Alcator C-Mod Weekly Highlights June 24, 2002 Plasma operations resumed on Alcator C-Mod last week. Four run days were scheduled and completed. A total of 23 plasma shots were produced, with a startup reliability of 27%, which is low but not atypical for the first week of operations after a major vent. The operation was devoted to tokamak clean-up and conditioning; several diagnostic systems were also brought online. Work also continued on ICRF, DNB, and Lower Hybrid Systems. Plasma operations are planned to continue this week. Operations ---------- Following a week of vessel conditioning (bake and ECDC), plasma operation resumed last Tuesday. The purpose of the week's runs was to begin the process of discharge conditioning of the tokamak in preparation for physics operations. We also began discharge development for several new equilibrium configurations, including higher triangularity and near-double null plasmas, called for in the experimental plans for this campaign. In addition, we began bringing diagnostics online, and verifying control and data system operation. We also began to gain experience with the new Linux-based computer systems under actual runtime conditions. As is typical of plasma operations after a major vent, the plasmas produced early in the week were short, disruptive, and characterized by high resistivity and radiated power fraction, with frequent injections. Density was typically higher than the programmed value (~1e20/m3), with most of the source coming from the walls rather than the gas valves. As the machine began to clean up, 800kA discharges were produced that lasted until their programmed rampdown, and control of the density improved. The initial discharges were programmed to be "standard" lower single null divertor configurations. Once full-length plasmas were obtained in this configuration, we began to modify the equilibrium toward a double null and to increase the triangularity, taking advantage of the new inner divertor assembly which allows more range for the lower x-point position. The purpose of these equilibrium changes was both to begin development of new discharge shapes required for upcoming physics experiments and to enhance the clean-up of the vessel by moving the strike points onto previously unscoured tiles. By the end of Friday's run, full-length discharges were produced in a near-DN configuration (SSEP~5mm). The H/D ratio, one important indicator of machine condition, was down to ~0.3 or less. However, progress was limited by rather poor startup reliability. This may simply be due to inadequate wall cleanliness; we are also investigating the possibility that the "sparker", an invessel spark-gap system in line-of-sight to the tokamak chamber which provides some initial ionization to assist breakdown at low loop voltage, may have malfunctioned. ICRF System ----------- New RF Cell rack cabinet wiring was verified. Some minor changes are being made for PLC operation of stub tuner/phase shifter. Stub tuner/phase shifter controllers are both working with local control. PLC operation for #1 has been done. PLC operation for #2 will be completed soon. Final jumper configuration of the new fault system was done. We have started on J port fault circuit testing and entered and tested proper setpoints for phase balance. Voltage limit settings were calculated and entered and will be tested. Power threshold settings for J port were tested. Delay from phase balance fault to fault system output (in cell) was measured for several channels and is less than 2 uS. J fiber optic receiver boards in power room electronics were modified to support failsafe signal operation from cell electronics and tested. PLC to fault board trip signals were tested and verified. Several problems were found and corrected. Software modifications associated with use of the new cPCI data system for RF operations are ongoing. Lower Hybrid MIE ---------------- Work continued on improving the response time of the body current protection circuits for the LH klystrons. Tests of a new breadboard circuit indicated less than 2usec delay. New, faster response, current transducers have been ordered. DC power supplies for the flow-meter instrumentation have been ordered. Mechanical work is proceeding on carts 1 and 2. DNB Systems ----------- The beam was operated throughout the week during the conditioning discharges. Beam conditioning shots were run between C-Mod shots. The beam was synched with the firing of C-Mod shots. A recent survey spectrum of the beam showed lines from hydrogen only, attesting to its cleanliness. The MSE system has acquired data from both beam-into-plasma and beam-into-gas shots, and the initial results look very encouraging. A set of optical channels is being configured for the MSE to look at the background plasma emission (i.e. non-beam-induced) to improve the signal-to-noise even more. Diagnostics ----------- The two new off-axis tangential xray spectrometers were successfully tested during plasma operation. The Electron Cyclotron Emission diagnostics, including Michelson, GPC and GPC2, are all operational and taking data, but not yet recalibrated. The FRCECE diagnostic also observed ECE signals during several shots. Work continued on the amplifier and power supply system for the Active MHD Spectroscopy Antenna. Travel and Visitors ------------------- Ian Hutchinson, Joe Snipes, Miklos Porkolab, Eric Nelson-Melby, Remi Dumont, Martha Redi, and Stewart Zweben attended the Conference on Plasma Physics and Controlled Fusion of the European Physical Society in Montreaux, Switzerland, last week. Hutchinson presented a paper "Plasma Mach-probe with Unmagnetized Ions"; Snipes presented a paper "Structure of the Quasi-Coherent Mode in Alcator C-Mod"; Nelson-Melby (now at CRPP, Lausanne) presented "Phase Contrast Imaging of Mode-Converted Ion Cyclotron Waves in the Alcator C-Mod tokamak"; Dumont (PPPL) presented a paper titled "ICRF wave propagation and absorption in plasmas with non-thermal populations"; Redi (PPPL) presented a paper titled "Gyrokinetic calculations of microinstabilities and transport during RF H-modes on Alcator C-Mod"; and Zweben (PPPL) presented a paper titled "Edge turbulence imaging on NSTX and Alcator C-Mod." David Mikkelsen (PPPL) visited C-Mod for discussions with Amanda Hubbard, Perry Phillips, and Alan Lynn about using ECE Te diagnostics and Bo sweeps to determine temperature gradient scale lengths.