Alcator C-Mod Weekly Highlights May 29, 2007 FY2007 weeks of research operations: tentative target: 15 weeks Completed: 4.91 weeks Operations ---------- Plasma operations continued at Alcator C-Mod last week. Three run days were completed. A total of 69 plasma discharges were produced with a startup reliability of 83%. This week's experiments supported miniproposals in the Lower Hybrid Physics and Operations (cryopump) areas. Friday's run marked the first use of the C-Mod upper divertor cryopump to control plasma density. These experiments contribute to our third quarter milestone "investigate particle and density control in tokamak plasma operation". Plasma operation is planned to continue this week. Operation Details ------------------ The runs on Wednesday and Thursday supported lower hybrid physics studies. Efforts to increase the lower hybrid power and improve the coupling resulted in our first demonstration of lower hybrid-driven H-modes. Coupling during the H-mode phase exhibited ~30% reflection coefficient, compared to ~20% for the L-mode target. The net lower hybrid power at the L-H transition was about 600kW. Additional data were also obtained for MP#468 "LH-driven fast electron diffusion time measurement"; this experiment comprises part of the thesis research of an MIT graduate student. Square-wave modulation of the lower hybrid power was employed to study fast electron diffusion, as measured using spatially-resolved hard xray emission. Data were obtained with different phasing (N-parallel spectra) for varied target density and plasma currents. On Friday we successfully commenced plasma operation of the new C-Mod upper-divertor cryopump system. The C-Mod cryopump features a unique design with slotted inputs to the baffled upper chamber, designed to permit pumping in a range of plasma configurations. Initial no-plasma tests demonstrated a pumping speed of 10,600 l/sec for D2 gas, which exceeds the benchmark of 9700 l/sec obtained last September with the full-scale prototype in a test chamber. We then proceeded to plasma operation, and demonstrated density control in upper null, double null, and lower single null configurations. Pumping throughputs of ~70 Torr-l/sec were obtained for lower single null discharges with the magnetic balance parameter SSEP set to -6mm, providing excellent density control at nebar~1.6e20/m^3. The pumping throughput for a similar upper null discharge (SSEP=+15mm) was estimated at ~140 Torr-l/sec. A strongly lower null biased configuration, SSEP< -25mm, exhibited negligible pumping. These results are very encouraging and suggest that the upper chamber tile geometry and baffling structures are performing as intended. ICRF Systems ------------- Further testing of the J-port antenna suggests the neutral pressure limit, previously observed at ~0.3 mTorr, has degraded to ~0.1 mTorr. Multipactoring in D2 did not improve the antenna performance. Below 0.1 mTorr, high power operation is possible, but little power can be coupled for higher pressures. Higher pressure tends to occur after boronization which could explain the observation of degraded performance after recent boronizations. Lower Hybrid System -------------------- New water loads were successfully installed on several channels. Work was done to repair a fault in a lower hybrid high voltage supply module. The HVPS was then successfully tested and run for numerous high power pulses this week. Lower Hybrid was used on two days last week in support of mini-proposals. It was necessary to reset some of the Coupler Protection System (CPS) trip levels to facilitate operation at 120 degree (low n-parallel) phasing. Long Pulse Diagnostic Neutral Beam System ------------------------------------------ All DNB systems are now operational at full parameters. A small change in the beam current interlock has allowed us to safely increase the beam current to over 6.5A. Diagnostics ----------- On Saturday the tokamak was backfilled with D2 for Raman calibration of the Thomson Scattering diagnostic. The wide view CXRS poloidal optics were removed from the F-top port for recalibration on the bench. The new spatial calibration will make it possible to rotate the optical system by 90 degrees so that the DNB position and width can be measured. This facility will be used to check the alignment of the optical system with the beam. Travel and Visitors -------------------- Steve Wukitch made a presentation at the ITER Hot Spot and Faraday Screen meeting via teleconference on May 24th. John Wright represented C-Mod at the IAEA Steady State Operations Technical meeting in Daejeon, South Korea, during the week of May 14. He presented an overview of LH operation on C-Mod as well as the state of modeling and simulation, using material provided by Ron Parker, Greg Wallace, Andrea Schmidt, Paul Bonoli, Randy Wilson and Ernie Valeo. The presentation was well received with several questions and comments. The similarities of the operating C-Mod system to the planned KSTAR LH system were noted. There was some discusion of the status of LH on ITER (1/4 power system being considered). Stewart Zweben (PPPL) was at MIT last week to work on data analysis of the SOL turbulence MP and the new divertor GPI images with Jim Terry and Brian LaBombard. He also discussed simulations for the SOL turbulence MP with Bruce Scott (IPP-Garching), who is visiting PPPL. Randy Wilson (PPPL) was at MIT to participate in lower hybrid experiments which were led by MIT graduate students on Wednesday and Thursday. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly