Alcator C-Mod Weekly Highlights September 17, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 17.4 weeks Plasma Shots: 2435 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 103 plasma discharges were produced with a reliability of 93%. The runs supported research in the MHD, Lower Hybrid Physics, and Transport Physics topical areas. Two hours of run time were lost on Wednesday because of a problem with the turning gear hardware of the MIT alternator, which provides prime power for the C-Mod magnet power supplies. The system was repaired, and eight hours of run time were completed, out of a planned ten hour extended run. Two hours of run time were lost on Thursday morning due to a data system problem; the run day was extended by one hour to partially compensate for the lost time. Plasma operations are planned to continue this week. Program Planning ---------------- The C-Mod Experimental Program Committee met on Monday to discuss the run schedule for the remainder of this month. A decision was made to change the operating frequency of FMIT#3 and 4, which drive the field-aligned J-port antenna, to 50 MHz for the last week of September. Additional run time was allocated to several experiments supporting student thesis research. This Monday was designated for plasma operations, and additional run-time, including possible extended shifts and Saturday operation, was scheduled on a contingency basis, to allow for completion of essential experiments. Operation Details ----------------- Tuesday's experiment supported MP#675 "Initial disruption mitigation experiments with two gas jets". This experiment comprises part of the thesis research of an MIT graduate student and also informs the design of the ITER disruption mitigation system. The run day included a scan of plasma stored energy at constant current, in order to determine the effect of stored energy on toroidal radiation asymmetry in gas jet mitigated disruptions, and a scan of stagger time between the two gas jets to verify the effect of using multiple, toroidally separated, gas jets on disruption mitigation (rapid shutdown) for plasmas with high stored energy. I-mode target plasmas with Wmhd up to ~130kJ (volume-averaged pressure ~1 atm) were employed for this study. These data are being analyzed. Wednesday's experiment was dedicated to MP#705 "Current profile measurements in plasmas with strong Lower Hybrid current drive", which comprises part of the thesis research of an MIT graduate student. A scan of LH phase from 75 to 115 degrees, at near constant plasma conditions and net LH power, was completed. We also did a LH phase scan within a long pulse at constant LH net power and plasma conditions. MSE performed well throughout the run and with the Intra-Shot Calibraion system (ISC) actuating every shot. The error bars on the MSE data were small (<0.1deg in the MSE frame.) These data will allow us to compare to models of LH current drive. Thursday's run was dedicated to the first investigation of the influence of poloidally varying impurity density on flux-surface averaged radial transport. Strong in/out asymmetries have been demonstrated in C-Mod driven by centrifugal force and poloidal electric fields sustained by ICRF heating. These asymmetries are predicted to have an influence on radial turbulent transport. In previous experiments in support of MP#692 "The influence of ion temperature profiles and toroidal rotation profiles on the formation of ITBs in I-mode plasmas", control of these in/out asymmetries was demonstrated using resonance layer scans. Thursday's run made use of target plasmas developed for MP#692 to obtain L- and I-mode plasmas, with molybdenum laser blow off (LBO) injections, that were configured to have varying levels of in/out impurity asymmetries at nominally fixed ne,Te and a small range of q95. These targets were produced by varying Bt to alter the minority resonance location and Ip to adjust the q95. For LFS heating, strong HFS impurity accumulation was observed, while for HFS heating, centrifugal effects dominated and higher concentration on the LFS was seen. Detailed analysis of these data is in progress. Two experiments were carried out during a ten hour extended run day on Friday. The morning experiment was dedicated to MP#710 "Impurity transport in non-inductive plasmas", which comprises part of the thesis research of an MIT graduate student. The goal of this experiment was to observe and diagnose plasmas in which a known non-recycling impurity (calcium) is injected into a tokamak plasma with reduced Ware-pinch (increased non-inductive current drive), and to observe sawtooth-free transport in the core. Scans of LH power with upper null target plasmas at Ip=500 and 600kA were completed. At 500kA, the variation in Vloop was from 0 to 0.9V, while at 600kA a smaller range was covered, but with more refined steps. Current profile data to support the analysis were obtained by the MSE diagnostic. The second experiment on Friday was devoted to MP#713 " Impurity Transport in H-modes of Varying Collisionality", which also supports the thesis research of an MIT graduate student. The objective of this run was to explore the transport of injected CaF2 in a variety of I-mode conditions, in order to augment the scan already performed in EDA H-mode, and to investigate the change in impurity transport around the L-I threshold. Data were obtained in LSN plasmas with Ip=1 and 1.3MA, above and below the L- to I-mode transition threshold. These data are being analyzed. ICRF Systems ------------ The ICRF system was run in support of experiments on Tuesday, Thursday, and Friday last week. The FMIT#2 crowbar trigger circuit failed on startup on Tuesday. The problem was traced to a defective relay which was faulting intermittently. The relay was replaced, and the transmitter was back in service on Wednesday. The new ARRA-funded ICRF FFTs were installed in the E antenna matching network. The transmission line network and the FFTs are being purged with dry N2 to reduce the water vapor pressure to acceptable levels. Lower Hybrid System ------------------- The LH system was used in support of runs on Wednesday and Friday last week. LH pulses of up to 1.0 s provided steady conditions for current profile and impurity transport measurements. These runs provided important thesis data for two graduate students. Design work on power supplies and controls for a 4.6 GHz double stub fast ferrite tuner (FFT) is in progress. The double stub FFT has previously been proven with manual tuning of the coil currents. The proposed system will determine the VSWR of the microwave circuit and use an automated control algorithm to optimize the circuit match. Diagnostic Neutral Beam Systems ------------------------------- The diagnostic neutral beam was successfully used in support of several experiments last week. Diagnostics ----------- Two (of three) channels of the MSE real-time background polychromator were operated last week for the first time. These channels are dedicated to the measurement of partially-polarized background light at wavelengths near those of the MSE spectrum. Measured signal intensities were consistent with expectations, and we observed clear responses to plasma behavior such as transitions from L- to I/H mode. Subsequent to these tests, the bandpass filters for the two channels of the background polychromator were replaced with filters chosen to pass the MSE-pi and MSE-sigma lines. We observe that the sigma line is twice as bright as the pi line and has a polarization angle offset by 90 degrees; both observations are consistent with expectations from atomic physics. An upgrade to the inner wall GPI diagnostic was installed last week. It consists of an APD array with 30 views, and is similar to the system for the outboard side. The new system saw first light on Thursday, and we are now working on calibrations. Initial detector operation was at room temperature, and we plan to eventually run at 15C. A few more adjustments need to be made to the system itself before it is fully operational. A new IF (Intermediate Frequency) section was recently installed on the C-Mod CECE diagnostic at A-Port as an upgrade to the existing fixed channel system. This new system features two additional fixed frequency filters and two remotely-tunable, variable frequency filters (6-18 GHz ). The entire CECE system is now operating as an 8-channel system, with the four channel pairs used to collect fluctuation data simultaneously at four spatial locations. The variable frequency YIG filters add versatility to the CECE system, allowing for operation over a wider range of magnetic fields and also for the possibility of correlation length measurements. The new system collected its first data on Tuesday and successfully measured edge fluctuations in ICRF heated plasmas using the newly installed YIG filters. Planned experiments using the upgraded system include the study of the radial variation of electron temperature fluctuations during a single plasma discharge (L and I-mode). Additionally, radial correlation length measurements will be attempted using the tunable YIG filter system. Travel and Visitors -------------------- Paul Bonoli and John Wright attended the annual workshop of the SciDAC Center for Simulation of Wave-Plasma Interactions that was held in Boulder, CO from September 11-13, 2012. The meeting was also attended remotely by Abhay Ram and Jungpyo Lee at MIT. On Friday, Steve Wolfe participated remotely in the Dry Run for the Conceptual Design Review of the ITER Plasma Control System. Wolfe is chairman of the review panel for the CDR, which will be held Nov 13-15 in Cadarache, France. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly