Alcator C-Mod Weekly Highlights Feb 21, 2012 FY2012 weeks of research operations Target: 17 weeks Completed: 4.7 weeks Plasma Shots: 648 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 104 plasma discharges were produced with a reliability over 92%. Runs supported experiments in ITER scenario development, Lower Hybrid physics, Transport physics, and Boundary physics areas. Plasma operations are planned to continue this week. Operation details ----------------- Tuesday's run was devoted to MP#690 "Implementation of new impurity seeding systems". The purpose of this experiment was to characterize the plasma response to the impurity seeding lines which were installed under the BC and HJ divertor sections during the last vent. The target plasmas were 800kA EDA H-modes similar to those studied previously. N2 puffing through the new divertor seeding tubes was compared to seeding using the midplane valves employed previously. We confirmed that divertor seeding does lead to a higher ratio of divertor to core radiation than the midplane puff. Seeding from the BC section, which is adjacent to a closed port was somewhat superior in this respect to seeding through HJ, which is near an open port. While core Mo levels were affected by the N2 seeding, it was not clear whether divertor or midplane puffing had a stronger effect on lowering the Mo. Actuator lag and decay times for the divertor and core nitrogen signals were documented for all of the seeding locations with puffs of 0.1 sec duration. Divertor probes showed clear evidence that the N2 seeding resulted in a shift from sheath-limited to high recycling conditions (decrease in Te, increase in ne), with some decrease in pressure. Complete divertor detachment was not observed with seeding rates up to 20 Torr-liter/sec. The seeding also led to changes in the pedestal turbulence, including an increase in the quasi-coherent (QC) mode frequency. These effects are undergoing further analysis. Wednesday's run was devoted to MP#633 "Lower Hybrid current drive induced rotation at different magnetic geometries". This MP comprises part of the thesis research of an MIT graduate student. The goal of Wednesday's experiment was to confirm the relation between ohmic rotation reversal and the LHCD rotation reversal observed in previous experiments. Difficulties with LHCD coupling prevented us from performing the prescribed density scans at the required power levels. A current scan was performed at a density of nebar~7e19/m^3, and the low current case (400kA) did not reproduce the rotation reversal previously obtained with higher LHCD power. This observation suggests that the rotation reversal may be sensitive to Te or collisionality. Several hours of run time were lost due to network difficulties which impacted the LHCD control system, and to a power system issue. On Thursday we carried out MP#672 "Nonlocality and possible correlation with rotation reversal". "Cold pulses" were generated by laser blow-off (LBO) injection of CaF2 to cool the edge, and the appearance of non-diffusive propagation, as indicated by an increase in the central temperature, was examined for correlation with the rotation reversal characteristic of the transition from Linear (LOC) to Saturated (SOC) Ohmic Confinement regimes. Core and edge turbulence diagnostics were employed to identify changes in the fluctuation spectrum associated with the modified transport properties. Shot to shot scans of density spanning the LOC to SOC transition were performed at two currents, 1.1MA and 0.55MA, and dynamic density ramps were obtained at 0.55, 0.8, and 1.1MA. Rotation reversals and changes from diffusive to non-diffusive cold pulse propagation were observed and documented. New results from the Gas Puff Imaging (GPI) diagnostic show a fast fluctuation propagating in the electron diamagnetic drift direction which appears only in the SOC regime, which complements the LOC feature observed with the PCI. These data are being analyzed. Friday's run was dedicated to MP#667 "Commissioning of new SOL probes". The goal of this run, which supports the thesis research of an MIT graduate student, was to get a base data set of SOL parameters measured with the A-port Scanning Probe (ASP) Ion Sensitive Probe (ISP). The use of an ISP to measure plasma potential has been demonstrated previously, but the use of this instrument for ion temperature measurements has not been robustly benchmarked. The probe is designed to be scanned up to the LCFS in ohmic discharges. We successfully scanned the ISP in to locations with Te>70 eV. Initial results at low density show a well-resolved ion temperature profile, with magnitudes greater than twice the local electron temperature, as expected at low collisionality. At higher density it was found that power supply limitations prevented sufficient biasing of the ISP guard electrode to reject ions. Modifications to overcome this limitation are being planned. In addition to characterizing the ISP, the Inner Wall (WASP) and F-port (FSP) scanning probes were used to collect time series data for studies of SOL turbulence. Physics ------- A paper "FIR Polarimetry Diagnostic for the C-Mod Tokamak" by J.H. Irby , W.F. Bergerson , D.L. Brower, W.X. Ding, E.S. Marmar, and P. Xu has been published in Journal of Instrumentation 7, C02033 (2012), and is available online at http://dx.doi.org/10.1088/1748-0221/7/02/C02033. ICRF System ----------- We have begun reassembly of FFT#2 after repairing the arc damage. The next major step is to pressure test the cooling water lines to verify the assembly is leak-free. In preparation for the delivery of the new ARRA-funded FFTs for the D- and J-port antennas, two electronic enclosures have been prepared to serve as models of the production versions of (a) the Temperature and Sensor Chassis and (b) the Control and Overload Chassis. We have completed the final preparations for repairing the field aligned antenna. Lower Hybrid System ------------------- The LH system was used for the C-Mod runs on Wednesday and Friday. Pulse length was extended to 750 ms for several pulses on Wednesday. Previous pulses were limited to 500 ms due to limitations of the previous transmitter protection system (TPS). Persistent breakdown events in front of the LH launcher were observed during this period of operation at power levels of 200kW and above. The cause of these faults is being investigated, but may require work in-vessel during the planned up-to-air period to resolve. Work on the LH-2 control system continues. The low power microwave drive setup (attenuator and phase shifter) was calibrated using a real time 10 kHz control loop. Lookup tables were created to check the calibration of the system. The calibration check yielded similar results to the tests in April of 2011. A low power feedback loop setup is under construction. Travel and Visitors -------------------- Ahmed Diallo (PPPL) visited last week to begin a collaboration on pedestal research with Jerry Hughes. They had a brief meeting with a number of C-Mod staff and students to discuss ideas for a new mini-proposal, which requests run time in the summer of 2012. While on-site Ahmed also discussed the status of the O-mode correlation reflectometry with Arturo Dominguez, and the extended use of M3D to model NSTX discharges with Linda Sugiyama. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly