Alcator C-Mod Weekly Highlights December 7, 2001 We continued the C-Mod machine inspection this week. Work on the new DNB, the Lower Hybrid MIE Project, and new inner divertor also made progress. Physics ------- The reliability of fluctuation-induced particle flux profiles computed from the A-port scanning probe have been analyzed for a series of ohmic discharges. The measured flux profiles and the cross-field heat convection profiles that they imply have been compared to separate estimates of the cross-field particle and heat flux profiles. These separate estimates are based on global particle balance (obtained from measurements of ionization profiles and flows towards the divertor), sound speed flow towards divertor surfaces, and heat fluxes arising from parallel electron conduction in the scrape-off layer (SOL). Similar to results reported from other experiments, it is found that the fluctuation-induced particle fluxes significantly exceed the flux estimates from particle balance models. The implied heat loss from the fluctuation-induced convection also appears to be too high; it would exceed the total input power in most discharges. Also, in discharges where the probe could be inserted well inside the LCFS, the radial-outward fluctuation-induced particle flux was found to continue to increase in magnitude, in apparent contradiction to satisfying particle continuity. These observations and others suggest that the fluctuation-induced particle flux estimates made by the C-Mod probe should not be interpreted as indicative of the transport in the unperturbed plasma. An alternative explanation is suggested: the measured fluctuation-induced particle flux is simply the cross-field plasma flow that fills-in the presheath zone formed by the probe. Two separate estimates of effective cross-field particle diffusivities in the presheath zone have been made. The magnitude of these diffusivities are found to be in the range of typical diffusivities seen in the far scrape-off layer (based on global particle balance estimates), lending some support to this new interpretation of the measurement. However, the presheath diffusivity does not appear to be directly related to the diffusivities found in the unperturbed plasma. Thus the fluctuation-induced particle flux 'measurement' made by the probe appears to provide no direct information on the transport level in the unperturbed plasma. Inspection ---------- The OH coaxes have been successfully silver plated in preparation for reassembly. New TF spring plates are being fabricated in our shop. These plates are used to hold the TF finger joints in place and their replacement is a routine part of the machine inspection. A new hydraulic system for inserting the plates is being developed. Lower Hybrid MIE Project ------------------------ Following resolution of remaining LHCD launcher design chits, requisitions have been signed at PPPL for bids on a variety of components. These presently include 5 front couplers (one spare), the front coupler flange, the interface flange, the upper and lower forward stacked waveguides, and the stacked waveguide flange. An RFQ is out for the fabrication of a three-channel full power launcher prototype for bench testing. All packages for procurement this fiscal year will be sent and received by early January 2002. Many of the special materials have been purchased and are in house (titanium). DNB and Related Diagnostics --------------------------- A support for the new DNB has been partially designed and will soon be ready for review. The support will bring the beam centerline up to the C-Mod centerline and properly distribute its weight onto the supporting diagnostic stand. The software to integrate the beam firing into the C-Mod shot sequence and allow test shots while avoiding collisions with C-Mod shots is in preparation. Modifications to the program used for the previous beam have been identified. The vacuum system design for the beam is largely complete and parts are being selected for order. A large number of vacuum parts from the previous beam will be recycled. Preliminary plans for cooling water, N2 gas and LN2 supply have been made. Next week, as we complete some of these tasks, we will be adding the DNB gas and vacuum control systems to those projects already in progress. Inner Divertor -------------- All divertor backing-plates and the associated Rogowski coils and thermocouples are in place on the inner wall. We have begun installation of the C-plates and additional thermocouples. We expect to have all major divertor components installed within the next two weeks. Travel and Visits ----------------- Martha Redi, PPPL, visited CMOD this week, continuing work simulating microinstability turbulence of C-Mod RF ITB experiments. Montgomery Grimes and Peter Koert visited Backnang, Germany last week to discuss two major rf components with a vendor. The circulators for the Lower Hybrid Project were reviewed, and plans for the loan to MIT of a fast ferrite tuner for our ICRF program were discussed. Steve Wolfe and Ron Parker participated in the DIII-D Physics Advisory Committee (PAC) meeting in San Diego.