Alcator C-Mod Weekly Highlights Nov 4, 2002 Physics Operations continued at Alcator C-Mod last week. Four run days were scheduled and three were completed; the start of the scheduled run on Tuesday was delayed and only about two hours of operation were achieved on that day. Experiments were carried out on Transport Physics and Advanced Tokamak Scenarios. Progress also continued on the Lower Hybrid fabrication project. In addition, the C-Mod Quarterly Progress Review was held on Thursday, Oct 31. Operations ----------- Plasma operations were conducted on Tuesday through Friday. A total of 71 plasma discharges were produced, with a startup reliability of over 75%. Physics Operations are scheduled to continue this week. The start of the run on Tuesday was delayed until after 3:00PM due to an instrumentation problem at the alternator. During the delay, the RF Group completed work related to the changeover of transmitter#3 (J-port antenna) from 70MHz to 78MHz. Once plasma operation began, the RF Group verified the operation of the J-port system at the new frequency setting, which had been the highest priority goal of the run day. Additional RF system evaluation tasks planned for this run have been rescheduled. The run on Wednesday was devoted to an evaluation of the effect of neutrals on H-mode pedestal parameters, as described in MP#320 and 321. The object was to document the dependence of pedestal width on the height of the density pedestal, at constant current and with the pedestal temperature held fixed by varying the RF heating power. Several techniques were employed to vary the pedestal density, including changing the L-mode target density, maintaining the gas fueling during the H-mode phase, and varying the location of the fueling source. Attempts to extend the study to an inner wall limited H-mode, by moving an established lower-null diverted plasma onto the limiter after the H-mode was established, resulted in H-L transitions shortly after the plasma became limited. For the range of pedestal density actually obtained, preliminary examination of the data indicates that the pedestal width is not sensitive to the height. Detailed data analysis is necessary before a final conclusion can be reached. Experiments to determine the boundary between ELMfree, EDA and ELMy regimes were carried out on Thursday, exploiting the new shaping capabilities provided by the modifications to the inner divertor. The goal was to map out the regimes in the q/delta plane. Most of our previous work was at delta ~ 0.4 and found a threshold in q95 in the range 3.5 - 4. Preliminary analysis, for low triangularity, supports our earlier work that found that EDA was harder to obtain for delta < 0.35. The new work found q threshold for obtaining EDA at delta ~ 0.3 was in the range of 4.5 - 5.5. At the higher triangularities, the quasi-coherent mode didn't disappear until q95 was dropped below 3.5-4, about the same as the q threshold for our standard triangularity. Friday's run was in support of MP#307, dealing with generation of high bootstrap current fraction in ITB discharges. The off-axis ICRF minority heating was employed to generate the Internal Transport Barrier in EDA H-mode discharges. The proton minority resonance was located on the high-field side by operating at 4.5T with ICRF frequency of ~80MHz. The idea of the experiment was to study the bootstrap current associated with the steep density gradient in the ITB region at reduced total plasma current. We found that radiation rose significantly in these discharges, resulting in hollow electron temperature profiles. While the sawteeth disappeared and the current profile may well have become hollow in these discharges, it is likely that the dominant effect was the resistivity profile rather than bootstrap current in these highly radiative, low temperature discharges. ICRF Systems ------------ The J-port ICRF system is now operating at 78 MHz. The transition from 70 MHz to 78 MHz required modification of the resonant loops, control system, and transmitter tuning. An arc in E-port stub tuner was found and repaired. The antennas were then tested into plasma up to 1 MW each before continuing with the physics runs. DNB Systems ----------- The diagnostic neutral beam operated for each operational day. The BES, CXRS, and MSE diagnostics were also operated. The BES diagnostics continued investigation of the QC mode. In essence, the mean frequency of the mode matches that observed by the PCI diagnostic; the mean poloidal wave number of ~1.5 cm^-1 agrees with earlier magnetic-probe measurements; the mode is well localized radially, and appears to hop between frequencies during the 50 ms interval of the beam. The initial CXRS ion temperature measurements were presented for the outer third of the plasma radius. Lower Hybrid MIE Project ------------------------ Fabrication continued on the RF/Circulator support structure. Parts for the HVPS crowbar circuit were installed. The fifth (spare) LHCD coupler has been brazed and tested vacuum tight. It has been sent to an electroplater for fixtureing work. The four previously finished couplers are being profiled to the plasma shape and will then be plated. Travel and Visitors ------------------- Amanda Hubbard attended a meeting of the FESAC Development Path panel at LLNL. A quarterly progress review was held at MIT on Thursday, Oct 31. Participants from DoE included Rostom Dagazian at MIT, Warren Marton and T.V. George remotely from Germantown and Greg Pitonak via video link from PPPL. Presentations were made by Ian Hutchinson, Earl Marmar, Jim Irby, Ron Bravenec, Bill Rowan, Steve Scott, Howard Yuh, Bob Granetz, Steve Wukitch, Randy Wilson and Peter Catto. The viewgraphs will be posted to the C-Mod website shortly, with links at http://www.psfc.mit.edu/cmod/sciprogram/quarterlies.html.