Alcator C-Mod Weekly Highlights June 2, 2003 Plasma operations continued at Alcator C-Mod last week, with three run days scheduled and completed. Runs were carried out in support of MP#326, "High Power ICRF Operation", MP#318, "High Performance Operation at 5 tesla", and MP#327, "Developing low density EDA H-mode targets for LHCD". Plasma runs are scheduled to continue this week. Operations ---------- A total of 85 plasma discharges were produced over three run days last week. All tokamak systems performed well, with overall startup reliability of about 95%. The ICRF system operated at high power (>4mW) all three days, using all three antennas, with peak power exceeding 6MW for short pulses. High radiative power fraction and elevated H/(H+D) ratio were observed, and may have contributed to the limitations on the pulse length of the highest power operation. A fresh boronization was carried out beginning Friday evening through Saturday afternoon. Physics ------- Similar high frequency modes in the core of RF ITB plasmas have been identified by Martha Redi, with the GS2 code, and by Alan Lynn (U. Texas) from ECE measurements. Nonlinear gyrokinetic calculations predict an apparent geodesic acoustic mode at 77 kHz in a low field case at the trigger time. The ECE heterodyne measurements of 80 kHz were made during ITB in a high field case. Operation on Friday continued experiments begun last year on development of low-density EDA H-mode target plasmas for Lower Hybrid current drive, in support of the future Advanced Tokamak program. H-mode plasmas were produced with Ip=600kA and sustained RF power at the 4MW level. EDA, as evidenced by observation of the Quasi-coherent mode on reflectometry, BES, and fast ECE and magnetics, was obtained with target densities down to ~9e19/m^3, while ELM-free H-modes, with no QC mode, were produced at a lower target density, ~6e19/m^3. At lower RF power the QC-mode was absent even with the 9e19/m^3 target. The performance of these low-density H-modes was good, with H-factors around 2 or above. ACCOME modeling of these plasmas will be carried out; we anticipate they will show good LH current drive efficiency. ICRF Systems ------------ The primary focus of the RF operations was to provide high power into plasma. Short pulses (~0.12 sec) at 5 MW into 1MA H-mode discharges were achieved, and peak powers over 6MW were briefly obtained with plasma currents up to 1.2MA. The systems operated well into low current, higher toroidal field discharges that were difficult to match to during the last run campaign. Improvements to the demodulator circuits that detect rf power levels and drive fault detection circuitry resulted in better reliability of the rf systems this week. Up to 2.7 MW has now been coupled through the j-port antenna, although more conditioning is required for full performance operation. Lower Hybrid System ------------------- High power testing of the prototype circulator was begun this week. Apparent waveguide faults limited peak power to approximately 100 kW. The fault location has not been identified and may not be in the circulator. We are in the process of tracking down the source of this fault and also a gas leak in the circulator that may eventually limit performance. The PXI Interface chassis, SFOL chassis, Logic chassis and patch panels were installed into the LH Active Controller racks and a temporary ethernet was run to the rack area. Fiber optic cables were pulled into place for CPS and Low Power Microwave Rack; termination is still required. The lower hybrid launcher forward stacked waveguide completed its 150C bake cycle last week. New locations with blistering and peeling were found, resulting in a decision to disassemble the guide and replate. The lower hybrid grill couplers have been successfully stripped and are now being prepared for plating. An electrode assembly has been designed and fabricated at PPPL in cooperation with MIT and the vendor. DNB System and Diagnostics --------------------------- DNB performance was very good last week, with beam currents averaging 4.5 Amps at voltages of 49.5 kV. No gas system problems have re-appeared. The computer CRT in the cell was changed out for a spare flat panel display. Work continues on fiber links between the local Russian beam camac and the C-Mod DNB camac (which is recorded in MdsPlus). The fiber links are being made up from spare and used AFOL boards. Diagnostics ------------ The 132 GHz reflectometer channel has become operational. Iniital data look very good. The auto power spectra show the expected specular reflection, MHD activity, and a broad background of density fluctuations. The coherent scattered power, which is an indication for the density fluctuation level varies when the plasma evolves and at times there is even a weak cross correlation visible with the 88 GHz channel. The NSTX X-ray imaging crystal spectrometer was installed on C-Mod 5/24. Everything went very well. Tom Holoman and Doug Lebrie moved the equipment from NSTX to MIT on Friday morning and put it on the C-Mod platform in the afternoon. On Saturday, John Rice, Ken Hill, Doug Lebrie and Manfred Bitter installed the spectrometer on the horizontal (lower-hybrid) port and aligned the crystal. The next steps are to install the detector and connect the Korean PC-based data acquisition system to MDSplus. The spectrometer will then be ready to take data. Travel and Visitors ------------------- Ken Hill, Doug Lebrie, Brent Stratton, and Manfred Bitter (PPPL) visited to work on the installation of the NSTX X-ray imaging crystal spectrometer on C-Mod. Bob Childs and Rui Vieira were at PPPL on 5/30 for ongoing evaluation of the LH launcher following the 150C bake cycle tests carried out last week.