Alcator C-Mod Weekly Highlights September 13, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 16.0 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 2599 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed. A total of 102 plasma discharges were produced with a startup reliability of 91%. Runs supported research in the Lower Hybrid Physics and MHD Physics areas. Last week's operation concludes the FY10 C-Mod Experimental Campaign. No plasma operations are planned for this week. Program Planning ---------------- A C-Mod/NSTX Pedestal Workshop was held on September 7--8 at PPPL. The purpose of this 1.5-day workshop, which was co-chaired by Jerry Hughes (MIT/C-Mod) and Rajesh Maingi (ORNL/NSTX) was to gather and discuss ideas for FY11 research on both C-Mod and NSTX that could advance the predictive capability for the H-mode pedestal, i.e. the goal of the FES FY11 Joint Research Target. Twenty-four talks were presented by experimentalists, theorists and modelers from several institutions, including GA, LANL, LLNL, MIT, NYU, ORNL, PPPL and U. Wisconsin. The ideas discussed at this workshop will provide input for the planning of the FY11 run campaigns on both devices. Talks will be archived at the meeting website: http://www.psfc.mit.edu/research/alcator/pubs/Cmod_NSTX_Wkshp_2010/ Operations Details ------------------ Wednesday's run was devoted to MP#604 "Effect of Elongation on the Generation of Disruption Runaways". The purpose of this experiment is to test the hypothesis that low-elongation, limited configurations have a greater propensity for current quenches with significant runaway electron (RE) populations. Lower hybrid current drive (LHCD) was used to seed the plasma with superthermal electrons prior to the disruption. Reproducible disruptions were generated by two different methods: massive gas injection (MGI), with 50 bar of a mixture of 85% He, 15% Ar; and tungsten injections using laser blow-off (LBO). The gas jet shots at the lower elongations showed a clear dump of RE's at the end of the current quench. The tungsten LBO injections did not show any signs of RE's in the CQ. Although decreasing the plasma elongation to 1.5, 1.3, 1.15, and finally 1.0 resulted in clear signs of LH-seeded suprathermal electrons existing in the current quench, the runaway population was not enough to affect the time evolution of the current quench in a significant way; in particular, no runaway plateau was observed during the current quench. This result contrasts with previous experiments in several devices, including DIII-D and FT-U. These experiments contribute to the completion of the C-Mod Plain English Target "Study of runaway electron dynamics during disrptions". On Thursday, we ran MP#617 "Locked mode rotation studies and development of a novel spectral calibration technique for HiReX-Sr". The main purpose of this experiment is to study the time-history of the toroidal velocity and ion temperature profiles during locked-modes as well as to investigate unlocking of the mode using ICRF heating. Diagnostic-neutral-beam (DNB) blips were also used to ascertain the edge and core toroidal velocities from boron charge-exchange spectroscopy and compare these with the results from Doppler shift measurements of H- and He-like Argon using the HiReX-Sr spectrometer. Data were collected in ohmic target shots and with modulated ICRF at power levels of 1, 2, and 3MW . An interesting aspect of the experiment was the successful un-locking of the mode during long (500 ms) and short (50-100 ms) pulses without transitions to H-mode. The data are being analyzed, and results will be used in planning further studies of heat and momentum transport. Friday's run supported MP#613 "LHCD Effectiveness in high Te and confinement plasmas". Experiments were carried out in L-mode and I-mode plasmas using USN geometry. We coupled LHCD into high Te USN plasmas with ICRF powers up to the L-H threshold, at two currents: 800 kA and 900 kA. It was found that LHCD could be combined with ICRF from J and E port antennas, at N||=1.9. However, reflections increased, and became more noisy, particularly with E-port. We obtained a good dataset with which to compare LH effects at different core and pedestal Te, up to values approaching the highest feasible on C-Mod. These data complement the 1.1MA L-mode dataset obtained previously, Analysis will require further processing of hard X-ray (HXR) spectra and careful comparison of achieved densities, given the known non-linear dependence. Preliminary comparison of non-thermal ECE signatures during LHCD indicates that any increase in the hotter plasmas is modest. ICRF Systems ------------ Modified driver input cavities have been machined and flange fabrication is in process. These cavities are required for the installation of the new solid state Intermediate Power Amplifiers (IPA). Lower Hybrid System -------------------- The LH system was used in support of C-Mod runs on Wednesday and Friday. Wednesday's experiments required running the LH system with low elongation, inner wall limited plasmas. The four-way splitter performed as designed in redistributing power between the middle waveguide rows and top/bottom waveguide rows in these atypically shaped discharges. The LH system was operated with a larger than normal launcher to limiter gap (greater than 1 mm) to prevent protect the launcher during the gas jet triggered disruptions in these experiments. Friday's run yielded additional experience in operating the LH system in conjunction with the J- and E-port ICRF antennas in L-, H-, and I-mode. The J-port antenna was seen to have little or no impact on LH reflection coefficients, while the E-port antenna caused a measurable, but manageable, increase in LH reflection coefficients. Diagnostics ----------- One of the new FIR detectors developed under ARRA funding using planar diode technology is now being used in the C-Mod Polarimeter diagnostic. This detector is a factor of 10 to 100 more sensitive than corner cube detectors being used elsewhere in this diagnostic. We expect to replace all the corner cube detectors with the new planar diode ones over the next few months. Travel and Visitors ------------------- Allen Sanderson from the University of Utah SCI Institute met with scientists from the Alcator and PSFC Theory groups to discuss using the VisIt ( http://wci.llnl.gov/codes/visit/ ) visualization tool to design and implement synthetic diagnostics. Jerry Hughes and Michael Churchill traveled to Princeton, NJ to participate in the C-Mod/NSTX Pedestal Workshop on Tuesday and Wednesday. Other C-Mod scientists participated remotely from the PSFC. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly