Alcator C-Mod Weekly Highlights September 10, 2012 FY2012 weeks of research operations Target: 18 weeks Completed: 16.4 weeks Plasma Shots: 2331 Operations ----------- Plasma operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 107 plasma discharges were produced with a reliability of 95%. The runs supported research in the ICRF Physics, Boundary Physics, and Advanced Scenarios topical areas. A fresh boronization was carried out over Thursday night. About two hours of run time was lost on Wednesday in order to replace fuses in the TF power supply. Friday's run was extended by one hour. Plasma operations are planned to continue this week. Operation Details ----------------- Runs on Tuesday and Wednesday were dedicated to MP#682 "Continued studies of RF induced electric fields in the SOL" and MP#708 "RF sheath rectification in plasmas heated with a field-aligned ICRF antenna". The purpose of these experiments, which comprise part of the thesis research of an MIT graduate student, was to determine the variation of the impurity penetration factor from the SOL to the core as a function of the strength of the ICRF-induced electric fields and to assess any difference on the ICRF-induced electric fields and plasma potentials between discharges heated with the field-aligned and regular ICRF antennas. We completed a scan of plasma current, Ne seeding and antenna phase (dipole and monopole). The penetration factor of puffed nitrogen impurities was found to be independent of the strength of the ICRF-induced electric fields, and also independent of the heating scheme, dipole vs. monopole. The largest electric fields and plasma potentials were observed in plasmas using the Field Aligned J-port antenna in monopole phasing. In plasmas heated with an ICRF antenna in dipole phasing, the ICRF-induced potentials and electric fields are the strongest on magnetic field lines that are "poloidally enclosed" by the ICRF antenna limiter box. In dipole phasing, the poloidal profile of the ICRF-induced electric fields and plasma potentials shows peaking near the corners of the ICRF antenna limiter box. In monopole phasing, the ICRF-induced electric fields and plasma potentials no longer correlate with field line mapping to the antenna, and values are very high on all probes. There is also evidence of ICRF field enhancement seen on the Langmuir probes mounted on the inner divertor. In this case the core Mo content begins to correlate with the ICRF-induced plasma potentials, yet the nitrogen penetration factor remains unchanged. Neon seeding has no effect in these monopole phasing cases, while for dipole phase, there was a measurable decrease with seeding in plasma potentials and impurity source rate at the antenna. Thursday's experiment was devoted to MP#722 "Assessment of LHCD efficiency and advanced scenario prospects of high Te I-mode plasmas". The objective of this run was to couple LHCD into a high Te I-mode at moderate density, and compare with colder plasmas at the same density, where we typically see density limit effects. We successfully obtained LHCD in I-mode and hot L-mode plasmas at N||=2.2, 1.9, and 2.6, and also obtained ohmic comparison shots in the same range of density and lower Te. These data are being analyzed. Friday's run continued the experiments, begun on Aug 29, devoted to MP#712 "Density range and dependence of I-mode in Rev Bt LSN plasmas". L->I and I->H thresholds were documented over a range of currents up to 1.3MA, and densities up to nebar~2e20/m^3. High performance I-modes, with Wmhd>200kJ. ~1.5 atmosphere and H98y2~1.1, were obtained with P_ICRF up to 5MW. Pedestal temperatures over 1keV and central Te0>7.5 keV were documented in these cases. Steady high density I-modes, nebar>1.7e20/m^3, were obtained by adding additional fueling into I-modes formed at lower target density. This is above the density at which we could make a steady I-mode starting with a cold target. The tentative interpretation of this result is that the existence of a hot pedestal and strong Weakly Coherent Mode is favorable for avoiding the H-mode transition at higher density. These data are being analyzed. ICRF Systems ------------ A new FMIT#2 driver tube was installed over the weekend with the tuning completed on Monday (Labor Day). On Tuesday morning, FMIT#1 Final Power Amplifier (FPA) tube expired after one discharge. The tube was replaced Tuesday night, but was found to have an abnormally high flow rate with a significantly reduced pressure drop (~75 psi versus expected 100 psi). Upon inspection it was found that a manufacturing error was allowing water to bypass the anode cooling channels. The water diffuser dome was replaced, and the water pressure and flow rates were returned to normal. All four transmitters were utilized during I-mode experiments on Thursday and Friday with maximum power ~5 MW. The first of two new ARRA-funded ICRF FFTs was bench tested in preparation for installation in the cell. The pair of FFTs will be installed in the E antenna matching network. The second FFT awaits welding of brackets for the leveling jacks prior to bench testing. The pair of FFTs will be installed in the E antenna matching network. Lower Hybrid System ------------------- The LH system was used during Thursday's run to investigate the physics of LHCD in hot I-mode discharges. Previous attempts to operate the LH system under these conditions were unsuccessful due to very low density (<< cutoff density @ 4.6 GHz) at the launcher. The density at the launcher was increased sufficiently by adjusting the outer gap and adding a local gas puff during the LH pulse. A layout for the collector over temperature system (COTS) thermocouple transmitter box has been designed. One box for each cart will consolidate thermocouple readings from each klystron into a single cable bundle carrying signals from all 4 klystrons. A prototype waveguide double-stub LHRF fast ferrite tuner (FFT) was tested with an unmatched load. The power reflection coefficient was reduced from 88% to less then 2% by manually adjusting the current in the bias-field coils. Diagnostic Neutral Beam Systems ------------------------------- After some consultation with the power supply engineer from the original manufacturer of our DNB system, it was determined that the faulting problems that were limiting the beam current in recent weeks were all caused by an over-current indication in one of the eight inverters of the high-voltage supply when operating at high power. This inverter has been turned off, resulting in good beam reliability at the full 7 amps of beam current and nearly full voltage. The beam will be run in this configuration so that it is available for runs in the near future. Troubleshooting of the faulty inverter will be done during the next extended maintenance period. Diagnostics ----------- The MSE real-time background polychromator has been completely assembled and aligned. This system will provide a real-time measurement of the polarized background emission using exactly the same viewing sight-lines as are used for the beam-induced MSE signal itself. It will also provide a simultaneous measurement of the sigma- and pi- MSE lines, thereby doubling the effective MSE intensity. The high-energy density physics group at the PSFC is working on developing a neutron spectrometer that could fill the need to measure the neutron spectrum at low energy (~0.1-3 MeV) at OMEGA and the NIF. The detector is based on the n+6Li -> T+alpha reaction. It consists of a thin Lithium Fluoride foil sandwiched between two plastic CR-39 nuclear track detectors. This detector was fielded at Alcator C-Mod on Friday, during a run which generated a total of 4.4e14 D-D neutrons (2.45 MeV). The goal of this test was to see if we can determine the energy of the tritons and alphas generated in the n+6Li reaction to high enough accuracy, based on track diameter, eccentricity and contrast on the CR-39, for reconstruction of the incident neutron spectrum. Travel and Visitors -------------------- John Rice and Matt Reinke attended the EU/US TTF meeting in Padova Sept.3-6. Reinke gave a poster and an oral presentation on high-Z impurity asymmetry physics, and discussed possible cross-machine intrinsic rotation studies with people from JET and ASDEX-U. John Rice presented a poster on Non-diffusive heat transport, rotation reversals and energy confinement saturation in Alcator C-Mod Ohmic plasmas. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly