Alcator C-Mod Weekly Highlights Dec 12, 2005 FY2006 weeks of research operations: Planned: 12 weeks (tentative) Completed: 4 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Two run days were scheduled and completed. A total of 49 plasma discharges were produced with a startup reliability of about 70%. Experiments were carried out in support of research in the ICRF Physics and Transport Physics topics. Following completion of the experiments on Tuesday, we began warming up the tokamak in preparation for a clean vent to re-install the Lower Hybrid launcher. C-Mod will be in a maintenance period this week. Access to the Cell may be restricted during the week to allow high power testing and calibration activities in support of the Lower Hybrid project. Physics and Run Details ----------------------- Monday's run was in support of MP#441, submitted by R. Maggiora of the Politecnico di Torino in collaboration with the C-Mod ICRF Physics Group. The purpose of this experiment is to provide data for benchmarking of the TOPICA ICRF antenna modeling code. The four-strap J-port antenna was employed for these experiments. Data were obtained in 0-pi-0-pi and 0-0-0-0 antenna phasing under a variety of plasma conditions with plasma currents between 0.6 and 1.2MA, density between 1.2 and 1.9e20/m^3, in L-mode and H-mode. Edge and SOL parameters were monitored using edge Thomson scattering and scanning Langmuir probe diagnostics. Two experiments were carried out on Tuesday; both of these experiments supported the thesis research of MIT graduate students. The experiment on Tuesday morning investigated the effects of impurities on the 4-channel compact neutral particle analyzer (CNPA). He, Boron, and Mo impurities were injected into low density discharges by puffing (He), or running the same shot inner wall limited (for Boron and Mo). Relative impurity concentrations were monitored with various diagnostic systems, including HIREX, VUV, CXRS(w/ DNB), and fast diodes. Strong puffs of He increased the the CNPA passive count rate noticeably without affecting the bulk plasma parameters. The effects of Boron and Mo injections were not as obvious and require further detailed analysis. Additionally, significant active CNPA data was collected using the Diagnostic Neutral Beam. This active data will allow the core ICRF heated fast hydrogen temperatures to be extracted reliably. Tuesday afternoon's run continued the experiments begun last Friday in support of MP#437 on poloidal rotation effects, from the Transport research group. These experiments are designed to establish baseline results with the recently installed NeSoXs soft x-ray rotation diagnostic in standard field discharges, for later comparison with reverse field experiments. Higher than normal levels of neon and argon puffing were employed in order to maximize the signal to noise of the rotation measurements. On Tuesday the lower single null cases examined previously were extended to include inner-wall limited configurations. Steve Scott(PPPL) and Jinseok Ko (MIT) have evaluated the effect of spherical aberration on the narrow passband filters for the Motional Stark Effect (MSE) diagnostic on C-Mod. Analytic and ray-tracing calculations indicate that spherical aberration can generate off-normal optical rays at the filter used by MSE which would shift the filter's passband significantly to the blue, i.e. by an amount large compared to its nominal width. This effect can be compensated by shifting the position of the fiber bundle beyond its focal point, closer to the collimating lens. Measurements of the actual spot size of the fiber-bundle image at the collimating lens confirm that this effect was properly implemented in the original design of the MSE PMT/filter system. We conclude that spherical aberration of the collimating lens of the PMT/filter assembly is not an issue in the analysis or interpretation of MSE data. Diagnostic Neutral Beam System ------------------------------ The DNB operated into C-Mod plasmas last week with excellent performance and near 100% reliability. Beam parameters were 50kV, 6.8-7.0A at pulse lengths of 150 mS. The optical pyrometer interface chassis has been installed in the beam control rack and connected to camac. The pyrometer was previously operating with a temporary connection to camac. The new chassis has a local temperature readout and over temperature trip levels that will interface with the new beam interlock system. Several problems with the calorimeter amplifier and interface panel have been found and repaired, and the amplifier has been re-installed in the beam rack. Cabling that connects the amplifier signals to camac is still being worked on by the electronics shop. ICRF Systems ------------ Following completion of last week's plasma operations, FMIT Transmitter #4 was disassembled for cleaning and maintenance. We are also replacing the ferrite on the base of the tube with material better suited to our frequency range. The slide tuning rods between the screen and grid capacitors now have new finger stock and end stops. The anode cooling channels were inspected and verified to be clear of obstructions. Reassembly of the transmitter should be completed this week. Lower Hybrid System ------------------- The klystron body current monitoring system is now operational for all tubes, enabling us to resume high voltage operation. Drive leg calibrations have been completed on 9 of the 12 legs. Preparations continue for high power calibrations of the forward and rear waveguides, probes, directional couplers, and water loads, which are planned to begin this week. Travel and Visitors ------------------- Steve Wukitch and Rui Vieira visited GA last week to observe the DIII-D antenna configuration and the work being done on the RF Fast wave antenna. They also took the opportunity to view the DIII-D in-situ cryopumps presently exposed by the lower divertor modification work. Nineteen C-Mod physicists, including collaborators, attended the U.S. Burning Plasma Workshop at the Oak Ridge National Laboratory last week. Earl Marmar, Chair of the Workshop Program Committee, opened the meeting and chaired the first Plenary Session. Bruce Lipschultz presented the talk on Boundary Physics issues in the Plenary Session on Wednesday afternoon, Amanda Hubbard co-chaired the Integrated Scemarios Breakout Session and Joe Snipes co-chaired the Energetic Particles Breakout Session. Vugraphs from the workshop are available through the U.S. Burning Plasma Organization website http://www.burningplasma.org/agenda _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly