Alcator C-Mod Weekly Highlights August 16, 2010 FY2010 weeks of research operations Base Target: 13 weeks Base Completed: 12.4 weeks ARRA Target: 5 weeks ARRA Completed: 5.0 weeks Plasma Shots: 2211 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed. A total of 69 plasma discharges were produced with a startup reliability of 88%. Runs supported research in the Boundary Science and Lower Hybrid Physics topical areas. A fresh boronization was carried out on Wednesday night. Plasma operations are planned to continue this week. Physics ------- Simulation results of a discharge with lower hybrid current drive using the LHEAF code have been compared to the experiment. The full wave calculations exhibit a wide spread of the parallel wave number spectrum which fills the kinematically allowed domain. This effect ultimately results in a broader driven current profile compared to ray tracing codes. Comparison of the current profile inferred from the full wave simulations was found to have better agreement with the experiment than those based on ray-tracing. Operations Details ------------------ Wednesday's run supported MP#570 "Boundary layer heat transport experiments in L-mode plasmas", which contributes to the 2010 SOL Heatflux Joint Research Target. Density scans were completed at 7.9 tesla at two values of current, 0.8MA and 1.2MA. Data were obtained with the IR camera, embedded probes and thermocouples to monitor the heatflux profiles on the divertor plate, and with scanning probes to map out the upstream profiles. Gas puff imaging (diodes and camera) for turbulence measurements was routinely performed. We carefully monitored the heat flux profiles on the outer divertor as reported from embedded Langmuir probes and surface thermocouple sensors. In the sheath-limited regime, the profiles reported by these two separate measures agreed. An e-folding length of 2.8 mm in the common zone was a typical value for the 800kA dataset. As the divertor transitioned into the high recycling regime, heat flux profiles from the LPs and TCs started to disagree, with the LPs reporting higher heat fluxes, and the e-folding lengths observed by both measures got longer. It is clear from the thermal sensor data (surface TCs, tile TCs, calorimeters) that the LP data is not to be trusted for heat flux analysis in this regime. This behavior has been noted previously in 5.4 tesla shots at 0.55, 0.8 and 1.04 MA at similar densities normalized to the Greenwald density. The results at the higher current (1.2MA, 7.9T) were similar with regard to the divertor footprints recorded by the surface TCs and the Langmuir probes, but with the power e-folding length in the common zone being ~ 2.2 mm at the 1.2 MA sheath-limited conditions. Thursday morning was devoted to further conditioning and testing of the Lower Hybrid system, particularly at high n-parallel, along with reconditioning of the ICRF antennas following overnight boronization. The experiment on Thursday afternoon supported MP#468 "LH-driven fast electron diffusion time measurement", which employs modulation of the LH power. This MP contributes to the thesis research of an MIT graduate student. The production of the desired modulated waveform was demonstrated during the morning conditioning activity, but at higher target density and lower phasing than called for in the run plan. During the afternoon, modulated data, albeit with some faulting, were obtained at 135 degree phasing (n||~2.9) at nebar~1.2e20, along with some additional data at n||~3.1. Friday's run was devoted to MP#613 " LHCD Effectiveness in high Te and confinement plasmas". Varying levels of ICRF power were applied to 1.1MA, 5.4T, USN plasmas with nebar near 1.2e20/m^3, which is just above the density at which fast electron generation is observed to fall non-linearly with density in ohmic targets. Lower hybrid power at 550kW with 90 degree phasing was applied during the ICRF pulses. Up to 2.5MW of ICRF from the J-port antenna was coupled to these L-mode discharges. Central Te0 was increased to 4.5keV, but Te(r/a~0.9) increased only to ~460eV at this power. Preliminary analysis of non-thermal ECE emission shows that the typical non-linear decrease with density is seen. The fall-off is at higher ne than observed in LSN with 800 kW of LH in recent experiments. Comparing different ICRF power levels, there seems to be a modest trend toward higher ECE radiation temperature with higher Te. Hard X-ray camera data were obtained on all shots, and await analysis for a more accurate estimate of the effect. We were unsuccessful in obtaining data in the DN configuration, but did succeed in obtaining ohmic target data at 1.1MA in the inner-wall limited configuration, which will be useful for comparison with 800 kA shots obtained last week. The same trends were seen, with significantly higher non-thermals at high ne than in USN, and moderately higher non-thermals at the higher Ip. ICRF Systems ------------ Vacuum conditioning of the J-port antenna was carried out prior to plasma operation last week, to test the transmission network and to increase the voltage handling. The antenna conditioned more rapidly than in previous attempts since the last vacuum break, but the antenna is still evolving significant gas. We operated into plasmas in piggyback fashion to continue plasma conditioning. In USN discharges, we coupled 2.5 MW from J-port, but had difficulty with screen supply issues at 3 MW. The screen supply will be checked for proper operation during the next maintenance day. Four of the six remaining waveguide sections needed for the SOL reflectometer have had flanges brazed and are ready for installation The remaining flanges will be brazed once final measurements are made. ARRA Activities ---------------- The electronic racks for the new ICRF Fast Ferrite Tuner (FFT) systems, purchased under ARRA funding, have arrived onsite. Lower Hybrid System -------------------- The LH system was used in support of C-Mod runs on Thursday and Friday. Attempts to run LH power modulation experiments on Thursday were met with difficulty due to high reflections at the desired n|| (2.3) and spurious faults detected by the coupler protection system during the fast changes in LH power. Friday's experiments explored the efficiency of LHCD in high temperature, high density discharges. The LH system was operated in conjunction with the E- and J-port ICRF antennas. DNB Systems ----------- The Diagnostic Neutral Beam was operated into plasma in support of experiments on Thursday and Friday. Diagnostics ----------- The PPPL/MIT diagnostic team for the C-Mod Motional Stark Effect (MSE) diagnostic has implemented a new "numerical lockin" software procedure that is resistant to drifts in the frequency and phase of the underlying hardware, namely the photo-elastic modulators. This new software eliminates problems observed in calibration data taken during June. The orientation of the transmission axis of a linear polarizer used in the MSE calibration has been calibrated with respect to gravity with an accuracy of a few hundredths of a degree, and the calibration analysis has been completed. MSE is now measuring magnetic field pitch-angle profiles in Alcator C-Mod with an absolute calibration. Initial efforts are focused on determining the magnitude of measurement errors due to thermal drift. First data from the C-Mod polarimeter was obtained last week. Initial analysis indicates good agreement with simulations generated from efit and Thomson scattering density profile data. The measured rotations scaled with plasma current and density as expected. Small shifts in the FIR laser wavelength caused by the C-Mod magnetic fields resulted in high vibrationally induced noise levels. This problem is being addressed. Travel and Visitors ------------------- Aaron Bader traveled to Oak Ridge National Labs to work with Fred Jaeger and Lee Berry on running simulations to model Compact Neutral Particle Analyzer (CNPA) fast ion data. Fred demonstrated how to set up and run simulations, and Aaron informed him and the rest of the SCIDAC group about the diagnostic capabilities and limits. They also continued working with Bob Harvey on a new version of CQL3D that includes a synthetic NPA diagnostic that can be used to compare with CNPA signals. Lastly, they went over the framework to allow time-dependent simulations to model the rise of the CNPA signal through steady state and turn-off. Yijun Lin taught courses on "ICRF theory and experimental applications on tokamaks" at the 4th Chinese Summer School on Plasma Physics in Shanghai, China. On Monday 8/9/2010, Arturo Dominguez traveled to PPPL to discuss plans to use the FWR2D code to analyze data from the correlation reflectometry system (a C-Mod-PPPL collaboration). He had meetings with Gerrit Kramer, Ernest Valeo and David Mikkelsen during which they discussed the use of FWR2D, as well as the possible ways to compare the modeling with GK simulation results using GYRO. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly