Alcator C-Mod Weekly Highlights January 25, 2010 FY2010 weeks of research operations Base Target: tbd Base Completed: 2.5 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 889 Operations ----------- Plasma operations resumed at Alcator C-Mod last week, following completion of a maintenance interval including a clean (up-to-argon) vent. Tokamak operation was re-established with no difficulty. Three run days were scheduled (W-F) and completed. A fresh boronization was performed on Wednesday night. A total of 82 plasma discharges were produced with a startup reliability of over 80%. The runs supported experiments from the Boundary Physics, H-mode Scenarios, and MHD areas. This week will be a scheduled maintenance week. No C-Mod operation is planned. Operation Details ----------------- The experiment on Wednesday supported MP#587a "Parallel structure and dynamics of blob/filaments". The gas puff imaging (GPI) array viewing the outboard midplane at the AB split-limiter was magnetically mapped to the flush-mount probes of the outer divertor module at F-port. The experiment was designed to investigate the correlation of I_sat and V_floating signal fluctuations at the mapped probe to the GPI emission, in order to determine whether the filaments/blobs extend from the midplane, past the X-point, to the divertor targets. We were able to magnetically map (according to EFIT) divertor probes 9 and 10 to the GPI view with the shortest possible connection length (~2.5 m); this mapping occurs at low q_95 of ~2.7. This q_95 was readily obtained at fields of both 5.44 T and 3.5 T. Control room analysis indicated that the cross-correlation with I_sat was inconsistent. On some shots, when the mapping showed magnetic connection, we did see cross-correlations (for 3 ms duration data sets) of as much as 45% at lag times on the probe signals of 50 to 100 usec. However, a clear correlation was not observed on other shots with nominally the same mapping. In order to establish whether or not the correlation tracked the mapping, we established Ip ramps to sweep the field-line connected to either probe 9 or probe 10 across the GPI field-of-view. These data are being analyzed. During two dischages we biased the mapped probe with a 10 kHz triangle waveform in order to see if we could affect the GPI emission. We saw no modulation on the GPI views in these cases. Thursday's run was devoted to MP#574 "ITER-like discharge studies at reduced B_T"; Chuck Kessel (PPPL) served as Session Leader for this experiment. During this run we developed a new discharge configuration at Bt=2.7 T, Ip=0.6MA (q95~3) in an ITER-like shape. The optimized startup with initiation at low field and early X-point formation was employed, with the current ramped to the flattop value in 350msec. ICRF heating was provided using the second harmonic proton minority scenario. For most discharges the ICRH power was between 2 and 3MW at f~80MHz. The goal of these studies is to obtain the ITER demo parameters of n/n_G~0.85 and beta_N~1.8 in the half-field configuration. Progress was made toward this goal, with n/nG up to 0.75 and beta_N~1.5 in repetitive ELM-free H-mode discharges. However, we were unable to access EDA or type-I ELMy conditions. The target density was scanned between 1.0 and 1.7e20/m^3, and q95 was varied over a narrow range from 3.0 to 3.4. Friday's run was devoted to a combination of MP#432 "TAE's and RF tail losses" and MP#590 "Reinforcing characterization of driven Alfven modes". One goal of this run was to investigate the possibility of exciting Alfven eigenmodes using modulation of the ICRF, and compare the ICRF modulation drive with the drive from the active MHD spectroscopy (AMHD) antenna operated in the same frequency range as the modulation. Additional goals were to look for fast ion losses induced by driven AE's, using the Compact Neutral Particle Analyzer (CNPA) and to look for effects on heating efficiency due to the driven AE's. Inner-wall limited equilibria were employed for this study. A range of fields from 5 < B_t < 6T and of density 1 < nebar < 2e20/m^3 was explored. Amplitude modulation was applied to the 80.5 MHz drive of FMIT#1 (D-port antenna), with the modulation frequency swept during the pulse in the range of 200 to 800kHz. Additional unmodulated heating was provided from the J-port antenna operating at 78MHz. The modulation was clearly observed on the magnetics probes. These data are being analyzed, along with corresponding data for shots in which the excitation was provided by the AMHD antenna. Fast ion data was obtained on the CNPA, with no clear evidence of fast ion losses or changes in heating efficiency. ICRF Systems ------------ In support of TAE experiments, we tested simple amplitude modulation on FMIT#1 (D antenna). We were able to sweep from 100 kHz to 1MHz within a single RF pulse with a relatively simple set up. The amplitude modulation however decreased above ~500 kHz due to mixer saturation. The AM techinque offers the advantage of a distributed drive throughout the plasma whereas the Active MHD antenna provides drive at the plasma edge. We will develop a linear amplitude modulation circuit for future experiments. Lower Hybrid System -------------------- We continued to develop an ultrasonic technique for cleaning the launcher vacuum windows. Preparations are being made to conduct high power tests of windows that were successfully cleaned. Travel and Visitors ------------------- John Rice attended the kickoff for the CMTFO (Center for Momentum Transport and Flow Organization) at UCSD on Jan.11-12, and presented a talk on C-Mod rotation results. Yunxing Ma visited Gary Staebler at General Atomics from 1/11/2010 to 1/14/2010. The purpose of this visit is to install TGLF and XPTOR codes on Loki cluster at the PSFC and use them to model the transport physics in fusion plasmas. The installation has now been successfully tested on DIII-D and C-Mod cases. Stewart Zweben (PPPL) visited MIT and took more data with the ultra-fast GPI camera at 390,000 frames/sec, in which the 'blob' motion in the C-Mod scrape-off layer could be clearly resolved. He also continued to analyze the convective flow speed of SOL turbulence in order to relate this to the observed SOL widths. Chuck Kessel visited the PSFC last week to discuss running TRANSP with Catherine Fiore, discuss advanced scenarios with Amanda Hubbard, discussions with Steve Scott, and to circulate an IAEA synopsis for comment by the C-Mod team. He was also session leader for the experiment on Thursday devoted to his MP#574 "ITER-like discharge studies with low Bt". Matt Reinke, John Rice and Jim Terry met with a group from the Harvard-Smithsonian Center for Astrophysics and the Chandra X-ray Observatory at the MIT Kavli Institute for Astrophysics on Thursday to discuss plasma spectroscopy and areas of common interest and collaboration. Matt presented an overview of C-Mod spectroscopic tools and experiments. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly