Alcator C-Mod Weekly Highlights October 13, 2009 FY2010 weeks of research operations Base Target: tbd ARRA Target: 6 weeks Completed: 0.8 weeks Operations ----------- Plasma operations resumed last week at Alcator C-Mod. Four run days were scheduled. Computer and RF transmitter issues resulted in minor delays during operation on Tuesday and Wednesday, and Friday's run was shortened due to a delay in bringing the alternator up to speed. Overnight boronization was performed prior to Tuesday's experiement. A total of 76 plasma discharges were produced last week, with a reliability of 73%. The runs supported experiments in the ITER H-mode Scenarios, Boundary Physics and Transport areas. Plasma operations are planned to continue this week. Operations Details ------------------ Tuesday's run was devoted to MP#575, "ITER-like discharge rampdown scenarios". A baseline discharge was used that featured early xpoint formation in the ramp-up phase, developed previously under MP#577. The target flattop parameters were Bt=5.4T, Ip=1.3MA (q95~3.2), H-mode with P_rf=2-3 MW, with the L-mode target density nebar~2.1e20/m^3. Neon seeding was employed during flattop for some discharges. Ramp downs were obtained with different levels of ICRF during the ramp down phase, including L- and H-mode conditions. The rise in li during ramp down was reduced in the H-mode cases. Difficulties in obtaining reproducible starting conditions prevented us from performing a systematic scan of ramp down behavior, but a range of conditions was produced for comparison with simulations. It was found that the early diversion ramp up scenario is more susceptible to vertical instability than previously recognized, resulting in a number of early terminations. Wednesday's experiment continued the series of runs devoted to MP#570 "Boundary layer heat transport experiments in L-mode plasmas", which supports the 2010 OFES Joint Facilities Milestone "... measure the divertor heat flux profiles and plasma characteristics in the tokamak scrape-off layer in multiple devices to investigate the underlying thermal transport processes..." The major goals of this run were to obtain good-quality IR images of the divertor footprint, using a fixed strike-point program and scanning plasma current and input power; and to sweep the heat flux footprint across divertor Langmuir probes, including the probe on the divertor nose. Data were obtained at three target plasma conditions with varying RF power:Ip=0.5MA, nebar=7e19/m^3, Ip=0.8MA, nebar=1.4e20/m^3, and Ip=1.1MA, nebar=2e20/m^3. On Thursday we carried out MP#582 "Edge Turbulence and Blob Formation Study with GPI Measurement". This experiment comprises part of the doctoral research of an MIT graduate student. The purpose of the run was to develop a dataset of "blobs" for statistical studies using the newly installed 2D fast Gas Puff Imaging (GPI) diode array. The goal is to investigate the rapid changes of the poloidal propagation direction of edge turbulence and their correlation to blob ejection, the poloidal blob size - radial velocity relationship, and the spectral properties of the edge turbulence leading up to an L-to-H-mode phase transition and comparison to the developed H-mode turbulence. These experiments were carried out in ohmic L- and H-modes with a wide range of field (2.6 < B < 6.8T), current (0.4< Ip < 1MA), and density (0.3 < nebar < 1e20/m^3) . The run succeeded in obtaining the desired datasets. Preliminary assessment indicates that the turbulent fluctuation feature localized to the edge (rho~0.98, ~1-1.5cm inside the separatrix) propagates in the electron diamagnetic direction in all conditions obtained. This behavior is regularly seen in L-mode plasmas. A variation of the propagation speed very similar to that found previously was observed, but now with new information about the accompanying variation of the turbulence spectrum. The spectra of this edge feature shows a very strong dependence on plasma density (Greenwald-fraction), occasionally exhibiting a broad peak rather than following a power law decay. Friday's run continued experiments under MP#579 "Characterization and optimization of I-mode". Less than a half-day was accomplished because of the time required to debug a fault condition in the alternator drive motor electronics. We completed a q95 scan (varying current at fixed Bt=5.6T) at fixed nebar~1.4e20/m^3. I-modes were obtained up to 1.35MA (q95~3.0), although the H-mode threshold was found to be lower than for similar cases (at higher density) obtained in 2008. These experiments are planned to continue. Physics -------- Dennis Whyte presented an analysis of dynamic particle balance and fuel retention with cryopumped discharges. Generally the shot-averaged results agreed with the more accurate static particle balance. However a surprising result was that the real-time retention rate trended to near zero in current flattop with stationary plasma conditions. Detailed analysis, and comparison to similar experiments on NSTX and DIII-D, are being prepared for the 2009 Joint Facility DOE milestone. ICRF Systems ------------ The coaxial transmission line for the J-port antenna was reassembled to allow for current drive phasing of the antenna. All antennas are now operational. Lower Hybrid System ------------------- We continue to test the e-beam welding process to be used for the waveguide to transformer and transformer to coupler welds. A spare transformer and copper flange have been welded onto a WR187 waveguide, and the welds have been successfully leak-checked. The vendor will visit MIT this week to review the entire welding process and confirm plans for production welding. The cart that supports the launcher and allows it to be positioned radially in-vessel is now complete and ready for installation. A double flange extension has been set up and is ready to be used to leak-check the launcher in the lab before installation on C-Mod. Travel and Visitors ------------------- Orso Meneghini and Syun'ichi Shiraiwa participated in the COMSOL conference held in Boston Oct. 8-10. They presented an oral and a poster, and submitted a paper, describing their work on lower hybrid full wave simulations using the COMSOL software. They were awarded the "Best Poster Award" at the conference. Chuck Kessel (PPPL) visited MIT on Tuesday and served as Session Leader for the run on MP#575, "ITER-like discharge rampdown scenarios". Stewart Zweben was at C-Mod last week to help take GPI measurements for the runs, and to discuss the results from MP#561, which concerned the relationship of the SOL turbulence to the SOL width. He also began to test the new GPI cameras intended for installation during the next run period. Bill Rowan (UT-FRC) presented a seminar entitled "Simulation of turbulent impurity transport in Alcator C-Mod" at the the UT Plasma Physics Seminar. Matt Reinke attended the ADAS Workshop from October 4th-7th at Schloss Ringberg in Germany where he presented an overview of C-Mod spectroscopy and impurity transport studies. He discussed low-energy charge exchange modeling with collaborators from Madrid that should enable better understanding of C-Mod thermal C-X results. Jerry Hughes and John Rice attended the joint meeting of the ITPA Topical Groups on Pedestals and Transport & Confinement, held at PPPL October 5--7. Jerry presented highlights of recent L-H threshold, pedestal and ELM research on C-Mod, as well as a summary of progress and plans of TC-3, which is an ITPA multi-machine joint experiment addressing the low-density limit for H-mode access. Finally, he presented an update on plans for implementation of an expanded L-H transition database to be hosted at MIT, and led a working discussion with tokamak database experts and device representatives about the detailed implementation of this database. John presented summaries of the status and plans of Joint Experiments TC-9 (Scaling of intrinsic rotation with no external momentum input) and TC-14 (RF-driven rotation). Amanda Hubbard also participated in the meeting from MIT using remote conferencing tools. Robert Granetz attended an ITPA/IEA MHD meeting in Culham last week. The sessions concentrated on disruption issues, including toroidal and poloidal asymmetries of radiated power during gas jet mitigated disruptions, as well as disruption-generated runaway electrons. C-Mod recently had two run days devoted to studying the asymmetry characteristics, and the results were presented at the ITPA meeting. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly