Alcator C-Mod Weekly Highlights April 20, 2001 Invessel installation of the modified J-Port ICRF antenna is expected to begin this weekend. Invessel work on DNB related diagnostics was completed, and we continue to work on the LH MIE Project and power systems. Physics ------- We continue to analyze the dynamics of off-axis ICRF-induced core transport barriers. In these discharges, the barrier formation follows the transition into an EDA H-Mode, at slightly reduced toroidal magnetic field, which places the hydrogen minority resonance for 80 MHz ICRF to the high field side. In typical cases, the density begins to peak, with a strong gradient in pressure approximately out to the half minor radius. The peaking of both majority and impurity densities can lead to radiative collapse and subsequent disruption. In a series of experiments performed in December, 2000, we used our tunable RF sources to apply heating at 70 MHz from the 4-strap antenna, while using the 2 dipole antennas at 80 and 80.5 MHz respectively. After the formation of the core transport barrier, application of the lower frequency RF, for which the minority resonance is close to the magnetic axis, can lead to the arrest of the density and impurity buildup. In some cases, the peaked pressure profiles are maintained over many confinement times, while the discharge continues to exhibit sawtooth activity. An MPEG movie of the density profile evolution, which demonstrates a number of the interesting properties of one such discharge, can be downloaded from www.psfc.mit.edu/people/marmar/denmovie2.mpg The movie shows the density profiles at 1 msec time intervals, along with the ICRF powers, in MWatts, at the two different resonance locations. TRANSP analysis of these discharges, including the measured ion and electron temperature profiles, is ongoing. Recombination profiles in the Alcator C-Mod divertor have been investigated using the tangentially-viewing divertor CCD camera, the visible spectrometer, and the flush-mounted and scanning Langmuir probes. It has been observed that during a density scan, the recombination rate on the outer divertor leg remained small until it detached, and then increased and remained constant as the density was increased beyond the detachment threshold. The recombination rate in the private flux zone increased until detachment of the outer leg, when the recombination rate was reduced to near negligible levels and continued at that level as the density increased. The recombination rate on the inner leg decreased until the core density was approximately 1.7 X 10^20 /m^3 (which is before the outer leg detaches) when the recombination rate increases with increasing density. After comparing the 2D recombination profiles from various densities, it is hypothesized that the cause of the plasma in the private flux zone and density dependence of the recombination rates in the three regions, may be due to an ExB drift caused by a parallel Grad(Te) term. Total particle current entering the private flux zone through the outer divertor leg estimated by recombination profiles and probe measurements, and the expected particle current due to the ExB drift quantitatively agree to within the experimental error. The recombination profile of a high density, reversed field discharge yields qualitative agreement with the hypothesis of the ExB drift. Further investigation is continuing using the B2-Eirene code. ICRF Systems ------------ The striplines, which were baked over the long Holiday weekend, were removed from the oven on Tuesday. Following a final polish and cleanup, they are now ready for installation invessel. Work concentrated this week on the installation of 0.03" copper plated steel covers needed as part of the rf current return path. These covers were e-beam welded to the Faraday screen modules and then polished to eliminate any chance of arcing in this critical area. This process, together with some other small tasks, took more time than expected, and we have lost some ground on our schedule. We expect to complete antenna installation, and invessel cleanup and documentation in the coming week. Gerd Schilling has taken the old stripline and septum support components back with him to PPPL. Randy Wilson plans to assemble them in the PPPL rf test stand and collect data on rf breakdown effects. They will also be able to apply a magnetic field during the tests using a set of PLT magnets. These tests are aimed at improving our understanding of the behavior of the original feedline configuration in C-Mod. The new J-Port pressure gauge and mount has been installed. The gauge will be ready for operation following installation of the controller, cable, and link to CAMAC. Work continued on the phase fault system. All the needed arc/phase detector modules have been upgraded with new faster phase detector chips. System testing and calibration of the phase balance boards with their associated arc/phase demods are ongoing. Diagnostics ----------- Both of the CXRS shutters have been tested by manually pulling on the shutter control wires. All in-vessel fiber optics and guide tubing for the shutter actuation wires have been spot welded in place and clamped down. The exterior vacuum vessel work is also nearly complete. Only the attachment of the wires to the feedthroughs remains before all flanges can be torqued down. We have begun an upgrade to our ratiomatic gauge controllers used for pressure measurements on C-Mod. A prototype was tested on C-Mod during the last run period. A new PC board incorporating the changes is being being developed. Lower Hybrid MIE Project ------------------------ The detailed electrical design for the transmitter cart and equipment racks has been completed. Work can now begin on the waveguide components. The first TPS (transmitter protection system) PC board has been released for bids. Design has begun on the safety key-lock interlock system. A test stand for simulating arcs on the lower hybrid coupler became operational this week. A 3kW magnetron source at 2.45 GHz drives a waveguide arc that will be used to test arc detection electronics. One of our new serial fiber optic links (SFOL) has been packaged and is ready to undergo testing in the LH phase and amplitude control system. Programs have been written to do open loop tests with the SFOL, I/Q detector, and vector modulator, after which the closed loop tests will begin. Power Systems ------------- New CAMAC and current and voltage transformers have been added to our hi-yard diagnostics. These signals will allow us to better monitor our power systems, particularly the new lower hybrid power supply. Travel and Visits ----------------- Ed Synakowski from PPPL came to C-Mod on 4/20 for discussions on the off-axis ICRF-stimulated internal transport barrier discharges and to present the PSFC Seminar on "The NSTX Program and an Initial Look at Local Physics." Gerd Schilling (PPPL) spent the week here working with us on the J-Port antenna modifications.