Alcator C-Mod Weekly Highlights July 16, 2001 Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed. A total of 27 plasmas were produced, with a startup reliability of about 85%. The primary focus of the week's operation was recovery from the previous week's vent, and reconditioning of the ICRF antennas. Physics operation is scheduled to continue this week. Operations ---------- Following the up-to-helium "clean vent" on Friday, July 6, the vessel was pumped down and a 120C bake was begun. On Sunday morning a low current, low voltage, glow discharge was started in helium at approximately 25 mTorr. On Monday the glow current was increased to 3000 mA at about 300 volts at 10 mTorr. The bake was dropped to 60C on Monday afternoon. These conditions were continued until Tuesday morning when the glow was stopped for several hours for antenna conditioning. The glow was resumed overnight at 3500 mA, 270 V, and about 12 mTorr. The glow was stopped early Wednesday morning and the vessel temp was dropped back to 35C and cooldown of the magnets to operating temp was begun. ECDC in D2 at 2e-4 Torr was run for about 1 hour before beginning plasma operation on Wednesday. The glow cleaning regimen described above differs from our usual post-vent cleanup procedure, which has emphasized baking and ECDC in deuterium. The high quality plasmas produced during recovery shots on Wednesday and Thursday indicate that the helium glow, combined with a sustained high temperture bake, is a highly effective technique. Good plasmas were readily obtained, and the initial H/D ratios were only about 0.3, compared to H/D>1 observed following our previous vent. Plasma operations on Wednesday and Thursday were limited as the engineering staff investigated and remedied a noise problem which caused TF crowbars following several successful shots. However, after only a relatively few conditioning discharges the H/D ratio had dropped to the 10-20% range, adequate to carry out a boronization on Thursday night. The vessel was boronized with about 2000 Angstroms of boron, followed by 2 hours of ECDC in helium at 5e-5 Torr, and 1 hour of ECDC in D2 at 2e-4 Torr. Machine operation continued on Friday, with H/D ratios at the beginning of the day below the 5% level. The main emphasis of this run was reconditioning of the ICRF antennas. Both E-port (80MHz) and J-port (70MHz) antennas were brought up to the 1MW level; this is actually the first time that the J-port antenna has been operated into plasma at 70MHz in its four-strap configuration. The D-port antenna experienced repetitive fault indications that may have been due to a faulty demodulator circuit in the instrumentation. ICRF conditioning should be completed early this week, followed by physics operation. Physics ------- This week we obtained first "movie" images of edge turbulence with the Gas Puff Imaging diagnostic using the Princeton Scientific Instruments PSI-3 camera viewing parallel to B. This camera can capture 12 frames at up to 1 million frames per second. Nice images of the motion of edge turbulence in the radial vs poloidal plane were taken at 100,000 frames per second and above. Diagnostic Neutral Beam Systems ------------------------------- The beam was operated at full voltage and current through the week with no significant problems. The sharing of arc current among the four filaments (which form one electrode for the plasma arc) was checked. Arc current is equally shared to within trimming adjustments that can be made via filament current (same as filament temperature) adjustment. We further conclude that it would be useful to measure this sharing on every shot. We experimented with an operating mode in which the arc is run for longer times for conditioning while we are also generating beams. This resulted in the beam not firing into C-Mod on a couple of shots and will therefore not be used again. Arc conditioning will continue before and after regular operation. This week we expect to increase the length of time that the arc can be turned on during arc conditoning beyond the present 300 ms. An experimental plan for MSE calibration was developed and we await the opportunity to do the calibration which will also further our understanding of reionization. CXRS measurements continue to progress with observation of small signals. BES is able to observe fluctuations in ambient D-alpha emission. Data was taken with beam and will be analyzed and tested for the effects of parasitic fluctuations. ICRF Systems ------------ We completed the reconfiguration of the coax transmission line and tuning the FMIT#3 and #4 to 70 MHz. Before tuning FMIT#3, we had to repair a broken mechanical switchgear. The current transmitter tune is not ideal. The match between driver and final is not fully optimized, but should be adequate at the power levels required for near-term experiments. The arc detection system testing was completed and initial antenna conditioning was completed for the J-port antenna at 70 MHz. The decoupling is not as complete as it was for the 78 MHz configuration; however, it did not limit the operation of the antenna into plasma. Antenna matching calibration factors were obtained in the initial operation and about 1 MW was injected into L-mode plasmas successfully. The D and E-port antennas were operated successfully after the up-to-helium. D-port was fairly unreliable and a malfunctioning fault detector was found and thought to be the cause. The module has now been replaced in preparation for this week's runs. Lower Hybrid MIE Project ------------------------ Work continued on the ladder-logic PLC program. A new design has been developed for the coolant pump controller interface. This design has significant improvements over the initial preliminary design which eliminate a PLC, I/O rack, and dedicated optical interface circuit. These changes reduce the project costs by over $10,000. Circuit design and PC board layout for the Lower Hybrid Timing was completed. This board will provide properly synchronized 6 and 48 kHz clocks for the LH control and data acquisition system. Inner Divertor Fabrication Project ---------------------------------- An independent CMM check on one of the C-Plates confirmed the vendor's data that all 89 critical dimensions were within the required dimensional tolerance. Work on the probe box parts is 90% complete. A design modification is being carried out to add new probes for the bottom portion of the C-Plate and modify 2 probes at the top in order to remove less material from the C-Plates. Mock-up building is moving forward. Material (0.020" thick SS strips) for "lining" under the BP coils has arrived, and we will complete the strips and install them on the mockup wall this week. Manufacture of the Rear Girdle Plates has resumed, following a vendor vacation. There are two more operations: Milling the bottom of the plates and wire EDM of the inside radius removal of the corners. Completion of these milling operations is promised for this week. Travel and Visitors -------------------- Thawatchai Onjun, a graduate student from Lehigh University, is visiting the C-Mod group for two weeks. He has been looking at experimental edge pedestal data, and is comparing this with his model. Gerrit Kramer was at MIT 7/9-11, working on the MSE diagnostic. Stewart Zweben brought up the fast camera 7/11 and stayed through 7/13, installing the camera and obtaining the new images mentioned above. Gerd Schilling was at MIT for the week, mostly helping the with diagnostic neutral beam conditioning. Earl Marmar was at U. of Wisconsin, Madison, on Tuesday and Wednesday last week for a meeting of the Next Step Options Program Advisory Committee.