Alcator C-Mod Weekly Highlights Dec 4, 2000 Last week was a maintenance week at Alcator C-Mod. No plasma runs were scheduled. Engineering tests were carried out on Friday to verify that all systems are operational following the buss reversal for resumption of normal field and current operation. One plasma shot was produced at the conclusion of tests of the individual coil systems. Principal activities during the maintenance week included glow discharge cleaning and work on the FMIT#3 transmitter. Plasma operations will resume this week. Analysis -------- Local flexing of the vacuum vessel wall during disruptions is measured on C-Mod with a "rangefinder" laser interferometry system. During the second half of this year's operation, the laser beams were shifted to two retro-reflectors that straddle the weld at the bottom of the inner cylinder. The data from this location has now been analyzed and compared to the previous rangefinder results in the disruption database. Differential radial motion of up to 35 micrometers has been seen at this new location, and this is roughly consistent with previous finite element stress/strain calculations done last year. The rangefinder measurements and finite element calculations are the basis for the effort to strengthen the inner wall with the new girdle/divertor hardware. ICRF System ------------ In preparation for operation of the 4 strap antenna at 78 Mhz, we observed some arcing damage on the input cavity of transmitter #3. It was determined that the external cylinder was significantly out of round (~1/8") and hourglass shaped. We suspect that during original manufacturing the longitudinal slots in the cylinder were cut after final machining. To improve the cylinder, strong ribs were attached to make the cylinder true, after the wall was jacked out. We spent time aligning the tube more carefully, but were unsuccessful in installing alignment pins to ease the assembly. Finally, the load element had been on the block capacitor teflon. There was arcing here as well as in the tune element. We cut back by 1" and radiused the teflon. After reassembly the transmitter was tested in dummy load and obtained approximately 1.75 MW. We are now ready to exercise the four-strap antenna in plasma. Machine Conditioning -------------------- On Friday, 11/24, we began a glow discharge cleaning campaign that extended into Monday, 11/27. The discharge was run current limited at about 3 amps, with 230 to 260 volts on the electrodes. The fill gas was helium kept between 18 and 19 mTorr. A decrease in the electrode voltage required to maintain a constant current over the run period indicated the surfaces were cleaning up. When the GDC was stopped on Monday the RGA indicated factors of 4 to 10 improvement in all of the impurity lines. Mass 28 was least affected, dropping from about 1.5e-9 to 2e-10. Other lines such as 14, 18, 20, 32 etc were reduced to noise levels. The chamber was baked at 60C during the entire GDC campaign. One overnight, and several shorter periods of GDC were carried out since 11/27. In each case the pressure was reduced somewhat and the current increased as much as possible without changing the ballast resistors. Currents of about 3500 mA were obtained at up to 450 volt electrode potentials. The discharge was operated arc free at pressures down to 2.0 mTorr. Overnight ECDC was run on 11/30 to 12/01 in D2 at 2.e-4 Torr. The standard diborane lines came back up to within a factor of 5 to 10 of their pre-GDC levels after about 1 hour of ECDC. ECDC was continued over the last weekend, in preparation for operation on Monday, 12/4. DNB System ----------- Baking hardware (pads and thermocouples) were installed on the DNB vacuum vessel. The PLC control hardware for baking was setup, and we are awaiting completion of the PLC programming. Though the beam has been conditioned to full voltage and current over the past couple of weeks, it was fired into the vacuum vessel for the first time since the duct was modified. There was a significant pressure pulse associated with the beam firing possibly due to beam interaction with the duct wall. We are continuing to fire the beam at low voltage and current to assure this is a conditioning phenomenon before returning to work at full voltage and current. Divertor Modification Project ----------------------------- 2,000 Tile screw keepers have arrived last week. We need only 1,080 keepers for production, and the rest are spares. Random fitting of tile screw keepers into tile prototypes shows that they meet the design intent. These parts arrived one week ahead of scheduled shipment date. Travel and Visitors ------------------- Dr. Joachim Roth and Dr. Martin Balden from Max-Planck Institut fuer Plasmaphysik, Materials Research Division visited the Alcator C-Mod last Thursday. The primary purpose of the visit was to tour the C-Mod facility and to discuss with members of the edge physics group aspects of first-wall design, plasma-facing materials and their performance, and edge physics transport results. On Thursday Nov. 30, 2000 Paul Bonoli attended the DoE Workshop on US - Tore Supra collaborations. The meeting was held at the DoE headquarters in Germantown, MD. The meeting was attended (in part) by Jean Jacquinot and M. Chetalier from Tore Supra, Wendell Horton from University of Texas, Ron Stambaugh from GA, John Hogan from ORNL, and Erol Oktay, John Willis, Rostom Dagazian, and Walter Sadowski from DoE. Dr. Bonoli spoke about the possibility of C-Mod - Tore Supra collaborations involving a general exchange of information in the following four areas: (i) Hard x-ray measurements during LH current profile control experiments in C-Mod to determine the spatial and energy distribution of fast electrons. (ii) Long pulse and cooling issues related to the C-Mod divertor and the rf antenna systems (LH and ICRF). (iii) LH launcher design (to be coordinated with PPPL). (iv) Internal transport barrier physics. Bruce Lipschultz spent Tues-Thursday (11/28 - 11/30) at GA. The primary purposes of the visit were two-fold: (1) discuss C-Mod and DIII-D data on the subject of radial transport in the SOL; and (2) discuss techniques used for measurement of Ti and v_flow using spectroscopy. On Tuesday, he gave a talk entitled 'Wall recycling in Alcator C-Mod; Is it occurring or not?'. This covered a variety of results from C-Mod on radial transport. Considerable time was spent looking at DIII-D probe data with Rick Moyer and Jose Boedo, and Thomson data with Dennis Whyte. They found a considerable number of discharges that exhibited flat density profiles in the far SOL. Whyte, Boedo, Peter Stangeby, Gary Porter and Bruce spent considerable time disscussing the DIII-D miniproposal that has been proposed by Dennis for studying the SOL transport under conditions of varying denity and gaps between the separatrix and the wall. Finally, Bruce discussed the fitting techniques for CXRS spectroscopy with Richard Groebner, and the hardware characteristics of the DIII-D equipment with Keith Burrell. This information will be useful in both our current analysis and the potential new system for poloidal CXRS that we are considering.