Alcator C-Mod Weekly Highlights July 10, 2000 Operations ---------- Alcator C-Mod is now in a scheduled maintenance period, which will last until September. Following the last plasma discharge on June 30, the C-Mod Cryo System was shut down, and the warm up of the machine was begun. By Thursday of last week all machine components were well above 0 C and preparation for invessel work was begun with a backfill of nitrogen gas and D2O. The heavy water is expected to react with the boron hydrides on the vessel wall and allow safe manned entry into the vacuum vessel without adding large amounts of hydrogen to the wall inventory, as would occur when venting to room air. The removal of the hydrogen and the reduction of the H to D ratio to a point suitable for ICRF heating experiments has been a major delay in obtaining research grade plasma discharges in the past. We hope this new technique will shorten the startup process somewhat. Entry into the vessel is expected on Wednesday of this week following two days of Thomson scattering calibrations. Preparations for invessel work are proceeding well as we are nearing the short machine vent scheduled for this week. The work will focus on the modification of the 4 strap antenna, and consists mainly of enclosing the sides of the antenna in order to increase its heating efficiency. In addition, we will substitute all molybdenum protective tiles with boron nitride ones on all antennas (including the two 2 straps). The new tiles are ready to be installed after completion of a high temperature bake early last week. A final MIT local review of the changes to the j-port antenna was held last Thursday in which all aspects of the invessel changes were discussed. Disruption forces, new plate design, location of and hold down hardware for the BN tiles, capacitor layout and soldering techniques, and plasma shields were all discussed. As time allows, small invessel tasks, such as light calibrations for BES, MSE, and camera views, will be completed in parallel with the antenna work. Flux loops will be examined, connections on the second Lyman alpha detector will be checked, as well as for a emissive probe, and a flux probe. Finally we will try to install the new fluctuation imaging system built by S. Zweben (PPPL). Preparations for our first period of long pulse operation continues. Simulation of coil temperature rises during the planned 3.5 second flattops indicate a large safety margin. Calculations of power supply component junction temperatures are proceeding and have indicated which components will need to be monitored. The design and procurement of instrumentation needed to monitor these critical components is proceeding. Upgrades to the C-Mod glow discharge system are underway. Fast IGBT switches are being installed to allow for current rollback control during arcs. This change will eliminate damage to invessel components during glow discharge campaigns. In addition, improvements to the gas control system will allow more automatic operation of the glow system, and the addition of RF chokes to the leads to the invessel glow paddles will eliminate RF pickup by diagnostics near the glow system electronics rack. Progess is also being made in the upgrades to the cryostat and igloo we plan to make during the machine inspection late next Fall. Preliminary designs have been completed. These changes will allow for the easy removal and installation of the horizontal ports to fix leaks, much better thermal insulation of the ports from liquid nitrogen, and new instrumentation ports through which to bring cabling out of the igloo. ICRF System: ------------ We began an inspection of D-port coax for evidence of arcing. This antenna did not condition well this past campaign and would trip consistently on a specific pair of faults (DC0 arc detect and phase balance). The resonant loops and a portion of the 9" coax have been inspected without finding any significantly damaged component. The inspection will continue this week. The J-port antenna modifications were reviewed last week. The plan is to RF ground the antenna structure, ground the end front tile and the side plate, and replace the Mo protection tiles with BN. We will begin with an inspection of the antennas at first invessel opportunity. Analysis of heating experiments with the J-port antenna configured as a dipole (center strap pair) show heating efficiencies ~70-75% (similar to D and E-port antennas). This suggests that proper RF grounding of the antenna should improve its 4-strap performance. DNB System: ----------- The DNB was operating most of the last week of plasma runs. However, a minor fault in the HV system led to an early shutdown in beam operation. Initial measurements of the magnetic field near the beam were completed and will be used to verify the effectiveness of the magnetic shielding of the beam. Preparations are now underway for realignment of the beam, improvement in monitoring of the beam plasma source, improvements in vacuum gauging, recalibration of the thermocouple system that monitors beam profile and energy, tests of the CXRS window transmission at F-top, and tests of CXRS fiber transmission. These tasks will be completed prior to restart of C-Mod operations. The BES diagnostic measured good signal levels from the hydrogen DNB during the week of 6/12. The signals were up to factors of 2-3 over background. The absolute intensities of the beam signals were compared with our model and agree within a factor of two. Matt Sampsell (UT-FRC) was at MIT that week to coordinate the BES measurements. A poster was delivered at the HTPD Conference in Tucson describing the BES model used to interpret the data. Lower Hybrid System ------------------- Testing of the first seven klystrons (all those presently in-house) has been completed. Four are in nominal operating condition; two (S/N 112 and 115) experienced high voltage breakdown in the vicinity of the flying leads; and one has a shorted filament. Physical inspection of tube 115 revealed that the cathode and heater leads showed signs of cracking, which could lead to the observed breakdowns. A plan has been devised to repair these klystrons, and parts for the repair hve been ordered. We are coordinating with Princeton for the shipment of the remaining nine klystrons, which are presently at PPPL. Diagnostics ----------- David Winslow (UT-FRC) completed installation of the Texas Scanning probe. It has now been successfully reciprocated both during ECDC and tokamak discharges. Mechanical limitations in the probe drive system prevented measurements near the separatrix. These will be corrected during the current break and it will be ready for the next section of the campaign in September. The UT-FRC ECE system was returned to operation. As noted in previous reports, repairs were required mainly due to blockages of the waveguide and faulty regulation in a power supply. These repairs have now been completed. By the end of C-Mod operation in June, 30 of 32 channels were back in operation and measuring electron temperature. Malfunctioning components in the remaining two channels have now been replaced. With the recent installation of the Diagnostic Neutral Beam, the Neutral Particle Analyzer had to be temporarily removed from the tokamak. It has now been reinstalled in its new position on B-port. It is an E||B type analyzer, with 39 channels for each species (H and D). Its new view will look at the center of the discharge, from slightly above the midplane, nearly perpendicular to the magnetic field. It will be used for characterization of the tail population, and help us determine the hydrogen concentration during ICRF heating. The Tangential Two Color Interferometer (TTCI) began operation near the end of the run period. This single chord diagnostic monitors the plasma edge density at 0.88 m and uses a diode pumped frequency doubled laser to provide the two colors for vibration subtraction. The transitions into and out of H-mode are clearly seen, are of the expected magnitude, and correlate well with other edge diagnostics such as the edge diode bolometer array and visible bremsstralung. The cause of a low frequency drift in the phase measurement must still be determined and fixed. Typical C-Mod pedestal densities result in peak phase shifts of 0.03 to 0.05 fringes, depending on pedestal location and width, so vibration subtraction requirements are quite severe. Travel and Visitors ----------------- Cynthia Phillips was at MIT last week (July 5-7, 2000) working with Paul Bonoli as part of the C-Mod/PPPL collaboration. We are carrying out TRANSP modelling of the 4.5 T density barrier discharges that were achieved on C-Mod (with emphasis on shots 1000607007 and 1000607008). We are using the new upgraded FPPRF ICRF package in TRANSP, which now includes the TORIC ICRF code. The results of this transport analysis will be reported as part of an IAEA poster presentation in Sorrento Italy in October, 2000. Ricky Maqueda (LANL) was onsite working with the LANL fast camera during the last week of operation. Stewart Zweben (PPPL) was at C-Mod during the last week of operation making preliminary measurements in preparation for his fluctuation imaging diagnostic.