` Alcator C-Mod Weekly Highlights Feb. 13, 2006 FY2006 weeks of research operations: Planned: 14 weeks Completed: 4 weeks Operations ---------- Plasma operations resumed at Alcator C-Mod this week with restart and commissioning activities. A total of 22 plasma discharges were produced during three run days, with a startup reliability of 55%. By Friday, startup reliability had increased to over 80%. Power system tests were begun on Tuesday and completed on Wednesday, followed by two and a half run days devoted primarily to machine conditioning, clean-up, and diagnostic setup and calibration (MP#298a). In addition, during Thursday and Friday's runs we successfully operated the Lower Hybrid system using the recently re-installed launcher with stainless steel coupler assemblies. Up to 200kW (source power) was injected into the target plasmas on Friday. A new adaptive sequence was successfully implemented using the new Digital Plasma Control System (DPCS) to force the current in the OH1 and OH2 coils to swing to a programmed reverse current value following a failed startup or early plasma termination, before ramping down to zero. This programmed sequence, which supercedes the less controlled behavior obtained with the hardware "fizzle detector", is intended to insure a reversal of the JxB forces on the coaxial OH bus connections, resulting in more consistent contact resistance in these joints. Full length (~2sec) plasmas with Ip=800kA, Bt=5.4T were readily produced during last week's runs. This indicates that machine cleanliness is reasonably good following the recent manned-access vent. The H/D ratio was measured to be around 0.25, still too high for minority proton ICRF heating, but typical for this early stage of the conditioning process. During last week's operation, it became apparent that LN2 cooling of one leg of the toroidal field magnet (C-Right vertical) was inadequate, resulting in delays in cooldown following shots which significantly impacted the shot cycle. A minimally-invasive borescope inspection is planned for Monday, following which a course of action will be determined. Physics ------- During power system testing on Wednesday, a series of three shots were used to characterize the transfer function of the chopper supply (EFC) used for vertical stability control in C-Mod at frequencies up to 2kHz. Open-loop control of the EFC voltage was provided by special programming of the DPCS waveforms, and both the response of the supply and the magnetics diagnostics were monitored. The results will facilitate development of more accurate control system simulations, and also help characterize the roles of power supply and plasma-induced noise in the vertical stability control loop. Experiments on the CMX RF testing facility at the PSFC have been carried out to test the effect of surface roughening on multipactor effects, which have previously been identified as a limitation in the C-Mod ICRF antennas' power handling capability at high neutral pressures. Test electrodes which have been sandblasted with 50um alumina beads were observed to suppress multipactor-induced breakdown at pressures up to 14mTorr after brief glow cleaning and bakeout; electrodes which had not been treated with sandblasting experienced multipactor breakdown at 0.8mTorr. The DC voltage hold-off of the sandblasted electrodes was degraded by 4% or less, compared to a mirror-polished electrode. As a result of these successful experiments, a proposal to apply sandblasting to critical locations on the C-Mod antenna feeds is under consideration. This work was carried out by Tim Graves, an MIT graduate student, as part of his doctoral research. DNB System ----------- High voltage inverter testing was completed, and the new inverters were installed in the modulator rack. During testing of the beam HV system faulting was observed on two modules at voltages above 35kV. The faults were determined to be due to problems in the transformer tank. Following consultation with the power engineer from Budker Institute who designed the system, we inspected the tank and found several shorted diodes and MOV's. The damage has been documented, and further consultations with the Budker engineer is in progress. A source for replacement components has been identified, and we have ten spares on hand. A new load resistor fuse assembly was tested and appears to be functional for normal operation. Lower Hybrid System -------------------- Drive leg calibrations were completed for seven legs at 40kV beam voltage. Work on the remaining legs was begun. Conditioning of the klystrons is continuing. On Friday we operated klystrons #1-5 into plasmas. The 90 degree CD phasing setup was used. The position of the coupler relative to the protection limiters was varied, and density data obtained using the coupler Langmuir probes. Coupling was better for the more forward coupler location, as expected. Up to 200kW (source power) was employed during these experiments, corresponding to ~4kW per waveguide at the coupler. ICRF System ----------- All four transmitters are ready for operation. No ICRF operation was attempted during last week's runs. Travel and Visitors ------------------- Stewart Zweben(PPPL) was at C-Mod last week to work on the design of the new lower divertor gas puff imaging system. One goal of this system is to determine whether there are 'blobs' in the vicinity of the lower divertor (as there are near the outer midplane), and if so, whether the blobs move outward in major radius or in minor radius. This can potentially have a significant impact on radial transport to the divertor. Last week Stewart worked on a code to calculate the location of the optimum line of sight to view along the B field near the lower divertor. Eric Edlund, an MIT graduate student working with Prof. Miklos Porkolab on the Phase Contrast Imaging diagnostic, visited PPPL during the week of Jan 30 - Feb 3. He was working with Gerrit Kramer and Nikolai Gorelenkov on developing a synthetic PCI routine to interface with the outputs from the NOVA-K code. Paul Bonoli and Amanda Hubbard participated in the IAEA FEC2006 US paper selection committee meeting in Germantown, MD, last week. Robert Granetz attended the combined US/Japan MHD workshop and the ITPA MHD meeting last week in Naka. Of particular interest was the new disruption database (DDB), which now has several thousand shots from four major tokamaks(including 2000+ from C-Mod), and will soon have entries from four additional machines. The first use of the DDB will be to determine the specification for the fastest expected current quench in ITER. There were also discussions concerning expansion and upgrades to the DDB. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly