Alcator C-Mod Weekly Highlights July 6, 1999 The scheduled maintenance period continued at Alcator C-Mod last week. No plasma operations were scheduled. This week is also a maintenance week. Cryogenics and HEAT systems will be turned on, and the machine cooled down in preparation for resumption of operations. Physics and Analysis -------------------- Further investigation of the rate of RF antenna phase balance faulting, including all RF runs from 1998 and 1999, has yielded a good picture of where the plasma should be for reliable RF operation. (Phase balance faults are difficult to deal with because unlike arc detection faults and voltage limit faults, there is nothing the RF system can do operationally to reduce the faulting rate). Defining top (bottom) to be the closest distance from the last closed flux surface (LCFS) to the top (bottom) of the RF limiter, and midplane gap to be the distance from the middle (Z=0) of the RF limiter to the LCFS, and asymmetry to be top minus bottom, it has been found that there are two regions in midplane gap vs. asymmetry space where the RF runs with the lowest probability of encountering phase balance faults. One region is centered around asymmetry +0.33 cm, midplane gap 1.4 cm, (so that the plasma is slightly down from center) and the other is centered around asymmetry -0.07 cm, midplane gap 0.91 cm (plasma is nearly up down symmetric but closer to the outboard limiters). RF operation becomes very poor when asymmetry exceeds +/- 0.9 cm, or midplane gap exceeds 1.3 cm for negative asymmetry or 1.7 cm for positive asymmetry. A relative calibration was performed on the high-resolution (Auburn/U.Texas) ECE system. A small ramp in the toroidal field was introduced near the end of a discharge which resulted in adjacent ECE channels scanning the same plasma volume. Under the assumption that the plasma does not change during this ramp, a relative calibration of the 32 channels was derived. The resulting profile was then absolutely calibrated by normalizing it to the other Te diagnostics. A small anomaly was observed near the transition from the gradient to the sawtoothing region. Other ramped discharges were investigated to make certain this was not an instrumental problem. No problems were found in these other discharges and the anomaly was attributed to the smoothing required to remove the time dependance of the sawteeth. Reviews ------- A Cost and Schedule Review for the proposed C-Mod Lower Hybrid Current Drive System was held at MIT on June 30-July 1. This project is planned as a collaboration between MIT and PPPL. The panel (T. Bigelow (ORNL), F. Soldner (JET), R. Pinsker (GA), T. Intrator (LANL), and W. Ferguson (LLNL)) heard technical presentations by Miklos Porkolab, Ron Parker, Ian Hutchinson, Paul Bonoli, and Joel Hosea (PPPL). A quarterly review of the Alcator C-Mod programme was held at MIT on July 2. Rostom Dagazian and Mona Bradford from DoE attended in person. For off-site participants, the viewgraphs were posted to the web using our show-station, and telephone audio links were provided. David Mikkelson, from PPPL, gave his presentation remotely, also using the show-station technology. The agenda from the review follows. A Engineering and Operations 1) Jim Irby -- Status of the ICRF systems and the diagnostic neutral beam B Recent Results - Experiment and Theory 1) John Rice -- Toroidal rotation and impurity transport in ohmic H-Modes 2) Earl Marmar -- Evidence of neoclassical impurity transport effects in the H-mode pedestal 3) Amanda Hubbard -- Experimental tests of temperature profile stiffness 4) David Mikkelsen -- Testing theoretical predictions for the critical ion temperature gradient length in C-Mod 5) Paul Bonoli -- Advanced tokamak modeling ICRF Systems ------------- FMIT #3 and #4 were each operated up to 150kW into the J-port antenna last week. A transmission line arc, which had limited the J-port conditioning program, was diagnosed and repaired. Dry N2 gas in the transmission line was replaced by SF6, which allowed higher voltages (up to 28kV) and powers to be obtained. In addition, FMIT#4 also required some retuning to eliminate self-oscillation. The remaining test for FMIT#3 and #4 is to operate simultaneously into vacuum with 250 kW total through the antenna. FMIT#2 and #1 were successfully high potted to 35 kV with the transmitters fully assembled. Initial tuning and testing of FMIT#2 into the dummy load was begun. After reducing the high voltage to 24 kV (due to a suspicion that the cavity was breaking down due to the humidity), 1.5 MW was obtained. A couple of problems were also identified. The RF was gating off without cause. There was also breaker trip that is either a result of a faulty overload signal or a malfunction in the breaker. These are being investigated. The arc detection and voltage limit protection circuits were characterized and upgraded. The MIT arc detection modules respond within 5.4 usec to an excess in the ratio of reflected to forward power. The PPPL demodulator response time was improved from 80 usec to 4 usec. The time required to shut the RF off once an arc or voltage signal is received is now less than 1 usec. The improved response time will reduce any damage associated with an arc. Travel and Visitors -------------------- Joe Snipes presented a talk at the CRPP in Lausanne on "Fast Particle Driven Modes in Alcator C-Mod". Randy Wilson visited last week to help with J-port antenna conditioning following the lower hybrid cost and scheduling review.