Alcator C-Mod Weekly Highlights May 8, 2006 FY2006 weeks of research operations: Planned: 14 weeks Completed: 9.9 weeks Operations ---------- Plasma operations continued at Alcator C-Mod last week. Three and a half run days were scheduled and completed. A total of 70 plasma discharges were produced with a reliability of 81%. Experiments were conducted in support of research in the Transport and Integrated Scenarios for ITER areas, and for ICRF conditioning and troubleshooting. Overnight boronizations were performed on Monday and Wednesday night. One between-shot boronization was performed during the run on Wednesday. Plasma operations are planned to continue this week. Run summaries ------------- Run time on Tuesday and Thursday was devoted to MP#445, "Ion temperature measurements in ITB plasmas". The purpose of this experiment is to understand the triggering mechanism for producing off-axis ICRF internal transport barriers. The question of whether ITB formation can be explained in the framework of suppressing ITG turbulence via increasing ion temperature scale length to the critical scale length is the target. This idea is tested by varying Bt and thus shifting the RF resonance location on a shot-to-shot basis. This experiment is part of the doctoral research of an MIT graduate student. ITB's were produced at different fields, and Ti profile data was obtained with and without ITB's for comparison. Wednesday's run was devoted to MP#451, "Structure and Stability of large Type I ELMs". The experiment employs an equilibrium shape developed during non-dimensional identity experiments with JFT-2M, which was found to be conducive to the appearance of large discrete ELMs. The purpose of these experiments is to investigate the structure and stability of these ELM events. Key diagnostics for this study include D-alpha fast-diode arrays with GPI, fast magnetics, ECE - especially FRCECE with high resolution in the pedestal, Edge TS, DALSA VB profiles, PCI, divertor probes, and the PSI4 fast camera with wide-angle view. In Wednesday's experiment we were successful in producing discrete ELM's, confirming our earlier observations. We were not able to do a ICRF power scan because the RF was limited to 2.3 MW (except for the last shot). We did succeed in getting good measurements of the edge pedestal and can preliminarily estimate the ELM energy loss at ~10% of the total pedestal energy. These ELMs do not destroy the pedestal, as shown by an edge Thomson scattering measurement that occurred during an ELM crash. We also got good measurements of the ELM precursor with the fast magnetics and of the ELM dynamics using the optical diagnostics. This experiment will be continued at a later date, particularly to obtain data with both steadier, and higher power RF. The first half of Thursday's run was scheduled for completion of MP#448,"Comparison of Small ELM regimes on C-Mod, MAST and NSTX", which is the C-Mod portion of the ITPA Joint Experiment PEP-7. Rajesh Maingi from NSTX was at MIT and served as co-Session Leader for this experiment. Unfortunately, the J-port antenna had difficulty recovering from the overnight boronization which preceded this run. The available ICRF power was therefore less than was employed during the previous day devoted to this experiment, April 7. After a half day it was decided to reschedule this experiment and turn the run over to MP#445, which did not require power from J-port. In view of the difficulties experienced with various ICRF systems this week, Friday was devoted to troubleshooting and further conditioning of the ICRF. IN the morning multipactor and RF glow cleaning of the J-port antenna was performed with static gas fill and low magnetic field using the ECDC supply). This was done in an effort to condition the antenna following the boronization on Wednesday night. Standard vacuum conditioning on Wednesday had been unsuccessful in obtaining reliable power from the J-port system. In addition, a new transmission line configuration with quarter-wave transformers was installed on E-port. The purpose of this modification was to reduce the voltage at the stub tuner and phase shifter assemblies, which has been a limitation on the maximum power available from this antenna. A similar installation on the D-port transmission line was quite effective in this regard. Plasma operations began about noon. The conditioning effort on J-port appears to have been successful, permitting RF pulses in the 2.5-3MW range and only a few faults with J-port alone. The modification to E-port prevented us from obtaining a good match to the plasma load; after returning the transmission lines to the previous configuration, we recovered satisfactory matching. Diagnostics ----------- The Alcator calibration ion/neutral source have been refurbished and is now working up to ~20keV, thanks to the efforts of an MIT graduate student. More conditioning is needed to get it up to 50keV. The Compact Neutral Particle Analyzer is now extensively calibrated, with energies from 12-132keV, using the beam from the ICF group. The new calibration results matched the old one (12-60keV) almost perfectly. ICRF System ------------ The D-port system (FMIT#1) operated successfully during all C-Mod runs last week, at powers up to 1.3MW. E-port operated successfully on Tuesday and Thursday, with power up to ~1MW into plasma, limited by high voltage in the transmission line elements. Quarter-wave transformer installations on Wednesday and Friday, intended to reduce these voltages, were unsuccessful, and the design of these components is being revisited. Following reassembly of the FMIT#3 phase shifter and stub tuner, J-port operated successfully on Wednesday at power levels of ~1.5MW into plasma. On Thursday J-port recovery from the preceding night's boronization was poor. RF glow cleaning of the antenna on Friday morning appears to have been beneficial, and plans are being made to implement this technique on a more routine basis. During the run on Friday the "Crowbar Ready" interlock on FMIT#4 opened and HV was lost. The fault was traced to a burned out ignitron heater element, which was successfully replaced. The unit was returned to service and the run continued. DNB Systems ----------- Work to complete the installation and testing of the new inverter drive boards for the long pulse DNB was completed this week with the successful operation of the beam at 50 kV and 6A. The beam was operated into C-Mod plasmas during Fridays run with nearly 100% performance. Lower Hybrid System ------------------- We finished modifications to the Coupler Protection System boards to add the jungle gym couplers. 60 channels of circuits were modified. All circuits were tested on the bench and trip levels for 30% Ref/For power were set. These 12 boards were installed and tested in place by injecting pulses simulating reflected power at the cabinet back panel connectors. All 60 circuits were tested successfully, and the system should be ready for plasma experiments in the coming week. Travel and Visitors ------------------- Dr. Stewart Zweben was on-site for the week working on the design of a new in-vessel imaging system for diagnosing turbulence. He organized a vendor demonstration of the Photron APX-RS fast camera which is being considered for addition to our set of fast camera diagnostics. Amanda Hubbard attended the Fusion Energy Workshop sponsored by the Stanford Univ. Global Climate and Energy Project, held in Princeton, NJ May 1-2. She gave a talk entitled "The Advanced Tokamak: Goals, prospects and research opportunities" and participated in panel discussions on opportunities for GCEP to support fusion energy research. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly