Alcator C-Mod Weekly Highlights December 13, 2010 FY2011 weeks of research operations Target: 15 weeks Completed: 5.80 weeks Plasma Shots: 701 Operations ----------- Plasma operations continued last week at Alcator C-Mod. Four run days were scheduled and three were completed. The run on Tuesday was delayed due to a problem bringing the MIT Alternator, which provides primary power for the C-Mod magnet systems, up to full speed. The problem was determined to be due to faulty contacts on four auxiliary switches. The contacts were refurbished and C-Mod operations resumed around 4:00PM. The alternator performed normally for the remainder of the week. A total of 113 plasma discharges were produced with a reliability of 86%. The experiments supported research in ICRF and Transport Physics, H-mode Scenarios, and Diagnostic Development. A boronization was carried out before the run on Friday. Plasma operations are planned to continue this week. Operations Details ------------------ Work on MP#619 "Effects of ICRF and LH power on SOL density profiles", which employs the SOL reflectometer system, was continued during the abbreviated run on Tuesday and a half-day session on Wednesday. This phase of the experiment concentrated on SOL effects associated with high power ICRF heating in L- and H-modes. Dynamic outer gap scans were performed, resulting in ICRF coupling to plasmas with gaps up to 12cm, using the E-port antenna with the fast ferrite tuner (FFT) matching system. Analysis of the change in loading resistance compared to the distance to the evanescent cut-off layer is underway. A series of shots were used to study asymmetries in the SOL using the reflectometer horns located at the top, middle, and bottom of the LH launcher structure at C-port. Shots over a wide range in density were also obtained with ICRF power from the E-port antenna. A scan of ICRF power in ELM-free H-modes was also obtained, using both E- and J-port antennas. Differences between the two antennas in their effects on the SOL density profiles observed at C-port were documented. Also on Wednesday, a half-day experiment was devoted to MP#628 "Off-axis ICRF ITBs in I-mode Target Plasmas". The goal of this experiment was to determine whether off-axis ICRF heating would result in formation of internal transport barriers in I-mode discharges, similar to the ITB's commonly produced in this manner in C-Mod EDA H-mode discharges. I-mode plasmas were successfully produced in USN (unfavorable grad-B drift direction) plasmas at toroidal fields from 4.2 to 6.3T, with the ICRF minority proton resonance on both high and low-field sides. There was no evidence for any ITB formation below 4.6 T or above 6.1 T, for which ITBs had previously been observed with favorable drift and in EDA H-mode plasmas. No peaking of density or temperature, or any reduction in core rotation was observed. Further analysis of the profile and fluctuation data in these discharges is underway. Thursday's run was devoted to MP#579, "Characterization and optimization of I-mode". The goals were to extend and improve the characterization of I-mode profiles and fluctuations. We successfully obtained core and edge Ti, V_tor, and V_pol profiles, using CXRS and X-ray spectroscopy (HIREX) for I-mode discharges at 1.1, 1.2, and 1.3MA. Same day diagnostic calibration shots were obtained for Thomson scattering and HIREX_SR systems to reduce measurement uncertainties. These data extend the detailed dataset obtained at lower currents to higher pedestal temperatures. Analysis of the Er well depth in these high performance I-modes is of considerable interest. Friday's run was dedicated to MP#575 "ITER-like rampdown studies". This MP supports the ITPA Joint Experiment IOS-2.2 on ramp-down effects. We studied ramp-downs from ITER-like discharges at Ip=1.3MA, Bt=5.4T (q95~3) with EDA H-mode at the end of the flattop phase (EOF). Two ramp-down rates were used, corresponding to ramp times of 0.5 and 1.25 sec; cases with a ramp time of 0.75sec had been obtained previously. ICRF power in the ramp-down was varied from 0 to 1.5MW. All cases remained in H-mode for substantial fractions of the ramp-down time. Density and stored energy were reduced along with current during the H-mode phase of the ramp. Detailed analysis of the ramp-down dynamics is underway, and the results will be compared with TSC simulations. ICRF Systems ------------ In preparation for plating of the new ARRA funded rotated four-strap antenna, a large Inconel-625 sample was plated by our vendor, and the specimen passed tape peel and 265C vacuum bake tests. The coating was determined to be at least 0.003" thick (microscopic examination). The specimen had a rough surface near where the plating solutions were pumped and 1/16" diameter pit that resulted from poor activation (location was used for diagnostic purposes). This test emphasizes that the plating process requires special attention to detail. The stainless steel current strap plating is underway and will help to establish plating parameters for the large complicated strap geometry. During C-Mod operations last week, up to 5.8 MW was coupled to the plasma in I-mode discharges. Further experiments were done to investigate long distance coupling with ICRF, with the SOL density profile monitored by the SOL reflectometer. For discharges with higher density, coupling could be maintained for outer gaps as large as 12 cm and the measured profile suggests the cutoff moved radially less than a centimeter. For lower densities, the coupling was less successful and the cutoff layer appeared to move further away radially. We have received a new Final Power Amplifier tube (ARRA funded) tube. High pot tests are planned for this week. Lower Hybrid System -------------------- The lower hybrid system was not required for C-Mod experiments last week. The launcher position was varied as needed for experiments involving the SOL reflectometer. Work on the fourth klystron cart continued. Troubleshooting of the drive signal for klystron #7 continued. Travel and Visitors ------------------- Chuck Kessel (PPPL) visited MIT on Friday and served as Session Leader for the day's experiment. Greg Wallace attended the IAEA Steady State Operations meeting in Vienna Mon-Wed last week where he gave a presentation on "Lower Hybrid Current Drive Studies on Alcator C-Mod." Miklos Porkolab attended the Fusion Power Associates (FPA) meeting in Washington, DC, where he presented a talk on "Continued Research Through the Next Decade on Existing Tokamaks is Critical to Make Magnetic Fusion a Viable Energy Source"; the vugraphs can be found at http://www.psfc.mit.edu/research/alcator/pubs/FPA.Porkolab.Final.2010.pdf At the meeting, Miklos also received a Distinguished Career Award from the FPA. The citation reads: In selecting Prof. Porkolab, the FPA Board recognizes his decades of career contributions to fusion research and development, including his pioneering contributions to wave-particle interactions, plasma heating and diagnostics development and the leadership he has provided to the MIT and world fusion programs. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly