Alcator C-Mod Weekly Highlights February 8, 2010 FY2010 weeks of research operations Base Target: tbd Base Completed: 3.3 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 971 Operations ----------- Plasma operations resumed at Alcator C-Mod last week. Three and a half run days were scheduled. Experiments supported research in the Transport and Boundary/Edge topical science areas, as well as the Diagnostic Development task. A total of 82 plasma discharges were produced with a reliability of 85%. During Tuesday's run, three hours of experimental time were lost due to problems with a driver circuit for the main gas fueling valve and with the breaker feeding the EF3 poloidal field power supply. An analog bypass circuit was installed to replace the valve driver and the breaker was replaced with a spare unit. Research operations are planned to continue this week. Operations Details ------------------- Tuesday's experiment in support of MP#593 represented the first attempt to trigger edge-localized modes on C-Mod by rapidly displacing the plasma equilibrium in the vertical direction, a technique that has been demonstrated on TCV, ASDEX Upgrade, JET and NSTX. Encouraged by successful modeling with the Alcasim control system simulator, a z-position modulation scheme was implemented. In the experiment we found that a high power 800kA EDA H-mode could be jogged vertically with a dZ of approximately 1cm, and with rise and fall times of 2--10ms. These jogs were maintained at a repetiton rate of 25Hz with no deterioration to the H-mode for 500ms. In ELM-free target discharges at Ip=1.1MA, our initial attempts to jog the plasma resulted in vertical instability. In none of these attempts were ELMs observed. We plan to continue this experiment in the coming week, concentrating mainly on lowering edge collisionality at modest current (Ip<=1MA). Wednesday's experiment supported MP#419 "Fiducial wall pumping measurements for comparing operating conditions". The purpose of this run was to ascertain the differences in wall pumping rates and retention dynamics when using the upper cryopump as compared to shots without cryopumping. A large set of experiments previously performed on wall pumping (fuel retention), using the static particle balance method, showed that retention scaled proportional to incident ion fluence and was likely occurring at divertor surfaces which were primarily cleaned of B layers. In contrast, analysis of previous experiments that tested the upper cryopump showed that most often the retention rate during flattop, diverted stages was approximately zero, in apparent contradiction to the physics interpretation of the static experiments. The intent of Wednesday's experiment was to explore the effect of cryopumping with added information gained from the dynamic particle balance. In low-density, Upper Single Null (USN) discharges the integrated retention was consistent with previous results, ~10 Torr-L retained out of ~20-30 Torr-L of fueling. However, at higher density, nebar>1.3e20/m^3, the edge recycling and cryopump pressure increased throughout the flattop despite constant core plasma parameters; this led to a constant rate of wall retention during this phase of the discharge. This result is inconsistent with our usual experience that the recycling is set by the core parameters. Furthermore, the cumulative retention in these higher density discharges was low (~5Torr-L), which contrasts with previous results in unpumped discharges which showed the retention to increase with increased recycling fluence to the wall. A shot-by-shot scan of the magnetic balance from strongly USN to marginally LSN indicated that the cumulative retention did not vary much, but the dynamic balance showed increasing wall retention as the equilibrium approached double null. Further analysis of these data is in progress. Thursday's run was devoted to MP#596 "I-mode and ELMfree H-mode Edge Fluctuation Measurements Using Gas Puff Imaging (GPI)". Attempts to study the electron diamagnetic directed (EDD) edge feature in ELM-free H-mode discharges were hampered by low signal to noise ratios, although further analysis of these data may be possible. In the second half of the experiment, I-mode discharges were produced in order to investigate the characteristic "quasi-quasi-coherent" (QQC) mode using the fast 2D GPI system; previously this mode has been observed using magnetics, PCI, and reflectometry. Preliminary results from the GPI data indicate this feature is characterized by k_pol < 2 cm^{-1}, 150kHz < f < 250kHz, and propagation in the EDD sense. GPI data were obtained during slow L->I transitions and on the radial location and profile of the QQC fluctuation. The I-mode edge in these plasmas also featured branching in the ion-diamagnetic-direction. The high performance I-modes obtained in the course of this experiment also exhibited ELM behavior. This discharge sequence also resulted in a new C-Mod record for stored energy (251kJ), with volume average pressure of 1.76 bar, during a transient ELM-free H-mode following an I-mode phase. A half-day experiment on Friday was devoted to MP#534, "Foil/AXUV bolometer calibration in helium plasmas". Argon and nitrogen seeding into helium majority plasmas was used. In addition to bolometer calibration, these discharges were also used to calibrate the effective crystal size in the HIREX-SR spectrometer looking at the He-like Ar transition. Spatial calibrations were carried out using a dynamic outer gap scan, with and without masking of the HIREX-SR crystal. Injections of tungsten using the laser blow-off system were used to isolate W lines on the new XEUS spectrometer, and to obtain impurity confinement times. The nitrogen-seeded discharges enabled comparison of the NV emission using the Ly-alpha cameras and CXRS spectrometer, and were also used to observe the He-like spectra with the XEUS. Lower Hybrid System -------------------- We continue efforts to qualify the clean-up procedure for the new LH launcher. High power testing, up to 125kW (corresponding to a power density of 300MW/m^2), was successfully carried out on previously contaminated window samples which had been cleaned using a chemical solution. High power testing was also carried out to qualify sliding terminations inserted into the guides for possible use in evaluation of the launcher prior to installation. Travel and Visitors ------------------- Paul Bonoli and Bruce Lipschultz traveled to Princeton Plasma Physics Laboratory last week where they served on the NSTX Program Advisory Committee. The Advisory Committee Meeting was held from February 3-5, 2010. On Friday, Congressman Edward Markey, (Massachusetts, 7th district) visited the PSFC, where he toured the C-Mod facility and had discussions about fusion research with Earl Marmar, Miklos Porkolab and Martin Greenwald. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly