Alcator C-Mod Weekly Highlights June 9, 2003 Alcator C-Mod successfully completed all four scheduled runs last week. Work progressed on the lower hybrid and ICRF systems. Plasma operation will continue this week. Operations ---------- Tuesday's run was devoted to recovery from boronization that had been applied the previous Friday. Wednesday's run investigated ICRF induced fast ion loss and its effect on edge radial fields and the H-mode transition. We also studied the mapping between probes and the gas puff imaging system. Thursday was spent developing high power rf operation scenarios, with up to 5 MW of power coupled to the plasma for 0.5 s and over 6 MW for shorter periods. On Friday we continued our JET/C-Mod similarity studies. Startup reliability was 96% this week, excluding Tuesday's boronization recovery day. Startup reliability was 100% on both Thursday and Friday during which 64 physics quality discharges were produced. The successful operation of the non-axisymmetric control coils has motivated us to add more capability to this new system. A second TMX supply is being installed in the power room that is expected to become operational before the end of the current campaign. Physics ------- Analysis of fast dynamics at the L-H transition, carried out in collaboration with Benjamin Carreras, ORNL, during his recent visit, has revealed some interesting results. The primary diagnostic was the high resolution, low noise FRCECE diagnostic (U. Texas). By using discharges with off-axis RF heating, the effects of sawtooth heat pulses were minimized. On several channels, there is a two-stage transient; an initial, near-linear 'jump' in Te over a period of order 1 ms, followed by a slower, apparently diffusive response at all radii. Somewhat unexpectedly, the biggest jump in Te is at about 87.9 cm, 1.5 cm inboard of the separatrix; the prompt response extends from 86-89 cm. A model by Carreras and del-Castillo-Negrete gives qualitatively similar behavior. The simulations indicate that the scaling of the size and rate of the initial 'jump' with P-P(thresh) would be the best test of the model; as power is raised above the threshold, the size of the jump should get larger, and the duration shorter. It is therefore planned to investigate the dynamics more systematically using a controlled power scan. A miniproposal on "Edge Minority Heating for H-mode Control" was run by Stewart Zweben, PPPL, on C-Mod. A scan of the H-minority resonance position was made from the low-R edge to the high-R edge at 78 MHz (i.e. B = 3.7 - 6.7 T) with RF power varying in the range 0.2-2 MW. Analysis of the results is in progress. Ultra-fast camera measurements of the 2-D radial vs. poloidal structure of the edge turbulence using the PSI-4 camera were also obtained. Good images of the radial and poloidal propagation of edge blobs were obtained at 250,000 and 500,000 frames/sec. Lower Hybrid Project -------------------- A high power testing facility is being planned for the lower hybrid launcher. A major part of this facility will be an absorber lined chamber into which 60 kW of power from the launcher can be safely dumped. Simulations of the coupler and its radiation pattern in the chamber are being performed and will be used to design the chamber. More equipment was moved into the Lower Hybrid Active Controller racks. The ethernet connections were made for both the CPCI and PXI chassis, and communications for both systems were successfully tested. Mounting shelves and wireways arrived and were installed. The CAMAC timing system was moved to the new racks and fiber optics and cables for the timing system were reconnected to allow tests to continue. Work continues in building the LH TC/Heat/Cooling system PLC rack. Wiring continued on heater control chassis. SFOL Tx board layout is nearing completion. Labeling convention issues for the LH patch panel were resolved for the directional couplers. The probes will be installed in accordance with our existing convention, as confirmed in discussions with Nevell Greenough, PPPL. ICRF Systems ------------ Steady (~0.5 sec) 5 MW ICRF EDA H-modes were achieved this week for the first time on Alcator C-Mod. Even higher RF power H-modes were achieved for shorter pulse lengths. RF pulse lengths and the total power level were paced by D and J-Port antennas. The D-Port dipole antenna can currently be used for reliable long-pulse operation at power levels up to about 1 MW. An issue with the D-Port control circuitry, which will be resolved during the next maintenance day, prevented operation at higher powers. The E-Port dipole antenna is operating reliably at powers levels of 1.5 MW or above for long pulses. The J-Port 4-strap antenna is now sufficiently conditioned to allow for 2 MW, long-pulse operation, and a new record of 3.2 MW was coupled through this antenna for shorter (80 ms) pulses. The J-Port antenna continues to improve as more conditioning is done. We continue to make improvements to the ICRF control system. Power calibration measurements have been completed for both the D-Port and E-Port transmission line systems. The measured powers from several diagnostics now agree to within 5-10%. The calibration factors involved in predictive tuning have also been refined and shot-to-shot tuning has become more reliable. Noise problems in the CPCI digitizers have been isolated and are being eliminated as maintenance time becomes available. Travel and Visits ----------------- Martin Greenwald attended a workshop in Reston, Virginia "DOE Science Networking". The workshop was charged by OASCR to draft a five year roadmap for networking for Office of Science programs. Manouch Farkhondeh, Jan Van der Laan, G. Townsend Zwart, and William Graves from the MIT Bates lab visited Peter Koert and Charley Schwartz to discuss some applications of ferrite tuners. They have some interest in the development of tuners on a free electron laser project. Our discussions may develop into a shared knowledge base for the materials needed for our ICRF tuners. Paul Bonoli visited PPPL on June 4-6, 2003 to attend a Program Advisory Committee Review of the OFES Sci-Dac Projects. Included in this meeting was a review of the rf Sci-Dac Initiative. Paul Bonoli also had discussions with Cynthia Phillips, Stan Kaye, and J. Manickam on the implementation of new Fokker Planck and ICRF heating modules in TRANSP. Brent Stratton, PPPL, was at MIT last week, working on the 2-D imaging high resolution X-ray spectrometer system which is on loan from NSTX. The PC-based control and data acquisition system was integrated into the C-Mod network, and synchronization with C-Mod shot cycles was successfully implemented.