Alcator C-Mod Weekly Highlights September 17, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 14.7 weeks The up-to-air period continued at C-Mod last week. Besides in-vessel work, a phase calibration of the lower hybrid launcher was carried out, and a borescope inspection of the E-Left TF arm, leg, and sliding joints was completed. Analysis of data from the last run campaign continues. Physics ------- On-going analysis of spectra from the new spatially resolving x-ray crystal spectrometer, an MIT/PPPL collaboration, has demonstrated the ability to calculate Te profiles from argon soft x-ray line ratios. At moderate temperatures, and core Te < 2.5 keV, these results are in very good quantitative agreement with profiles measured by Thomson scattering and ECE. This work helps to verify excitation rates generated by Ming F. Gu, LLNL, using his Flexible Atomic Code which will now be used to calculate charge state density profiles from the experimental spectra. In the near future, time evolving spatial profiles of highly ionized argon charge states will be made routinely available to aid impurity transport and impurity peaking research on Alcator. This diagnostic has also shown that in discharges with high temperature (Teo > 4 kev), and low density (ne ~ 1e20/m^3), there can be significant separation of Ti and Te in the central portion of the plasma. These results are part of the thesis research of two graduate students, Matt Reinke, and Alex Ince-Cushman. Further experiments performed for MP 468, as part of the thesis work of Andrea Schmidt, confirmed trends in LHCD deposition that had been seen previously, and also expected, based on physical principles. The first trend is that higher n|| phasing leads to broader hard xray profiles, which indicates more off-axis current driven. This has now been shown for both low and high plasma current cases (500 kA and 800 kA). The second trend is that higher total plasma current leads to broader hard xray profiles, indicating that more off-axis current is driven. This result has now been demonstrated for both n||=1.6 and n||=2.3. Liang Lin, another C-Mod graduate student, has been analyzing turbulence measurements during ITB plasmas. The upgraded phase contrast imaging diagnostic has shown, when density peaks, turbulence in the frequency range of 20-60 kHz mainly propagates in the electron diamagnetic direction and is not edge-localized. These two features are consistent with TEM in ITB plasmas predicted by modelling carried out by Darin Ernst. Further gyrokinetic analysis is being carried out. In-vessel Work -------------- Monday and Tuesday of last week were spent purging the vessel with room air to neutralize heavy boron hydride compounds and make the vessel safe for entry. On Wednesday we had the first personnel entry, and an extensive set of in-vessel pictures were taken to document the status of the vessel walls and diagnostics. Steve Scott, PPPL, and Jinseok Ko, a C-Mod graduate student, have begun inspections, calibrations, and tests of the MSE diagnostic. Using a fixed reference polarized light source mounted inside the C-Mod torus, they demonstrated that credible changes in the temperature of the Photoelastic Modulators (PEMs) do not affect the angle measured by MSE. However, they did observe that heating the optics 'cannister' by ~25 Celsius changed the measured angle by up to 0.5 degrees in the MSE frame of reference, which corresponds to somewhat more than 1 degree in inferred pitch angle of a magnetic field line. They also observed some melt spots on the thin stainless-steel shim stock that seals the optics canister from unwanted stray light. These observations suggest that heating of the MSE optics canister is contributing to the lack of reproducibility of the MSE measurements. Further experiments and calculations are in progress to better characterize this issue. In-vessel measurements were made to verify that the lower hybrid launcher position calibration made during the last up-to-air was still valid at the end of the run campaign. TF Inspection ------------- We continue to investigate the TF ground-fault that appeared near the end of the last run campaign. The fault cleared approximately two weeks before the end of the campaign, but we continue to work to understand its cause. A small diameter borescope was inserted inside the retaining cylinder and used to inspect the TF arm, leg, and spring-plates near the fault location. Nothing that could be associated with a fault condition was found. We have also monitored the TF magnet resistance to ground during warm-up, but have had no recurrence of the ~1 Ohm resistance seen during plasma operation during several discharges. We will continue to monitor the TF resistance during the up-to-air period. Lower Hybrid System ------------------- The latest set of 4-way splitter test data, with power levels of up to 200 kW, using the new detector diodes, indicates very good agreement with our simulation code. These results give us confidence in our ability to design the splitters for the new lower hybrid launcher. Following the position calibration, the lower hybrid launcher was removed from the C-Horizontal port so that measurements of relative phase across the columns of the couplers could be obtained for comparison with data taken during the last up-to-air. Results were very similar to those obtained earlier, except for one of the 24 columns, which displayed discrepancies several times larger than the others. The phase-shifter for this column had been replaced during the run campaign, which may explain the larger differences. Travel and Visits ----------------- Stewart Zweben, PPPL, was at C-Mod last week to continue analysis of the X-point GPI data and to discuss calibration and hardware issues. The angle of the X-point 'fingers' in the (R,z) plane was found to vary significantly over the field of view, but not much between various types of shots (USN,LSN,DN). The auto-correlation times and spectra are also not much different between the X-point region and the outer mid-plane GPI region. Arturo Dominquez visited Gerrit Kramer and Chung Kung at PPPL last week to make measurements of the attenuation of the curved reflectometry waveguides at 140 GHz. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly