Alcator C-Mod Weekly Highlights December 6, 2010 FY2011 weeks of research operations Target: 15 weeks Completed: 5.07 weeks Plasma Shots: 617 Operations ----------- Plasma operations continued last week at Alcator C-Mod. Four run days were scheduled and completed. A total of 123 plasma discharges were produced with a reliability of 95%. The experiments supported research in Lower Hybrid Physics, H-mode Scenarios, MHD, and Diagnostic Development. Boronizations were carried out before the runs on Tuesday and Thursday. Plasma operations are planned to continue this week. Operations Details ------------------ Two experiments were carried out on Tuesday. The first continued the series of runs in support of MP#564 "Power requirements for high confinement H-modes and the role of radiated power spatial distribution". In this experiment we began assessment of N2 seeding compatibility with ELMy H-modes, employing the high delta-lower ("JFT2-M-like shape") used for most of our prior ELM studies. Initial discharges, used for recovery from boronization, were moderate density EDA H-modes. These exhibited quite good performance and stationary operation. The N2 puffing rate was optimized at a series of target densities, and seeded ELMy H-mode conditions were accessed. The second half of Tuesday's run was devoted to MP#578 "Experimental tests of EPED1 pedestal prediction in ELMy H-Mode". This MP supports the OFES Joint Research Target on Pedestal Physics. This experiment continued the exploration of the effect of shape changes on ELM and pedestal behavior. Efforts to increase the elongation over k~1.5 while maintaining the lower X-point and strike points in the suitable range were not successful. However, the seeded ELMy discharges obtained earlier in the day should provide additional conditions for the EPED studies. The first part of Wednesday's run was devoted to MP#612 "Expanded exploration of LHCD Density Limit". The goal of this run day was to explore the possible synergistic effects of combining double null discharges with reduced inner wall gaps. Previous runs have shown that these two changes can have positive effects on fast electron generation with LHCD at high density. We successfully established a low density (nebar~8e19/m^3) DN equilibrium with a large inner gap, but were unable to maintain density control while dynamically decreasing the gap toward an inner-wall limited configuration. Increasing the density to nebar~1.3e20/m^3 in the DN configuration resulted in undesired H-mode transitions, often followed by disruption. An alternate approach will need to be developed to address the goal of this experiment. The second half of Wednesday's run continued MP#553 "Creation of reversed shear current profile using LH on low Ip plasmas". We began from a reference 400kA discharge developed during the previous run under this MP, and attempted to optimize the ITB parameters by variation of the LH phase and power, and by adding additional heating with ICRF. The ICRF (J-port antenna) ran satisfactorily in combination with the LH in these low current discharges, but did not show a positive effect on the LHCD. High frequency MHD activity, apparently associated with fast electrons, was observed on many discharges. Data are being analyzed. Thursday's run was dedicated to MP#614 "Pedestal modifications with lower hybrid in EDA H-mode". This experiment contributes to the 2011 OFES Joint Research Targeg on Pedestal Physics. All discharges were at 5.4T, 600kA, and employed LHCD with n||=1.9 . We were able to reproduce the density "pump-out" and increase in pedestal temperature observed previously in marginally upper null equilibria with the cryopump operating. The effect was also observed without the cryopump, but was not as strong. Lower single null discharges with the cryopump cold also exhibited this behavior, apparently just as strongly as for the upper null, but with the cryopump warm the effect was weak or non-existent. Several experiments were carried out on Friday. The main experiment on Friday morning was devoted to MP#534 "Foil/AXUV Bolometer Calibration in Helium", which employed highly radiative discharges in He main ion plasmas seeded with argon for the purpose of bolometric calibration. In addition, piggyback experiments were carried out addressing LHRF in helium discharges; scoping of possible target plasmas for MP#622 on "Molybdenum nano-filament growth"; and calibration of the HIREX_SR diagnostic. An L-H transition was observed in a 1MA, nebar~2e20/m^3 He majority discharge with input power ~3.5 MW (ohmic+ICRF). The HIREX_SR calibration shots involved a dynamic scan of the outer gap from 2 to 10cm. One ohmic shot and one ICRF-heated shot were produced with this scan; in the ICRF case, J-port and D-port antennas were able to maintain coupling up to a gap of 4.1cm, while the E-port antenna using the Fast Ferrite Tuner (FFT) feedback for matching maintained coupling for the full programmed pulse length, up to a gap of 8 cm. An attempt to produce I-mode discharges in helium was unsuccessful with Pnet up to 3MW. On Friday afternoon we carried out an exploratory experiment to determine our capability for measuring the disruption heat-flux onto the outer divertor targets with high (i.e. 1 ms) time resolution. With sufficient time resolution, the heat flux onto the targets during the thermal quench may be discriminated from the heat flux during the current quench. Previous study has shown that the disruption heat-flux depends on plasma current, presumably through the loss of magnetic energy (the current quench). The experiment employed unmitigated programmed downward-going VDE disruptions at 1.1 and 0.8MA with varying plasma kinetic energy (L and H-modes). The principal heat flux diagnostic was the IR camera, viewing the instrumented ramped tile section of the outer divertor. ICRF Systems ------------ All four of the inconel straps for the ARRA-funded rotated ICRF antenna have been cut from the inconel block. Two straps have had one side properly rounded (full radius) and additional clamps have been purchased to allow two straps to be cut in parallel (one side takes approximately 3 days). The first braze test results were satisfactory. We have ordered additional braze material and the stainless strap and inconel block have been sent to the vendor to be copper plated to validate the plating process. The purpose is to get the specific plating process for these large parts verified before plating the current straps. Inconel for the side, top and bottom plates has been ordered as well. We are waiting to hear from the Faraday rod bending vendor on the schedule for bending five of the more difficult Faraday rods. We have proceeded to procure the boron carbide (resistive) washers and nichrome ribbon plus magnesium adhesive. The former is the preferred design for the resistive washer and the latter is a backup. The antenna systems were working well in support of a variety of experiments last week, with up to 5 MW coupled. During one experiment, E antenna, with automatic matching, maintained 1 MW coupled to the plasma with an outer gap as large as 8.5 cm. This is the largest gap with which we have ever successfully operated on C-Mod. The reflectometry data is being analyzed to provide more information on the density profiles at the antenna. The FPA Screen supply in FMIT 3 failed during the Tuesday run when one of the 3 phase 480 fuses cleared. Fuse replacement resolved the problem. We have measured the DC0 forward (used in power feedback) and reverse paths to document the impact of notch filters in the measuring circuit. All the notch settings were found to be slightly out of adjustment but the impact on normal ICRF operation is minimal. Operation using Amplitude Modulation, however, is significantly impaired, and retuning has been scheduled. Lower Hybrid System -------------------- The LH system was used for runs on Wednesday, Thursday, and Friday. Friday's run marked the first test of the LH system in helium discharges. No problems were encountered with operation in He. Progress was made on preparations for the 4th LH klystron cart. Two new klystrons are now ready for acceptance tests at the vendor's facility. Troubleshooting of the vector modulator and serial fiber optic link continued as part of the effort to determine the cause of output power variations from klystron #7. Travel and Visitors ------------------- Dennis Whyte presented "Implications of Plasma-Material Interactions" to the annual meeting of the Fusion Power Associates in Washington D.C. Dennis Whyte attended the first meeting on materials requirements for Fusion Nuclear Science Pathways in Gaithersburg, MD. Steve Wukitch visited our vendor's site in Backnung, Germany for the final design review for the new fast ferrite tuners; several issues were favorably resolved. In the lastest design, the phase shift range is expected to be 110 deg at 80 MHz and 100 deg at 50 MHz, as requested. The thermal analysis shows that up to 5 MW circulating power can be handled for up to 3 sec without issue. For longer pulses we will need to reduce the circulating power by implementing more specific prematching. The current pre-matching in the E antenna transmission network has been successful at maintaining the circulating power below 5 MW, and we expect to be able to have good pre-matching for the new fast ferrite tuners. Steve Wukitch also participated in ITER ICRF antenna feedthrough design activities. David Mikkelsen (PPPL) was at MIT all week to discuss turbulence simulations and plan a particle transport experiment. Stewart Zweben was at C-Mod last week to discuss GPI plans and begin to look at the archived data from the recent run. An initial analysis of the fraction of GPI light outside the separatrix (in the midplane view) showed some evidence of 'quiet times', similar to that seen recently in NSTX. The modulation frequency of this feature was in the range ~ 5-10 kHz, similar to NSTX, but the amplitude was considerably lower. The next step is to determine whether these quiet periods were correlated with local poloidal zonal flows. Stewart also reviewed with Jim Terry the PPPL property at C-Mod in order to prepare an updated loan agreement, and brought back to PPPL items which are no longer in use at MIT. Professor Francesco Volpe from the University of Wisconsin - Madison visited MIT for the entire week. He worked with Luis Delgado-Aparicio (PPPL) and held discussions with several C-Mod scientists regarding C-Mod results and potential collaborative experiments. On Friday he presented a PSFC Seminar entitled "Disruption avoidance by magnetically guiding locked modes towards stabilizing electron cyclotron current drive". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly