Alcator C-Mod Weekly Highlights August 27, 2007 FY2007 weeks of research operations: Target: 15 weeks Completed: 13.6 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Four run days were completed, supporting experiments in the Integrated Scenarios, Lower Hybrid Physics, and Transport Science areas. A total of 96 plasma discharges were produced, with a reliability of 83%. Two of the runs were conducted by Session Leaders at other sites, using remote conferencing tools. One overnight boronization was carried out on Monday night, prior to the first plasma run of the week. There has been no recurrence of the ground-fault condition on the toroidal field magnet. We continue to monitor this situation. Plasma operation is planned to continue at Alcator C-Mod this week. Operation Details ----------------- Tuesday's experiment supported MP#514 "Influence of magnetic topology on the H-mode pedestal". This experiment examined the detailed dependence of H-mode pedestal parameters on magnetic balance. We generally observed significant reduction in H-mode pedestal density as the discharges transitioned from LSN to USN. Typical values observed for n_e,PED were 2.6e20/ m^3 in LSN and 2.0e20/m^3 in USN. The lower confined density was correlated with reduced Prad, but, contrary to our expectations and prior experience, this did not always result in a higher temperature pedestal. A degree of hysteresis in the density pedestal response was also observed in sweeps from LSN to USN and back, with n_e,ped mostly recovering its original LSN value. Pedestal and global parameters seemed to be extremely sensitive to magnetic balance with SSEP in the range of 0 to +4mm. With SSEP of +2mm a long steady H-mode could be maintained with fairly low density and low radiated power. The run on Wednesday continued previous joint experiments between DIII-D and C-Mod on intrinsic toroidal rotation in non-dimensionally matched discharges. This effort is part of an ITPA Joint Experiment. The experiment was led by John Degrassie (GA) using remote collaboration tools. This run focussed primarily on a C-Mod shot designed to correspond to the H-mode conditions in DIII-D shot#129125. The target C-Mod parameters were B=5.75T, ne=2.3e20/m^3, Te0=3.1keV, q95=4.75. Once this match had been obtained, additional discharges were produced with variations in individual non-dimensional parameters, in order to test the sensitivity of the observed rotation. In particular we obtained data with increased collisionality, lower q95, and lower BT (which tests the dependence on location of the ICRF power deposition. Additional shots neighboring the target in parameter space were also obtained, and these will also be useful as these datasets are analyzed. Attempts later in the day to obtain matching C-Mod data for a DIII-D Ohmic H-mode case were unsuccessful, although data were obtained which will contribute to the overall rotation database. The run on Thursday was devoted to MP#495A "Alfven Cascade studies with ICRH and LHCD ". The purpose of these experiments, which comprise part of the thesis research of an MIT graduate student, is to investigate the excitation of Alfven Cascades in the current ramp phase, utilizing ICRH for heating and a fast ion source, and LHCD to maintain a reversed shear q-profile. The LH system was operated at medium and high n|| to achieve off-axis current drive, and the relative timing of the ICRF and LH was adjusted to try to maintain q>1. The main result was that, contrary to expectations, the cascades are not observed in the presence of the LH. Similar shots with and without LH show significant changes in RSAE activity, though TAE activity is, at least qualitatively, unaffected. Initial results from hard x-ray data show that LH current is being driven on-axis in all cases. This may explain the loss of RSAEs. The slight amount of central reversed shear present in the ICRF-only discharges may be destroyed by the on-axis current drive. Further investigation into the damping of LH waves in the early plasma in the presence of ICRH and a current ramp is necessary to complete this study. Friday's run was devoted to MP#499A "Parametric Decay and Density Limit for LHEH/LHCD". The run was led (from Japan, despite the 13 hour time difference) by Prof. Yuichi Takase (University of Tokyo) using remote collaboration tools. The purpose of this experiment was to investigate the dependence of parametric decay instability on edge (inside/outside the LCFS) density and temperature under ITER relevant conditions, i.e., in diverted configuration at ITER magnetic field. Comparison was made between L-mode and H-mode conditions, and for different LH wavenumber spectra. These experimental results provide important information for determining the frequency for the ITER LHCD system. A density scan with LH at 90 degree current drive phasing (n_parallel = 2.3) indicated that interaction with electrons disappears around nebar = 1e20/m^3. Intense parametric decay is not observed until the density is around nebar = 1.5e20/m^3, substantially higher than where interaction with electrons disappears. For the same density, parametric decay is weaker in H-mode, and the spectral shape indicates that the maximum growth rate occurs at a region where the local temperature is higher. A similar result was observed at 60 degree phasing. A reciprocating Langmuir probe detected large amplitude RF signal at low densities. The wave amplitude decreased rapidly at densities nebar > 1.5e20/m^3. The radial profile of RF wave amplitude was not monotonic, but increased and decreased during a radial scan, indicating that the wave field is localized radially. Further analysis is in progress. Program Planning ---------------- A meeting of the C-Mod Experimental Program Committee was held on Monday, August 20. Two miniproposals for experimental time during the remainder of the FY07 Campaign were approved. The status of outstanding "top priority" experiments was reviewed. An update on the present status of the facility, and possible options for the scope and duration of the upcoming vent was provided. A straw-man schedule incorporating two changes of the ICRF frequency (to 70 and 50MHz) was also presented. A decision was made to extend the current 80MHz operation at least into the week of August 27 in order to accommodate high priority experiments that rely on this configuration. A decision on subsequent frequency changes was deferred. Lower Hybrid System ------------------- During the run on Thursday, the circuitry associated with the optical arc detector on klystron #10 malfunctioned. The run was completed with klystrons #9-12 disabled. Following the run, extensive troubleshooting of the Transmitter Protection System (TPS) identified a failed IC which had caused the problem. The component was replaced and the arc detector circuit successfully tested. The Lower Hybrid system was returned to normal service in time for the run on Friday, and performed nominally in support of this experiment. Long Pulse Diagnostic Neutral Beam System (and associated Diagnostics) ---------------------------------------------------------------------- Measurements of the width of the diagnostic neutral beam at two locations, separated by 2.2m, were shown to be consistent with simulation. The beam width is needed for interpretation of beam-based diagnostics, for example, to specify spatial localization and averaging of CXRS measurements. The beam width was shown to be consistent with a beamlet divergence angle of 0.8 degrees. One measurement was made in the beam line with a calorimeter, the other in the plasma using the wide-view CXRS diagnostic. These two widely-spaced measurements were compared via a simulation which represented the beam density profile as a sum of many non-interacting beamlets each emanating from an aperture in the acceleration grids. The measured angle is somewhat larger than the beam manufacturer's specification of 0.6 degrees. The MSE group received repaired photoelastic modulators (PEMs) and re-installed them into the MSE optical system. By measuring the amplitudes at sum and difference frequencies at 38, 42, and 46 kHz we can determine the actual retardance imposed by the PEMS. This analysis indicates that the retardance is at the requested value and that it remains constant throughout a run day, thereby confirming that the PEMs are operating normally. Nevertheless, following the repair we continue to observe significant variability in the pitch angles (of order several degrees) measured in a series of reproducible plasmas. Two conjectures remain viable to explain the observed lack of reproducibility: contamination of the MSE signal by thermal Halpha light, and motion of the MSE optics during a plasma and/or between plasma shots. We expect delivery of a set of steep-edge optical filters in early September, which should greatly reduce the intensity of the unwanted thermal Halpha light. Motion of the optics is also under investigation. Digital Plasma Control System ------------------------------ An Analog Output module on the Digital Plasma Control System (outputs #17-32) was found to be malfunctioning during the run on Thursday. The module was replaced with a spare and will be returned to the manufacturer for repair. Signals from the FRCECE electron cyclotron emission diagnostic (P. Phillips, U. Texas) are now available as inputs to the DPCS. The initial application will be real-time detection of locked modes, based on changes in the statistics of sawtooth behavior as observed in the ECE signals. This work comprises part of the thesis research of an MIT graduate student. Splitting the signal had no effect on the FRCECE diagnostic. Travel and Visitors -------------------- Ken Hill and Manfred Bitter visited Alcator C-Mod on Friday to discuss analysis techniques for the curved x-ray crystal spectrometer with Ming Feng Gu (LLNL) and Alex Ince-Cushman and John Rice (PSFC). The collaboration on this diagnostic was highlighted in the most recent Princeton Plasma Physics Digest: "X-ray Crystal Spectrometer Makes Debut at C-Mod: New Technique a Major Advance for ITER", http://www.pppl.gov/polImage.cfm?doc_Id=347&size_code=Doc _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly