Alcator C-Mod Weekly Highlights April 14, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 11.4 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 93 plasma discharges were produced with a reliability of 84%. The four runs supported five primary experiments and one piggyback experiment, including research proposed by the Lower Hybrid Physics, H-mode Integrated Scenarios thrust, Rotation Physics thrust, Boundary Physics, and Diagnostic Development group. One overnight boronization was carried out prior to Wednesday's experiment. Plasma operations are planned to continue this week. Operation Details ----------------- Tuesday's run was devoted to MP#545 "LH wave damping in the presence of energetic ions". The goal of this experiment is to assess the interaction of LH wave and energetic ions generated by ICRF. Parasitic absorption by energetic ions via perpendicular ion Landau damping is an issue for ITER, where the choice of LH source frequency (~5 GHz) is based on avoidance of damping on fusion alphas. These experiments in C-Mod are designed to investigate this interaction using the ICRF-driven minority tail to simulate the alpha particles. The experimental technique proposed was to observe the decay rate of the fast ion tail, observed using the Compact Neutral Particle Analyzer (CNPA) with modulated ICRF heating with and without lower hybrid injection. Hard Xray spectra and non-thermal ECE emission are used to monitor the LH interaction with fast electrons. Results for proton minority heating (f=78MHz) will be compared with later experiments employing mode-conversion electron heating (MCEH) with the ICRF at 50MHz, which does not produce an ion tail. On Tuesday, two fiducial discharges, one with constant ICRF power (2MW) and one with a power ramp from 0.5 to 3MW, with the LH power fixed at 0.9MW, were documented for comparison with the MCEH experiment. Discharges with modulated ICRF and constant LH were obtained at different ICRF power levels, target density, and LH phasing. The primary experiment on Wednesday was MP#496, "High performance operation at Ip>1.2MA". Up to 4MW of ICRF power was successfully coupled to LSN and USN plasmas with Ip=1.35MA and Bt=5.4T (q95~3.2). Some data were also taken at Ip=1.2MA. The LSN configuration resulted in ELM-free H-modes, with increasing radiative power fraction leading to back-transitions. In some cases the increase in radiative power was initially moderate, with both Prad and density tending to a steady state, but terminating with a rapid increase in radiation due to influxes of iron and molybdenum. The upper null cases exhibited an enhanced L-mode phase, with low density, controlled radiation, and good confinement (H98>0.9) prior to the L-H transition. The subsequent H-modes were also ELM-free, with increasing radiated power. A piggyback experiment was also carried out on Wednesday in support of MP#546 "ITER ramp-down scenario demonstration", which is part of ITPA Joint Activity SSO-5. The purpose is to demonstrate safe and slow termination of the discharge in a divertor shape, to allow control of the plasma shape, vertical stability, and safe termination of the burn phase (in ITER), without consuming more OH flux after the end of the flattop phase (EOF). Diverted rampdowns from 1.35 MA to <0.1MA were successfully demonstrated beginning from the LSN equilibria with rampdown rates of ~2MA/sec and 4MA/sec, including extension of the ICRF heating into the rampdown phase in some cases. The elongation was reduced during the rampdown to improve vertical stability. Thursday's run supported MP#537 "Momentum impulse from rapid SSEP changes". The goal was to follow changes in the momentum through the plasma edge as the plasma is moved up and down in such a way as to change the magnetic balance from LSN to USN and back. The change in topology has previously been documented to result in an change in the intrinsic rotation of both the SOL and the core plasma. Data were obtained at different currents (0.8 and 1MA) and densities (nebar=1.4 and 1.7e20/m^3), and with different values of the inner gap (10 and 7 mm). Two experiments were conducted on Friday. The first half-day was devoted to MP#547 "High-field side SOL flow momentum coupling experiments" which is designed to critically test the hypothesis that it is the high-field side SOL flows which are specifically responsible for coupling parallel momentum into the confined plasma, causing the toroidal rotation of the core to change according to magnetic x-point topology. The experiment was carried out with USN equilibria in which the gap between the LCFS and the inner nose of the lower divertor was systematically varied over the range of flux which typically contains the bulk of the SOL flow (~5 mm mapped to the outboard midplane). Gaps of 0, 2, 5, 6, and 10mm were obtained while recording flow profiles with the scanning probes. The variable nose-gap spacing had the desired effect on the high-field SOL flows. With zero nose-gap the parallel flows on the HFS became stagnant and the local pressure profile exhibited a very sharp fall-off. However, the nose-gap modulation produced no clear change in the toroidal plasma rotation as inferred from soft X-ray measurements (HIREX) or the outer CXRS profile. Taken together, these data indicate that there is no systematic dependence of core toroidal rotation on whether the near-sonic SOL flows pass along the HFS leg. This result implies that the SOL momentum does not couple to the confined plasma along the HFS leg; if such a coupling mechanism is occurring, it is happening elsewhere in poloidal angle. The second half of Friday's run supported MP#534 "Foil/AXUV bolometer calibration in helium plasmas". The main purpose of the experiment was to create radiative plasmas with a variety of impurities to cross-calibrate the foil and AXUV bolometers. Upper null helium plasmas were employed with the cryopump used to steady concentrations of injected impurity gases. High power ICRF heating (up to 5.5MW) was used to support the total radiated power with large impurity content. Data were obtained with different levels of argon and krypton impurities. Molybdenum radiation was higher in these helium L-mode discharges, compared to similar experiments in deuterium majority plasmas. This work comprises part of the thesis research of an MIT graduate student. ICRF Systems ------------- All four ICRF transmitters were operational for plasma experiments this week. On Friday up to 5.5MW of ICRF power was coupled to the plasmas with pulse lengths of 1.1 sec. Travel and Visitors -------------------- David Mikkelsen (PPPL) spent the week at MIT working with Martin Greenwald and Alex Ince-Cushman on shot selection for studies of density peaking in H-mode plasmas. Several C-Mod plasmas with varying degrees of density peaking will be analyzed by TRANSP, and then simulated by GYRO to learn whether the peaking can be understood in terms of changes in microturblence. He also reviewed Liang Lin's GYRO simulations of a C-Mod ohmic density scan, and suggested a series of sensitivity studies. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly