Alcator C-Mod Weekly Highlights May 9, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 7.5 weeks Operations ---------- Four run days were scheduled and carried out last week. However, the presence of excessive dust, spectroscopically identified as containing titanium, prevented the production of research quality plasmas. Out of 137 attempts, only 41 plasmas lasting over 200msec were produced. Based on the spectrosocopic measurements, as well as invessel fueling during, and deuterium pressures after plasma discharges, it appears that the dust is primarily titanium deuteride. Evidence points to its having been formed during a backfill of the vessel to 500 Torr of D2 over the weekend, for a planned calibration of the Thomson scattering diagnostic. Scattering from dust was observed during the calibration. It is suspected that the high pressure D2 reacted with exposed titanium alloy in the lower hybrid launcher assembly. Much smaller quantities of dust were observed following previous calibration backfills. Test chamber exposure of titanium samples, from the same alloy used in the couplers, to high pressure deuterium showed no such effects. On Monday night a fresh boronization was carried out, with the ECDC resonance location concentrated in the vicinity of the outboard limiters and antennas, and out to the outer vessel wall. The range of the boronization was restricted in order to investigate the importance of the antennas and limiters relative to the divertor and inner wall as sources of molybdenum during high power plasma operation. The possibility that the localized boronization exacerbated the formation and mobilization of the titanium-bearing dust is being considered. Four days of plasma operation (Tuesday-Friday) were spent in attempting to recover normal plasma operation. Conditions improved gradually, as evidenced by reductions in titanium concentrations and in-vessel deuterium fueling. The runs included a twelve hour run on Wednesday. On Thursday morning an attempt was made to remove some of the dust by flowing helium at a pressure of about 200 Torr into the pumping system. While monitoring by laser scattering indicated a reduction in the concentration of suspended dust particles, there was no significant change in plasma behavior when operation resumed on Thursday afternoon, relative to the state at the end of the run on Wednesday. By the end of the run on Friday, full-length (>1.7 sec) plasma shots with current of 750kA had been obtained, indicating substantial progress toward recovery. On Friday night a vacuum leak developed through the lower hybrid launcher. The chamber has been backfilled with inert gas (argon) to inhibit the influx of air, and preparations are being made to remove and evaluate the launcher after warming up the machine. The coming week will be a maintenance week, with no plasma operations. Long Pulse Diagnostic Neutral Beam ---------------------------------- The beam was operated to condition the source for approximately fifty shots of two second duration with a 50% duty cycle. Beam performance improved with each shot, and routinely operates at 40 kV, ~5 A beam current. More conditioning is required to reach the full beam voltage of 50 kV. The DNB was operated during C-Mod shot cycles on Friday. Initial operation was synchronized with the shot cycle, but the beam was fired into the DNB calorimeter with the gate valve to C-Mod closed. We also operated the beam in between C-Mod shots with gas pulses only to determine the effect of the beam gas on the C-Mod pressure. Once the interlock system has been tested, the beam will be ready to to begin operation into C-Mod plasmas. Lower Hybrid System -------------------- Analysis of data collected during the dedicated lower hybrid runs on May 31 and April 14 indicates excellent agreement between theoretical coupling and the experimental results, using the actual density in front of the launcher, as measured by Langmuir probes embedded in the coupler. Travel and Visitors ------------------- Ian Hutchinson visited PPPL on Wednesday to present a talk entitled "The Nuclear Renaissance" about the state of nuclear power in the US and the world, and the relationship between fission and fusion. During the week of April 22, Steve Scott and Howard Yuh visited PPPL to participate in the calibration of the beam-into-gas calibration of the NSTX MSE system. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly