Alcator C-Mod Weekly Highlights Sept. 6, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 18.4 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Two run days were scheduled and completed. A total of 36 plasma discharges were produced with a startup reliability over 86%. An additional 14 programmed beam-into-gas with field (no plasma) shots were carried out during Tuesday's experiment. C-Mod has now completed research operations for the FY2005 campaign. A total of 2022 plasma discharges were produced during the campaign. Run Details ----------- Tuesday's experiment was primarily devoted to MP#426, "Tests of Flourine and Molecular emission contamination of MSE signals and calibration". Beam-into-gas MSE calibration shots were carried out with both deuterium and helium gas targets, in order to test the conjecture that molecular emission from D2 might be perturbing the calibration. On-line analysis indicates that this conjecture is false. Following the beam-into-gas tests, the second part of the miniproposal, dealing with the possible effect of flourine line emission on MSE signals, was carried out. Fluorine is a naturally occurring impurity in C-Mod plasmas, and beam excitation of intrinsic fluorine lines could affect the pitch angle measured by the MSE diagnostic. In order to evaluate this hypothesis, MSE measurements were made in low-density ohmic plasmas with and without freon gas puffs from an inner wall capillary. The freon puff provides a source of fluorine, as well as carbon, which has also been suggested as a potential source of line emission in the MSE passband. The fluorine levels were monitored using VUV spectroscopy. The results do not support the conjecture that either fluorine or carbon emission is interfering with the MSE pitch angle measurement. The addition of Freon perturbed the plasma slightly: Zeff increased by 1.5; the radiated power doubled; Ohmic heating power increased by 10%, and the electron temperature profile steepened. Interestingly, the addition of Freon increased the central temperature (Teo = 1.9 -> 2.4 keV) while slightly depressing the edge temperature. The plasma internal inductance as analyzed by EFIT increased by 0.13, indicating a more centrally peaked current profile. The magnetic field pitch angle as measured by MSE increased by 13-14% at the plasma half-radius (r=a/2), while little change was observed near the plasma edge. A piggyback experiment also carried out on Tuesday supported MP#434 "Xenon spectral line identification". Xenon was puffed late in the discharge as part of a 5:1 D2:Xe mixture. 2MW of ICRF power was injected to increase the temperature sufficiently to produce higher xenon charge states. Lines of lower charge states of Xe, predominantly Xe XIX, were observed near 108 Ang and used to monitor the xenon concentration. It was demonstrated that at the concentrations employed in these experiments the Xe does affect the global Prad in regions with Te<1keV. Xenon retention in the wall was shown not to be a major issue, as the brightness of the xenon lines decreased by a factor greater than five on a shot with the xenon puff removed. Lines from higher charge states, Xe XXXI and above, which had been previously observed in JT-60, were not sufficiently bright to be measurable in these experiments. Wednesday's experiment was devoted to the tests of between-shot boronization using glow discharges, as detailed in MP#430. These experiments marked the first use of the glow discharge boronization procedure on C-Mod. Comparisons of glow boronization employing a variety of parameters (pressure, current, duration) were made with between-shot boronization using the ECDC technique developed earlier. We found that, at least for the conditions tried on Wednesday, glow boronization has little effect on subsequent plasmas. In addition to not decreasing Prad in h-mode, glow boronization has a much smaller effect than ECDC boronization on gas evolution from the walls at breakdown, with little adjustment of prefill puff required to avoid fizzles with the glow boronization. Long Pulse Diagnostic Neutral Beam ---------------------------------- The DNB operated into C-Mod on Tuesday, for approximately 28 shots, with 23 50ms long pulses, and five shots that faulted after approximately 35 ms. The beam was operating at 50kv and a nominal current of 6.2 A. Following Tuesday's run, the beam was succesfully operated for one second duration pulses, modulated 100ms on/100 ms off. A cathode assembly with a heater strap that has been redesigned to allow much improved compliance to thermal strains is being fabricated as a spare. The cathode presently in the beam source is working well and does not yet need to be replaced. ICRF Systems ------------ E-port and J-port ICRF systems were successfully employed during both C-Mod experiments last week. A plan for operation of the J-port four-strap ICRF antenna with the Faraday screen removed has been developed. Design and fabrication of required components is proceeding, in preparation for a possible test in FY06. Lower Hybrid System ------------------- After successful 3-window and 24-window prototype braze tests, the first stainless steel lower hybrid coupler was brazed on Thursday. Based on the initial inspection, the braze of some of the windows does not appear to have been successful. Further evaluation, including leak testing, will be carried out early this week. Calibration of the launcher phase and amplitude control system continued last week. Travel and Visitors ------------------- Dr. Petr P. Deichuli and Mr. Grigory Shulzhenko from the Budker Institute for Nuclear Physics in Novosibirsk, Russia have completed their visit to C-Mod. Working closely with Dexter Beals, they successfully improved the reliability and increased the current of the long pulse diagnostic neutral beam. Dr. Howard Yuh of Nova Photonics visited C-Mod on Tuesday and participated in the MSE experiments. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly