Alcator C-Mod Weekly Highlights Mar 1, 1999 Operations: ----------- Plasma operations continued on Alcator C-Mod last week. Four run days were scheduled and completed. Five physics experiments were carried out, along with continued conditioning and development related to the D- and E-port ICRF systems. A total of 85 plasmas were produced, with a reliability of better than 85%. Tuesday's run was devoted to study of ohmic H-modes. Two experiments were carried out, both aimed at comparing behavior in ohmic H-modes with that in the more commonly studied ICRF H-modes. Specifically, the goals of the experiments were: (1) Measure plasma rotation in ohmic H-modes to determine whether or not the plasma rotation is due to RF specifically. (2) Measure pedestal characteristics in ohmic H-modes to determine if there are any differences due to RF fields in the edge region. Ohmic H-modes were obtained at 1.0 MA and 0.8 MA, with a toroidal field of 3.5 tesla. Toroidal rotation in the co-current (ion) direction was observed during the ohmic H-modes. The stored energy and corresponding velocity increases are very similar to the values found during some ICRF heated discharges, suggesting that the rotation is associated with stored energy (or related some related parmeter), regardless of the mechanism. Similar rotation for these discharges has been inferred from magnetics measurements. The soft x-ray pedestals in these ohmic H-modes are similar to those in ELM-free H-modes with RF heating. At 1 MA current, there are no measurable differences between RF- heated ELM-free H-modes and ohmic (ELM-free) H-modes in terms of the width or position of the x-ray emissivity pedestals. Wednesday's run was primarily aimed at developing more reliable simultaneous high power operation of the D and E port ICRF systems. Operation above 2 MW was achieved, but with faulting. FMIT#1's power feedback performed quite well (maintained power within ~15% of requested for varying loads). Numerous faults continued throughout the run. Vacuum conditioning was performed following the run and it was found that the antennas were essentially unconditioned. This was rather surprising since previous experience suggests that the vacuum condition could be maintained for weeks of plasma operation. This incident suggests the vacuum condition had become suspect after a few days. It does however offer the prospect that the problems with the RF can be solved with further vacuum conditioning. Also on Wednesday, the plasma was moved vertically for evaluation of the edge Thomson scattering system. Data were obtained on the edge Thomson system and the soft x-ray array looking at the pedestal near the top of the plasma. Some piggybacking with krypton puffing in support of the investigation of high Z screening was also accomplished. Measurements were made with the divertor RGA and HIREX. Data for atomic physics investigations of highly charged krypton were also obtained on these shots, with both the McPherson and HIREX. The krypton appeared to prevent access to EDA H-mode at ICRF powers up to 2 MW, with type III ELMs being the norm. The purpose of Thursday's run was to carefully measure conditions just before the LH threshold in a range of conditions. We started with the standard 800 kA, 1.6e20/m3 discharge for a reference. Both scanning probe data and Bt sweeps were obtained shortly before the transition. A current scan was carried out at fixed density, up to 1.2 MA. The LH threshold was at 1.2 MW RF at 1.0 MA and only 560 kW at 1.2 MA. Again, L-mode Te profiles were obtained close to threshold. It appears there is at most a weak Ip dependence of the Te threshold. We will have to look more closely at this data, since sawteeth have a significant effect. The remainder of the run consisted of a density scan at 1.2 MA. Several high density points, up to nebar=2.5 e20/m3, were obtained. A surprise was that even a small density increase, to 2e20, caused a large increase in the RF power required (to 1.75 MW from 0.5). This effect may have been related to the large gas fuelling required, and high neutral pressures. The threshold temperatures seemed quite constant with density. 2.2 MW was needed at the highest density. Data were obtained on several edge diagnostics, including the soft x-rays and edge Thomson scattering, in both L and H-modes. Friday's run was in support of two experiments: MP#227, H-mode Ion Fueling, and MP#229, Secondary Divertor Experimemts. During previous runs it has been observed that the lower divertor neutral pressure "pumps out" during the high dn/dt phase of an H-mode transition, particularly when the transition leads to an EDA H-mode. Apparently, the H-mode is fueled, at least in part, by neutrals in the lower divertor region. The goal of MP#227 is to determine whether the fueling results simply from ionization of these neutrals above the x-point region inside the last-closed flux surface or if the fueling involves parallel plasma flows from the divertor to the core scrape-off layer. The F-port and A-port scanning probes (FSP,ASP), which are outfitted with Mach probe sensors, were used to look for changes in the parallel plasma flows during the high dn/dt phase of L to H-mode transitions. Data were obtained in the first half of the run at 0.8MA and densities of 1.6 and 2.0e20/m3. Additional relevant data was obtained during the second half of the run, which was primarily dedicated to investigations of the effect of the secondary separatrix. Recent analysis of flows into the divertor chamber and ionization in the main chamber indicates that most of the plasma in the main chamber recycles onto main chamber surfaces, i.e, the limiters, the walls, and the top surfaces of the lower divertor. Particle recycling fluxes in the lower divertor chamber are large but well separated from the particle recycling that occurs on the walls in main chamber. At first glance, this is not too surprising since the closed lower divertor chamber is presently optimized for power flux handling not particle flux handling. However, it has also been found that cross-field particle transport radially increases with distance from the separatrix in the SOL. This prompts the question whether the upper divertor, with its relatively open geometry, could be used to influence the main chamber and divertor recycling balance, or if the cross-field transport so high (relative to parallel flows to the divertor) as to maintain the same coupling between midplane neutral pressure and core plasma density, independent of divertor geometry. A related question of operational interest is whether the neutral pressure in the upper divertor is high enough for a cryopump located there to be used for density control. In these experiments, we investigated the relationship between neutral pressure in the upper divertor and SSEP, the separation of the separatrices associated with the upper and lower x-points. A series of discharges were run at moderate and low plasma densities and with varying upper secondary separatrix position (SSEP). The principal diagnostics were upper divertor, lower divertor, and midplane neutral pressures, and the F-port and A-port scanning probes (FSP,ASP) to monitor SOL plasma conditions and flows. For ne~1.6e20/m3, the upper divertor pressures were found to vary with SSEP from around 1 mtorr with SSEP>15 mm to 20 mtorr with SSEP~0 mm, i.e double null. These results imply that a cryopump in the upper divertor could be used for dynamic density control. Detailed analysis of the probe data is underway. RF Systems: ----------- Measurements of FMIT#4's input cavity indicate that this cavity had a problem. The transmitter, which will be used with the J-port four-strap antenna, has been completely disassembled for inspection. The same isolation capacitor failed in a manner similar to that observed in June 1998. Several arc tracks were found in the vicinity of the load isolation capacitor. The most significant was located under the the collar, which makes up part of the isolation capacitor, where the teflon insulator had been arced through. To fix the problem, we need to replace the teflon tubing (which we have) and repair the Al coax. The resassembly should be complete by Friday. Measurements of test resonant coax configurations have been completed on J-port. Determination of the proper lengths for 75 MHz and 78 MHz will be completed shortly. Manufacturing of the coax can begin this week. Diagnostic Neutral Beam: ------------------------ Technical progress on the DNB remains satisfactory. The schedule is presently four days behind the baseline due to underestimate of time required to lay out circuit boards for low-end electronics for the high voltage modulator-regulator. Physics Analysis: ----------------- After the first boronization of this campaign, we observed the following changes in the carbon and oxygen impurities: The carbon concentration was reduced from the pre-boronization level by a factor of ~2.7, and the oxygen concentration was reduced by a factor of ~6. These measurements were made spectroscopically by looking at the impurity resonance lines. While the carbon and oxygen emissions were decreased by the boronization, the atomic deuterium emission from the divertor region increased after boronization, typically by a factor of ~2.5 (at the same plasma density). The first Enhanced D alpha H-modes of the 1999 campaign had some interesting fast magnetic fluctuations that clearly increased during EDA and decreased during the ELM-free portions of the discharges. Broadband activity increased during EDA below 100 kHz and again between 150 and 200 kHz. There were also coherent modes observed throughout most of the EDA phase with steady frequency that varied from shot to shot. These coherent modes were typically between 20 kHz and 80 kHz. In some cases, there were two simultaneous modes at 47 kHz and 80 kHz during EDA. These modes were observed on the inner wall. The outboard coils could barely make out these modes on some shots, perhaps indicating an anti-ballooning structure or that the mode rational surface was closer to the coils on the inner wall than to the outboard limiter coils and that the mode amplitude decreased rapidly with distance from the mode rational surface due to high poloidal mode numbers. On some shots, there were also high frequency magnetic fluctuations at 300 kHz that were frequency modulated by the sawteeth. In these cases, there were also observed simultaneous 800 kHz modes on the outboard coils that were sampled at 2 MHz. Analysis of these fluctuations is ongoing. Travel and Visits: ------------------ Bob Granetz, Jay Kesner, and Jesus Ramos attended an MHD Crititcal Issues Workshop at PPPL from Wed to Fri. While there, they saw Energy Secretary Bill Richardson dedicate the NSTX, which just recently began operation. Martin Greenwald and Tom Fredian attended a workshop on software sharing in the MFE community at PPPL last week. The main area of discussion was the sharing of analysis applications (such as TRANSP and EFIT), database applications (such as LOGBOOK and LOCUS) and graphical tools (such as REVIEW+, FAS and IDL/RPLOT). This software sharing has become feasible since many sites have standardized on the MDSplus data system, a common relational database interface and the IDL data scripting language. On Monday (Feb. 22) Paul Bonoli was at PPPL for their Science Focus Group Meeting on Wave Particle Interactions. He gave a 30 minute talk on non-inductive current drive for Advanced Tokamak Applications. Dave Johnson from PPPL reported on a new design for the X-point Thomson Scattering diagnostic.