Alcator C-Mod Weekly Highlights August 22, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 16.7 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Two run days were scheduled and completed. A total of 44 plasma discharges were produced with a startup reliability over 80%. The two run days supported three experiments, including MiniProposals from the RF Physics and Transport areas and the Operations group. Research operations are planned to continue this week. Run Details ----------- The first half of the run on Thursday was devoted to MP#389 "Compact Neutral Paricle Analyzer (CNPA) Testing and Calibration". This experiment, which comprises part of the thesis research of an MIT graduate student, concerns diagnosis of ICRF-generated hydrogen tail ions by pulse-height analysis of charge exchanged neutrals, including both passive measurements and active charge exchange measurements using the diagnostic neutral beam (DNB). For these experiments, two diagnostic systems, at F-port and a newly-installed two channel detector at C-port, were employed. ICRF minority heating was carried out for target plasmas at 600 and 800kA and Bt=5.2T, using the E-port and J-port antennas oprating at frequencies of 80 and 78MHz respectively. Density scans were carried out at both currents. Passive fast ion signals with high count rates were observed. The spectra were energetic, with some distributions almost flat between 100 and 400keV. In addition, clear active charge exchange signals were obtained using the DNB. The CNPA signal was found to be sensitive to edge neutral pressure. The second half of Thursday's run was dedicated to MP#430 "Localization of the boronization effect" from the Operations group. Three between-shot boronizations were performed to assess the relative effectiveness of wide versus narrow scans of the ECDC resonance location and of 100% vs 50% duty factors in the boronization discharge. All post-boronization discharges exhibited good initial H-mode performance, but these were limited by impurity injections. The 100% duty factor cases required longer pump-out times before attempting the next shot than did the 50% example. The run on Friday was devoted to MP#400a, "High k, high frequency fluctuations in electron transport dominated regimes" from the Transport Science group. The purpose of this experiment is to look for changes in density fluctuations, particularly those at short wavelength and high frequency ,in low density Ohmic regimes, where transport is known to be dominated by the electron channel. The goal is to correlate electron transport levels with fluctuations at scales that might correspond to predicted instabilities. Density scans were carried out in both inner-wall limited and lower single null equilibria, although the lowest density regime was only accessible in the divertor discharges because of high disruptivity in the limter configuration. Initial assessment of the density fluctuations observed by the PCI (Phase Contrast Imaging) diagnostic did not indicate the presence of significant high-k fluctuations. High frequency magnetic fluctuations which had been previously observed during current rise and fall in single null plasmas, were observed throughout the limiter discharges, suggesting possible damping of the modes due to edge magnetic shear in diverted cases. ICRF Systems ------------ During the planned ICRF system conversion to 78 MHz, Uptegraff #3 power supply (27 kV, 150 A) developed a short between the HVDC and LVDC taps. To continue with J antenna operation, the antenna was reconfigured as a dipole and resumed plasma operation on August 18th using only FMIT#4. J-port operation as a two strap antenna has reliably supplied 1.5 MW of power to the plasma. Repair of the uptegraff supply will begin following the current run period. Lower Hybrid System ------------------- Finite element analysis of the 24-window prototype is complete. Stresses were found to be at allowable levels, and we are proceeding with preparations for the braze test. Two of the four new stainless steel couplers are in house with two more expected this week. Enough new alumina windows are in house to braze all the couplers . Inspections and bake tests of the windows indicate very high quality material and metal coatings. Preparations for phase and amplitude measurements continue in collaboration with PPPL. Long Pulse Diagnostic Neutral Beam System ----------------------------------------- Two Russian scientists from the Budker Institute arrived on Monday night for a two-week stay, during which they will recondition the DNB back up to full performance. They have been running the beam during C-Mod runs, including into the plasma on Thursday, in the evenings after C-Mod shuts down, and over the weekend. The IR pyrometer for monitoring inboard wall heating by the long-pulse DNB has been calibrated and is ready to install. Travel and Visitors -------------------- Ted Biewer traveled to PPPL to participate in an experiment (XP505) on NSTX, which was run Tuesday and Wednesday with Rajesh Maingi of ORNL/PPPL and Hendrik Meyer of UKAEA. Tuesday's run was dedicated to locating the L-H power threshold in NSTX plasmas as the plasma shape was varried between double-null (DN), lower-single-null (LSN), and upper-single-null (USN), where the auxilliary power was supplied by co-current Neutral Beam Injection (NBI). The plasma shapes which were developed on Tuesday, using rtEFIT control, were then used on Wednesday to explore the L-H power threshold, where the auxilliary power was supplied by the HHFW RF-antenna. In both NBI and HHFW heated cases, the lowest L-H power threshold was found in balanced DN, though the power threshold was lower in NBI heated plasmas (factor of 2). The power threshold increase in LSN by about a factor of 2 in both cases, compared to DN. And the power threshold increased in USN by about a factor of 4 in NBI heated plasmas, compared to DN. USN in HHFW heated plasmas was not attempted due to the lack of runtime, and the indication from the above trends that the amount of power required would be greater than that which could be supplied by the RF antenna, for this shape. Analysis of data, particularly edge flow velocity, is pending. Dr. Petr P. Deichuli and Mr. Grigory Shulzhenko from the Budker Institute for Nuclear Physics in Novosibirsk, Russia arrived Monday evening for a two-week stay. They will be working on the Long Pulse Diagnostic Neutral Beam. -- Stephen M. Wolfe M.I.T. Plasma Science and Fusion Center Room NW17-101 175 Albany Street Cambridge, MA 02139 telephone: (617) 253-5510 fax: (617) 253-0627 e-mail: wolfe@psfc.mit.edu _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly