiments, which comprise part of the thesis research of an MIT graduate student, make use of the newly-commissioned laser blow-off (LBO) impurity injection system to investigate the dependence of the impurity flux as a function of local plasma parameters. The experimental plan, which was continued on Friday, includes systematic scans of safety factor, input power, and plasma density. Upper single null discharges with B=5.4 tesla were employed, using the cryo-pump to aid density control. CaF2 is injected at up to five times during each plasma discharge, while the RF power is varied in a staircase fashion to produce a variation in plasma temperature at each density and current level. The half-day run on Wednesday produced a complete dataset at a single density, nebar~1e20/m^3, and four values of plasma current between 600kA and 1.2MA. The density scan, including data up to 2e20/m^3 and down to 0.7e20/m^3, and the same range of current and RF power, was completed during Friday's run. Initial results indicate that, in addition to an increase in impurity particle confinement time (tau_p) with current, tau_p appears to be a decreasing function of temperature. A weak dependence on density is suggested by the data at the higher densities. These results are undergoing further analysis. The experiment on Thursday was intended to be devoted to MP#419, "Fiducial wall pumping measurements for comparing operating conditions", to investigate differences in wall pumping rates and hydrogenic retention dynamics when using the upper divertor cryopump, compared to unpumped shots. However, these plans had to be changed after it was determined that a key diagnostic, the pressure gauge used to measure the amount of gas supplied from the main fueling plenum, was inoperable. The remainder of the day was therefore dedicated to an alternate run, a continuation of MP#579, "Characterization and optimization of I-mode". Data were obtained on I-mode triggering and H-mode avoidance. It was demonstrated that the triangularity at the active (upper) x-point is a significant parameter for I-mode access, which is easier at higher triangularity. Use of off-axis ICRF heating to reduce the sawtooth amplitude was found to be helpful in H-mode avoidance, as the I-mode is often terminated by an H-mode transition induced by the sawtooth heat pulse. The "QQC mode" fluctuations characteristic of I-mode discharges were simultaneously observed on reflectometry, magnetics, and phase contrast imaging (PCI) diagnostics. A discharge at 1.3MA, 5.8T and about 7MW of total input power (including ohmic) exhibited the highest performance levels so far obtained in I-mode, with plasma stored energy up to 220kJ and volume-averaged pressure up to 1.6 atmospheres, comparable to our best H-mode performance. Program Planning ---------------- A meeting was held on Tuesday, Dec 1, to discuss plans for installation of the new lower hybrid launcher and associated operations and experimental scheduling. A tentative schedule for the installation, including a clean vent of the machine in January, was agreed upon. Longer term issues, including the timing and scope of a potential manned-access vent in the spring, were discussed. A meeting of the C-Mod Experimental Program Committee was also held on Tuesday. One Mini-Proposal was considered and approved for scheduling. Priorities for experiments to be conducted before and after installation of the lower hybrid launcher were discussed. ICRF Systems ------------ The J-port antenna matching network, and transmitters #3 and 4, have been reconfigured for 78MHz operation, following completion of 50MHz experiments. All three ICRF antennas were operated successfully in support of experiments last week. For the I-mode discharges, reliable power was obtained up to 6 MW and launched power briefly reached 7 MW. The strap design for the rotated ICRF antenna is in its final stages. An SLA(stereolithography) plastic model has been produced of the rotated strap design. We discussed the proposed Fast Ferrite Tuner (FFT) specifications with Wolfgang Arnold of Advanced Ferrite Technologies and plan to refine the specifications to allow better system performance. Further discussions are planned for the upcoming week. Lower Hybrid System -------------------- All components required for installation of the new lower hybrid launcher are now in-house, and assembly of both the LH couplers and the waveguide interconnection system (jungle gym) are in process. The assembly of the LH II couplers, which includes the WR187 waveguides plus the 4-way-splitter, is proceeding. The link below is a picture of the launcher showing the coupler grill on the left and the interface to the vacuum flange on the right. http://www.psfc.mit.edu/research/alcator/photos//POWER_SYSTEMS/RF/lower_hybrid/LH%20Second%20Launcher/New%20LH%20Launcher_20091204_001.JPG The LH-SOL reflectometer vacuum waveguide run manufacturing and cross calibration were completed. This system is now ready for fit-up into the LH launcher assembly. Assembly of the jungle gym is taking place in the C-Mod cell. All support bars, brackets and clamps have been fabricated for the initial fit up of waveguides from the alignment plate to the jungle gym, and these waveguides are in place. The first two of the split directional couplers and DC blocks were installed for fit up. The link below shows the current status of the launcher support stand and jungle gym. The launcher will attach to the stand on the left where an alignment plate is currently being used to position the waveguides. http://www.psfc.mit.edu/research/alcator/photos//POWER_SYSTEMS/RF/lower_hybrid/LH%20Second%20Launcher/P1010482.JPG Diagnostics ------------ A new vacuum ultraviolet spectrometer was integrated into the C-Mod shot cycle this past week. The X-ray Extreme Ultraviolet Spectrometer (XEUS) is part of a collaboration with LLNL. The spectrometer housing and grating are provided by Ed Magee and Peter Beiersdorfer at LLNL and the detector and vacuum systems are from MIT. The XEUS uses a spherical, varied line spacing grating to allow flat-field imaging over a wider spectral range than the older McPherson VUV spectrometer, and will be used for impurity transport and monitoring. Calibrations are ongoing. On Friday, December 4, we collected the first data from the new J-Top CNPA (Compact Neutral Particle Analyzer). This diagnostic is designed to view neutralized fast ions at energies above 100 keV and complements an existing CNPA on F-Top. The new instrument views roughly from R=.68 to R=.78 m, providing greater radial coverage than the F-Top diagnostic. Using detectors and amplifiers supplied by David Brower and Weixing Ding, UCLA, the single chord mock-up of the Alcator C-Mod polarimeter became operational last week. The mock-up fully simulates the optical path as it will be on C-Mod, and preliminary results indicated good signal levels. The system operated well at an IF frequency of 2 MHz which will allow for both very good Faraday rotation measurements and the measurement of fluctuation levels. The enclosure for the lower optical table arrived last week. This enclosure, together with the upper optical table enclosure and beam-line will allow the entire system to operate in a dry atmosphere with very low attenuation of the FIR beams. ARRA activities ----------------- We have now completed the FY2010 milestone of five weeks of additional research operation funded under the ARRA. This operation supported sixteen miniproposals and resulted in production of approximately 600 plasma discharges. The milestone was accomplished nearly four months ahead of schedule. Travel and Visitors ------------------- Amanda Hubbard, Earl Marmar, and Miklos Porkolab attended the Fusion Power Associates meeting in Washington, DC last week. Amanda presented a talk entitled "U.S. Burning Plasma Organization: Supporting U.S. Scientific Contributions to ITER" on behalf of the BPO. Earl presented an "Overview of the Alcator C-Mod Program and Recent Results", and Miklos presented a more general talk on "Plasma Physics and Fusion Energy". Prof. Sergei Krasheninnikov (UCSD) visited the PSFC on Dec. 2 and 3. During his visit he had wide ranging and useful discussions with C-Mod and PSFC Theory scientists on a variety of topics in edge physics and plasma-wall interactions. He will continue contacts with C-Mod staff on various blob/filament issues. On Dec 2, he presented a Special Seminar on "Blobs and Dust". Dr. Jong-Kyu Park (PPPL) visited MIT on Friday and presented the PSFC Seminar on "Ideal Perturbed Equilibria in Tokamaks". During his visit he also discussed ongoing collaborations on non-axisymmetric field effects and locked modes with Steve Wolfe and Ian Hutchinson. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly