Alcator C-Mod Weekly Highlights January 31, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 10.4 weeks Plasma Shots: 1249 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and three and a half were completed. The experiments supported five miniproposals in the Transport and Boundary physics areas. A total of 104 plasma discharges were produced with a reliability of 88%. An overnight boronization was carried out prior to the run on Wednesday. Plasma operations are planned to continue this week. Operation Details ----------------- The run on Tuesday was devoted to MP#632, "q dependence of rotation reversal density". This run continued the experiments begun on Jan. 5, 2011, with the goal of determining the current and magnetic field dependence of the density at which a spontaneous reversal of the toroidal rotation is observed in ohmic discharges, and to document changes in turbulence associated with the reversal. The dataset was extended to include reversals at fields of 3 and 7 tesla, as well as at 5 T. The run was stopped two hours early due to a data system problem. Wednesday's run was devoted to MP#614, Pedestal modifications with lower hybrid in EDA H-mode. This MP contributes to the FY11 OFES Joint Research Target on pedestal transport. The goal of this run was to further explore and understand the pedestal modifications that are observed in some EDA H-modes upon application of Lower Hybrid power. The run was begun with an attempt to reproduce a 600kA, 5.4 T discharge which had exhibited density reduction and increase in pedestal temperature with 500kW of LH power. Much of the day was spent trying to reproduce this result. Late in the day it was discovered that, due to an error in the requested gas system setup, the main fueling plenum had been filled with helium instead of deuterium, resulting in mixed species plasmas with majority helium ions. The data are being analyzed, but it is unlikely that these discharges can be directly compared with the majority deuterium plasmas from previous runs in support of this MP. The data may be useful for investigation of species effects on H-mode behavior. Two experiments were carried out on Thursday. The morning was dedicated to MP#635 "Effects of RF heating on the edge/SOL poloidal velocities". This experiment comprises part of the thesis research of an MIT graduate student. In this experiment we successfully reproduced RF-heating phenomena previously observed in the SOL by gas puff imaging (GPI) and related them to specific parameters. All antenna combinations (E, D, J, D+E, D+J, E+J) were tested. A strong negative radial electric field was observed with all individual antennas (E,D and J). The radial profile of this electric field observed by GPI at the A-B limiter location is different for each antenna. A flow striation pattern, alternating between electron (EDD) and ion (IDD) diamagnetic drift directions from the wall towards the core, which had previously been only rarely observed, was clearly produced with antennas D+J operating at a minimum power of 800kW. The combination of E+J produced a similar effect, and a weaker phenomenon was observed using only the J-port antenna at the same power level. The size of the outer gap seems to have a strong effect on this striation pattern. The second half of Thursday's run continued experiments under MP#587a, "The Parallel Structure and Dynamics of Blob/Filaments". Correlations were documented between divertor probe (I_sat) signals and the intensity fluctuations in the 2D outboard mid-plane Gas Puff Imaging (GPI) view. These cross-correlations, up to 75%, were observed using filaments nominally connected by field-lines mapping through approximately one half toroidal turn. The EFIT-based field-line mapping of the probes was typically 10mm outside the rho location of the point on the array where the cross-correlation was maximum. Also the mapping in the Z location of the correlation maximum was not as expected from EFIT; about a 5% modification in the pitch angle (increase in Bt or decrease in Bp) would be required to reconcile the field-line mapping to the observed correlation. We were unable to identify strong cross-correlations between a probe and the array for higher q discharges with a nominal separation of 1.5 toroidal transits. The run on Friday was devoted to MP#633, "Lower hybrid rotation scans in different magnetic geometries". This experiment, which comprises part of the thesis research of an MIT graduate student, is designed to document the dependencies of co- and counter-current rotation observed during lower hybrid current drive. Scans of plasma current were carried out in upper single null (USN) and lower single null (LSN) configurations, with plasma density nebar~8e19/m^3 and Bt=5.4 T, and P_LH=800kW. The USN scan confirmed the scaling of delta-V, the change in rotation observed with application of LHCD, with current. At high current (800kA), delta-V is in the counter-current direction, and reverses to co-current at lower currents. The zero rotation point was determined to be near 550kA. The counter-current rotation appeared to saturate with increasing current above 800kA. For LSN equilibria, a reversal in rotation direction to co-current was also observed, but at significantly lower current values. The apparent reversal point for LSN was Ip~400kA, with co-current rotation observed at lower currents, down to 300kA. Detailed analysis of the data is in progress. ICRF Systems ------------ We completed switching the J-port ICRF system to 78 MHz, and successfully operated all antennas into plasma in support of experiments last week. The ARRA-funded rotated four strap antenna fabrication made good progress last week. The "Z-plate", the center plate covering the feedthroughs and Faraday screen support, is being fabricated, and the top/side/bottom plates have been water jet cut. The top/side/bottom plates will now be sent to an external machine shop for fabrication. The short strip lines have been machined and all EDM cuts on all the current straps have been completed. The first strap has been test fit and measured with the CMM arm in preparation for brazing. The strip lines will be detailed and the straps will be glass bead blasted to remove EDM residue prior to brazing. The braze processs will be tested again before the production brazing begins. The production plating tank at our vendor has been completed, and the stainless steel strap and inconel drop have been stripped as part of the verification of the stripping and re-plating procedure. A second inconel drop has had a pocket detail cut into it to test the impact of plating on this detail. The boron carbide resistors are in-house and their resistance has been confirmed to be within specification (~1.2 ohms). The TZM side tile design was modified to follow open field lines as in the D- and E-port antenna design. This change required that corner tiles be shortened to avoid potential excessive heating, as has been observed on the J antenna corner tiles. Lower Hybrid System -------------------- The LH system was used in support of runs on Wednesday and Friday last week. The LH system performed nominally on both days. Noise in the PIN diode supply circuits was found to be a possible source of power spikes on several klystrons. Progress on the TPS upgrade continues. New interface boards were tested, with positive results, and the fan/power supply circuits were revised. Travel and Visitors ------------------- Dennis Whyte and Paul Bonoli attended the NSTX Program Advisory Committee Meeting at PPPL on January 26-28, 2011. They are both currently members of the NSTX PAC. Dennis Whyte also attended the Fusion Nuclear Science Pathways Assessment meeting at UCLA Jan 24-25. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly