Alcator C-Mod Weekly Highlights Jan 23, 2012 FY2012 weeks of research operations Target: 17 weeks Completed: 1.8 weeks Plasma Shots: 331 Operations ----------- Plasma operations resumed at Alcator C-Mod last week. Four run days were scheduled and two and a half were completed. A total of 50 plasma discharges were produced with a reliability of 67%. Runs supported experiments in the Diagnostic Development, Transport Physics, and Lower Hybrid Physics tasks. Run time was lost on Thursday due to a vacuum problem, and a boronization planned for Thursday night and a boronization recovery and ICRF antenna evaluation experiment planned for Friday were postponed in order to investigate and remedy the problem. The cryopump was successfully used for density control during Wednesday's experiment, marking the first operation of the cryopump during the FY12 experimental campaign. Plasma operations are planned to continue this week. Physics ------- The paper "Scaling of H-mode threshold power and L-H edge conditions with favorable ion grad-B drift in Alcator C-Mod tokamak" by Y. Ma, et al., has been published in Nuclear Fusion, Vol 52 (2012) 023010; the paper is now available on-line at http://stacks.iop.org/0029-5515/52/023010. The paper "Search for zonal flows in the edge turbulence of Alcator C-Mod" by S.J. Zweben, J.L. Terry, M. Agostini, R. Hager, J.W. Hughes, D.C. Pace and the Alcator C-Mod Group, has been published in Plasma Physics and Controlled Fusion. Operation details ----------------- Tuesday's experiment was devoted to MP#607, "Optimize the Gate Timing for the Thomson Scattering Data Acquisition System" and MP#296, "Confirmation of Thomson scattering density calibration using electron cyclotron emission cutoffs". The first Thomson scattering signal from the upgraded TS lasers was obtained during the run. The main purpose of this run was to verify the correct timing of Thomson signals and data acquisition gates during plasma operation, and to begin optimizing alignment of collection fibers with respect to the laser beams. This information will allow us to optimize integrator gates to maximize S/N for the system as a whole. Efforts to correctly align collection optics with the beam image appear successful, although the collected photons are smaller in number than expected, indicating losses somewhere in the system. Maintenance days will be used to identify and remedy these losses. Tuesday's run ended with two ECE cross-calibration discharges needed to obtain initial density calibration coefficients for the core TS. During the two subsequent run days, the Thomson system was available with two lasers, each operating at 50 pulses per second, up to the full length of plasma flattop duration. On Wednesday we carried out experiments under MP#609a "Dependence of momentum and particle pinch on collisionality, R/Ln and q". This MP supports ITPA Joint Experiment TC-15, and employs various modulation techniques to separate diffusive and convective terms in the momentum transport. Wednesday's experiment, which was led by Tuomas Tala of VTT (Finland) aimed to exploit modulation of the transport regime between L-mode and I-mode, but conditions were not conducive to achievement of I-mode, so modulation of the ICRF power in upper-null L-mode targets was performed. Modulation of the rotation was observed with an RF power modulation amplitude of 1.7MW (either 0-1.7MW or 1-2.7MW) and disappeared at a lower amplitude of 1.2MW. A scan of density from 0.85 < nebar < 2.2e20/m^3 was carried out at fixed q95, and scans of plasma current (q95) were carried out at the intermediate and low density. Finally, an attempt was made to reproduce and extend previous results based on modulation of the geometry between lower and upper null, but these experiments exhibited high disruptivity. Thursday's run was devoted to MP#605, "LH2 Lower hybrid Launcher and Conditioning". This was the first plasma run day of the 2012 campaign for the LH system. This was also the first day of operations for the Transmitter Protection System (TPS) upgrade. Overall the run was very successful despite a delay to fix a vacuum leak. The new LH limiter, which moves with the launcher, seems to have somewhat reduced reflection coefficients, resulting in nearly fault free operation. There were no TPS faults during the run, and very few Coupler Protection System (CPS) faults; the CPS faults occurred mostly when the launcher was in its most retracted position, due to out-of-range reflection coefficients. Moving the launcher in helped reduce the number of faults. Power was limited to about 300 kW due to lack of klystron calibration at higher beam voltage and also lack of time to push power higher. ICRF System ----------- We successfully operated the E-port antenna with the mechanical tuning elements last week. We plan to operate in this configuration until the Fast Ferrite Tuner FFT#2 is repaired. We completed disassembling FFT#2 and have documented the damage from arcing and nearly 5 years of operation. In addition to the arc damage, we found damage to the permanent magnet, blocked cooling lines on the electromagnet, and restricted flow on some paddle (center conductor) cooling lines. All the cooling lines have been restored to nominal flow and the arc damage has been polished out to an acceptable level. Alan Binus and Stephen Wukitch discussed repair options with the manufacturer, and we plan to remove two rows of ferrite tiles. This should not adversely affect the tuner performance. We plan to monitor the condition of the tiles after each campaign to document their status. Steve Wukitch met with a vendor's representative to discuss additional testing of plasma sprayed tungsten material on antenna components. Lower Hybrid System ------------------- The LH transmitter protection system (TPS) upgrade is now operational. A few issues still need to be addressed before operation at high power for long pulses. The empirical collector heat capacity must be determined based on downstream thermocouple time response. Also, bi-directional communication between the TPS and the coupler protection system (CPS) must be established to ensure that a persistent fault condition is not generated by asynchronous TPS and CPS faults. Diagnostic Neutral Beam System ------------------------------- The beam operated normally into plasma last week, with data being taken by the new spectral diagnostics in the beam duct to measure the beam component fractions and spatial profiles of each component. Diagnostics ----------- The waveguide for the FRECE high frequency channels was repaired during the week of January 9. These channels continued to work through last week. The low frequency channels operated intermittently at the beginning of the week. This appears to have been due to waveguide problems similar to those identified for the high frequency waveguide. The low frequency waveguide was repaired. The low and high frequency channels were calibrated on Thursday. As planned, a new blocking bar system was installed in the core CXRS diagnostic in place of an interference filter to improve detection for the Fast Ion Diagnostic. Data were acquired with this system for the first time with ICRF and with LH. Analysis of the BES spectrum and analysis for fast ions is proceeding. The MSE push-pull system was successfully repaired before plasma operations on Wednesday. The Intershot Calibration System (ISC) is now operating on a routine basis in a semi-manual mode: between C-Mod plasma shots, an operator must manually trigger the ISC system to acquire a calibration point. We are writing software to evaluate the effectiveness of the ISC system in compensating for calibration drift of the MSE diagnostic over the course of a run day. The C-Mod Polarimeter has been successfully upgraded from one to three chords. Strong signals have been obtained on all probe and reference detectors, and noise levels of somewhat less that 1 degree have been obtained. The signal-to-noise is limited primarily by spurious reflections in the optical system which are being reduced by the addition of absorbers and realignment of some components. Travel and Visitors -------------------- Julien Hillairet and Mélanie Preynas, who are visiting MIT from the Tore Supra group at CEA/IRFM in Cadarache, France, presented PSFC seminars, entitled "CEA/IRFM and TORE SUPRA 2011 key facts - RF aspects" and "Nonlinear effects in the Lower Hybrid (LH) wave coupling with tokamak plasmas" respectively, on Tuesday, Jan 17. M. Preynas also participated in the Lower Hybrid experiment on Thursday. Dr. Orso Meneghini also participated in Thursday's experiment using remote collaboration tools from Italy. David Brower and Weixing Ding, UCLA, visited last week and worked with Will Bergerson, Peng Xu, and Jim Irby on the C-Mod Polarimeter as part of the MIT/UCLA collaboration. All are working to commission the new 3 chord system. Tuomas Tala from the VTT Technical Research Center of Finland visited from Jan.12-19 and was session leader for MP 609a on Wednesday. On Tuesday January 17, Brian LaBombard traveled to Aachen, Germany to attend a program committee meeting for the upcoming Plasma Surface Interaction Conference (PSI20). This conference will be held in Aachen on May 21-25, 2012(https://www.congressa.de/PSI2012/). Bruce Lipschultz traveled to Juelich Germany to attend the ITPA SOL/Div meeting January 16-19. While there he gave three talks for the C-Mod team. The topics were 1) Divertor heat flux widths in C-Mod; 2) Divertor heat loads (profiles and time dependence) on Alcator C-Mod; and c) The operational issues associated with tungsten tiles in Alcator C-Mod. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly