Alcator C-Mod Weekly Highlights August 1, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 14.25 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Five run days were scheduled and completed. A total of 158 plasma discharges were produced with a startup reliability over 90%. The week's experiments included miniproposals from the RF Physics and Transport topical science groups and the Advanced Tokamak thrust. Plasma operations are planned to continue this week. Run Details ----------- Monday's run continued the evaluation of D(He3) minority heating at 50 MHz (MP#390). We completed a scan of He3 concentration and identified the optimum minority puff duration under standard conditions. From the break-in-slope analysis, the optimum 3He fraction is ~5-7%. For concentrations less than this, the heating effectiveness was lower compared to E antenna (80 MHz, H minority heating) at similar power levels. Another observation is that -90 degree phasing produces the best plasma response in heating and neutrons. Heating phasing (180 degrees) is similar but +90 phasing was significantly reduced. The trips in H-mode suggest the He fraction remains relatively constant across the H-mode transition. We will need further analysis to confirm this observation. These experiments complete the C-Mod FY05 Program Execution Agreement Task #8, "Compare ~50MHz D(He3) minority heating to ~80MHz D(H) minority heating at B~5 tesla", originally due Sept. 30, 2005. Tuesday's run was in support of MP#370, "Large volume ITB's at 2.8T with 50MHz heating". This experiment, which was aimed at increasing the volume inside of the "foot" of the internal transport barrier, was successful. ITB discharges, were produced at currents of 0.45, 0.5, 0.6, 0.7 and 0.8 MA at 2.85T. Off-axis heating to trigger the ITB's employed D(H) minority heating at 50MHz. The higher current ITBs had density peaking across the entire plasma radius, indicating a foot location near the edge pedestal, the purpose of the run. The lower current (higher q) cases had the foot at smaller minor radius. A few ITBs were obtained with low field side ICRF, and also had narrower foot locations. Central heating, inside the ITB, was accomplished using 2nd harmonic minority (H) heating at 80 MHz. At the end of the day, there were a few H-modes (no ITB) produced with 2nd harmonic heating alone. On Wednesday, we carried out the second day devoted to MP#425 "Toroidal rotation in dimensionally similar discharges with no momentum input", submitted by John DeGrassie (GA, DIII-D) through the Transport topical science group. DeGrassie again served as co-Session Leader using remote collaboration tools provided by the Fusion Collaboratory. Experiments were carried out at several values of q95, beta, nu* and rho* for comparison with previous data from DIII-D and with the prior C-Mod experiment. These cases employed RF heating at both 80MHz and 50MHz, with both D(H) and D(He3) minority heating scenarios. Thursday's run was in support of MP#378 "Mode Conversion Flow Drive Study in D(3He) Plasmas Using J-antenna at 50 MHz" from the RF Physics group. The approach was to use the HIREX high-resolution xray spectrometer diagnostic to look for changes in the rotation profile during field ramps with the mode conversion layer near 0.61m (r/a~0.3). The four-strap J-port antenna (50 MHz) was employed with both heating and current drive phasings. No significant rotation effects were observed, although some features are still being analyzed. The preliminary conclusion of this experiment is that no driven flow large enough to be detected by our present diagnostic was produced. The run on Friday was devoted to MP#356 "Mode Conversion Current Drive Experiments", also from the RF Physics group. The goals of this run were to measure net current drive from mode converted waves using differences between co/counter phasing and to investigate the effect of phasing on the sawtooth period. Upper single null and inner-wall limited target plasmas were employed in order to remain in L-mode at higher power levels. The best target conditions were obtained in inner-wall discharges with nebar~1e20/m3 (ne0~1.5e20/m3), Te0~4keV, and He3 concentration ~20%; under these conditions Zeff was relatively high, at ~4. Initial comparison of the loop voltage in co- and counter-CD phasings was inconclusive, and further work is planned to better optimize the target conditions and increase the current drive efficiency. Better results were obtained on the sawtooth control part of the experiment. By sweeping the mode conversion layer through the inversion radius with a toroidal field ramp, we were able to increase the sawtooth period to 10 ms in co-CD phasing and decrease it to 4 ms in counter-CD phasing. This result is consistent with local changes in the shear at the q = 1 surface from currents driven by the mode converted waves. ICRF Systems ------------ The ICRF system was successfully employed in all five experiments last week. The J-port antenna (FMIT#3 and 4) coupled up to 3MW at 50MHz, and the E-port dipole was used to provide H minority heating (both fundamental and second harmonic) at power up to 1.5MW. Long Pulse Diagnostic Neutral Beam ---------------------------------- A new tantalum heater strap was fabricated and installed on the DNB cathode heater assembly. The cathode has been reassembled and installed onto the DNB plasma source. The rebuilt cathode was operated up to full heater current of 125 Amperes for thirty minutes. The coolant for the source showed good flow rates of ~20 gallons/hour with the new booster pump, and the metal parts of the source remained relatively cool to the touch throughout the heater tests. The re-assembly of the external source housing and gas system is nearly completed. A new mass flow controller electrical panel has been installed. Gas system leak checks, followed by start up of the arc source and subsequent operation of the beam source with high voltage are scheduled to begin early this week. A spare replacement heater assembly made by the Budker Institute in Novosibirsk was delivered to MIT on Friday. Dr. Petr Deichuli and Mr. Gregory Shulzenko are planning to come to MIT, arriving around August 15th. While they are at MIT, they plan to install the rebuilt cathode and tune the DNB. Travel and Visitors ------------------- Bob Ellis, Gerd Schilling and Randy Wilson attended the weekly C-Mod Lower Hybrid teleconference on 7/26. Experimental progress and modeling of the new non-titanium coupler were discussed. Gerd Schilling was at MIT 7/27-29 to help with the repair and reassembly of the Diagnostic Neutral Beam ion source. Beam conditioning is expected to resume early in the week of 8/1. Steve Wolfe participated in the Sorters' meeting of the Program Committee for the APS DPP05 Meeting, held at the American Center for Physics in College Park, MD, July 28-29. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly