Alcator C-Mod Weekly Highlights March 22, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 8.6 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 1495 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. Experiments supported research in the ICRF Physics, and Transport topical science areas. A total of 95 plasma discharges were produced with a reliability of 90%. A fresh boronization was performed on Monday night. Part of Thursday's experiment featured operation at 7.2T and Friday's experiment was carried out at 7.8T. Research operations are planned to continue this week. Operation Details ----------------- Tuesday's experiment was devoted to MP#521a "ITB formation - Ti and rotation profile effects", and made use of the J-port antenna with ICRF sources at 70MHz. The purpose of this experiment is to understand the triggering mechanism for producing off-axis ICRF internal transport barriers. ITB plasmas were produced using both low- and high-field side off-axis heating, at Bt~5.7 and 3.9T, respectively. Fine scale scans of the resonance location were carried out in order to identify the critical condition for onset of the ITB. Only a few fully-developed ITB cases were obtained, but rotation profiles for both ITB and non-ITB conditions were recorded. These data are being analyzed, and additional experiments addressing this topic are planned. The run on Wednesday was dedicated to MP#535a "Rotation inversion vs density and current, in limited and diverted L-mode plasmas", which addresses toroidal rotation phenomena observed on TCV and on C-Mod in low density ohmic discharges. Basil Duval (EPFL) served as Session Leader for this experiment. The goal of the experiment was to check the toroidal reversal with limited and diverted configurations and to attempt to move between the configurations to observe changes. Reversal of rotation as a function of density was observed in (LSN) diverted configurations, and was demonstrated to be reproducible. The reversal was not clearly observed in the inboard limited configuration, possibly because it was not possible to access sufficiently low density. A change from Counter- to Co-current rotation was observed when changing between limited and diverted equilibria. A scan of q95 from 3 to 4.5 confirmed the scaling with plasma current. A reversed transition was observed at q95=3.6 by allowing the density to decrease after the initial part of the discharge. The data obtained support the conclusion that the toroidal velocity reversal in C-Mod is consistent with that observed on TCV. Additional experiments have been proposed. Thursday's experiment was devoted to a continuation of MP#597 "ICRF Mode Conversion Flow Drive at 8 Tesla". We observed mode conversion flow drive at 70 MHz/7.2 T D(He3) plasmas. Not surprisingly, the conditions for effective flow drive are similar to previous observations at 50 MHz and 78 MHz. The rotation is sensitive to He3 level, and maximizes around 10-12% He3 (as inferred from the mode-conversion radial location measured with PCI). Rotation speed increases as the density is lowered; the maximum rotation change observed for these conditions was about 70 km/s at 2.7 MW power. Unlike previous experiments, this run was carried out with dipole phasing of the four-strap J-port antenna, rather than at +90 degrees. With the dipole phase, the rotation does not increase monotonically with Ip, but is maximized at Ip~1 MA level, which is similar to previous observations at -90 degree phase. Furthermore, the rise time of the rotation was much longer (up to 300 msec) than had been observed at +90 degree phasing. We also produced some discharges at 4.6 T in order to compare results with D(H) minority heating at 70 MHz. The change in rotation for these D(H) minority shots clearly follows the delta(W)/Ip Rice scaling while the rotation at 7.2 T with optimum He3 level exceeds the Rice scaling by a factor of 2. On Friday, we carried out MP#601a "Experimental Study of ITG Stiffness and Threshold (8T and 70 MHz)". This MP supports the ITPA Transport and Confinement Joint Experiment TC-13, the purpose of which is to measure ion threshold and stiffness by varying gradually the ion heat deposition profile from 100% on-axis to 100% off-axis using multi-frequency ICRH, and to compare with theory for transport model validation, with the aim of improved extrapolation to ITER. This experiment is also part of a collaboration between C-Mod and JET under IEA Large Tokamak Facility Implementing Agreement. Dr. Paola Mantica (Istituto di Fisica del Plasma CNR-EURATOM, Milano, Italy) served as co-Session Leader using remote collaboration tools from Milano. The experiment was conducted at Ip=800kA (q95~6.3) in order to minimize the effect of sawteeth. On- and off-axis ICRF D(He3) heating at 80.5 and 70MHz was employed, both in the minority and mode conversion regimes. Dominant ion-heating was obtained in the minority regime, while direct electron heating and flow drive were observed in the MC regime. The MCFD was used to assess the effect of changing the velocity shear. The RF power was modulated to facilitate the analysis. Time-dependent profiles of Ti and velocity were obtained using the HIREX_SR diagnostic. We also documented one discharge with strongly ramping plasma current (0.8 to 1.2MA in 250msec) to assess the role of changing the magnetic shear on the ion profile stiffness. ICRF Systems ------------ Conversion of the J-port antenna and FMIT#3 and 4 transmitters to 70MHz operation was completed on Monday, in preparation for the week's experiments. During Friday's experiment, The FMIT#2 (E-port antenna) driver stage experienced a fault and was removed from service. Upon disassembly, we found damage to the filament connection in the tube socket. Repair of the socket is underway. Lower Hybrid System -------------------- We continued testing of the new Coupler Protection System (CPS). Tests of the CPS have been completed for all ten klystrons that will be used to drive the launcher. The CPS fault information extraction software has also been completed. Any detected CPS faults are being reported to the MDS tree, and a Graphical User Interface for displaying this information has also been developed. The new launcher was placed in the vacuum test stand last week and leak checked. A leak was found in one of the sixteen waveguides at the rear of the launcher where it attaches to the main vacuum flange. The vacuum windows near the front of the launcher, which were recently etch-cleaned, are leak free. Diagnostics ----------- The FIR lasers for the new polarimeter diagnostic were installed in the cell last week. They were successfully operated for the first time during plasma operations on Friday. Tests of other optical components will continue next week. Graduate Studies ---------------- Two Alcator graduate students, Jason Sears, MIT Department of Electrical Engineering and Computer Science, and Brock Bose, MIT Department of Physics, successfully defended their PhD thesis work last week. Jason's thesis title is "Alfvén Eigenmodes in Alcator C-Mod"; Brock's thesis title is "Plasma Turbulence following Li Pellet Injection into Alcator C-Mod Plasmas". Travel and Visitors ------------------- Paul Bonoli and Martin Greenwald attended a Planning Workshop for the Fusion Simulation Project that was held in Boulder, Colorado from March 15-18, 2010. Basil Duval from EPFL (Lausanne, Switzerland) visited from Mar.16-19, to discuss TCV/C-Mod similarity experiments; he served as Session Leader for the run on Wednesday devoted to his MP#535a. Joe Snipes (ITER Organisation) visited MIT last week. He presented the PSFC Seminar on "Plasma Control in ITER" on Monday, and held discussions with C-Mod scientists on a variety of ITER-related topics. He also served on the committee for the thesis defense of an MIT graduate student on Friday. Will Bergerson, UCLA, visited MIT on March 15 to discuss the polarimeter diagnostic with Peng Xu and Jim Irby. Dr. Bergerson will relocate to MIT later this Spring to work full time on the polarimeter as part of an MIT/UCLA collaboration. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly