Alcator C-Mod Weekly Highlights October 10, 2006 FY2007 weeks of research operations: Tentative target: 14 weeks Completed: 0 weeks Physics ------- Ongoing H-mode analysis has been focusing on scalings of the pedestal over an expanded parameter range, in particular higher Bt (up to 7.9 T) and Ip (up to 1.7 MA). The new data confirm that there is not a systematic scaling of pedestal widths with either Ip or Bt. Pedestal pressure gradients and heights are also independent of Bt, leading to approximately constant stored energy. The biggest apparent difference in high Bt H-modes is in the pedestal density, which in the range 5-8 T shows a decrease with Bt, or equivalently with q95, at constant Ip. Pedestals in a lower Bt range had scaled more closely with Ip. The lower ne leads to high temperature pedestals at 8 T, typically ~ 600 eV, and consequently lower collisionality. This may well explain the absence of steady EDA H-modes to date; while weak QC modes are seen in some discharges, most are ELM-free and therefore transient. One interesting discharge at low nu* showed large ELMs. These regimes will be explored with higher power in the next campaign. Stewart Zweben, PPPL, worked on the analysis of a GEM edge turbulence simulation of a C-Mod limited discharge (done by Bruce Scott of IPP Garching), and a BOUT edge turbulence simulation of a C-Mod diverted discharge (done by Maxim Umansky of LLNL). These simulations are being compared with data from the GPI and Langmuir probe diagnostics for these specific cases. Operations ---------- Preventative maintenance of the Hi-Yard 13.8 kV breakers was completed last week. These breakers feed all C-Mod high power systems either directly or through the alternator. New carbon brushes for the alternator field slip rings have been machined to spec and installed. The alternator is now back on turning gear. The F-Port horizontal flange was removed last week and is being modified to allow for the planned 7 degree toroidal rotation of the DNB. Lower Hybrid System ------------------- We continue refurbishment work on the klystrons. Preparations are underway to conduct another braze test as we prepare to manufacture new klystron cathode ceramic assemblies. The upcoming test will use new ceramic and Kovar components. We also continued work on the klystron test stand. The high speed switch has been tested at low voltage and has been connected to the 10:1 pulse transformer in order to generate high voltage pulses of up to 50 kV into a dummy load. This test stand will allow the operation of the klystrons to be quantified before installation in the C-Mod cell. ICRF Systems ------------ Fabrication of the new crowbar units needed for FMIT#1 and #2 has been completed, and testing of the units has begun. Interface wiring for the new units in the transmitter crowbar cabinets has also begun. Long Pulse DNB and Related Diagnostics -------------------------------------- An analysis of the effect of radial electric field on the pitch angles measured by MSE using measurements of poloidal rotation and ion temperature from CXRS has begun (MIT, PPPL, UTexas collaboration). Maintenance of the DNB high voltage power supply continued last week. Hi-pots of the transformer varistor and diode stacks discovered four diodes that needed to be replaced. A simulation of the stack is being developed to better quantify the transient behavior of this system. Travel and Visits ----------------- On September 27, Gordon Michaels, who was attending a conference at MIT, toured several facilities at the PSFC, including Alcator C-Mod. Mr. Michaels is the Chief Technology Officer for Energy and Engineering Sciences at Oak Ridge National Laboratory, and is working to gain a better understanding of the DOE Fusion Energy Sciences Program and how ORNL can help it in the future. Yijun Lin attended the US-JPN-EU workshop on RF technology and US-JPN workshop on RF physics in fusion plasmas in Nara, Japan last week (09/24-09/30). He gave a talk on 'ICRF and plasma facing components interactions in Alcator C-Mod.' Randy Wilson, PPPL, gave two talks, one on the LH system, and another on LH experimental results. John Wright also attended the meeting and gave a talk on edge issues in RF and the use of synthetic diagnostics for comparison between simulation and experiment. Dr. Joan Decker from CEA - Cadarache is visiting the PSFC and will be working with Ron Parker, Andrea Schmidt, and Paul Bonoli on the analysis of lower hybrid current drive (LHCD) experiments on Alcator C-Mod. He will be using the 2D Fokker Planck code, DKE, and a synthetic diagnostic in that code for hard X-ray emission. Dr. Yves Peysson, also from Cadarache will arrive at MIT on Oct. 9, 2006 to take part in this collaborative effort. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly