Alcator C-Mod Weekly Highlights October 13, 1998 Engineering: Assembly of Alcator C-Mod continues. Final alignment of the TF core and OH stack has been completed along with installation of the lower wedge plate. The upper wedge plate, which is needed for the alignment, is also installed, but only temporarily. It must be removed before the machine is lifted to its final position in the center of the cell to reduce the load on the crane. The OH2 lower coax was retorqued several times over several days to insure that all components have relaxed to their final state. Resistance checks of the feltmetal contact resistance indicated proper installation of the coax in terms of contact area and pressure. These resistance checks of both the upper and lower coaxes will be repeated throughout assembly. Ultrasonic cleaning of the TF arms continued last week. Both the feltmetal and the plated surface ends of the arms have been processed through water and ethanol baths to removed any remaining solder flux or plating chemicals. Production soldering of the TF legs was begun over the weekend after a problem with solder flow in the shoulder region of the leg was solved. A small difference in the radius of curvature in the shoulder region of the leg compared to that of the arms caused this problem. Changes to the fixturing that holds the feltmetal to the leg together with changes to the press fixturing that bends the feltmetal were required. Production fabrication of the new spring plates was also begun last week. Enough spring plates have already been produced to install the lower arms. The lower arms are the first ones to be installed and must all be in place before the machine is moved to the center of the cell. The power systems group has begun bringing C-Mod power supplies back online. All gate drives for the EF2U, EF2L, EF4, and EFC supplies have been checked and are operational. The EFC control system has also been checked and made operational. We will continue checking gates, SCRs, fuses, bus instrumentation, and CAMAC over the next few weeks. We continued development of the DNB for C-Mod. Fabrication of the low capacitance transmission line, transition boxes, and cable tray for the cell installation continued. The firing board and the regulator board for the accelerator supply were completed and installed. Design for the analog conditioning board and the fast logic fault board continued. The water content of the oil in one of the supplies was reduced to an acceptable level using the oil dryer. After implementation of a nitrogen blanket for this supply, we will be ready to condition the oil in the remaining supply. The PLC control systems for the new transmitters are debugged and operational. We have moved on to completion of the new CAMAC systems. Arrival of the new PPPL fast wave antenna is expected next week. We continue fabrication and installation of important invessel components. The new edge X-ray array was installed and calibrated last week. Fabrication of divertor bypass flapper components continues as does installation of the bypass conduit and cabling. The new F-side flange arrived in house after fabrication at U Texas. This port supports the DNB and its many diagnostics. The new K-side flange has been machined and welding of components to the flange is proceeding. Both new flanges will soon be sent out for grinding and polishing of the vacuum seal surface. Design of the rangefinder invessel components is nearly complete. Our standard 5/16" invessel studs will be modified to contain the 0.2" diameter retro-reflectors. These studs will be driven onto the vessel wall with a stud gun in the standard manner. Tests indicate that the retro-reflectors easily survive this rather severe treatment. Physics: Jim Reardon has continued his analysis of data from the loop probes on the inner wall, opposite the E-port antenna. His results indicate that the RF power detected by the probes during minority heating depends on the minority concentration in the D(H) scenario but not in the D(He-3) scenario. In D(H), for which the single-pass absorption is expected to be high, the detected RF power decreases monotonically with increasing [H]. This agrees well with the analytic single-pass transmission calculation, and the output of the FELICE code (run with radiating boundary condition at the high- field side). In D(He-3), for which the single-pass absorption is expected to be low, the measured RF fields are much larger than in the D(H) scenario, and independent of [He-3]. Measurements of Stark broadening of the D^0 8->2 line have been analyzed. The measurements are made along a number of viewing chords through the divertor region. When a number of detached divertor discharges over a range in density and heating power were examined it was found that the measured Stark density averaged over either the inner or outer divertor scaled as nebar^0.75xP_SOL^0.3. This scaling can be roughly explained if one assumes that the pressure in the divertor region is a fraction of the upstream pressure (detachment), that the divertor Te in the emitting regions is essentially constant (0.5 eV), that the upstream density and Te scale as nebar and P_SOL^2/7 respectively. Measurements of D_gamma/D_alpha ratios have also been analyzed. This ratio has been used previously by McCracken and Isler to determine whether or not the Balmer series emission is recombining. We applied the technique to a number of shots and found a wide range of cases. For the detached H-mode plasmas, the D_alpha is primarily excitation-dominated (although high-n lines are mostly recombination-dominated). Ohmic detached plasmas indicated the opposite - much higher values of D_gamma/D_alpha indicating that D_alpha is dominated by recombination. This knowledge was used to interpret flow measurements from detached H-mode shots. We have Doppler flow measurements of He-II and D_alpha. The relative velocity is ~5e3 m/sec. Because the D_alpha is dominated by excitation (thus a neutral velocity) we have measured the relative velocity of ions and neutrals and determined that the transfer of momentum is efficient. Visits and Travel: A quarterly review of the C-Mod program was held at MIT on 10/6. Rostom Dagazian represented DOE, Bill Rowan from U Texas discussed the DNB and related diagnostics, Randy Wilson from PPPL discussed the new antenna and other PPPL/MIT collaborations. Sefano Bernabei also from PPPL, discussed plans for the C-Mod lower hybrid installation. Steve Wolfe discussed FY99 operating plans, Jim Irby discussed machine reassembly, and Miklos Porkolab and Paul Bonoli discussed lower hybrid performance under various operating scenarios. Earl Marmar reviewed budget issues. Bob Granetz gave a seminar at Dartmouth last week on "H-modes and the Edge Pedestal in Alcator C-Mod".