Alcator C-Mod Weekly Highlights November 5th, 2007 FY2008 weeks of research operations: Target: TBD Completed: 0 weeks Physics ------- A Monte Carlo simulation of boron deposition onto the Surface Science Station's (S^3) quartz microbalances (QMB) during boronization has been developed by Roman Ochoukov, a C-Mod graduate student. Simulation results are consistent with boron ionic deposition at Ti ~ 1-2 eV. Finite Larmor radius effects allow deposition on surfaces parallel to the magnetic field. Ion heating at the Upper Hybrid (UH) resonance is found to be critical to explain observed boronization results. At the highest pressures, with the largest deposition rates, the boron ions become effectively de-magnetized, and the deposition on the S^3 QMBs is independent of orientation to the magnetic field. This result implies that previous deposition estimates based on QMB data and ion travel along field lines were low by as much as a factor of ten. Peak deposition rates as high as 3 nm/minute for surfaces at the UH resonance, regardless of orientation to the magnetic field, can be expected, implying an approximately 1.4 um boron thickness after 8 hours of boronization. Operations ---------- Following a successful hi-pot of the TF magnet, re-assembly of the TF bus has begun. Work on the new CRYO system thermocouple scanner has been completed and this system is now ready for operation. New and reconditioned breakers were installed for the alternator last week. Testing of these systems has begun. In-vessel Work -------------- The primary activities last week were in-vessel calibration and alignment of diagnostics in preparation for pump-down. MSE, CXRS, NESOX, bolometry, video cameras, pellet imaging, hard x-ray, visible bremsstrahlung, HIREX SR, and polarimeter systems have been prepared for pump-down. The vessel has also been carefully vacuumed and wiped down with both de-ionized water and alcohol to remove boron deposits from the last run campaign. Lower Hybrid System ------------------- The vendor continues to make progress on the rebuild of the #117 klystron. Fabrication of the new gun assembly is nearly complete. Tests of the klystron cavities indicate they are well tuned. A decision will be made soon on whether a new collector will be required or the old one can be repaired. The vendor continues to evaluate three other klystrons for rebuild. The lower hybrid launcher has been re-installed in the vessel horizontal port, torqued to specification, and the primary and secondary vacuum seals leak-checked successfully. Re-assembly of the waveguide runs feeding the 1st launcher is nearly complete, and the launcher's radial positioning system has been calibrated. ICRF Systems ------------ Ongoing installation of new demodulators concentrated on FMIT#3 and #4 last week. Instrumentation racks are undergoing a general wiring cleanup, and the cabling from preamplifiers to the transmitters is being improved. We continued refurbishment of phaseshifter and stub tuner components for the D-Port antenna last week, and they are now installed in the cell. Motional Start Effect Diagnostic -------------------------------- The Motional Stark Effect (MSE) diagnostic was re-assembled, re-installed, and re-calibrated last week by Jinseok Ko (MIT/PSFC) and Steve Scott (PPPL). Stress was imposed onto the in-vessel MSE optics canister using a band clamp, to determine whether the new, strengthened 'rings’ that support the MSE lenses isolate them from external stresses. We observed no change in the measured polarization direction as external stress was applied, confirming that the rings do not transmit stresses to the lenses. This result indicates a large improvement over the old design. The effect of thermal stresses on the MSE performance (through thermal stress-induced birefringence on the internal lenses) with the new ring design was also evaluated by heating the MSE optics canister in the vicinity of the lens positions. Although the rings no longer transmit their thermally-induced stress onto the lenses, we nevertheless observed significant changes in polarization angle as heat was applied. This effect is now understood to be the consequence of temperature gradients that develop inside the lens itself during the heating process, which then generates stress and birefringence. The lens near the 1st focal plane of the MSE optics can cause as much as 50 times the polarization rotation as those lenses where the rays do not form a focus and the entire surface of the lens is used. During the upcoming FY08 run campaign, we will rely on more extensive post-shot and between-shot MSE 'beam-into-gas’ calibration shots to compensate for the temperature-induced changes in polarization angle. Redesign of the optical system to reduce/eliminate the effects of temperature induced stresses has begun, with the aim of implementation prior to the FY2009 campaign. Travel and Visits ----------------- Stewart Zweben, PPPL, was at C-Mod last week to continue analysis of the x-point region GPI imaging data, and to plan hardware upgrades for the next run. Bruce Lipschultz attended the EU task force meeting on Plasma Wall Interactions in Madrid Spain, October 29-30, 2007, held at CIEMAT. He presented a review of the US boundary physics program plans for approximately the next 5 years based on contributions from the major tokamaks and labs involved. Bruce also attended the meeting in order to foster better communication between the US and the EU, particularly with respect to BPO work. Lastly, the gathering was used to plan the agenda for the next ITPA SOL/divertor group meeting to be held in January. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly