Alcator C-Mod Weekly Highlights August 8, 2005 FY2005 weeks of research operations Planned: 17 weeks Completed: 15.25 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 126 plasma discharges were produced with a startup reliability of 92%. The week's four runs supported seven experiments, including research carried out by the RF Physics and Transport topical science activities, the Burning Plasma thrust and the Operations Group. Boronizations were carried out over Tuesday and Wednesday nights. Plasma operations are planned to continue this week. Run Details ----------- The run on Tuesday continued the experiments in support of MP#356 on Mode Conversion Current Drive. Additional data were obtained on the modification of sawtooth period by localized co- and counter-current drive near the q=1 surface. These data will contribute to the doctoral thesis of an MIT graduate student. On Wednesday we carried out the low-field experiments called for in MP#410, "Extension of Pedestal Scalings" from the Transport Science group. These experiments employed 50MHz ICRF heating in the D(H) minority heating scenario, at B~3.5T, and second harmonic H heating at 80MHz at 2.6T. The run was quite successful in exploring the parameter space available at low magnetic field. H-modes were obtained in a range of Bt from 2.6 to 4.4T, and a range of Ip from 0.4 to 1.0MA. Edge safety factor (q95) ranged from approximately 2.6 to 6.1. Profiles will be fitted and stored in the database, along with data from the previous run days of this MP, for further analysis. Trends in pedestal parameters with Bt, Ip, and/or q_95 will be examined in both EDA and ELM-free regimes. The run on Thursday was primarily devoted to MP#390, "Evaluation of D(He3) heating". This aspect of the heating experiments was carried out in H-mode discharges under freshly boronized conditions. The heating with the D(He3) minority scenario at 50MHz was found to be very similar to that with D(H) at 80MHz, as was the case in L-mode discharges studied previously. Results with combined 50MHz and 80MHz heating at powers up to ~4MW were comparable to results obtained previously in purely D(H) heated plasmas using 78 and 80MHz sources. Based on these results, we have decided to continue operation with the J-port ICRF system configured for 50MHz for at least an additional week. Also on Thursday, additional data were obtained in support of MP#417, "Plasma performance, boronization and ICRF heating effectiveness with high-Z antenna guards and all-metal surface". SOL profiles were first obtained under freshly boronized conditions and then after significant degradation of the boronization effect. These data will be compared with the profiles measured under the same plasma conditions prior to the first boronization of the machine earlier in this experimental campaign. The last 9 shots on Thursday continued the experiments on Mode Conversion Current Drive (MP#356). The He3 concentration was adjusted to place the current drive on-axis at a field of 5.7T (nHe3/ne~20%); additional shots were taken with the mode conversion slightly to the high-field side. Data were obtained with both co- and counter-current drive phasings. More analysis will be done to determine the significance of the small but systematic loop voltage differences that were observed. On Friday morning we carried out another half-day experiment in support of MP#398 "Complete identity experiments with JET on error field locked mode thresholds", which is the C-Mod contribution to ITPA Joint Experiment MDC-6, which is aimed at developing improved predictions of the locked mode threshold for ITER. Tim Hender (UKAEA) served as remote Session Leader using H323 video communications tools from JET. David Howell (UKAEA) also participated in the experiment remotely. Threshold data were obtained using two orientations of the C-Mod non-axisymmetric coils at fields of 4.1 and 6.3 tesla. These results will be combined with those obtained using a nearly orthogonal applied field configuration during earlier experiments to obtain an improved threshold scaling for C-Mod, which will then be correlated with results from non-dimensionally similar experiments on JET and DIII-D. Friday afternoon's experiments were in support of MP#419 on wall-pumping and hydrogen isotope retention. A density scan at fixed current of 800kA and a scan of pulse length at fixed density were carried out. Wall-pumping at 800kA was found to be greater than that observed in previous experiments at 1MA. As in the 1MA experiments, the pumping was lower at low density and increased as the plasma density was raised. At the highest densities in the 800kA discharges, divertor detachment was observed; detachment did not result in decreased wall-pumping. A scan of pulse length was carried out at 800kA and nebar~1.4e20/m^3, with the end of flattop varied from 0.5sec to 2 sec. For non-disruptive discharges the net wall pumping was smaller at .5 and 1.0 sec EOF cases than with EOF=1.5sec; the shot with EOF=2.0sec disrupted during rampdown at a current of 200kA, and produced a net outgassing of the walls. Experimental Program --------------------- The C-Mod Experimental Program Committee met on Monday, with remote participation from U. Texas and PPPL. Eight new MiniProposals were reviewed, and seven were approved. C-Mod MiniProposals can be read online at http://www.psfc.mit.edu/research/alcator/program/cmod_runs.php?miniproposals Plans for the remainder of the FY05 Experimental Campaign, particularly with regard to the choice of frequency of the J-port ICRF system, were discussed. Long Pulse Diagnostic Neutral Beam ---------------------------------- The DNB source re-assembly has been completed. The mass flow controllers and gas connections are complete, and all data connections between the mass flow controllers and the C-Mod camac have been re-connected. Arc conditioning started on Monday, and the arc performance increased quickly to full current at over 300 Amperes. The source was successfully hi-potted to 47 kV without arc using the beam high voltage supplies. Several beam shots were fired under mass flow control, and the beam performance appeared to be quite good, reaching over four amperes within a few shots. Lower Hybrid System -------------------- New LH couplers fabricated from 304L stainless steel are being manufactured. The first coupler was received at the PSFC last week and the remaining three are scheduled to be delivered by mid-August. Ceramic windows for the couplers have been manufactured and a shipment of 160 windows was also received at the PSFC last week. Shear tests indicate that the moly-manganese metalizing and nickel and copper plating has shear strength well within that required for successful bonding of the ceramic to the stainless waveguide. A 24 window prototype has been sent out to be cleaned and plated, and ceramic windows are scheduled to be brazed into it toward the end of this week. Assuming successful brazing of the prototype, the new couplers will be brazed in late August or early September, and the LH antenna will be ready for reinstallation into Alcator C-Mod in mid-September. Meanwhile, the removal of the antenna from Alcator C-Mod has permitted a low-power calibration to be made of the antenna system from the input of the splitter to the input of the couplers. The results showed somewhat larger variations in the waveguide phasings than in the previous calibration, which had only been from the splitter to the joint between forward and rear waveguide assemblies; however the calculated n-parallel spectra are nearly identical to those inferred from the previous calibration and thus the relatively minor phase errors found in the new calibration are inconsequential. Travel and Visitors --------------------- Perry Phillips (U. Texas) visited MIT last week to install the repaired local oscillator for the low frequency channels of the FRCECE heterodyne electron cyclotron emission diagnostic. The LO now has an isolator to protect it from high reflected powers. The unit is now operational with all channels working, except for channels 1 and 2, which have a connection problem in the filter/amp circuit boards. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly