Alcator C-Mod Weekly Highlights August 3, 2009 FY2009 weeks of research operations Target: 9 weeks Completed: 2.2 weeks Discharges: 376 Operations ----------- Plasma operations continued on Alcator C-Mod last week. Four run days were scheduled and completed. A total of 96 discharges were produced, with startup reliability of 77%. The runs continued the ICRF commissioning process, and also supported two miniproposals, in the Boundary and ICRF physics topical science areas. The runs on Tuesday and Wednesday continued conditioning of the ICRF systems. The J-port antenna was operated up to 3MW into plasma, and combined power from all three antennas up to 4MW was successfully coupled. Some PLC control issues remain for the J-port system, and a work-around was implemented while these are being addressed. H/H+D was observed to rise during operation from 6%, which is acceptable for H-minority heating, to as high as 25%, resulting in low single-pass absorption. One and a half run days on Thursday and Friday morning were devoted to MP#562 "Characterization of the decay time of the fast-ion tail". This experiment comprises part of the thesis research of an MIT doctoral candidate. The experiments aim to investigate an observation made last year using the Compact Neutral Particle Analyzer (CNPA) that indicated a faster decay time for higher energy tail ions (150-280keV) than for lower energy components (80-150keV), contrary to expectations if charge exchange, rather than transport, were the dominant loss mechanism for fast ions. Moreover, the observations have consistently higher than expected signals corresponding to the more energetic tail populations. The present experiments provide data to benchmark simulations incorporating fast ion orbit effects and diffusion. Square-wave modulated ICRF is employed to permit phase-sensitive detection of the evolution of the tail signals observed on the CNPA. Scans of plasma current and density were performed, using both deuterium and helium majority plasmas. Indications of a dependence of the decay time on current was observed in helium at lower density. These data are being analyzed. The second half of the run on Friday was devoted to obtaining preliminary information related to MP#563 "Toroidal asymmetry of impurity seeding in support of ITER divertor gas injection requirements", proposed in collaboration with the ITER organization. The goal was to perform some scoping studies on the toroidal variation of divertor radiation in response to nitrogen puffing. N2 was introduced through two toroidally separated divertor capillaries, and the drive pressure adjusted to optimize signals. NII and NIII emission was observed spectroscopically at the K-port divertor. The initial experiments were carried out in helium plasmas, which permitted use of the Lyman-alpha arrays, also located at K-port, to provide spatially resolved measurements of NV emission. This also permitted an assessment of the detachment threshold in helium plasmas. Despite the 144 degree difference in the toroidal location of the capillaries, these preliminary results indicate no significant difference in the resultant nitrogen level at the viewing location. We successfully determined the satisfactory range of drive pressure and pulse timing for use in future experiments addressing this MP. These discharges also provided data for MP#534 regarding the sensitivity of the divertor Lyman-alpha detector system to NV radiation. Plasma operations are planned to continue at Alcator C-Mod this week. ICRF Systems ------------ Commissioning of the ICRF systems continued last week, with up to 3MW delivered through the J-port antenna, and up to 2.5MW from D- and E-port antennas. The ICRF system was operated in support of experiments on Thursday and Friday. Lower Hybrid System ------------------- Work continued on the fabrication of the new launcher's four-way splitters. Three more splitters returned from the vendor last week with e-beam welding of the forward wavegueide/window assembly completed. One of the twelve assemblies exceeded the allowed waveguide distortion specifications and will be replaced. DNB System ---------- Troubleshooting of the DNB arc power supply continued. Diagnostic Systems ------------------- An absolute calibration of the ECE diagnostic was completed. Travel and Visitors -------------------- Ron Parker hosted a visit by Dr. Young-soon Bae from KSTAR and a graduate student, Ms. Heejin Do from Pohung University. They were interested in learning details of the new C-Mod LH coupler design with an eye toward adopting our design concept for use in KSTAR LH experiments. There were also discussions about possible C-Mod - KSTAR collaborations. Greg Wallace visited PPPL on Tuesday and Wednesday. He presented a seminar "Observations of Lower Hybrid Wave Interactions in the Scrape-Off-Layer of a Diverted Tokamak". Randy Wilson (PPPL) visited MIT last week to work with Paul Bonoli on simulations of C-Mod lower hybrid current drive experiments. Tom Fredian and Joshua Stillerman met with Gabriele Manduchi from the RFX experiment (Padova, Italy) to discuss MDSplus software development, including object based interfaces, refinement of long pulse ideas and a new event notification scheme. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly