Alcator C-Mod Weekly Highlights March 13, 2006 FY2006 weeks of research operations: Planned: 14 weeks Completed: 4.9 weeks Operations ---------- Plasma operations resumed at Alcator C-Mod last week after a brief maintenance interval during which the LN2 cooling line to one of the TF vertical turns was repaired. Four run days were scheduled, and 3.5 days were completed; the run on Thursday had to be delayed for several hours in order to repair a key diagnostic. A total of 63 plasma discharges were produced last week, with a startup reliability of 68%. Research activities last week included two run days devoted to Lower Hybrid coupling studies and two to MHD topics. Specifics of these runs are presented below. The repair of the LN2 cooling line to the C-Right vertical TF leg was successful in restoring the magnet cool-down time to less than ten minutes following a standard 5.4 tesla pulse. A piezo-electric gas puff valve used for plasma fueling was found to be malfunctioning during Wednesday's run. The valve was successfully replaced following the run, and operated normally during the remainder of the week. Plasma operations are planned to continue this week. Run Details: ------------ Runs on Tuesday and Friday were devoted to Lower Hybrid coupling studies (MP#418). These experiments also comprise part of the doctoral research program of an MIT graduate student. The goals of these experiments were to measure the LH coupling, i.e. reflection coefficients, for different antenna phasings and plasma density at the grill mouth, and to characterize the plasma density at the coupler as a function of coupler location and outer gap. These goals were accomplished. Control of the lower hybrid antenna phase during a shot was demonstrated. Results of the phase scans were qualitatively consistent with expectations, and these data are being analyzed. Clear signatures of electron tail generation were observed on multiple diagnostics, including the HXR hard xray camera and ECE; analysis of the non-thermal ECE emission is also being carried out by an MIT graduate student. Wednesday's run was mainly devoted to MP#439, "Scaling of gas jet induced disruption timing with q95". The goal of this experiment is to test the prediction of the NIMROD modeling that the timing of the thermal quench would be sensitive to the location of the q=2 surface relative to the penetration distance of the neutrals. A scan of q95 (by varying B_T) was carried out with the intent of moving the q=2 location. In the experiments, we observed no consistent variation in the timing of the thermal quench relative to the injection time for 1MA ohmic target plasmas, nebar~1.4 to 1.7e20/m3, as the field varied from 5.4 to 7.1T. Lower density target plasmas, nebar~0.5e20/m3, did exhibit a small dependence of the quench time on B_T. Dennis Whyte (U. Wisc) participated remotely in this experiment. These experiments contribute to high priority ITPA research on disruption mitigation. Thursday's run was devoted to MP#404a, "Parametric Dependence of Moderate n Alfven Eigenmode Damping Rates". Prof. Ambrogio Fasoli (EPFL, CRPP) served as co-session leader for these experiments. The goal was to determine damping rates of stable Alfven modes as a function of density and elongation in near-double null discharges. The experiment benefitted from recent improvements in the Active MHD amplifier circuitry and antenna cabling. Despite technical difficulties which limited the number of plasma shots, data over a range of density and kappa were obtained. Typical damping rates were in the 2-3% range, and n-numbers estimated to be between 3 and 10. These experiments contribute to ITPA high priority research on the physics of Alfven eigenmodes. Physics ------- At the weekly C-Mod Science Meeting, David Mikkelsen presented a status report on three user-oriented gyrokinetic stability and turbulence codes. These are intended for application to experimental plasmas, and software tools that automate data preparation and analysis exist. Several possible research topics were suggested. First results from the DIONISOS experiment at the University of Wisconsin-Madison for hydrogenic retention in Mo have shown an average retention rate (D/Mo) of ~0.05 %, an order of magnitude lower than what was seen on the "clean" C-Mod walls. A 1 mm thick plate of Mo at 300 K displays a square root dependence of retained fluence vs incident fluence, while a 25 um thick Mo foil at 370 K displays a linear dependence of retained fluence vs incident fluence. Factors such as flux density, surface temperature, incident ion energy and effects from surface boron will be studied in future work. These results form part of the thesis research of a graduate student at the University of Wisconsin. A coupling was detected between the quasi-coherent mode (QCM) near the separatrix and core fluctuations using a single high-resolution electron cyclotron emission diagnostic (FRCECE). The FRCECE system observes the quasi-coherent mode as a density fluctuation that shows up clearly on an autopower spectrum (~4% signal amplitude). Core (temperature) fluctuations are detected by cross-spectral analysis with the QCM since the amplitude is too weak to be observed with local spectral analysis. In the 60 to 70 kHz frequencies of the QCM a definite phase relation is seen in all 32 ECE channels from the very edge to the center of the plasma. The amplitudes are very small <1%. The nature of this coupling is not understood at this time. Diagnostic Neutral Beam System ------------------------------ Repair of the HV power supply for the DNB is proceeding. Final designs for the replacement MOV support structures were completed last week, and the supports were machined and mounted in the high voltage transformer tank. The new high voltage inverter control boards have been partially populated, and are slated for testing this week. All modifications of the DNB electronics for the long pulse interlock system have been completed and successfully tested. A four-fiber bundle has been prepared to connect between the DNB and the interlock board and will be installed on Monday. Lower Hybrid System ------------------- The lower hybrid system performed well during dedicated C-Mod experiments on Tuesday and Friday last week. Pulses with nearly 900kW of source power and pulse lengths up to 20msec were injected. Trains of up to seven pulses with different phasings were also injected during these experiments. The third harmonic arc detection system is now operational and was available for both lower hybrid runs last week. Evaluation of this new type of arc-detection system is proceeding. ICRF System ----------- All four ICRF transmitters were operational during C-Mod experiments last week. Antenna conditioning activities were carried out in "piggyback" mode on Tuesday, Thursday, and Friday. The D- and E-port dipole antennas have successfully coupled up to 2.2 MW (combined) and the J-port four-strap antenna up to 2.4MW. Additional conditioning is still required. The H/D ratio is still too high for optimum proton minority heating, but some ICRF-induced H-modes have already been produced. Travel and Visitors ------------------- Shinji Yoshimura, from the National Institute for Fusion Science, in Toki Japan, visited the PSFC from Tuesday, February 21 thru Tuesday February 28. He came to learn about the C-Mod Langmuir probe array, including the divertor probes, scanning probes and the new WASP inner wall probe. Dr. Yoshimura will be building and operating a scanning probe or probes for LHD to measure plasma profiles, and hopefully flows in their ergodic field region. He had discussions with C-Mod scientists including Brian Labombard and John rice and graduate student Noah Smick. He also toured the scanning probe hardware and probe electronics in the C-Mod Cell and was shown some probe data from past shots and some partially assembled probe tip hardware. Dr. Myunghee Choi from General Atomics visited the Theory Group at the PSFC last week from Tuesday (March 6) through Friday (March 10). She worked with John Wright and Paul Bonoli on the implementation of a parallel version of the TORIC full-wave solver (TORIC5) on the GA computing cluster - "DROP". They also worked on the formulation of the quasilinear diffusion coefficient using electric fields computed by TORIC. This diffusion coefficient is then used in the Monte Carlo orbit code ORBIT RF. This code is being applied to minority ICRF heating experiments in Alcator C-Mod. Perry Phillips (UT-FRC) visited the PSFC last week. He is in the process of repairing the local oscillator shared by the 16 FRCECE channels that cover the outer half of the plasma. The 16 core channels are working well and were used last week for various experiments including a LHCD experiment. Perry discussed the LO failure with Amanda Hubbard and Steve Wolfe. When the LO is replaced (in a few weeks), he will add power conditioning and magnetic shielding. Perry also modified the data acquisition for FRCECE so that he will be able to look for TAE modes using correlation with a magnetics signal and so that he will be able to look for AE modes during active MHD experiments by correlating with the active drive signal. Last week Stewart Zweben (PPPL) brought the PSI-5 ultrafast camera back to C-Mod (after it was repaired by PSI), and he and Jim Terry set it up and began to operate it successfully at the GPI view on C-Mod. Stewart also continued the analysis of GPI data taken in 2005 and the design for a new lower divertor GPI view on C-Mod. Randy Wilson (PPPL) was also at C-Mod last week, participating in the lower hybrid experiments. Prof. Ambrogio Fasoli (EPFL/CRPP, Lausanne, Switzerland) visted C-Mod last week. He co-led (with Joe Snipes) the TAE damping experiment on Thursday, and also held discussions with Joe, graduate student Jason Sears, and Ron Parker on the subject of Active MHD Spectroscopy and our collaboration with JET. Prof. Fasoli also presented a seminar on results of turbulence characterizaiton on his basic plasma physics experiment TORPEX. Bruce Lipschultz attended a short review of part of the UCSD research programs during the afternoon of March 2, following the Plasma Facing Components Meeting. That program involves materials studies in PISCES B and plasma research on PISCES A & other linear machines. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly