Alcator C-Mod Weekly Highlights April 2, 2007 FY2007 weeks of research operations: tentative target: 15 weeks Completed: 0.27 weeks Operations ---------- Startup and conditioning plasma operations continued at C-Mod last week. Four run days were scheduled and three and a half were completed. A total of 70 plasma discharges were produced with a startup reliability of 80%. The main goal of the week's operation was to continue clean-up of the machine and bring diagnostics and RF systems back on-line. The Lower Hybrid System and D-port ICRF antenna were both operated successfully into plasmas last week. We also successfully carried out the first research experiments of the campaign, with about eight hours on Thursday and Friday devoted to MP#464, "LHCD Coupling Optimization". These experiments, which comprise part of the thesis research of an MIT graduate student, aim to correlate the reflection coefficients for the Lower Hybrid Current Drive system with the local plasma conditions immediately in front of the launcher grill, with the goal of optimizing the coupling. The experiments last week were carried out at low power (~200kW) and explored variations in plasma shape and density, as well as launcher position. As expected from modeling, the reflection coefficients lowered as the local density (measured by probes 1.5mm in front of the grill) was increased and the shape was adjusted to provide more uniform density from top to bottom of the launcher. Reflection coefficients as low as 11% were obtained with the launcher positioned 1.5mm behind the adjacent guard limiters. Startup and Conditioning plasma operations are planned to continue this week. Planning -------- The C-Mod Experimental Program Committee met on Monday, March 26, to consider Miniproposals for experiments to be carried out during the Startup and Conditioning period. Six new or revised MP's were approved for possible scheduling during this period. Physics ------- First results from seven new outboard limiter poloidal field pick-up coils were presented at the weekly Science Meeting. The new coils are located +/-10 cm above and below the outboard midplane at four different toroidal locations between KA, AB, BC, and EF ports. They provide greatly improved toroidal phase information for determining low to intermediate toroidal mode numbers in the range of 1 <= n < 14. An example was shown for a large amplitude, low frequency disruption precursor oscillation that gave a very clear and linear n=1 toroidal phase. These coils significantly improve the toroidal mode number measurements since the previous limiter coils were located in closely spaced groups of three coils beneath limiter tiles on both the AB and GH limiters, which were good for moderate to high toroidal mode numbers, but made it very difficult to precisely determine low toroidal mode numbers. The new coils at approximately the same poloidal locations improve toroidal mode number measurements across the full range of mode numbers from 1 <= n < 75. The data are generally sampled at 2.5 MHz throughout the discharge. Collaborations -------------- Amanda Hubbard and Jerry Hughes co-led (with Geoff Maddison of UKAEA), by remote participation, an experiment on JET on 3/27. This was an ITPA experiment based on inter-machine proposal PEP-7, aimed at achieving a match of dimensionless pedestal parameters in devices of very different sizes, to check the profile shapes and H-mode regimes and instabilities as a stringent test of models. Discharges made previously (April-May 2006, MP 382) on C-Mod, with BT=7.9 T and q95=4.2, were used to set the target parameters. The run went well and preliminary analysis indicates pedestal conditions were close to matched, though not yet exact. A second shift has been allocated next week to complete the JET part of this experiment, including ICRF heating power scans. Lower Hybrid System ------------------- The lower hybrid system operated successfully into C-Mod plasmas last week. Initial system checkout and operation (MP#473), including verification of operation of the Coupler Protection System (CPS), were conducted in a series of runs on Wednesday through Friday co-led by Randy Wilson (PPPL). Up to 550kW of net power was launched into C-Mod plasmas, under conditions of improved coupling identified during the MP#464 experiments described above. ICRF Systems ------------ We completed the initial operation of the D antenna and began plasma conditioning D antenna up to 1 MW last week. We obtained 0.3 sec pulses at 1 MW with a few faults. Some control issues arising from the change over to the new cpci based digital I/O board and a failing gate board were addressed. We also had initial difficulty with obtaining a match due to an offset in the stub tuner position reporting, likely a result of the disassembly and reassembly of the unit during the previous week. We have begun retuning FMIT#3 and #4 (J-port antenna) to 78 MHz and installing the coaxial loops. We have completed the repair on the #4 stub tuner and reassembled. We plan to assemble the coaxial loops and complete the new gas system for the 9" coaxial lines and to complete the tuning of the transmitters and begin initial operational tests of the J antenna this week. Long Pulse Diagnostic Neutral Beam ---------------------------------- The beam operated into C-Mod plasmas for two run days last week for a total of approximately 20 plasma shots. Beam spectra show low water content and approximately 73% full energy fraction at the source. Beam parameters are currently 46kV, 5A for 1 second duration shots. A faulted current sense resistor in the the grid supply was replaced, eliminating most of the faulting problems observed during the previous week. Diagnostics ------------ All 32 FRCECE (electron cyclotron emission) channels are working and calibrated against the Michelson system with the exception of one channel. The amplifier/filter card on that channel will be replaced this week. An additional amp/filter card was also repaired and tested so that there will be a spare. The BES diagnostic was tested with plasma signals this week. With filters centered at 658 nm, fluctuations were observed between 100 kHz and 300 kHz. There were clear phase shifts between poloidally spaced channels as might be expected for measurements of plasma turbulence. The poloidal and toroidal Charge Exchange Recombination Spectroscopy (CXRS) channels which cover r/a < 1 were successfully tested with plasma signal. New fibers were installed to allow data from all channels to be acquired simultaneously. The Thomson scattering diagnostic has been re-installed, following a major space-saving reconfiguration during the up-to-air. Removal of obsolete hardware and infrastructure helped to accommodate the recent rotation of the DNB, and will make room for the addition of eight new polychromators from General Atomics, delivery of which is expected in April. Re-commissioning of the existing diagnostic components is almost complete. The hard X-ray array used for diagnosing the fast electron tail generated during lower hybrid current drive experiments is operational. All capillaries displayed in the control GUI of the NINJA localized in-vessel gas puffing system (used to support several diagnostic systems as well as for fueling) have been tested. The capillary at the A-B limiter location was used during plasma experiments last week. Five video views of the plasma (Wide1, Wide2, DANT, JANT, and DIV2) were operational during operations last week. The new inner-wall WASP scanning probe was exercised and is ready for initial plasma experiments. Travel and Visitors ------------------- Steve Scott (PPPL) presented an analysis of performance projections for the ITER MSE diagnostic at the 12th ITPA meeting in Princeton, NJ. Randy Wilson (PPPL) was at C-Mod on Wednesday through Friday to participate in the Lower Hybrid experiments. Catherine Fiore attended the semi-annual meeting of the APS Committee on the Status of Women in Physics at APS headquarters in College Park, MD. Troy Carter visited from UCLA on Thursday and Friday March 29-30. Thursday morning he met with Liang Lin and Eric Edlund to tour C-Mod. He met with Darin Ernst Thursday afternoon to discuss transport physics in the LAPD experiment at UCLA. Friday, he met with Jan Egedal to tour and discuss VTF (Troy did his dissertation on the MRX experiment at Princeton), and then with Martin Greenwald. After lunch he talked with the edge group (Dennis Whyte, Brian Labombard and Bruce Lipschultz). He gave the PSFC Seminar on measurements of turbulence on LAPD titled "Studies of Turbulence and Transport in a Linear Magnetized Plasma: Intermittent Turbulence and Transport Suppression Through Biasing" on Friday afternoon. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly