Alcator C-Mod Weekly Highlights Jan 10, 2012 FY2011 weeks of research operations Target: 17 weeks Completed: 1.3 weeks Plasma Shots: 281 Operations ----------- Plasma operations resumed at Alcator C-Mod last week. Three run days were scheduled and about 2.6 were completed. Some time was lost on Friday morning due to a computer problem. A total of 72 plasma discharges were produced with a startup reliability of 78%. Experiments supported ICRF testing and conditioning and research in the Transport topic. Four days of maintenance have been scheduled for this week. Plasma operation on Friday is contingent on progress of diagnostic preparation. Operations Details ------------------ Runs on Wednesday and Thursday were dedicated to MP#355c "ICRF Testing and Conditioning... ". The goals were to continue conditioning of the antennas and to condition the machine for Upper Null operation. The three antennas were operated simultaneously resulting in net power levels above 3MW into L- and H-mode plasmas. The hydrogen fraction during RF operation ranged from 10% to just above 20%. On Thursday the E-port antenna system began to exhibit faulting in the external coaxial transmission/matching line; this will be investigated during this week's maintenance period. On Friday we carried out MP#672 "Nonlocality and possible correlation with rotation reversal", authored by H. J. Sun (NFRI, Korea). The aim of this experiment was to investigate a connection between previously observed toroidal rotation reversals in ohmic plasmas near the density corresponding to the transition between Linear (LOC) and Saturated (SOC) Ohmic Confinement regimes, and the phenomenon of anomalous cold pulse response, including non-diffusive or "non-local" propagation, as reported on several tokamaks. Cold pulses were produced by the Laser Blow-Off (LBO) technique, injecting CaF2 to transiently reduce the edge temperature; up to four LBO injections per shot were employed. The evolution of the temperature response was monitored using ECE diagnostics, and toroidal rotation was measured by High Resolution X-ray Spectroscopy (HIREX). Scans of target density covering the range of confinement regimes from LOC to SOC at currents of 0.8 and 1.1 MA were conducted. With Ip=0.8 MA, at higher density (nebar > 1e20/m^3) in SOC, the cold pulse propagation was diffusive and core rotation was counter current. At lower densities (below 0.75e20/m^3) in LOC, the core Te perturbation following the injections showed an increase (with a drop at the edge) and the core rotation was co-current. Several shots with ramping density exhibited dynamic LOC/SOC transitions with consistent responses of rotation and cold pulse response in each regime. Ramping density discharges at 1.1 MA exhibited similar results, except that the LOC/SOC transition occurred at higher density. A more detailed analysis, which will need to account for the effects of sawteeth, is being undertaken. Physics -------- The final version of "ALCBEAM - Neutral beam formation and propagation code for beam-based plasma diagnostics," I.O. Bespamyatnov, W.L. Rowan, and K.T. Liao was published online in Computer Physics Communications, Volume 183, Issue 3, March 2012, Pages 669-676. A standard FIDA simulation code was ported to C-Mod. ("A Code that Simulates Fast-Ion Dalpha and Neutral Particle Measurements," W.W. Heidbrink, D. Liu, Y. Luo, E. Ruskov and B. Geiger, Commun. Comput. Phys. 10, 716 (2011)). The code is an alternative to ALCBEAM, the simulation that was written in-house. The ported code was modified to describe the C-Mod beam by specifying the beam shape, injection angle and injected species. Results from the beam penetration portion of the FIDA code were compared to those from the ALCBEAM code and found to be in reasonable agreement given differences in the penetration cross sections used in the codes and that the FIDA code lacks ALCBEAM's details regarding beam shape and transport to the plasma. ICRF System ----------- Work continues on the ARRA-funded Fast Ferrite Tuner (FFT) fabrication. Eleven (out of twelve) Electronic System component sub-chassis were assembled in preparation for component installation and wiring. A final shipment of testing components to our vendor is being readied, including a directional coupler, data acquisition unit, RF cables and adapters, demodulator, and fan supply. A stress analysis of the bellows/bolt connection assembly for the four-strap field aligned antenna feed has been carried out, and a revised design incorporating a Belleville washer to maintain compression of the connection has been developed. Lower Hybrid System ------------------- Testing of the transmitter protection system (TPS) upgrade continues. The collector over temperature system (COTS) component of the TPS was exercised with actual water cooling system signals. Beam current and beam voltage parameters signals were simulated with a waveform generator. COTS faults were generated as expected based on the assumed collector heat capacity and the actual water cooling parameters. Diagnostic Neutral Beam Systems ------------------------------- Last week the DNB was run as a deuterium beam injector in order to compare perveance and calorimetry measurements with recent results in hydrogen, and also to use D-D fusion neutron measurements to supplement the calorimetry. The on-perveance beam current was found to be at 5 amps, which is the expected value based on the hydrogen perveance at 8 amps (ratio should be sqrt(m_D/m_H)). Calorimetry showed a deficit of a factor of 2, similar to results in hydrogen. Diagnostics ----------- The "Shoelace" Quasi-Coherent Mode antenna underwent its first powered tests into plasma discharges on Thursday and Friday. The RF amplifier source was run with swept frequency between 95 and 115 kHz at 50-60 W total power, and over 45 W was coupled to the antenna through the matching network. A number of Mirnov coils registered fluctuations at the antenna fundamental frequency, as well as at several harmonics. For Mirnov coils far from the antenna, the fluctuation disappeared after plasma termination, even though the antenna was still being driven; for coils close to the antenna, the harmonics disappeared in the absence of plasma, while signal at the fundamental persisted. Travel and Visitors -------------------- David Pace (GA) completed his visit to MIT to work on preparations for the new Fast Ion Charge Exchange diagnostic. He also presented a report, "Correlation between ELMs and Beam Ion Losses during the Test Blanket Module Experiment at DIII-D", describing initial analysis of results from an experiment at DIII-D proposed by C-Mod scientists and carried out last year. Alessandro Bortolon (U.C. Irvine, on research assignment at PPPL) visited C-Mod January 4-5 to discuss FIDA measurements at NSTX and give a seminar, entitled "Fast Ion Redistribution and Alfvén Eigenmodes Destabilization in Presence of Low Frequency MHD in NSTX". He also spent time discussing C-Mod CXRS measurements of Boron Ti and velocities with C-Mod scientists. Dr Bortolon had made measurements at the TCV tokamak of core carbon ion velocities and had studied in-out asymmetries of the velocities; similar studies are being carried out at C-Mod. Julien Hillairet and Melanie Preynas from CEA/IRFM (Tore Supra) arrived last week for an extended visit at C-Mod. Dr. Hillairet will stay through Jan 20 and Ms. Preynas will stay through Feb 3. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly