Alcator C-Mod Weekly Highlights June 18, 2001 Plasma operations continued at Alcator C-Mod last week, as four run days were scheduled and completed. A total of 59 plasma shots were produced with a startup reliability over 90%. Physics experiments carried out included studies of the EDA H-mode, investigation of the role of molecular recombination in the observed continuum emission in the C-Mod divertor, and continuation of earlier work on disruption mitigation. Plasma operations are scheduled to continue this week. Physics and Operations ---------------------- The first boronization of the campaign was carried out Monday night-Tuesday morning, June 11-12, with an average deposition 2050 A. Approximately 3.5 hours of ECDC in helium at 5.0e-5 Torr followed boronization. Tuesday's run was devoted to boronization recovery and conditioning of the J-port four-strap antenna into plasma. The machine ran well, with no startup or other difficulties which have been associated with some previous post-boronization runs. The H/D ratio was in the 5% range, which is satisfactory for the hydrogen minority ICRF heating scenario. Antenna conditioning was making good progress, until the J-port system encountered a problem with an external phase-shifter, as described below. Wednesday's run was devoted to MP#294, Divertor Continuum Emission Studies; this experiment constitutes part of the doctoral dissertation research of one of our graduate students. The purpose of these experiments was to obtain high resolution images of D_gamma and continuum emission in the divertor at various densities, determine the spatial relation of the D_gamma and continuum emissivities, and to test the hypothesis of molecular recombination as the cause of the "anomalous" continuum emission in the divertor. Plasmas were run with the same density programming in deuterium, and with strong helium puffing, to vary the possible contribution of molecular deuterium to the observed continuum emission between 420nm and 430nm. The continuum signal was observed to decrease by about 25% during the mostly helium shots, while the deuterium lines dropped by approximately a factor of 4 to 10. This modest decrease in the continuum when compared to the significant decrease the deuterium leads us to believe that the continuum is not dominated by molecular recombination. The second part of the run consisted of a series of shots in which the separation between the separatrix associated with the upper x-point, normally the secondary separatrix, and that associated with the lower x-point, which normally defines the plasma boundary, was varied dymamically during the discharge. In some cases the ramp was extended until the plasma went from the usual lower single null to an upper single null. This was done to determine whether or not the emission band seen near the inner wall is associated with the inboard-most (secondary) separatrix. All results seem to indicate that the emission follows the inboard-most separatrix, suggesting different radial transport characteristics for the two regions separated by the inboard-most separatrix. The run on Thursday concentrated on the quasi-coherent mode and the transport which results from it. Related Miniproposals were #253 and #292. Ohmic EDA H-mode plasmas, which are more suitable for investigation with the fast scanning probes, were produced in the usual manner, by ramping the toroidal field down to induce an L-H transition and then bringing the field back up. Specifically, the goals were: to look for q-resonance effects; to compare the particle (and energy) flux convected by the fluctuation with local particle and power balance; and to verify the extent and radial structure of the quasi-coherent mode. The motivation for the q-resonance study stems from the tentative identification of the QC mode with the family of resistive ballooning modes, which would be expected to be localized to rational q-surfaces. The experiment varied the programming of the toroidal field ramps to produce a slow scan of the edge q, looking for reproducible changes in the QC mode amplitude, frequency, or wave number. Phase Contrast Interferometry (PCI) was the principal diagnostic for this experiment, while the scanning probe was used to collect detailed fluctuation data. No clear q resonance effect was seen. Changes in mode amplitude and frequency were observed, but the strongest correlation was with sawteeth. There are other excursions which are not correlated with sawteeth and these need to be carefully compared to the q time histories. Data were obtained from the probes, edge Thomson scattering, and from the Lyman alpha array for the transport studies. On one shot, the scanning probes apparently traversed the entire radial extent of the mode, which measured 1-2 mm, as had been found in earlier experiments. Ten shots were selected for further analysis. Friday's run was an attempt to extend the encouraging results of last year's neutral point experiments (MP 264) by programming the hybrid plasma control system (PCS) to jump to specialized feedback programming at the thermal quench, with the goal of extending the period of post-quench vertical stability. Thermal quench disruptions were induced by firing silver-doped lithium pellets into established discharges. We started off by reconfirming last year's neutral point result that post-thermal-quench plasmas show enhanced vertical stability when run about 3 cm above the midplane. Then we proceeded to try out two different PCS programming setups, which we hoped would promote vertical stability for even longer periods after the thermal quench. The alternate programming showed some promise, but overall was not as successful as had been hoped for. ICRF Systems ------------ E-port antenna conditioned very quickly and J-port was making good progress in reconditioning following boronization. (FMIT#1 water system was repaired this week and unavailable for plasma operation till 6/13.) Both antennas had clear indications of heating. However, it became clear that J-port needed to be voltage conditioned again; therefore, we decided to switch to vacuum conditioning. During conditioning, the FMIT#4 phase shifter began breaking down after it reached 25 kV. During the inspection, we found that the push/pull rod and a teflon insulator had become tracked. The push/pull rod material was replaced with G7, which has a lower index of refraction and dissipation factor. We have had very good success with replacement G7 rods in our other stub tuners and phase shifters. The tracked teflon insulator was of more concern because it is the second time an insulator at this location has shown this behavior. We have discussed this with the vendor, but have not identified a strong candidate explanation. The focus has been upon the center conductor and the teflon insulator itself. The center conductor was carefully inspected to ensure proper contact was made when fully assembled. Sharp edges were rounded and mating surfaces were blended (these could have enhanced the local electric field). Additional vent holes were added to ensure proper evacuation and purging of assembly. New teflon insulators were installed and the assembly was cleaned. All ICRF systems should be ready for operation this week. Diagnostic Neutral Beam System ------------------------------- The arc supply for the beam was tested in open loop. It is normally run as a regulated supply. We wanted to determine whether we could make an arc current with a flatter profile avoiding the initial overshoot and the slight increase in current toward the end of the pulse. There was not significant improvement and the arc supplies were returned to regulated mode. The beam appears to be fully recovered from the bake and is now operating reliably. For the BES diagnostic, the filters and the filter rotators were calibrated by illuminating them with a continuum source and observing the transmisson with a spectrometer. Some beam profile measurement were completed and BES will now return to fluctuation measurments. The CXRS diagnostics continued to make observations of helium spectra and analysis is underway. Inner Divertor Project ---------------------- Work on the new probe box assembly is proceeding, and is now approximately 70% complete. Mock-up building is moving forward, and a new Stainless Steel Wall was installed. Good thermal contact between the inner divertor and the inner wall is required for proper operation during high heat load conditions. The dimensional specs for the inner wall components are therefore very important. An inconel flame-spraying technique for correcting tolerance errors in the radius of the upper portion of some C-plates was tested on a sample piece. A sample of Inconel 625 plate was sprayed with a layer of Inconel 625, approximately 0.020" thick. Then part of the plate's sprayed surface was machined, leaving a layer 0.015" thick. The porosity was checked, and the plate was baked to confirm proper adhesion of the coating. The process produces a satisfactory result. Travel and Visitors ------------------- David Mikkelsen (PPPL) visited MIT last week to work on EFIT calculations of Alcator C-Mod plasmas and to discuss possible tests of profile stiffness. Gerd Schilling was at MIT 6/12-15, working mostly with the Diagnostic Neutral Beam group on aspects of beam operation. John Cary from the University of Colorado visited MIT on Thursday June 14, 2001. He had discussions with Paul Bonoli and Martin Greenwald concerning modelling needs at the Alcator C-Mod facility. Information from these discussions is being incorporated in the revised project plan of the National Transport Code Collaboration (the NTCC). John was told that the two most needed capabilities for C-Mod in an integrated transport modelling code would be an ICRF Fokker Planck package and a lower hybrid current drive module. John also discussed data acquisition and access issues related to MdsPlus with Josh Stillerman and Tom Fredian. Howard Yuh also gave John Cary and in-depth tour of the Beowulf cluster under development at C-Mod.