Alcator C-Mod Weekly Highlights Nov 12, 2002 Physics Operations continued at Alcator C-Mod last week, with five run days scheduled and completed. Experiments were carried out in the areas of transport physics, AT scenario development, RF physics, and extension of C-Mod operating regimes. Progress also continued on the Lower Hybrid Fabrication Project, and the diagnostic neutral beam and ICRF systems continued to operate in support of the physics program. Operations ---------- Plasma operations were carried out Monday-Friday, included an extended twelve-hour run day on Thursday. A total of 122 plasma discharges were produced, with a startup reliability of over 90%. The current C-Mod experimental campaign is scheduled to conclude this week. Monday's run was in support of MP#315, a joint experiment to produce non-dimensionally similar H-mode pedestals on C-Mod and JET. This run was a follow-up to experiments carried out at JET in early October. Good matches to JET equilibrium were obtained, and pedestal data for the analysis were collected for a range of RF power 1 0.5 on the inboard side to r/a> 0.6 on the outboard side. Rotation profile data are being analyzed. On Wednesday we investigated the question of how low in density we could obtain a steady EDA H-mode, for use in future LHCD scenarios. The experiment, described in MP#327, employed a current scan down to 580kA and RF power in the 2MW range. At the low current condition we were able to produce steady H-modes with acceptable radiative power fractions at nebar ~ 2.4e20/m3, with pedestal density ~1.7e20, both significantly lower than achieved at higher current. These parameters will be investigated using ACCOME and other current drive codes to assess their potential as scenarios for LHCD and AT experiments. Thursday's run began with experiments to extend density limit studies using an inboard gas puff to reach the density limit without ramping down current. We were successful in gas fueling to the empirical limit, nebar ~ 6e20/m3 at Ip~ 800kA in these experiments. The physics of the limit was investigated by deep insertion of the A-side scanning probe, which was able to follow the evolution of plasma profiles and fluctuations. While there was a gradual cooling as the density is raised, a bifurcation occurred between 0.5 and 0.8 x nG. This effect was seen as a sharp drop in Te, a somewhat smaller drop in density and a sharp increase in the floating potential. The role of the edge shear layer in stabilizing edge instabilities, and the importance of its supression in the density limit, will be a subject of future investigations. The second part of Thursday's run consisted of a comparative evaluation of the three ICRF antennas, all operating at a frequency of ~80MHz. The four-strap J-port antenna was configured as a dipole, with the outer two straps shorted, so all the antennas had essentially the same configuration. The purpose of this study was to assess the modifications made to each of the antennas during the last major vent, and to provide guidance for future improvements. The study began with a comparison of the voltage handling performance of the three systems in vacuum, with and without magnetic field, and then with different neutral pressures in the presence of magnetics field. These non-plasma cases were also used to obtain additional calibration of the MSE diagnostic. The heating efficiency of the three antennas were then compared in a standard target plasma, at Prf~1,2MW. Each resulted in an H-mode, with equivalent heating efficiency. Also during Thursday's run, piggyback experiments were carried out with the Active MHD spectroscopy system. The new wide band amplifier developed and built by the C-Mod Electronics Shop was employed for the first time. Stable modes in the TAE frequency range (300-500kHz) were probed, extending work done during a dedicated run in October. In addition, the low-frequency band was used to investigate interaction with low m/n MHD modes in the 3-10kHz range, particularly during the density limit experiments in the period just prior to the density-limit disruption. The results of these experiments are being analyzed. Successful operation of the Active MHD antenna during ICRF operation was also achieved, demonstrating the efficacy of recently installed filters in eliminating RF interference which had previously prevented the MHD antennas being connected during ICRF heating experiments. Friday's run was devoted to extending the demonstrated C-Mod operating space to higher plasma current, and to documenting the performance of the upgraded inner divertor design installed during the last vent. A major goal of this upgrade was to provide additional safety margin against disruption forces during operation at higher current and fields. The run was successful in increasing the plasma current to Ip>1.7 MA, at a toroidal field of 7.8T. Vessel motion during disruptions at this level was found to be well within allowable values. Many of the high current shots in this run developed locked modes which affected the performance and contributed to increased disruptivity. This effect will be the subject of additional study and analysis. DNB Systems ----------- The DNB operated reliably during all five run days this week. Preparations are being made to improve the performance of several beam systems over the upcoming maintenance period. Lower Hybrid MIE Project ------------------------ Installation of the HVPS crowbar system continued, as did construction of the waveguide/circulator support structure. Travel and Visitors ------------------- Gerd Schilling (PPPL) was at MIT 11/5-8, participating in ICRF experiments, APS prsentation dry runs, and collaboration discussions.