Alcator C-Mod Weekly Highlights March 26, 2007 FY2007 weeks of research operations: tentative target: 15 weeks Completed: 0 weeks Milestone status ----------------- Both FY07 second quarter sub-milestones are now complete: 1) Install cryopump in C-Mod and perform invessel cooldown tests. 2) Re-install LH launcher and ICRF antennas; resume C-Mod operations. Operations ----------- Plasma operations resumed at C-Mod last week. Four run days were scheduled and three were completed. The main goal for last week was to re-establish plasma operation and begin the process of cleaning up the machine. Diagnostic checkout, alignment, and calibration were also carried out. A total of 15 plasmas were produced, of which ten were full length (>1.8 sec) 800kA, diverted discharges. Startup reliability for the week was under 20%, but increased markedly as the machine began to clean up. The intermittent discontinuties of several magnetic flux loop signals, noted during the previous week's testing, were repaired on Monday. Monitoring of the low voltage resistance of these loops through the vacuum feedthrough revealed poor contact resistance, consistent with the presence of an oxide layer on the vacuum-side connector. A low energy (<.25 J) capacitive discharge was applied to the connectors, resulting in steady resistance readings of the expected values. No intermittency was observed on these signals during last week's operation. In addition to the startup and conditioning plasma operations Tuesday through Thursday, on Monday we carried out an initial scoping study (MP#470) on the use of vertical field to improve the localization of electron cyclotron discharge cleaning (ECDC). The motivation for this experiment is to assess the possibility of improving our boronization technique to deposit boron more efficiently in the areas where it is needed. Monday's experiment was designed to evaluate the effectiveness of applied vertical fields in localizing the ion flux in the vessel. We implemented the vertical field in increments of 2% of Bv/Bt. Above 4% the images of plasma light showed a clear sharpening at the resonance location, an indication that the plasma radial width was narrowed. The upper divertor probes indicated a radial width of the ion flux ~ 5 cm, roughly as expected. One of the main objectives of the experiment was to show that the application of vertical field would greatly reduce the plasma flux seen to surfaces well outside the resonance, with the hope of reducing unwanted boronization films on antenna structures. The probes on the LH launcher indeed showed that the incident ion flux essentially disappeared with the applicaiton of Bv/Bt >~4%. Based on probes and imaging, there was not much additional sharpening of the radial profile above Bv/Bt~4%. This may be a limitation set by the width of the RF power absorption (e.g. Upper Hybrid resonance). Nevertheless, a 5 cm flux width is probably sufficient for the purpose of localizing B ionic deposition during boronizations. Initial invessel cooldown tests of the cryopump system were also carried out on Monday, as reported below. Friday's run was cancelled to repair a vacuum leak at a high voltage feedthrough used for the sparker, which assists with pre-ionization for discharge initiation. The machine was briefly vented to argon and the feedthrough removed, following which the vessel was pumped down and baking and discharge cleaning (ECDC) carried out over the weekend. Plasma operations are planned to continue this week. Lower Hybrid System ------------------- The lower hybrid system is ready to begin low power pulses into tokamak plasmas for the purpose of instrumentation checkout and tests of the coupler protection system (CPS). ICRF Systems ------------- We completed re-assembly of D antenna coaxial transmission line and have closed E antenna transmission line to allow testing of the transmitters into the dummy load before we operate into plasma. Designs for the FFT support cradles are complete. We have completed inspection of FMIT#4 stub tuner and are beginning to refurbish the coaxial transmission lines. Cryopump System ---------------- Cool-down tests of the upper divertor cryopump were successfully completed on Monday. The liquid nitrogen (LN2) and liquid helium (LHe) circuits were simultaneously cooled to 77K by flowing LN2 into both. At the same time, vacuum chamber pressures and residual gases were monitored. The torus pressure showed a small reduction (6.5 --> 4.5x10-7 torr) with a corresponding reduction in partial pressures of water vapor and CO2. Helium gas was used to reheat the pump and to purge all cryogenic lines of LN2. The residual gas analyzer showed no significant change in masses 4 or 28 throughout the entire procedure. These tests demonstrate the pump is leak-tight down to cryogenic temperatures. Long Pulse Diagnostic Neutral Beam ---------------------------------- Beam operation has continued and the beam spectra show continued reduction in water content. The beam vacuum is excellent at pressures of mid 10^(-9) Torr. The beam power supplies and vacuum systems are all operating at full performance. The beam has been operated at 50kV with 5 to 6 A of beam current, but the beam current has not been as flat as desired over the length of a beam shot, and some excess noise has been observed. Several causes are being investigated, including cathode gas flow rate and third grid supply voltage. Diagnostics ----------- Both ECE grating polychromators and the Michelson were operational during C-Mod plasma operation last week. The baratron (capacitance manometer) pressure gauges at the E-top, E-bottom, B-bottom, G-side, and F-top port locations are operational. The F-top gauge has a 'sniffer-tube' to measure pressures in the cryopump gas box. The ratiomatic ion gauge used to monitor main chamber pressures during plasma operation has now been cross-calibrated against the G-side capacitance gauge. The 2pi AXUV bolometer diode is operational. The 2pi foil is also on-line, but was exhibiting rather low signal to noise during plasma operation last week. Travel and Visitors ------------------- John Rice and Matt Reinke attended the 2007 Atomic Processes in Plasmas (APiP) meeting in Gaithersburg, MD last week. Matt contributed a poster on the previous work done to calibrate the VUV spectrometer using data calculated from core bolometry and cooling curves. While there he talked with atomic physics modelers about improving our existing molybdenum, krypton and argon data and discussed how best to go about making tungsten density measurements given our limited VUV/SXR observation window. He also discussed a possible collaboration with NRL/NIST/PPPL on a novel, high resolution hard x-ray spectrometer to help diagnose emission from LH induced pertubations to the electron distribution function. Stewart Zweben was at MIT last week to start testing the new divertor GPI (gas puff imaging) diagnostic using the Phantom 7.3 camera. Initial results were encouraging: clear images of edge turbulence 'filaments' were obtained of the recycling light at the lower divertor surface (without any GPI gas puffing). These images were taken at 150,000 frames/sec with a 4 microsec exposure time, similar to the exposure time to be used with the local gas puff. These filaments had a complex space-time structure at least qualitatively similar to that seen near the outer midplane. About 30,000 frames were recorded for each shot, which is 100 times more frames per shot than possible with the previous GPI camera. Tests with a GPI gas puff will be done as soon as the machine is conditioned. Also, Stewart worked on fast diode data analysis from previous runs. Jay Kung (PPPL) continued his visit to C-Mod this week to complete installation of the swept-frequency reflectometer. It should be ready for plasma testing next week. Eric Edlund visited PPPL this week to collaborate with Gerrit Kramer on the stability analysis for reversed sheas Alfven eigen modes that were observed in C-mod. It was found that ICRH ions can supply sufficient drive to excite those modes, in line with the experimental observations. Eric also worked with Chuck Kessel to model the plasma start-up in C-mod with the TSC code. Three people from ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences) , director Prof. Jiangang Li, deputy director Prof. Baonian Wan, and Prof. Shijun Du, visited PSFC on Monday and Tuesday last week. They discussed collaborations with people from different divisions of PSFC, including C-Mod, plasma application division, and fusion technology division. They discussed potential collaborations with C-Mod in the areas of ICRF, LHRF, and edge/wall. Prof. Wan and Prof. Li gave a special double-seminar on Monday with the titles of 'Recent progress on HT-7 and EAST' and 'Present activities and future plan of MCF research in ASIPP'. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly