Alcator C-Mod Weekly Highlights September 28, 2001 The inspection of Alcator C-Mod continues. Progress was also made on LH, ICRF, and divertor systems. Physics ------- Data from power ramp experiments, including some new results from August 2001, have been analyzed in collaboration with Ben Carreras of ORNL. In these experiments, edge parameters are measured with high time and spatial resolution throughout a controlled L-H-L hysteresis cycle. An effort has been made to cast measurements in terms of flux vs gradients, in the forms suggested by models of the H-mode as a critical transition. The variation of thermal conductivity chi, in L and H mode, as a function of local gradients can then be directly assessed. Strongly non-monotonic flux-gradient relationships are indeed found. Results are consistent with a dependence of conductivity on temperature gradient, having a very sharp decrease above a critical value, which is independent of density. Other flux-gradient relations are also possible. These results were summarized at a recent C-Mod group meeting and presented at the recent IAEA TCM in Japan. It is hoped that they will stimulate further discussion with other theorists. Analysis of the edge turbulence data continues (MIT/PPPL collaboration). Using the Princeton Scientific Instruments "ultra-fast" camera, that takes 12 sequential 4 usec images in D_alpha light from the separatrix region, we are able to follow the evolution of the "blobs" of emission that have been seen previously in the C-Mod edge plasma. We believe that they reflect the "blobby" nature of the density there. The characteristic size of the brighter "blobs" is slightly less than 1 cm in both the radial and poloidal dimension (they have much longer characteristic wavelengths along the field). The characteristic lifetime of the brighter "blobs" is 10-50 microseconds. The time-sequenced images show clearly that these "blobs" move both radially and poloidally. Outside the separatrix the dominant radial propagation direction is outward. Characteristic outward speeds for the some of the brighter "blobs" are as much as 400 m/s, which means that they move a distance similar to their size during a lifetime. In contrast, some "blobs" move very little during their lifetimes. The poloidal velocities for the features that move poloidally are also as much as a few hundred m/s. Eddy-like motion of some features has also been observed. Examples of data from this diagnostic may be viewed at: http://www.psfc.mit.edu/people/terry/edge-turbulence-movies.html A sawtooth heat pulse propagation study has shown that plasma discharges (off-axis ICRF heated) with an internal transport barrier (ITB) may have a notch-shaped transport coefficient profile (Chi_e). The notched region corresponds to a discontinuity in the time-to-peak profile as shown in soft X-ray data. A code has been developed to calculate the Chi_e profile by solving a simplified 1-D transport equation. The ITB region also seems to move inward during the first 10-20 msec of the discharge, which may give a hint of the trigger mechanism. Further study is underway to check the ballistic effects in the heat-pulse study. Inspection ---------- All upper horizontal TF arms and vertical legs have been removed from the machine. The upper wedge and mounting plates have also been removed, giving full access to the upper OH coaxes. A detailed inspection of the TF joints, spring plates, and feltmetal is in progress. No significant problems have been found. Inspection of the OH coaxes will begin next week. Lower Hybrid MIE Project ------------------------ The HVPS has arrived in port (New Jersey). Shipment to Cambridge should take place next week. We continue to prepare the control system, dummy load, cabling, data links, interlock circuits, and other equipment needed to test the supply. Work continues on the TPS (Transmitter Protection System) for the LH klystrons. All HV faults are now indicating properly. HV enable outputs have also been verified for proper operation. The enable outputs have been shown to properly drive the fiber-optic transmitters. The control PLC ladder logic program has been modified so that it will communicate properly with the HVPS PLC when it arrives. We continue working with the contractors on the layout and specifications for the cooling water manifolds. The contractor is making progress on the cooling water piping. Wiring of the EMI/RFI control and protection chassis is proceeding as is the wiring for the transmitter control and supply cabinets. ICRF Systems ------------ We continue to test mockups of the new phase demodulators. A system using a downconvertor with an IF at 10 MHz has been shown to provide very good linearity. Modeling of the antennas and transmission system is proceeding. Inner Divertor -------------- Installation of the divertor components invessel revealed a .020" interference between the c-plates. We are currently making modifications to the plates, and expect to be ready for another trial fitup next week. Stud welding tests, and preps to tile support plates are moving forward. Leadership Award ---------------- Ron Parker has been awarded the Fusion Power Associates 2001 Leadership Award. The citation reads: Ron Parker, former director of the MIT Plasma Fusion Center and former leader of the ITER co-center in Garching, Germany, is recognized for his many scientific contributions to fusion research, his dedication to the production of high performance fusion conditions in the laboratory and his leadership and vision to the cause of developing practical fusion power." His awards states, "You have provided inspiration and technical guidance to the tokamak fusion program and leadership to those seeking to expedite the development of a practical fusion power system." Trips and Visits ---------------- Miklos Porkolab, Ron Parker, and Earl Marmar attended the Fusion Power Associates annual meeting in Washington, Sept. 25,26. Ron presented a talk entitled "Comparative Overview of Burning Plasma Experiments", and Earl presented a talk on the "Alcator C-Mod National Facility Advanced Tokamak Program." Stew Zweben was at MIT 9/24-26 for discussions on the analysis of the Gas Puff Imaging diagnostic results. Gerrit Kramer, Raffi Nazikian and Jay Kung came up on 9/28 to present their latest thoughts on upgrades to the C-Mod reflectometer diagnostic which would extend the measurement of fluctuations to the plasma core; several issues remain to be resolved. Gerd Schilling was at MIT 9/25-28 for discussions on programmatic issues, ICRF antenna modeling and the remaining 4-strap antenna modifications, and to participate in the reflectometer upgrade discussions. Bob Harvey from CompX visited the PSFC (Sept. 24-28). Bob worked with Paul Bonoli on theoretical and computational issues related to coupling the TORIC ICRF field solver to the 3-D Fokker Planck code CQL3-D. They also carried out simulations of lower hybrid current drive for C-Mod using plasmas typical of the density barrier mode produced with off-axis ICRF. The calculations were done with CQL3-D and the GENRAY ray tracing code using an EQDSK MHD equilibrium reconstruction for an actual C-Mod ITB discharge.