Alcator C-Mod Weekly Highlights March 24, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 9.4 weeks Operations ---------- Research operations continued at Alcator C-Mod last week, with four run days scheduled and completed. A total of 98 plasma discharges were produced with a reliability of 84%. The runs supported experiments by the Transport Physics Group, Lower Hybrid Physics, AT Integrated Scenarios , and the Diagnostics Group. Overnight boronization was performed prior to the plasma run on Friday. All tokamak subsystems performed nominally during last weeks runs, with minimal unplanned delays and interruptions. Plasma operations are planned to continue this week. Operation Details ----------------- Two experiments were carried out on Tuesday. The morning was devoted to calibration shots for the Motional Stark Effect (MSE) diagnostic, consisting of beam-into-gas tests with toroidal and vertical fields. Two additional shots were taken at the end of the day, in order to test shot-to-shot reproducibility. The observed pitch angle was observed to vary over time in a manner consistent with the hypothesis of thermally-induced stress in the internal optics. The temperature at different points on the optics cannister was monitored using thermocouples. Single shot pitch angle calibrations employing ramping fields corresponding to a range of 10 degrees were completed. A pressure scan from 0.25 to 3 mTorr was carried out in order to determine the effects of secondary neutrals on the measured pitch angles. A scan of toroidal field from 4.9 to 6.2 T was also performed to determine the change in polarization fraction admitted by the MSE bandpass filters. The remainder of Tuesday's run was devoted to a continuation of MP#500, "Development of Diverted AT Target Plasma with ICRF, LHCD and Current Ramp". One of the main goals of this run was to continuously operate the LH system from 0.15 seconds through the current ramp to about 0.5 seconds. This aspect of the run was successful and we achieved fairly continuous operation of the LH system at the 0.6-0.8 MW level in multiple shots. Application of high on-axis ICRF power (above 1.5 MW) during the current ramp was found to be detrimental to formation and sustainment of reversed shear plasmas. We saw strong evidence of current drive with LHCD during ramp-up and in L mode plasmas. As the plasma enters H mode in the second half of the pulse with higher edge densities, LHCD couples even at 120 degrees phasing, but so far there is no evidence of substantial current drive in the H-mode phase of these discharges with up to 0.7 MW of coupled LH power. Wednesday's run was dedicated to MP#532 "Sensitivity of edge pressure gradients, toroidal rotation and flow-shear profiles to magnetic x-point topology". The goal was to map out in detail the sensitivity of edge plasma pressure gradients and flows in ohmic plasmas as the magnetic topology is varied from lower single null (LSN) to upper single null (USN). Data were obtained by all scanning probes systems, edge Thomson scattering, gas-puff imaging diodes and fast camera, inner wall CXRS, core and pedestal CXRS, and the core X-ray spectroscopy (HIREX) systems for a range of densities spanning the range of edge collisionalities for which the flows have been observed to vary with x-point location. Initial observations indicate that the location of the perpendicular velocity shear layer clearly changes with topology, and may be related to the location of the secondary separatrix. The density also affects the magnitude and location of the shear layer. These data are being analyzed in more detail. The run on Thursday supported MP#515 "Effect of launched n-parallel spectrum on current profile", from the Lower Hybrid Physics group. The goal was to make clear measurements of the LH driven current using MSE, and examine the effect of n-parallel on the profile. The experiments were carried out at 6.2T, to improve LH accessibility. Phase was scanned from 60 to 120 degrees, with net coupled power levels up to 1MW. At low density (nebar<5e19/m^3) strong MHD activity associated with the LHCD was observed, depending on phase and LH power. MSE data are being analyzed and should give a good indication of the dependence of driven current profiles on launched phase. The run on Friday was dedicated to MP#537 "Er profile studies", which comprises part of the thesis research of an MIT graduate student. The goal was to study the radial and temporal behavior of the edge radial electric field before and during L-H transitions in order to characterize the role the field plays in the transition and the effect the structure of the field has on H-mode confinement. Experiments were carried out in both EDA and ELM-free H-modes. Data were obtained in EDA H-modes at different power levels, from just over the threshold power to Prf>3.5MW. ELM-free conditions were studied with ICRF power up to 2.5MW. A strong quasi-coherent (QC) mode was observed during the EDA discharges, and on several shots it showed a shift in frequency correlated with changes in stored energy. This change in frequency also correlated with changes in the poloidal velocity. ICRF Systems ------------- The FMIT#2 transmitter (E-port antenna) was placed back in service after replacing the FPA tube and repairing the grid regulator. The transmitter operated up to 1.8 MW into dummy load before operating into plasmas. A fault in the FMIT#3 FPA tube stopped operation of that transmitter late in the run on Friday. Preparations for installation of a replacement tube are underway. Long-pulse Diagnostic Neutral Beam System ----------------------------------------- The DNB operated successfully in support of plasma experiments throughout this week. Travel and Visitors -------------------- Pat MacGibbon (MIT) witnessed final testing of the refurbished LH klystron S/N#117 at the vendor site last week. The klystron performed as expected, and will now be shipped to MIT. Jim Irby witnessed acceptance testing at the vendor's site for the FIR laser system to be used in the C-Mod polarimetry diagnostic. The lasers are performing above specification in stability and power. The lasers have now been delivered to MIT and engineers from the vendor will visit this week to set them up and provide instruction on operating procedures. Professor Fukun Liu and Dr. Jiafang Shan, of the Institute of Plasma Physics Academia Sinica visited the PSFC last week for discussions related to the C-Mod Lower Hybrid System. Perry Phillips (U. Texas) was at MIT last week to work on the FRCECE electron cyclotron emission diagnostic. The RF unit for the 16 inner (higher frequency) channels was re-installed following repairs at the vendor, but did not perform properly during tests. The unit will be returned to UT-Austin for further work. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly