Alcator C-Mod Weekly Highlights April 18, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 5.5 weeks Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 98 plasma discharges were produced with a startup reliability of 86%. Eight experiments, based on MiniProposals from the Divertor/Edge, Transport and MHD Topical Science groups, the Operations and Lower Hybrid groups, and the Burning Plasma Support Thrust, were carried out. Two of these experiments were proposed and led by collaborators. Following the run on Friday, the first boronization of the 2005 Campaign was carried out. The boronization used 120psi of deuterated diborane, with the ecdc resonance location swept from 0.54m to 0.92m, a range which covers the divertor region and extends out to the outboard limiters, but does not include the inner or outer vessel wall. Following the boronization, which was completed at about 05:00 on Saturday morning, discharge cleaning was carried out in helium for a period of 24 hours. Plasma operations are planned to continue this week. Three experiments were carried out on Tuesday. The first concentrated on obtaining data in ohmic plasmas in support of MP#420 "Developing and applying a relative boron density measurement". The goal was to validate spectroscopic measurements of the level of boron in C-Mod discharges, for use in evaluating and understanding boronization. The boron density was measured by observations of intrinsic BII and BV brightness, and by CXRS (Charge eXchange Recombination Spectroscopy) of relative change in the level of BV normalized to injected D2. During this quarter day experiment we obtained data over a range of density 0.8 < nebar < 2.6e20/m3 in standard 1MA, 5.4T lower single null plasmas. The second experiment on Tuesday completed the pre-boronization part of MP#419 "Fiducial wall pumping measurements for comparing operating conditions". The level of wall pumping is an important factor in predicting and understanding the operation of a cryopump in C-Mod. Understanding wall pumping and its dependence on wall conditions (bare Mo or B layer, new boronization vs old) will have important implications for predicting the operation of an experiment like ITER. Previous experiments, mainly as piggyback on MP#417, had obtained data in a variety of ICRF-heated plasmas. The quarter day experiment on Tuesday concentrated on obtaining ohmic data. A range of density was explored. A surprising result of this experiment was that the wall pumping did not increase monotonically as the density was reduced. Rather, at the lowest densities, below nebar~1.4e20/m3, the observed wall pumping was very small, while at about that density there is an abrupt increase in wall pumping, to about 17 Torr-liters under the conditions of this experiment. At still higher density a general decrease in wall pumping was observed, similar to previous datasets. Reasons for the variability of wall pumping, including possible effects of disruptions and runaway electrons as well as variation in divertor plasma parameters with density, are being investigated. The final experiment on Tuesday involved cross-calibration of the core Thomson Scattering density measurement using cut-off of the second harmonic electron cyclotron emission, as described in MP#296 "TS/ECE density calibration". High density discharges were run at a relatively low toroidal field ~4T to obtain the data necessary for this procedure. Cutoff of the ECE emission was observed a different radii, essentially covering the entire profile. The results are being applied to the Thomson calibration. On Wednesday morning we carried out a half-day experiment in support of MP#348a, "Initial tests of the Boron injector", from the C-Mod Operations Group. A novel injector capable of introducing powdered boron (50-90 um grain size) into standard C-Mod plasmas has been installed, with the intent of providing an alternative method of boronization. The injector is capable of injecting 10 to 25 mg of boron per discharge, which is comparable to the replacement level required, based on the observed "lifetime" of a standard boronization: about 200 shots with a typical deposition of ~5000mg. The primary goal of this first experiment is to observe the effects of the injection on the plasma and monitor boron at various points in the plasma and near the walls. These goals were accomplished. The injected material was clearly observed on the video viewing the injection port and also by passive and active (CXRS) spectroscopic diagnostics. Little if any change was observed in core plasma parameters, indicating that the injected material was interacting primarily in the SOL. Between 1 and 3MW of ICRF was coupled to the plasma during most discharges to see if the boron had an effect on the H-mode threshold or plasma performance. None was observed. The second experiment on Wednesday was involved in looking for temperature profile evolution as the density limit is approached, in support of MP#333 "Density Limits and Turbulence" from the Transport Topical Science group. Ohmic plasmas at 6T, 500kA were run at densities up to the limit, with profile evolution monitored using ECE and Thomson Scattering diagnostics. The temperature profile is observed to shrink by about 3cm as the density is raised, and is then static for a considerable length of time before the plasma disrupts. It is also worth noting that the radiated power fraction can be static throughout the high density phase, in at least one case maintaining Prad/Ptot~0.5 for about 1 second. The data are being subjected to further analysis. Thursday's run was devoted to MP#418 "Lower Hybrid Commissioning at low power" from the Lower Hybrid Group. This experiment, the second run day devoted to this MP, was led by Stefano Bernabei, who visited from PPPL for the run. In the previous experiment, a phase scan was performed at three different coupler positions; as expected the best coupling was obtained near delta-phi~90 degrees. In last week's experiment, the scan was repeated using counter-CD phasing. Scans were performed at coupler locations of 1 and 3mm (relative to the LH limiter), and additional phases of 30 and 135 degrees were added to the scan in order to obtain a more complete scan. These data are presently being analyzed. Two related experiments were carried out on Friday. The first was a continuation of MP#398 "Complete identity experiments with JET on error field locked mode thresholds", submitted through the Burning Plasma Support thrust by Tim Hender (UKAEA). Dr Hender, along with David Howell (UKAEA) led the experiment using remote collaboration tools from the JET site. Locked mode threshold values were obtained in 4T discharges (q=3.2) in two different configurations of the C-Mod non-axisymmetric coilset (A-coils). Both configurations produce poloidal mode spectra similar to the JET EFCC coils, but are oriented at different toroidal phase, and therefore combine differently with the intrinsic error field. Use of these, and additional configurations, is expected to reduce the dependence of our results on the imperfectly known intrinsic error field. The main point of this MP is to obtain the toroidal field scaling of the locking threshold in C-Mod for this JET-like equilibrium, and thus complete the non-dimensional identity experiments begun last year. These experiments are part of ITPA/IEA Joint Experiment MDC-6, and are aimed at improving the prediction of the locked mode threshold in ITER. In addition to the experiments at 4T, we also on Friday began exploration of the threshold in the second of the two configurations at 6.3 T. These experiments will be continued at a later date. The second experiment on Friday also employed the A-coils, in support of MP#402 "Locked mode studies at different toroidal phase", from the C-Mod MHD Topical Science group. The purpose of this proposal is to improve the characterization of C-Mod's intrinsic error fields, and to determine the influence of sidebands in the mode spectrum on the locking threshold. On Friday we completed the evaluation of the locking threshold B21 (applied) field at nebar~2e20/m3, 1MA, 5.4T using a standard C-Mod lower single null equilibrium for a new A-coil configuration (the same one as used for the second part of the morning experiment), and also determined the locking density corresponding to a fixed value of the applied 2/1 field in this configuration. When combined with existing data and data in additional configurations, having different toroidal orientations and also different sideband compositions, these results should help to quantify the magnitude of the intrinsic error field as well as improve our understanding of the effects of the poloidal mode spectrum on mode locking. ICRF System ----------- The ICRF system was employed during one experiment last week. Up to 3MW of power was coupled in support of Wednesday morning's tests of the Boron injector. All transmitters and antennas performed nominally. Long Pulse Diagnostic Neutral Beam ---------------------------------- The beam duct connecting the new DNB to C-Mod was installed on Thursday. A helium backfill was employed to allow sufficient compliance in the bellows between the DNB gate valve and the vacuum chamber to complete the attachment. Following the duct attachment the DNB and tokamak were again pumped down to high vacuum. Travel and Visitors ------------------- Steve Wukitch, Yijun Lin, Alex Parisot, Ron Parker, Miklos Porkolab, Gerd Schilling, and Stewart Zweben presented papers based on C-Mod experiments at the 16th Topical Conference on Radio Frequency Power in Plasmas, April 11-13, Park City, Utah. Paul Bonoli, Valerie Izzo, and Brian LaBombard participated in the Sherwood Theory meeting April 11-13 in Stateline, NV. Martin Greenwald participated in the American Physical Society April meeting in Tampa, FL. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly