Alcator C-Mod Weekly Highlights November 8, 2010 FY2011 weeks of research operations Target: 15 weeks Completed: 2.83 weeks Plasma Shots: 339 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed, with some minor delays. A total of 71 plasma discharges were produced with a reliability of 87%. Experiments supported research in the Lower Hybrid Physics, Transport, and MHD topical physics areas. This week will be a scheduled maintenance period. No plasma operations are planned. Most of the scientific staff will be participating in the APS Division of Plasma Physics Meeting in Chicago. Operations Details ------------------ Two experiments were scheduled for Wednesday. The first was in support of MP#604 "Effect of Elongation on the Generation of Disruption Runaway Electrons Using LH-driven Seed". The start of this experiment was delayed for several hours due to computer network issues, following which we encountered difficulties in obtaining the required target plasma. Two 1MA inner wall limited discharges with fast electron seed population produced by lower hybrid current drive were successfully terminated using the gas jet. In contrast to a previous experiment at lower current, neither shot exhibited any evidence of runaway electrons in the current quench. However, because of differences in the target density, the results are not directly comparable. Two hours on Wednesday were devoted to a continuation of MP#595a "Analysis of the Radial Impurity Transport at the Pedestal Region". The purpose of this experiment is to analyze the edge transport of boron and fluorine. EDA H-modes were successfully produced at 600 and 800kA, with good data obtained from active CXRS using the DNB and from inner wall gas puffing. Thursday's run was devoted to MP#463 "Reaching zero loop voltage with LHCD". The goal of this experiment was to improve the documentation of the successful fully non-inductive discharges obtained in previous experiments, including the current profile evolution with good time resolution. The experiment was complicated by the appearance of locked modes in the low-density target plasmas on many discharges, which often led to significant MHD activity during the current drive phase, with significant impact on the current drive efficiency. Nevertheless, a substantial dataset including MSE data with different timing during the current evolution was obtained. These data are being analyzed. Friday's run was devoted to assessment of a modification to the configuration of the feed network of the LH2 launcher system, implementing a multijunction-like (MJ) modification to the "magic tee" power splitters. The modification involved replacing the dummy load at the fourth port of each magic tee with a short. Insertion of a quarter-wave phase shim was required to optimize the power splitting from one pair of columns. Operation with toroidal phasings of 75 and 90 degrees had low reflection coefficients and few nuisance trips. Operation at higher toroidal phasings (>105 degrees) resulted in very high reflection coefficients. Scans of the vertical position and the outer gap showed that the favorable launcher behavior with respect to sharing among the rows has not changed substantially with the MJ-like modification. Power is still distributed between rows of the launcher as the plasma moves. Power was increased up to 180 kW/klystron at 75 deg phasing. Net power with the modified magic-T's was 10-15% higher than for the unmodified system (with the same klystron power and reflection coefficient). Overall reflection coefficient is about the same as before the modification. The increase in net LH power comes from the recirculation of the reflected power. The launched n|| spectrum does not appear to be adversely effected by the recirculation of the reflected power. A Fourier transform of the forward power at the waveguide mouths is very similar to the unmodified spectrum with the exception of some small shifts in the side lobes. The current drive efficiency seemed to be as expected, based on the observed changes in loop voltage. ICRF Systems ------------ We continue to work on characterizing the ICRF control system. A new modulator board has been sent out for fabrication and will be tested prior to resuming operation. We have also been preparing an AM modulation network for installation in the D antenna control system for planned upcoming Active MHD experiments. A further refinement of the current strap fabrication process for the new rotated 4-strap antenna has been tested. We are addressing the surface finish quality and have been working with vendors of the wire EDM programming software to obtain an acceptable surface finish. Lower Hybrid System -------------------- The lower hybrid system was successfully run in support of plasma experiments during all three run days last week. A problem in the low power drive amplifier reduced the power available from one klystron during Friday's experiment. Troubleshooting of this circuit will be carried out during the upcoming maintenance week. Travel and Visitors ------------------- Steve Wukitch and Paul Bonoli participated in the 3rd ITPA MHD Working Group 3 video conference, dealing with sawtooth control for ITER. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly