Alcator C-Mod Weekly Highlights September 21, 2001 The C-Mod inspection continued this week with detailed measurements of TF joint resistances and the removal and documentation of spring plates. Work continues on ICRF, Lower Hybrid, and divertor systems. Physics ------- Transient transport analysis of sawtooth perturbations has been begun for ITB discharges. The analysis is based upon a time-to-peak technique described in Fredrickson et al, NF 26 (1986). The primary data is soft x-ray data since the ECE data is often cutoff. The radial profile suggests a discontinuity in the time-to-peak data when an ITB is present. The analysis also suggests that the barrier is initially near the plasma edge and moves into the core stopping near r/a~0.5. Further analysis and modelling is under way. The fluctuation data measured by the phase contrast imaging diagnostic shows no significant change in the density fluctuation level from 2 to 500 kHz. Since the PCI is a chordal measurement, we conclude that the fluctuations are not suppressed over a large cross section of the plasma during the ITB. The TORIC code for the Ion Cyclotron Range of Frequencies has been used to explore the interesting 2-dimensional structure of the mode conversion region in H-D-He3 plasmas, where the launched fast wave converts to an ion Bernstein wave near the center of the plasma. Results from the code can be compared to the Phase Contrast Imaging (PCI) data of that region that was obtained in last year's run campaign. High resolution TORIC simulations using 161 poloidal modes in the Fourier expansion of the RF electric field are now routinely run on the C-Mod cluster (using an alpha workstation, run-time is 36 hours). Many different toroidal mode numbers have been run, simulating the effect of the full antenna spectrum in C-Mod. Each individual mode is asymmetric about the midplane, and for toroidal mode numbers below ~15, the positive and negative modes are not mirror images. The sources of asymmetry include: the presence of Landau damping, which destroys the cold plasma symmetry, a shift in the poloidal mode spectrum due to evanescent gap at the edge, the size of which varies for different poloidal mode numbers, and the presence of a poloidal field, which causes the direction of the B field relative the gradient of B to be different above and below the midplane. In order to compare the TORIC results to the PCI data (which measures density fluctuations), the RF fluctuating density must be calculated using all three electric field components from TORIC. The most important result from this calculation has been to realize the dominance of the parallel electric field contribution to the fluctuating density near the mode conversion region. Normally these waves are studied considering only E+ and E-, the left and right-hand circularly polarized components, which are much larger than the parallel electric field. For this reason the PCI results were puzzling at first, because they did not seem to match the E+ and E- field pattern. However, by including the effect of the parallel electric field, the TORIC results are beginning to match the experimental measurements, and are pointing the way to what should be done in future experiments to understand the mode conversion process better. Inspection ---------- TF magnet joint resistance measurements were completed this week with all readings well within the acceptable range. Having completed these measurements, the removal of the magnet spring plates began. The spring plates provide the pressure required to maintain contact between the finger joints and the feltmetal pads. A new hydraulic tool was used to extract the spring plates, and record the force required as a function of plate insertion depth. These measurements will allow us to assess the uniformity and magnitude of spring plate pressure. Fixturing is now being setup to remove the TF arms and legs. A direct inspection of the feltmetal pads will then begin. Lower Hybrid MIE Project ------------------------ Work continues on the LH water cooling system. Piping (6" and 8") has been fabricated and installed from the basement into the power room. Holes for the piping have been cored from the power room into the cell. Construction of the concrete pad needed to support the HVPS has been completed by our contractor. Delivery of the HVPS is still scheduled for early October. Plans are being confirmed for transport of the supply from New York harbor to Cambridge, and for a crane to install it on the pad. Work continues on the TPS, optical transmitters and receivers, phase and amplitude control system, and PLC software and hardware development. Work is nearing completion on re-wiring the klystron filament power supply controller. Work is proceeding on wiring the three transmitter control and supply cabinets. Work is also proceeding on the wiring of the three transmitter EMI/RFI control and protection cabinets. The HV junction box for the HVPS has been mounted, including the interface box. The HV junction box optical interface and relay control board was designed and sent out for PCB fabrication. ICRF Systems ------------ Antenna modelling of modifications to the J-Port antenna suggests a more aggressive approach is required to reduce the electric field at the ground bridge. Driver grid and screen spark gaps were installed and set in FMIT#1-#3 (already installed on FMIT#4). The spark gaps protect the tubes and prevent the grid and screen capacitors from exceeding their design voltage during a tube arc. FMIT#3 door solenoid was also repaired. Work on the rf control and demodulator systems continues. Our vacuum test stand has been moved into the rf lab where access to rf power at the 100 kW level is available. Rf breakdown tests will be performed on critical components using this facility. Inner Divertor -------------- We continue the invessel fitup and associated measurements. Results from the fitup will be used to specify coating thicknesses for the C-plates. We expect the fitup to be completed in the next few days, after which installation of the new studs can begin. A small stud welder has been developed to allow installation of the inner wall studs without removal of the outer divertor. It is currently working well for new stud installation, but needs some development to operate reliably for the stud-on-stud application. We also continue work on new diagnostics for the divertor and relocation of old ones. DNB and Related Diagnostics --------------------------- Design of a pumping system to lower the duct pressure and hence reduce attenuation of the beam in the duct is underway. Some potentially useful components have been identified, and modifications to the F-port flange are being studied. The other occupants of the flange have begun their redesign process to accommodate various proposed modifications. The BES/MSE optical system was returned to PPPL where Ron Bravenec (UT-FRC) and Howard Yuh (PSFC) are working with PPPL engineers to understand its performance during the C-Mod campaign and to identify problems with the design. Work was done to measure throughput and focal lengths of the invessel optics used in the last run campaign. Results show significant degradation of internal optics throughput, assumed to be caused by plasma disruptions and glass dust on optical components. Howard and Ron discussed new upgrades/repairs of all MSE/BES hardware with Russ Feder (PPPL), the new mechanical engineer assigned to the project. Trips and Visits ---------------- Martin Greenwald was in Santa Fe last week to attend a meeting of the ESnet steering committee and to participate in a review of the ESnet program. From Sept 5 to 7, Amanda Hubbard attended the 8th IAEA TCM on H-mode physics and Transport Barrier Physics, at NIFS, Toki, Japan. She presented a poster on "Evolution of Pedestal Profiles through the L-H and H-L Transitions in Alcator C-Mod", and also one on "Double Transport Barriers in Alcator C-Mod" on behalf of John Rice. On Sept 10-12, she attended the first meeting of the International Tokamak Physics Activity (ITPA) on burning plasma transport, also at NIFS. Bruce Lipschultz was at JET from Sept 5 to 7 to work on JET data related to SOL transport and main chamber recycling. This collaboration is with Guy Matthews, Kevin Erents and Wojtek Fundamenski. He looked at SOL data from a number of shots, discussed improvements in diagnostics and potential joint experiments. He joined Brian LaBombard in Aix en Provence to attend the IAEA Technical committee meeting on Divertor Concepts organized by the Cadarache group. Brian LaBombard gave a talk on cross-field transport in the SOL. Bruce Lipschultz gave a talk on the origin of neutrals in the main chamber of C-Mod.