Alcator C-Mod Weekly Highlights June 30, 2003 Last week was a maintenance week at Alcator C-Mod. No runs were scheduled. Plasma operations will resume this week. The C-Mod Experimental Program Committee met at MIT on Thursday, with remote participation from PPPL, U. Texas, and GA. Thirteen MiniProposals, most for experiments to be carried out during the remainder of the present campaign, were considered at this meeting. A tentative run schedule has been developed. Operations ---------- No plasma operations were scheduled. The tokamak was maintained under vacuum and 60C bake as the engineering staff performed preventive maintenance tasks and worked on power system improvements. The Netware server PC for the Paragon interfaces to the engineering and diagnostic PLC's was replaced with a more reliable computer; the old server had experienced several crashes in recent weeks, necessitating reboots of all of the Paragon stations. A second TMX (slave) supply to increase the voltage/current available to drive the non-axisymmetric control coils was installed in the power room. This includes AC input wiring and high current DC wiring and connection at the slave supply, as well as control circuit wiring in both master and slave supplies. The PLC programming work to operate the second supply has also been completed. The original TMX master supply is still connected and ready for use during this week's operation. Some additional wiring and testing of the master/slave configuration into a dummy load will be required before the combined power supply is ready for use. Physics -------- At the Monday group meeting, Robert Granetz showed results of observations of beam heating by the diagnostic neutral beam in low density, low current ohmic discharges. The beam power into the plasma is of order 100 kW, primarily in the form of 50 keV hydrogen ions, injected perpendicularly to the magnetic field. This reasonably mimics the ICRF H minority heating scheme. The increase in plasma total stored energy due to the DNB was roughly 1 kJ, which is just discernible on the EFIT calculation of stored energy. Plasmas with locked modes generally show less efficient DNB heating, possibly indicating enhanced loss of fast perpendicular ions. With the new long-pulse DNB, expected in early 2004, it will be possible to modulate the beam throughout the discharge, allowing energy confinement analysis of ohmic, L-mode, H-mode, ITB's, etcetera to be systemically compared in a single discharge. ICRF Systems ------------- Initial current drive phasing tests for the J-port four strap antenna were successfully completed. The phase control settings were determined from vacuum measurements and agreed well with expectations. The J-port loops have been reconfigured for heating phase for this week's runs. Operation of the 500W Kalmus amplifier on transmitter #2 was verified to be qualified for long pulse (5 sec) operations. Lower Hybrid System ------------------- The location of the arcing in the circulator assembly has been determined by extensive testing to be near the center of the circulator itself. The unit is being crated and will be shipped back to the vendor for evaluation and repair. Preparations for power testing of the lower hybrid launcher are proceeding. All of the rigid elbows required for the test have been received. We also received nine out of twelve waveguides with flanges (12' long) required for the test. All of the brackets for holding the waveguides have been installed in the cell. DNB Systems ----------- The DNB turbopump which failed last week was temporarily replaced with a spare, and the beam is operational for this week's runs. The failed pump will be repaired and returned to service after the end of the present run campaign. Diagnostics ----------- Work continued on bringing up the NSTX imaging Xray Crystal Spectrometer. The crystal was rotated slightly to improve the centering of the argon spectrum on the detector. The crystal was also masked down to reduce the detector count rates to acceptable levels. Calculations indicate that the etendu of this spectrometer is much higher than that of the single chord C-Mod crystal spectrometers, consistent with the observation of symptoms of high count rate and detector/electronics saturation during C-Mod discharges. Measurements were made to better understand the apparent high deadtime fraction of the signal-processing electronics at high count rates, and to assess the individual contributions of the TDC and the KBSI interface box to this deadtime. Measurements on C-Mod plasmas with this instrument will continue during the upcoming week's operation. The 140 GHz channel of the reflectometer diagnostic system needed some small retuning to compensate for temperature-induced frequency drifts. The 130 GHz channel was also modified so that it can be tuned easily to allow for frequency drifts due to temperature changes. Travel and Visitors ------------------- On Monday, 6/23, Larry Dudek, Doug Loesser, Rich Hawryluk, Joel Hosea, Ned Sauthoff, Gerd Schilling, and Skip Schoen, all from PPPL, visited MIT for a meeting on the Lower Hybrid Launcher. Scheduling of the plating, brazing, and testing activities was discussed. Attending the meeting from MIT were Ian Hutchinson, Earl Marmar, Ron Parker, Miklos Porkolab, Jim Irby, Bill Beck, Bob Childs, and Rui Vieira. Manfred Bitter and Ken Hill (PPPL) were at MIT this week making further improvements to the NSTX imaging XCS. Gerrit Kramer was at C-Mod 6/23-25 working on the reflectometer diagnostic; he also installed his reflectometer data analysis program on the Linux cluster to facilitate analysis of fluctuation data.