Alcator C-Mod Weekly Highlights July 21, 2003 Research operations continued at Alcator C-Mod last week. Two and a half run days were scheduled and completed, bringing the 2003 Spring-Summer experimental campaign to a close. C-Mod is now entering a maintenance period. During the two experimental campaigns in fiscal year 2003, C-Mod operated for a total of 54 Research Days, exceeding our Level 1 Milestone target of 52 days (13 weeks times 4 operating days per week). A total of 1443 plasma discharges were produced with an overall startup reliability of 80.5%. Operations ---------- A total of 48 plasma discharges were produced with an overall startup reliability of 89% in support of three MiniProposals on Tuesday and Wednesday of last week. The final half-day of operation on Thursday involved power system only (no plasma) shots in support of an experiment to quantify non-axisymmetric error fields of the poloidal field coils. The new boron injector was operated for the first time during Tuesday's run. We hope the injector will reduce somewhat the need for boronizations and help maintain more constant wall conditions during run campaigns. This device is designed to inject boron at rates of up to 2 mg/pulse at pulse rates of up to 30 Hz. The initial tests were run at deposition rates too low to actually affect the plasma because of a still undiagnosed problem believed to have been caused while refilling the injector with boron before the run. However, the laser scattering system used to measure the boron deposition rate, the BIII detector used to monitor the boron light from the plasma, the injector control system, and the magnetic shielding around the injector, were all tested successfully during this first day of operation. Wednesday's run was devoted to MP#345, Mode Conversion Current Drive (MCCD) experiments, employing D-He3 plasmas at 8 tesla toroidal fields. This experiment marked the first 8T operation of this year's campaign. All systems performed nominally; startup reliability was 100% and all plasmas ran to full programmed length. Post-shot recool time was increased to 20 minutes, in accordance with previous practice. Following completion of operations on Thursday, the tokamak is being warmed up to room temperature. Minor maintenance, including a clean vent to repair leaks and replace some vacuum components, will be carried out over the coming weeks. The major activity during this period will be the scheduled inspection of the alternator which provides 13.8 kV power to the C-Mod power systems. Additional maintenance and upgrades to the power and RF systems will also be undertaken. Physics -------- On-going analysis of experiments conducted earlier this month indicates that in Internal Transport Barrier discharges with off-axis (HFS) ICRF heating, the barrier is maintained as the field is ramped up such that the resonance moves closer to the axis by 1-2cm. In these cases, the central density rise associated with ITB formation was arrested, but the barrier was not destroyed. DNB Systems ----------- The DNB was switched to deuterium for the first half of the week in order to accomplish MP 354, which was a study of fast ion confinement. This was the first time that deuterium had been used in the beam, and the changeover was uneventful. The beam was used to generate a perpendicular deuteron tail population in the plasma, and their confinement time can be estimated from the resulting increase in the 2.45 MeV neutron rate, although a full TRANSP analysis is required to get quantitative results. Measurements of the beam-target neutrons were made under a variety of discharge conditions with excellent signal to noise. The new A-coils were used to compare plasmas with and without locked modes at a number of densities, currents, and fields. Preliminary results suggest that locked modes do not enhance fast particle losses. This bears directly on the issue of ICRF heating efficiency, since that also involves a perpendicular ion tail. The DNB was switched back to normal hydrogen for the latter part of the week. The changeovers from H->D and D->H took only a couple of shots to reduce the previous component levels to the few percent level. ICRF Systems ------------ The initial mode conversion current drive experiment at 8T was successfully carried out on Wednesday. The mode conversion power deposition was clearly observed and localized near the plasma core and the location was controlled by both magnetic field and He3 concentration. Up to 2 MW were coupled from the J-port antenna in heating, co- and counter-current drive phasing. Initial tests of the two quarter wave transformers to mitigate high voltage in the phase shifter/stub tuner sections of the D-port transmission line suggested the position of the transformers was not optimal. Further measurements are required to optimize their location. Transmitter #2 developed a driver bias problem which precluded operation of the E-port antenna last week. Since RF operations were focussed on current-drive phasing experiments using the J-port antenna, research operations were not significantly impacted. The problem is under investigation. During operation in May and June of this year we completed the 3rd quarter target of the Level 1 C-Mod Milestone associated with the ICRF system performance: "Begin high power ICRF heating experiments with modified antenna systems and identify candidate target plasma conditions likely to lead to high temperature and low collisionality." Also in June, we successfully demonstrated 5MW operation for 0.5sec, which satisfies the first part of the target for the 4th quarter. Assessment of the stability and confinement properties of the resulting plasma is expected to be complete by the end of the fiscal year. Lower Hybrid Project --------------------- We continued to make progress on the tasks associated with the Lower Hybrid launcher replating this week. The forward waveguide plates have now been cleaned, stripped, replated, vacuum baked, inspected, and partially assembled at PPPL into a stack of 5 plates for RF loss measurements. Measured losses of 10 waveguide channels were in the range of 0.3 dB each, well within acceptable levels. A trial stackup (white glove handling, extreme cleanliness) of 25 of the plates for one complete half of the front waveguide will be performed next, and the losses will be remeasured. If successful, this will allow us to save schedule time and effort by eliminating the surface polishing steps. Chemical stripping of the copper from the front end coupler had also removed part of the braze material fastening and sealing the ceramic microwave windows, resulting in gaps and vacuum leaks. Rebrazing tests using different alloys are under way at our industrial vendor; prototypes will be available for inspection next week. We plan to procure a full set of backup ceramic bricks and titanium frame material in case a refabrication is necessary. The weekly MIT/PPPL videoconference to discuss technical progress and track the launcher schedule was held on 7/17. Travel and Visitors --------------------- Doug LaBrie (PPPL) visited C-Mod on Friday to retrieve the detector and data acquisition components of the NSTX high-resolution xray diagnostic, which had been being tested at C-Mod during operations earlier this month. An upgraded version of this diagnostic will be installed at C-Mod during the next experimental campaign.