Alcator C-Mod Weekly Highlights July 31, 2000 Invessel work on Alcator C-Mod was completed the middle of last week. After final cleanup, documentation, and diagnostic alignments the vacuum vessel was closed up and pumped down on Friday. Also on Friday, the vessel heaters were turned back on and a GN2 purge was begun of the cryostat and bus tunnel. A leak on a newly installed flange was found and fixed on Saturday, and more sensitive leakchecks were performed today. No additional leaks have been found. We plan to begin LN2 cooling of the machine structure and a 120C bake of the vessel later this week. Invessel Work ------------- The J-Port antenna is now rf shielded top and bottom, and rf currents are bypassed through ceramic capacitor arrays on the left and right sides. These changes should considerably reduce or eliminate the parasitic power losses to plasma surface waves, which we think have been responsible for the previously observed low heating efficiency. Both the J-Port antenna and the two older antennas at D- and E-Port have also had their molybdenum protective tiles replaced by boron nitride to reduce metallic impurity influx to the plasma. Alignment of the tangential two-color interferometer and new charge exchange analyzer, replacement of mirrors boronized during the last run period, extensive documentation using digital and 35mm cameras, vacuuming and wipedown, and final inspections were also completed last week. Pictures documenting the invessel status at pumpdown can be found at http://www.psfc.mit.edu/cmod/operations/EngImages/CmodImages.asp?ref=/INVESSEL/2000/Close_Up_Survey/ Other Engineering Activity -------------------------- Nine klystrons needed for our lower hybrid experiments which had been on loan to PPPL were returned to MIT last week. All 16 tubes from the Alcator C Lower Hybrid experiment are now back on site at the PSFC. The first of the nine returned tubes is now being prepared for installation into our klystron test stand. The first of two delrin insulators needed for repair of two other klystrons is now ready for installation. The J-Port external resonant loop, coupling the antenna to the transmitters, has been reassembled and checked for resonance in the two 80 MHz phasing configurations: (0,180,180,0) and (0,180,0,180). It has also been tested for operation at 70 MHz. Operation of the D- and E-Port antennas at their nominal 80 MHz combined with J-Port at 70 MHz will allow us to form the highly peaked discharges discovered in June by ICRF heating off-axis, while simultaneously using the lower frequency power for on-axis heating near the density peak. Chris Brunkhorst (PPPL) came to MIT 7/24-27 and worked on optimizing the tuning of the #3 and #4 ICRF transmitters that are coupled to the J-Port antenna for operation at 70 and 60 MHz. 70 MHz operation is needed for the off+on-axis heating scenario described above, while 60 MHz is intended for current drive experiments. Work continued last week on the DNB and associated diagnostics. The alignment of the beam was measured and adjusted. The field generated by the DNB dump magnets was measured. The transmission of the F-Top CXRS window before and after cleaning was measured. The tangential CXRS optics were backlighted to assure that no mis-alignments had occurred during the several months since installation. The upgrade to the glow discharge and boronization system is moving very quickly and should be ready for operation within two to three weeks. Upgrades to the power system instrumentation in preparation for long pulse operation are also progressing well. Physics ------- Very low density H-modes produced in June as part of an exploration of the low density H-mode limit have recently been analysed. Most of these discharges were ELM-free, with high pedestal temperatures and modest density rises. The edge collisionality was correspondingly lower than in most EDA H-modes. It is possible that this prevented access to the EDA regime. However, the discharges had q95 ~ 3.5-3.7, which is on the low side for EDA access, so this result is not conclusive. Several discharges showed isolated ELMS, which occurred at fairly high edge pressure and temperature, unlike our usual Type III ELMs. These may have been Type I ELMs, which are seen regularly on other machines but not normally on C-Mod. Further experiments to clarify the ELM type and the effect of density on the EDA/ELM-free boundary are proposed for September. Travel and Visits ----------------- Rejean Boivin travelled to INRS-Energie et Materiaux, located in Varennes Quebec, for the thesis defense of Irina Condrea, who did research on rotation measurements in L and H modes. He also attended, along with Martin Greenwald, the APS program committee meeting in Washington for the upcoming APS-DPP meeting in Quebec. Chris Brunkhorst visited from PPPL to work on improved tuning procedures for the #3 and #4 transmitters. Tom Fredian visited PPPL to analyze the use of the MDSplus data system on the NSTX experiment and make suggestions for improved performance and functionality. The system is installed and working well. A few fine tuning suggestions were made. Gabriele Manduchi from Padova (one of the original authors of MDSplus) visited Tom Fredian, Josh Stillerman and Stuart Sherman to discuss new MDSplus features and future development plans. David Winslow (UT-FRC) visited the PSFC during the vent to remove the Texas probe and get an absolute position calibration. After modifications and prior to beginning of the next campaign, the probe will be reinstalled to take data during the remainder of the campaign. Pablo Acedo (Carlos III University, Madrid) has returned to MIT for 6 weeks to continue our collaboration on interferometry. John White, Juergen Alex, and Walter Kaufmann from Thomcast Radio Systems, Inc. visited last week for two days of discussion on the high voltage power supply/modulator Thomcast will supply for our Lower Hybrid experiment. Items covered included contractual clarification, and technical issues such as electrical interfaces, testing procedures, protection circuits, and PLC software.