Alcator C-Mod Weekly Highlights Feb 6, 2012 FY2012 weeks of research operations Target: 17 weeks Completed: 3.0 weeks Plasma Shots: 464 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed. A total of 71 plasma discharges were produced with a reliability of over 82%. Runs supported experiments in the Diagnostic Development, Lower Hybrid Physics and MHD tasks. Maintenance days were scheduled on Monday and Wednesday. Plasma operations are planned to continue this week. Operation details ----------------- The experiment on Tuesday morning was devoted to MP#534 "Foil/AXUV bolometer calibration in helium". This run employed He plasmas seeded with Ar and heated with ICRF to achieve high radiated power to cross-calibrate the resistive bolometers and the AXUV diode measurements. Additionally, the He plasmas were used to demonstrate good "zeros" on the Lyman-a diagnostics, indicating scattered light or filter issues are not a problem. The second half of Tuesday's run was dedicated to MP#666, "Assessment of new limiter design and effects of LH wave coupling". Melanie Preynas, who is visiting C-Mod from CEA/IRFM (Cadarache, France) served as Session Leader for this experiment, and Orso Meneghini (GA), the author of the MP, participated remotely from San Diego. The purpose of this experiment is to perform a systematic study of how the new limiter configuration affects the coupling performance of the LH2 antenna. Experiments were carried out with the launcher positioned at different locations behind the main limiter. All experiments were performed with 90 degree phasing. A density scan was performed at the -3mm launcher position at a power of 200kW. A power scan up to 550kW was also performed. On Wednesday, we carried out MP#675 "Initial disruption mitigation experiments with two gas jets". The objectives of this experiment were to commission the second disruption mitigation gas jet and the toroidal radiation asymmetry diagnostic (DMBolo) and to begin to study the performance of massive gas injection disruption mitigation with two gas jets, especially as regards toroidal radiation asymmetry during the thermal quench. These experiments are intended to inform the design of the MGI system for ITER. We successfully commissioned the second (F-port) massive gas injection disruption mitigation gas jet system, and recomissioned the original (B-port) gas jet system. Timing and operational parameters were determined for each jet. A series of shots was taken with both jets firing into the same target plasmas, with relative timing varied so that the gas arrival times at the plasma were systematically scanned from -1.25 msec to +1.25 msec. Evaluation of the toroidal radiation asymmetry during the pre-thermal quench phase indicates a consistent dependence on the relative timing. Most of Friday's run was devoted to MP#687, "Transfer calibration of VUV/SXR spectroscopy". The goal of this experiment is to calibrate the VUV and SXR impurity spectrometers on Alcator C-Mod, using strong impurity seeding on Ohmic plasmas. With sufficient impurity levels, global measurements like PRAD, XTOMO brightness and the dilution can be used to estimate the absolute impurity density. This information can be used, along with impurity transport and radiation modeling, to provide a calibration constant for the spectrometers. Additionally, the results of this run can be used to determine Zeff which can be compared to a number of other methods (VUV/VIS/IR continuum, neoclassical resistivity). Calibration data were obtained for He, N2, Ne and Ar. The last three shots of the run were used for MP#296 "Confirmation of Thomson scattering density calibration using electron cyclotron emission cutoffs". Sufficient data were obtained to provide a preliminary calibration of the TS channels inside Rmaj~0.8m. ICRF System ----------- We are working with one of our vendors to eliminate intermittent turn-on issues with the FMIT#1 and #2 transmitters. We are preparing for fabrication of the second four-strap Field Aligned antenna, planned to be installed at E port to replace the existing dipole D and E antennas. Back plate, current strap, bellows, and Faraday shield component fabrications are being discussed with our vendors. Drawings and stock availability are being reviewed. We have separated the FFT#2's magnets and documented their status. We plan to repair magnet insulation and then begin re-assembly. Lower Hybrid System ------------------- The LH system was utilized during the run on Tuesday afternoon to investigate the impact of the new LH protection limiter mounted directly to the LH antenna. Reflection coefficients as low as 10% were observed when there was sufficient density indicated on the LH Langmuir probes. A fast time response thermocouple was installed at the cooling water return manifold to measure the water temperature as the klystrons are pulsed. The heat pulse is easily identified from the thermocouple output, but the thermocouple location (at the manifold end of a 6' hose connecting the klystron to the manifold) introduces an undesirable lag in the measurement. Components were ordered to install the thermocouple at the klystron end of the hose to decrease the time lag. Work continues on the LH-3 control system. IDL is running in real time on the control hardware and data were recorded on the digitizer. Controller outputs were tested to verify synchronicity, slew rates and timing of the outputs. Diagnostic Neutral Beam System ------------------------------- New results from the spatially and spectrally resolved beam profile diagnostic have been used to characterize the DNB performance. The profiles of the beam components were shown to be consistent with a beam operating near perveance, although the width of the profiles is still larger than the original vendor specification. The diagnostic was also able to show changes in the component shapes and concentrations during the brief high voltage excursions that are sometimes observed in beam pulses. From this data, it is clear that the component mix returns to normal soon after the voltage recovers and that the portion of the beam pulse which is at proper voltage can be used for analysis. These results demonstrate that the profile diagnostic can provide quantitative results. It can now be used to explore the beam performance issues including attenuation of the beam prior to entering the plasma and speculations about the ion optics. The DNB operated at normal parameters during Tuesday's experiment, and was not required for the experiment on Thursday. On Friday, the DNB was unavailable due to an internal fault. The cause of the fault will be investigated during scheduled maintenance on Monday of this week. Diagnostics ----------- The Wednesday maintenance day was used to finish correcting the polarization of the new 50Hz Thomson scattering lasers obtained for the Thomson scattering diagnostics. During Thursday's run both these lasers began operation together during shot cycles, providing effective 100Hz operation. Signals obtained from plasmas have so far exceeded performance requirements. Detection systems for both the core and edge Thomson systems, including alignment fibers, were successfully checked out and operated robustly. At this time, all components of the upgraded C-Mod Thomson scattering system are operational. A small number of discharges during Friday's run included density ramps at reduced toroidal field (4.0T) in order to obtain electron cyclotron emission cutoffs, which will be used to calibrate the density reported by several of the core Thomson channels. The core CXRS diagnostic was returned to the configuration used for measurement of ion temperature and impurity density using boron spectra. Data acquired while configured for fast ion measurements is being analyzed. The run on Thursday marked the first operational use of the new toroidal array of AXUV diodes ("disruption mitigation bolometry" or DMBolo). These diodes were absolutely calibrated during the "highly radiative plasma" run on Tuesday. Travel and Visitors -------------------- Jim Terry spent two days (Feb. 1 and 2) at PPPL attending an Independent Expert review of the US ITER Diagnostic program. The review will be used to prepare for a "Lehman Review" of several areas of the US ITER Project that is scheduled for May 2012. The five-expert-panel will produce a written response to the review's charge. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly