Alcator C-Mod Weekly Highlights March 14, 2005 FY2005 weeks of research operations planned: 17 weeks, completed: 1.6 weeks. Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled and completed, including 2.5 days of research operation and one half-day of RF conditioning. Operation is planned to continue this week. Operations ---------- Scheduled maintenance was carried out on Monday and Tuesday last week. Plasma runs were conducted W-F on a modified split-shift schedule, with tokamak operations between 10:30AM and 6:00PM. A total of 49 plasma discharges were produced, with a startup reliability of 67%. Runs were carried out in support of experiments proposed by the Divertor/Edge and Transport topical science groups and the Burning Plasma/ITER Support thrust. Two of these experiments were led by collaborators. Tokamak clean-up has continued to progress, with H/D levels below 5% obtained on many discharges last week. Significant ICRF power has been successfully coupled through all three antennas, and the Lower Hybrid group continued to inject microwave power into vacuum and into plasmas during piggyback operation last week. The run on Wednesday was the second run day devoted to MP#409 "SOL Phase space studies", which has the overall goal of mapping the relationships between the 'transport state' (profiles, gradients, fluxes, advection velocities, turbulence) and theory-based 'control parameters' (collisionality, fields, connection length, ...). Wednesday's run was aimed at completing the density scan in 5.4T, 0.8MA plasmas, and then beginning a dataset at 0.4MA at lower fields, 4T and 2.7T. Problems with uncontrolled density rises and injections during the first half of the run limited the amount of useful data obtained. Several lower single null plasmas (and one upper null) were successfully produced in the afternoon for the 5.4T, 0.8MA dataset. The lower field and current discharges were not attempted. On Thursday morning, we carried out a half-day experiment in support of MP#398, "Complete identity experiments with JET on error field locked mode thresholds". This miniproposal, authored by Tim Hender (UKAEA) and submitted through the Burning Plasma thrust, is part of ITPA Joint Experiment MDC-6 on error field effects and locked modes, and aims to establish the scaling of locked mode thresholds to ITER. Hender served as Session Leader and was assisted by David Howell (UKAEA), both participating in the run remotely from the JET site in Culham, UK, using H323 videoconferencing software for communications, and web-based logbook tools and MdsPlus for data access. The aim of this session was to continue the C-Mod/JET/DIII-D error field experiment begun last year by establishing the B-field scaling. The required JET/DIII-D identity shape at 4T, and q95=3.2, was established in 3 pulses. Non-axisymmetric field perturbations were applied using the C-Mod A-coil configure to produce a mode spectrum similar to that of the JET EFCC coils. Two opposing toroidal phases were employed, establishing an upper bound on the threshold in one sense and a lower bound in the other, at density between 1.5 and 2e20/m3. More data will be required to establish the threshold at this field, for comparison to the data obtained last year at 6.3T. Additional experiments at higher field, and with different toroidal phasing, are also planned in support of this MP. Operation on Thursday afternoon was devoted to ICRF antenna conditioning (MP#355), as reported in the ICRF section below. Friday's run was devoted to MP#405, "L-mode confinement time determined from fast SSEP change". This experiment is aimed at developing a new technique for measuring momentum transport. The basic idea picks up on the observations regarding the dependence of edge flow velocity on configuration (LSN, USN, balanced), and how this flow velocity defines a boundary condition for core flows. The concept is to change the equilibrium from LSN to USN in a time shorter than the momentum confinement time, and to observe the resulting variation in the core rotation profile, using HIREX xray doppler spectroscopy of seeded argon impurity ions. Ed Synakowski (PPPL) served as Session Leader for Friday's experiment. We were able to confirm the anticipated effect on core rotation as the configuration was varied within a single discharge. However, the fastest equilibrium change obtained using SSEP control was limited to 50-60 msec, which is longer than anticipated and longer than the expected L-mode transport timescale. The limitation may be due to slew rate restrictions on the power supplies used in this control scheme, which would imply that a different technique would be required to carry out such experiments in L-mode plasmas. There may still be value in using such an edge perturbation technique in H-mode or ITB plasmas, for which the momentum transport times are expected to be longer. ICRF System ------------ The D-port dipole antenna (FMIT#1) was successfully brought into operation during the second half of Thursday's run. An arc fault condition which had been restricting operation of this antenna was eliminated during vacuum conditioning. D-port antenna successfully launched up to 1.4MW for 0.4 seconds into plasma during the run. The E-port antenna (FMIT#2) coupled up to 1.5MW, and operated well overlapped with D-port. FMIT#3 and 4 (J-port four-strap antenna) have been successfully re-tuned to 78MHz and also operated into plasma on Thursday. Additional run-time will be required to fully condition this antenna. We are also investigating RF leakage which is affecting some diagnostics during operation of this antenna system. Lower Hybrid System ------------------- Substantial progress was made on the LH system last week, with numerous >100kW, 10ms pulses being launched both into plasma and into vacuum. Several different phase configurations were tried for the 8 x 4 columns being used with four transmitters. Phasing was varied from co-current to counter-current to balanced, and the launcher behaved as expected. Cart 2 was used in all tests last week. An extensive set of calibrations relating output from the vector modulators to power and phase delivered by all four klystrons in this cart was successfully completed. An additional cart (four more klystrons) is expected to be ready for operation this week. Work is continuing on CPS (Coupler Protection System) and other fault protection and monitoring systems. DNB System ----------- The installation phase of the new DNB is progressing well and is approximately 95% complete after about 11 working days. The vacuum tank with cryopumps was craned into position. Slight modifications to pads under the chamber stand legs were required to reduce the height by 25mm and provide for good horizontal alignment with the F-port beam duct. The high voltage transformer tank was craned into final position and mounted in a transverse orientation at the rear upper level of the DNB support stand. The ion source and grid assembly was leak tested and mounted at the rear of the chamber. All liquid helium fill tubes, fill valves and associated hardware were mounted on the chamber. The bending magnet leads and all water cooling tubes were attached. The vacuum pumps, valves and associated piping were mounted on the chamber, leak checked, and the pumping system was started. The base pressure dropped quickly to the low 10^-6 Torr range, and the assembled diagnostic neutral beam vacuum vessel was given a final successful leak test. A new water distribution panel that provides an additional two coolant channels (for a total of six) has been machined and is ready for assembly. The liquid nitrogen delivery lines have been re-attached and were used to fill the cryo pump jackets in preparation for liquid helium. The hydrogen gas delivery system has been assembled and passed a rough leak test. The 480V high power delivery system has been wired and energized all the way up to the input breaker. The lanthanum hexaboride hot cathode in the beam source has been heated up to temperature, and longer pulses with cooling monitoring will start this week. Beam camac shot cycle tests are underway, and low power testing of the source should begin soon with high power tests to follow. Travel and Visitors ------------------- Ed Synakowski (PPPL) visited C-Mod Thursday and Friday. He worked with John Rice, Martin Greenwald, and Steve Wolfe in performing a perturbative momentum transport experiment in ohmic L mode plasmas. Miklos Porkolab visited the Center for Integrated Plasma Studies at the University of Colorado on March 4, 2005, and presented the weekly seminar with the title: "Phase Contrast Imaging Studies of ICRF Waves and Turbulence in the Alcator C-Mod and DIII-D Tokamaks". _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly