Alcator C-Mod Weekly Highlights August 16, 2004 FY 2004 weeks of operation planned: 18 weeks, completed: 18.9 weeks. We are currently up-to-air with the primary goal of installing the lower hybrid launcher. In addition, a large number of in-vessel activities are ongoing in support of ICRF and diagnostic systems. Maintenance and upgrades to diagnostic and power systems are also in progress. Physics ------- A set of web based tools has been deployed which integrate information from mini-proposals, run plans and summaries, shot logs and run comments from the electronic logbook. Work has begun on adding shot summary information. The tools are aimed at improving the efficiency of post-run analysis. The pages can be viewed at http://www-cmod/cmod_runs.php Operations ---------- Preventive maintenance of the alternator power converters, vapor extractor, and lube oil filtration system continued last week. Upgrades to the bus instrumentation rack AC and DC power systems have been completed and are ready for testing. Preventive maintenance of the LN2 duct work has begun. Switchgear for the new long pulse DNB has been moved into the power room and work begun on mounting hardware and conduits for power connections to the DNB transformer. More EF1 upper conic heat shield modules and corresponding ceiling modules were removed from in-vessel for cleaning and installation of new lock-down hardware. Lower Hybrid System ------------------- A 3-window braze test last week resulted in three leak-tight windows. One window had a small void that was subsequently successfully filled with a re-braze operation using a lower temperature braze. We are now cleaning this sample and preparing do high power tests of all the windows. Preparations are also being made to braze a 24-window PPPL coupler. The braze wire and windows needed for this process were fired in a hydrogen atmosphere over the weekend to clean away any impurities that might affect the braze. A compression test of the rf gasket between the rear and forward waveguides indicates more compression would be useful. New thinner shims are being fabricated that will allow additional compression of the gasket. In addition, an rf gasket with more compliance has been fabricated and is undergoing stress/strain tests. We continued work on the new rf probes, klystron amplitude and phase control systems, and fiber optic links. ICRF System ----------- Three new 2 MW FPA tubes are expected to be shipped from the vendor by the end of August. Issues with new filaments supplied to the vendor by their supplier have delayed tube delivery. The controls and power subsystem requirements for the fast ferrite tuning system were reviewed and plans made to prepare this system for testing. The fast ferrite tuner should be capable of maintaining a match to the ICRF antenna during rapid variations in plasma loading. RF simulations of the new 4-strap antenna design have begun. Diagnostics ----------- The C-Mod MSE optical system includes 9 components which face along the radial magnetic field (six lenses, one vacuum window, and two photoelastic modulators) and two which are aligned with the vertical field. All but the PEMs are composed of low Verdet glass to minimize Faraday rotation. The PEMs are composed of fused silica with a Verdet constant of 3.84 rad/T/m. Previous laboratory measurements of the Verdet constants of spare MSE lenses suggested that most of the optical elements have a low Verdet constant of order 0.04, although one (spare lens L4) had a Verdet constant of 0.37. Previous calculations based on these Verdet constants and on the radial field expected at the position of each of these optical elements suggested that the polarized light measured by MSE could be rotated about 0.4 degrees in high-current C-Mod plasmas, which corresponds to a deviation in the actual magnetic field pitch angle of 2.3 degrees. About 70% of the expected rotation occurs in the PEMs and the remainder in lens L4. To measure Faraday rotation in the MSE optics, a small radial field (+/- 60 Gauss at the PEMs) was generated by passing +/- 300 amps through the EF4 upper coil. A linear polarizing film was positioned in front of lens L1 and the diagnostic was illuminated by an LED array positioned along the DNB trajectory. A change of 0.007 and 0.022 degrees was measured on two datasets between the +300 and -300 amp configurations. Scaling up to the radial fields expected in high performance plasmas, we would expect a Faraday rotation of 0.04-0.12 degrees (MSE frame of reference), corresponding to a pitch angle change of 0.2-0.7 degrees in the plasma frame of reference. These measurements indicate that the actual L4 lens installed in the machine is made of the proper low-Verdet glass. Furthermore, the effect of the PEM's is about half of what was expected, indicating that a more rigorous derivation of the PEM Faraday properties is warranted. The measured changes in polarization angle between the +300 amp and -300 configurations were only comparable to the drift in measured angle observed with zero EF4 current throughout the course of the experiment. Thus, there is order-unity uncertainty in these measurements. Further measurements are planned next week to reduce the drift and improve the overall measurement precision. Travel and Visits ----------------- Justin Burrus (GA) visited from 8/9-8/12 to collaborate on a common version of PreTransp, a set of IDL tools used to prepare data for the TRANSP code. Nick Setzer (U.Md) visited from 8/12-8/13 to work on an MDSplus aware version of the gyrokinetic stability code gs2. This work is aimed at making gs2 a "service" available on the fusion grid. Manfred Bitter and Sang Gon Lee from the Korea Basic Science Institute visited Alcator C-Mod from August 9-11 to calibrate a new X-ray crystal spectrometer, which consists of a spherically bent mica crystal and a CCD camera. The instrument will record spectra of helium-like neon at 900 eV, and it will be used to measure the plasma rotation in the edge of C-Mod plasmas. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly