Alcator C-Mod Weekly Highlights Nov 17, 2003 Plasma operations continued on Alcator C-Mod last week. Three run days were scheduled and completed, with a total of 37 plasma discharges produced. In addition to post-boronization conditioning of the ICRF antennas, experiments were carried out in support of Transport and Edge/Divertor Group research. Progress also continued on Lower Hybrid System development. Operations are scheduled to continue this week. Operations ---------- Only three run days were scheduled last week, owing to the Tuesday holiday. Startup reliability was in the 50-60% range, probably due to the effects of the recent boronization. There were significant delays each day due to data acquisition system problems related to the recent operating system upgrade of our Linux systems. There was also a delay on Wednesday in getting the alternator up to speed, and on Thursday related to a PLC problem on one of the PF supplies. The run on Wednesday was primarily devoted to RF conditioning of the D- and J-port ICRF antennas. Thursday's run supported a study of edge turbulence in H-mode plasmas (MP#332), using correlation between the fast scanning probe at F-port and the fast camera and D-alpha diodes at A/B looking at the same flux tube. Friday's run was primarily devoted to MP#335, "Edge plasma flows in upper versus lower x-point discharges". The primary purpose of these experiments is to investigate the dependence of the parallel plasma flows routinely observed in the edge plasma to the magnetic geometry. Good data were obtained on all three probe systems (ASP, FSP, ISP) in matched upper and lower null discharges. In addition, core rotation monitored by soft xray (HIREX) was observed to change from 10km/sec to 45km/sec (counter-current) when the geometry was changed from SNL to SNU. The double null portion of the experiment will require additional run time. Physics -------- Work continued on analysis of the ECE density fluctuation experiments. A better model for rays in TORAY has been implemented so we can accurately calculated the density effect on the ECE signal. We also obtained a revised version of TORAY which will allow more points in the temperature and density profiles to be used in the calculation. We need this ability to test the localization of the density refraction effects on the ECE signal. Initial tests of the prematching system installed on the D-Port ICRF network demonstrated that quarter-wavelength transformers can be used to lower the voltage in the matching network and control the range of impedances which a future fast matching network would have to address. Without affecting the antenna performance, the reflection coefficient was lowered from .8 to .5 and the voltage dropped by 10 kV, while up to 1.5 MW could be coupled to the plasma at D-Port. The inductive loading was also found to be significantly different between vacuum and plasma conditions, which indicates that measurements and numerical predictions in plasma are essential for practical designs. With this information, good agreement is found with transmission line theory models, and the system will be optimized to further reduce the reflection coefficient. Possible loss of fast ions in Alcator C-Mod has been studied by comparing the beam-target d-d neutron emission during injection of a deuterium diagnostic neutral beam (50 keV, 4 amp, radially-directed) into a variety of plasma conditions. TRANSP analysis which includes full profile effects (Ne(r), Te(r), flat Zeff) and classical orbit losses confirms previous global analysis that the measured neutron rates are a factor ~2 less than expected classically at high density (Ne0 = 2 - 4 x10^20 m^-3), with a smaller discrepancy at low density (0.5-1.0 x 10^20 m^-3). The discrepancy cannot be explained by measurement uncertainties in Te, Ti, Ne, Zeff or the neutron emission rate. In some plasmas, uncertainties in the attenuation of the beam through the edge neutral region and uncertainties in the impurity composition would be sufficient to explain the neutron deficit. The steady-state neutron emission can be matched in TRANSP by assuming a large fast-ion diffusivity of order 40-80 m^2/sec, but such simulations do not match the measured rise and fall times, which are best matched assuming Dfast=0. These results would be consistent with a situation combining classical fast ion behavior in the plasma itself and some mechanism, not yet identified, that causes the current of full-energy DNB neutrals incident on the plasma to be a factor ~2 less than expected. ICRF System ------------ The antennas were re-conditioned post-boronization. As in the previous 3 campaigns, J-port required more conditioning time than D and E-port antennas. Recovery time tends to improve with subsequent boronizations. The FMIT#2 driver screen by-pass capacitor failed prior to Wednesday's run and was repaired in time for operation on Friday. By the end of the week, the antennas achieved 5 MW for ~0.25 sec, with the pulse length limited by opportunity, as RF conditioning was piggy-backed at the end of shots. DNB Systems ------------ The arc slope correction circuit has proved useful in improving the beam pulse shape. Data was successfully taken with the new CXRS detector. The H-like boron spectrum which has been used previously for rotation and temperature measurement was observed with this new detector. We expect to install additional fibers during the coming week to increase the number of simultaneous channels. Lower Hybrid System ------------------- Further work on the Lower Hybrid launcher was completed this week. A site visit to the brazing vendor was very successful. The refurbished and modified couplers were examined and a trial fit of the ceramic windows displayed a good fit adequate for brazing. MIT plans to run additional prototype tests prior to brazing the first coupler. Presently enough windows exist to braze the first two couplers, with the balance of the ceramic windows now scheduled for delivery to the vendor on 11/21/03. The Forward Waveguide Assembly has been delivered to the machinist for final machining following completion of measurements at PPPL. The modified circulators have been shipped from the vendor in Germany. Travel and Visitors -------------------- M. Bitter, K. W. Hill (PPPL) and S. G. Lee (Korea Basic Science Institute) spent the week working at MIT optimizing the NSTX imaging XCS on C-Mod for throughput to avoid saturation of the detector/electronics system. Previously the detector was often saturated at a count rate of about 400 kHz, due to the intense argon emission from the plasma. Several techniques were attempted to reduce the system throughput. Ultimately, these allowed measurements, apparently without saturation, during the Ohmic phase of plasmas on Friday. Bill Beck and Bob Childs were in Seattle, Washington on Thursday and Friday last week working with Doug Loesser, PPPL, and the brazing vendor on the lower hybrid couplers.