Alcator C-Mod Weekly Highlights Feb 22, 1999 Plasma operations continued on Alcator C-Mod last week. Due to the Monday holiday, only three run days were scheduled, Wednesday through Friday. Operation following the first boronization proved more difficult than anticipated, with only 12 successful shots obtained on the first two runs. Reliability improved markedly on Friday, with 26 successful plasma shots obtained. EDA H-mode plasmas were reproducibly obtained at 800kA and 1MA, with performance similar to previous campaigns. The Transmitters #1 and 2 (D- and E-port two-strap antennas) were used to couple up to 2.8 MW into EDA H-mode plasmas at 800kA. However, the power feedback gain is insufficient to compensate for changes in total gain of the transmitter as the plasma loading changes, resulting in lower power and RF faulting for other target conditions. We attempted to modify the feedback during the run, but this was unsuccessful (resulted in power oscillations). This problem will be addressed during this week's operation. Temperature pedestal measurements made using small Bt ramps during EDA H-modes on Friday's run were quite consistent with those from the last campaign, with widths ~ 11 mm, and the foot of the pedestal located at the separatrix. The pedestal height was moderate (~ 350 eV) and varied with input power. Romik Chatterjee (UT-FRC), John Heard (AU) , and Perry Phillips (UT-FRC) completed the installation of the high spatial resolution ECE radiometer. Thirty of the thirty-two channels are operating with low noise, and shakedown data is being acquired at the "slow" 10 kHz rate. Qualitatively, this data is consistent with that expected of plasma temperature during current rise and sawteeth. Spatial localization of the channels is not consistent with expectations, and the cabling, filtering, and other problems which might contribute to this discrepancy will be investigated this week. Some CAMAC highway problems arose which interfere with use of the fast CAMAC channels. These do not affect shakedown yet and should be worked out in parallel with the rest of the shakedown activies this week. Romik Chatterjee returned to UT-FRC to assemble and test amplifiers -- two will replace the inoperative ones now installed, and the remainder will be spares. When these problems are resolved, development of the fluctuation measurement can proceed. Tuning FMIT#4 to 78 MHz is progressing slowly. We had Chris Brunkhorst (RF engineer from PPPL who has operated these transmitters to 80 MHz) visit and work with the MIT RF engineers on this problem. Further work with PPPL support is planned for the upcoming week. We also have contingency plans to begin operation of J-port antenna at 75 MHz so as to begin testing the rest of the system as soon as possible. The DNB high voltage power supply was operated successfully for the first time last week with up to 4 amps of current at 55 kV into a dummy load. Ten amps into the dummy load will be tried as soon as a modification to the dummy load is complete, and debugging of some of the instrumentation is finished. We continue to work on the DNB, new J-port antenna feedline installation, improving the reliability of the RF feedback systems, tuning of the #3 and #4 transmitters for operation at 78 MHz, installation of our new pump vent system, and installation of new diagnostics. Equipment for the lower-hybrid system is being returned from storage and options for system installation are being discussed. Physics: ------- The first H-modes gave interesing results for the two high resolution edge x-ray arrays, which view the soft x-ray emission of the edge plasma at the top and outboard midplane, In H-mode, a pronounced pedestal is seen on both arrays, as expected. The center of the pedestal is 10-12 mm inside the separatrix at the outboard midplane, consistent with previous measurements at the outboard midplane. However, it is located within 2 mm of the separatrix at the top of the machine. This implies that the x-ray emissivity varies significantly on EFIT flux surfaces just inside the separatrix. Previous measurements by J. Rice et al. found large up-down asymmetries just inside the last closed flux surface for hydrogen-like argon in Alcator C-Mod. Power-law fits to the reflectometry fluctuations power spectra show systematic differences depending on plasma conditions. If we assume flucuations power density S(f) is proportional to f^(alpha), the parameter alpha is significantly different in L mode plasma and H mode plasma. For ohmic L-mode, the spectra are usually flat for f < 100 - 150 kHz, -0.5 < alpha < 0.0, but steeper for f > 100 -150 kHz, where -2.5 < alpha < -1.5. In H-mode case (ELM free), the spectra show a robust 1/f (-1.1 < alpha < -0.9) characteristic for f > 1-10 kHz to the frequency where system noise dominates. In EDA H-mode, a frequently occuring coherent mode degrades this power law. The physics behind these power-laws and its effect on plasma transport is still under investigation. Radiated power measurements have been obtained in the last 2 weeks. The systems were all relocated during the last shutdown and numerous changes have been made. Three of the 4 systems are functional. 2 of these are global measurements, one using a standard foil bolometer, the other using a wideband silicon detector (AXUV diode series). The 3rd system is based on 2 arrays of wideband detectors (AXUV diodes). The diode single view agrees with the integrated radial emissivity profile, within a few percent. However, a significant difference (up to a factor of 2-3) is found between the standard foil bolometer and the diode. On a single impurity injection event, though, the difference amounts to approximately 30% only, indicating that a measurable amount of power is carried by neutrals (which are not measured by the diodes). Difficulties arise with neutrals as the plasma is not "optically thin" and careful inversion/analysis is required. Additional checks and calibrations are under way, and a future run in helium is planned which should help in elucidating the difference. Components of the currently unused divertor region Thomson scattering experiment have been adapted to measure electron temperature and density at the upper separatrix. This new edge system began operating during the week of February 9-12, obtaining appreciable signals from scattered photons. The signals from four spectral channels are used to compute electron temperature in each of 17 scattering volumes. Currently the accuracy of the results is unclear due to incomplete characterization of the transmission in each spectral channel. However, tests of the analysis code using modelled data indicate that once a reliable calibration is obtained, relatively accurate measurements of edge temperature, as well as relative density profiles, can be produced. Efforts to obtain the necessary calibration coefficients are proceeding. Travel and Visitors: -------------------- Chris Brunkhorst visited from PPPL to assist in tuning of the new transmitters. Bill Noonan, from the University of Maryland, visited C-Mod last week to work on the high resolution visible/UV spectrograph diagnostic. Gary Taylor (PPPL) continued to assist in ECE operations. The 38 channel GPC2 instrument is now fully operational. Romik Chatterjee and Perry Phillips (UT-FRC) visited to work on installation of the ECE radiometer diagnostic, as described above.