Alcator C-Mod Weekly Highlights Sep 5, 2000 Operations ---------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. The first boronization since the July vent was carried out on Monday night. Approx. 140 psi of diborane gas was used corresponding to about 2000 Angstroms of boron on the walls. The boronization was followed up with 2.5 hours of ECDC in helium at 5.e-5 Torr. C-Mod operation was run on the usual Tuesday through Friday schedule. A total of 94 plasma shots were produced, with a startup reliability of 90%. Two run days were devoted to experiments by the pedestal/H-mode physics research group, and two to ICRF conditioning and development. Plasma operation will continue this week. Physics -------- With its good energy confinement, moderate particle confinement and lack of large ELMs, the EDA H-mode could be a highly favorable regime for a tokamak reactor - if we can understand the underlying physics sufficiently to extrapolate into new regimes. The key to understanding EDA seems to be in the quasi-coherent fluctuations which have been observed by the reflectometer, PCI, and probes. These fluctuations seem always to accompany EDA and have been shown to drive significant particle flux. An experiment carried out this week employed the A-side fast scanning probe with a magnetic coil embedded in the probe head along with two Langmuir probes, and the fast framing tangential camera, which was set up with an Halpha filter and a hydrogen puff to observe the quasi-coherent (Q-C) fluctuations in ohmic H-mode EDA plasmas. Ohmic EDA's are chosen for these experiments to limit the incident heat flux on the scanning probe, permitting deeper penetration. A principal goal was to look for magnetic components of the quasi-coherent mode with the scanning probe. Q-C fluctuations were clearly seen on the reflectometer, electrostatic probes and on the magnetic probes, with frequency between 100 and 150kHz. Although the magnetic sensor itself never got closer than 1 cm to the separatrix, the amplitude of the signal reached 400 T/sec peak to peak. Accounting for the mode frequency, this corresponds to about .4mT or about 20 A/cm^2, which is of the same order as the total calculated edge current density. The electrostatic probes, which are part of the same probe head, saw the modes quite clearly - with similar time histories. The main distinction is that they only see the mode when they are immersed in it (deep penetration) while the magnetic probe needs only to be close to the separatrix. The conclusion is that the Q-C mode has a very significant magnetic component. On one shot the probe also observed some ELM precursors from type III ELMs. These had a longer decay length as seen on the probe scan, and were also seen with the wall magnetics, suggesting that they have a lower m number. These fluctuations were not seen on the floating potential, strengthening this conclusion. Thus we would conclude that the Q-C mode is distinct from ELM precursors. We also obtained D-alpha images with the gated tangentially-viewing camera for most shots. Clear differences among L-mode, ELMfree H-mode, and EDA were seen. Emission from L-modes is quite broad, extending 5-6 cm outside the separatrix. In ELMfree H-mode, the region of emission is much narrower and hugs the separatrix. During EDA, clear fluctuations are seen well outside the separatrix. These images suggest additional transport in the far scrape-off (consistent with profile measurements). They do not appear to be a visualization of the Q-C fluctuation itself. Thursday's run was devoted to documentation of pedestal parameters in ELM-free, i.e. non-EDA, H-modes with ICRF heating. Data were obtained at 1.0 and 1.2 MA, at a toroidal field of 5.4 tesla. RF power was <2MW, and the H/D ratio increased to >10% over the course of the day, resulting in less than optimum heating efficiency. On Friday's run it was found that between-shot ECDC in D2 resulted in stable H/D fractions of ~5%, which is preferred for efficient minority heating. A set of eight BP-loops mounted on the outboard limiter located between G- and H-ports were incorporated into the EFIT flux reconstructions, in an effort to improve the reliability of the inferred location of the last closed flux surface. Discrepancies between the EFIT reconstructions and other diagnostics of order 5mm, particularly at the outboard midplane have been previously noted. Inclusion of the limiter BP-loops, which are much closer to the plasma than the original magnetics diagnostics mounted on the outboard wall, did result in reconstructions which placed the separatrix further in by about 5mm, in better agreement with independent diagnostics. However, the results, and subsequent testing, indicated a discrepancy between the new and old loops at the level of ~2%; this discrepancy, which is well outside the nominal accuracy of 0.5%, was later determined to be due to a systematic difference in the calibration techniques used for the two coilsets. An independent check of the absolute calibrations is now in preparation. Dynamic outer gap scans carried out during the previous week give some independent indication of the correct location of the last closed flux surface by comparing magnetic fluctuations with the EFIT calculations of the outer gap. The amplitude of the outboard midplane magnetic fluctuations increases as the gap is reduced, then remains constant. We speculate that the fluctuations remain constant when the gap is in fact zero and the mode cannot get any closer to the coil. The time history of the RMS of the magnetic fluctuations was compared with the EFIT calculations using either just the original poloidal field pick-up coil set alone or including the integrated outboard limiter coils, as discussed above. The comparison indicates that the fluctuation amplitude is constant when the outer gap is zero for the EFIT calculation including the outboard limiter coils, suggesting that these reconstructions give a better estimate of the outer gap. However, questions about the coil calibrations must be resolved before a definitive conclusions can be drawn. ICRF Systems ------------- ICRF antenna conditioning resumed this week following overnight boronization on Monday. Hydrogen/deuterium ratios in the plasma were now reduced to the 5% level, allowing ICRF hydrogen minority heating experiments to be initiated on Wednesday. Inner-wall limited discharges were run in order to avoid L-H mode transitions, which complicate antenna matching and power balance analysis. D- and E-Port antennas conditioned up to 1.35 MW each relatively easily. The J-Port antenna began to fault above 0.8 MW, with accompanying impurity injections. A series of low power ramps at different plasma outer gap settings produced loading resistance measurements from the J-Port antenna that indicate similar values and power behavior to those obtained before the July modification, and different from the other two antennas. A further series of discharges with different outer gap settings and a constant power level of 0.8 MW from each antenna in sequence allowed a heating efficiency comparison of the antennas to be made. The J-Port heating efficiency, as indicated by central electron temperature increase, stored energy increase, and neutron rate, was lower than the other two antennas (roughly half). A phase change of the J-Port antenna from (0,pi,0,pi) to (0,pi,pi,0) resulted in severe faulting with ejections from many different points on the antenna. Further conditioning and a repeat of the antenna comparisons were carried out on Friday. The J-port antenna conditioned better compared to the Wednesday run, in the sense that the number of faults were greatly reduced. However, there was still quite a bit of glowing on the antenna and many impurity injections. Power ramps were used to try and get around this condition and these were successful in getting to higher launched powers (up to 1.2MW) but resulted in disruptions due to the injections. During this conditioning at a plasma current of 1 MA the disruptions started to occur at lower and lower powers. Eventually it was necessary to lower the power to 400kW (200kW each pair) to get the antenna to run cleanly. The current was then changed to 0.8MA and conditioning was repeated. The total achievable power was raised to ~1.2MW during this set of conditioning shots just as at 1MA. On Friday, performance of the J-port antenna was significantly better than that on Wednesday. The central temperature increase, stored energy increase, and the neutron rate were only slightly lower (~15%) for the J-port antenna compared to the other two antennas. DNB Systems ------------ The Diagnostic Neutral Beam was brought up during the last week. The modreg problems that caused DNB operation to cease a few days before the end of C-Mod operation in June were traced to relatively minor problems with power for the control system and with fuses for the main beamline power. An intermittent problem early in the week with one of the beam cryo pumps was associated with high ambient cell temperature. This did not limit operation and has not recurred. Apparent modreg cooling problems were traced to a faulty interlock and resolved. The beam plasma source was conditioned early in the week. The accelerator was then vacuum conditioned to approximately 47 kV. In the course of the run on Friday, the beam was operated into the plasma. Travel and Visitors: ---------------------- Stew Zweben (PPPL) came to C-Mod 8/31-9/1 to continue to work with the C-Mod physicists and computer engineers on optimizing and automating data acquisition for the new 2-D edge turbulence imaging diagnostic.