Alcator C-Mod Weekly Highlights July 29, 2002 Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. Conditioning of the ICRF Antennas continued, and Physics Operation began with one run day devoted to a Divertor/Edge Group experiment. Work also continued on the Lower Hybrid and Diagnostic Neutral Beam Systems. Plasma operation are scheduled to continue this week. Operations ---------- Plasma operations were carried out Tuesday-Friday last week. Startup reliability for the week was about 70%, with a total of 64 successful discharges being produced. Following Tuesday's run, a small leak was identified at one of the Thomson Scattering laser windows. This window was replaced on Wednesday; at the same time a failed ratiomatic ion gauge at G-side was also replaced. Since these components are located behind isolating valves, it was not necessary to break main vacuum to carry out these repairs. Three of the week's runs were dedicated to conditioning of the ICRF antennas. The target plasmas for these runs were mainly inner-wall limited equilibria, in order to remain in L-mode as the power coupled to the plasma increased above the MW level. The loading changes associated with L-H transitions can lead to increased faulting which detracts from the conditioning process. Thursday's run was in support of MP#313, submitted by the Divertor/Edge Group and Density Control Task Force. The goal of this experiment was to quantify the effective conductances of neutral gas through various pathways in and around the lower and upper divertor structures during plasma discharges. This information will help reconcile some of the puzzles with respect to divertor neutral leakage, and will also help us understand the behavior of gas compression in the upper chamber and allow us to anticipate pressure changes there in response to a future cryopump system. Both standard lower single null and upper single null equilibria were investigated in this experiment. The primary tools were the NINJA capillary gas puff system and divertor flappers, in conjunction with pressure gauges (in-vessel and ex-vessel) and the divertor TV camera system. The new miniature Penning gauge under the B-C divertor module ran better than expected, recording fast pressure changes in this region as the B-C flapper was opened or the B-C Bottom capillary was used to puff gas. The divertor camera (DIV2, viewing D-alpha) observed puffs from the B-C and C-D capillaries located above the B-C and C-D flappers. The camera also was able to record changes in local D-alpha light when the flappers were opened, giving us the possibly to 'calibrate' the flow through the flapper. In addition, the pressure under the B-C flap was recorded by the Penning gauge there, which should allow us to estimate the effective conductance through the open flap in the presence of plasma. Diagnostics ----------- The new tangential HIREX (High Resolution X-ray) spectrometer systems are operational, and observations of toroidal rotation profiles during ohmic and ICRF operation have begun. Following repair of digitizers in the core system, and replacement of the Thomson laser window on Wednesday, the Edge and Core Thomson Scattering diagnostics began taking data last week. The number of FRCECE radiometer channels was increased to the full complement of 32 this week with installation of the detectors that were recently repaired by the manufacturer. The detector problems were due to faulty manufacture. Calibration for recent shots was completed. ICRF Systems ------------ In this week, we ran RF conditioning on Tuesday, Wednesday and Friday. The D-port antenna has been conditioned up to 1.2MW and the E-port antenna to 1.6MW. The D and E modulator boards were modified to accommodate a soft-start after faulting, This modification has been shown to provide more reliable operation. We also changed the phase feedback input system for the four-strap J-port antenna. We added 8309 limiter boards in the feedback path of the controllers to improve the dynamic range of these inputs. We have also developed an operating strategy which allows the phase to lock on startup at low power and then ramp up to higher power. Using this scheme, the J-port antenna has been conditioned up to the 0.5MW level. Diagnostic Neutral Beam System ------------------------------ Work continued to identify the source of the arc current stability problems that have shown up since the power supply arcdown a few weeks ago. Since then, damaged components in several subsystems have been replaced. including fuses, voltage divider resistors, pulldown resistors, bleed resistors, protection diodes, and one arc charging SCR. The final repair was to replace a feedback resistor in the control circuitry for the arc supply. This appears to have remedied the instability in the arc current. The DNB was operated for approximately 10 shots over the weekend, with no faults beyond those normally encountered during conditioning. Additional conditioning shots will be run this week, with beam-into-plasma operation anticipated during C-Mod operation. Work continues on the upgrade to the beam cryo system. The cryo distribution panel for the cell end of the vacuum line has been assembled, and the fill station end to supply the vacuum line has been re-plumbed. Remaining work includes installation of the line through the last diagnostic lab, mounting the vapor vent in the diagnostic lab, and piping the vapor vent through the back wall to the outside. Lower Hybrid MIE Project ------------------------ The back panels for the EMI/RFI cabinets were modified to accept the coax filtering boards. Previously only the cabinet for Cart #3 had this board installed, but there were clearance issues with this panel. The new panel was retrofitted on the equipment rack for Cart #3. The new mechanical arrangement has no clearance issues. The old panel which was removed from Cart #3's cabinet is being modified to address the clearance issues. The 24VDC power supplies for the water system flowmeters arrived this week and are being installed. The schematic for the water interface is being modified to include the new power supplies, and a tech request was generated to provide 120VAC to these supplies. Travel and Visitors ------------------- Phil Ryan of Oak Ridge visited the RF Group on Tuesday to Thursday. He participated in the ICRF conditioning activities during C-Mod operations. Ricky Maqueda from LANL visited C-Mod during the week and re-installed the infrared imaging system. This system is now operational.