Alcator C-Mod Weekly Highlights May 5, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 13.8 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Four run days were scheduled and three and a half were completed. A total of 108 plasma discharges were produced with a reliability of over 90%. The runs supported experiments in the Lower Hybrid and ICRF Physics, Transport, Rotation, H-mode Integrated Scenarios, and Plasma Operations research areas. About five hours of run-time was lost on Friday due to a combination of power system and computation/data acquisition delays. Additional experiments planned for Friday afternoon had to be deferred. A total of about 90 minutes of experimental time was lost earlier in the week due to delays associated with modifications to the digital plasma control system. This week is a scheduled maintenance week at Alcator C-Mod. Plasma operations are planned to resume next week. Operation Details ----------------- Tuesday morning's run was devoted to MP#551 "Penetration of accessible LH Waves" and MP#541 "LH wave damping with plasma temperature". Density scans using ramped waveforms from 0.5 < nebar < 1e20 and 0.9 < nebar < 1.6e20/m^3 were obtained at 75 and 90 degree phasing with toroidal fields of 5.4 and 6.0T. The hard xray (HXR) emission as a function of density is close to a straight line with a negative slope, although closer inspection shows that there is a knee in the curve in each shot around nebar=6.5e19. This is where the slope was observed to change from something proportional to 1/n to a steeper slope in previous experiments. A scan from high to low density resulted in data which overlaid almost exactly on a scan from low to high density. A power scan with shots at 500kW and 800kW resulted in data consistent with the other scans at 700kW. The change to higher field did not increase the HXR count rate for any density at either phasing. Data from the day show remarkably little variation across different values of launched n|| and toroidal field when normalized to the square root of net LH power. Analysis of these results is ongoing. The experiment on Tuesday afternoon supported MP#541 "LH wave damping with plasma temperature". An ICRF power scan from 1MW to 4MW (counter-current phased MCCD) was conducted at 75 and 90 degree LH phasing. Te0 of up to 5keV during LH was obtained. Overlaps in the timing of the ICRF and LHCD waveforms of > 100msec provided sufficient data for analysis of profiles of HXR emission. Preliminary analysis indicates a wider HXR emission profile at the higher temperatures. The first half of the run on Wednesday was devoted to MP529 "Toroidal Field Dependence of the Low Density Limit of the H-mode Threshold with ICRF". We started the run with 50 MHz central H minority heating at 3.3 T and did a scan in density down to nebar = 7 x 1019 m-3. The low density limit was at about nebar = 8 x 1019 m-3 just as previously observed at 5.4 T. This was confirmed by running shots at 5.4 T with D and E port antennas operating at 80 and 80.5 MHz for central H minority heating and found the same results as at 3.3 T. This is in contrast to previous Ohmic H-mode results, which indicated that the low density limit decreased roughly linearly with decreasing toroidal field down to 2.2 T. These results imply that there may be a difference in the low density limit depending on the heating scheme. The second half of the run on Wednesday was dedicated to MP#517 "Experimental test of a Kalman filter for the vertical position". This work comprises part of the thesis research of an MIT graduate student. Subject discharges for the experiment were high elongation (kappa > 1.8) lower single null equilibria near the stability limit for C-Mod. A Kalman filter designed using simulations of similar equilibria was applied to the vertical position observer used for the fast vertical stability control. Initial experiments indicated a reduction of the rms amplitude of the control demand, but also a reduction in controllability at the the highest elongations, relative to the unfiltered system. Increasing the derivative gain in the feedback loop resulted in a decrease in the amplitude of vertical oscillations with the Kalman filter in the loop. Operation with the same derivative gain without the Kalman filter resulted in increased high frequency oscillations. The run on Thursday was devoted to MP#472 "Continued studies of turbulence in low density ohmic plasmas", which comprises part of the doctoral research of an MIT graduate student. The high-k (k_R < 60 cm^-1) configuration of the phase contrast imaging (PCI) diagnostic was employed to study turbulence and transport in low density ohmic plasmas where the main thermal energy loss is through the electron channel. At B = 5.2 T and 0.8 MA, the density was scanned from ~0.3 < nebar < ~1.3e20 m-2, which covers both "linear" and "saturated" ohmic regimes. Electron and ion temperature profiles were obtained for gyrokinetic simulation and transport analysis. Detailed analysis of the fluctuation measurements are in progress. The run on Friday morning was devoted to MP#535a "Rotation inversion vs density and current, in limited and diverted L-mode plasmas". Basil Duval (EPFL-Lausanne, TCV) served as Session Leader using remote communications tools. The purpose of this experiment was to map out the rotation inversion (where the intrinsic rotation changes direction) as a function of density and plasma current, and compare results with similar observations in the TCV tokamak. A density scan was first carried out in LSN discharges with current of 0.65 MA and a magnetic field of 3.0 T (q95 ~ 3). For a range of average densities between .35 and 0.7 10^20/m^3 no rotation inversion was found. Subsequently inner-wall limited discharges were run, and rotation inversions were observed with average density nebar~ .7e20/m^3. For higher and lower densities, no inversion was seen. The observed rotation inversion occurred inside of r/a=0.6 only, and seems qualitatively similar to the TCV results. Travel and Visitors -------------------- Alan Binus and Steve Wukitch visited the vendor for our high power ICRF amplifier tubes (4CM2500KG) to examine three expired tubes. We have agreed to perform some testing at C-Mod together with the calculations needed to investigate tube lifetime issues. While at the vendor's site, one tube was positively identified to have a cracked anode without additional melting and the vendor is attempting to identify the locations of leaks in the other two tubes. Ron Parker served on the DIII-D 5-year Program Review Committee at GA, April 28-30. Rui Vieira served on the DIII-D Facilities Review at GA on April30 - May 2. Catherine Fiore attended the semi-annual meeting of the Committee on the Status of Women in Physics in College Park on Tuesday April 29th. Catherine is the 2008 Chairperson. Amanda Hubbard participated in the 14th meeting of the Pedestal ITPA Topical Group in San Diego, April 30-May 2nd. She gave a presentation on "LHCD Effects on the pedestal in Alcator C-Mod". Jerry Hughes and Dennis Whyte participated remotely in parts of the meeting. Jerry made a presentation on H-mode pedestal modification by topology variation on C-Mod, and Dennis on "Exporing ELM control on C-Mod with n=1 perturbations from external coils". Stewart Zweben (PPPL) was at MIT last week to work on analysis of a BOUT run for comparison with the C-Mod edge turbulence imaging results, and to plan for future experiments. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly