Alcator C-Mod Weekly Highlights May 12, 2003 Plasma operations continued at Alcator C-Mod last week, with four run days scheduled and completed. A total of 56 plasma discharges were produced, with a startup reliability over 80%. The H/(H+D) ratio is below 10% and the RF systems have been conditioned up to 4.5MW total power (through three launchers). One run was devoted to initial studies of locked mode generation and suppression with the new non-axisymmetric control coils. Progress also continued on the DNB and Lower Hybrid Systems. Plasma operations are scheduled to continue this week, following the first boronization of the campaign. Three run days are planned. Operations ---------- The C-Mod tokamak operated for four days last week. Runs on Tuesday, Thursday and Friday were primarily devoted to RF conditioning and continuation of clean-up discharges. Tuesday's run was scheduled to start late to allow completion of work on the RF systems, and was further delayed by a computer outage and a TF instrumentation problem. Several shots last week were terminated prematurely due to noise on bus instrumentation resulting in spurious fault indications. Following two such events on Thursday, the run was suspended and a series of power system tests were run in an effort to determine the source of the noise; these tests, which continued on Friday morning, were inconclusive. Plasma operation was then resumed. The origin of this intermittent noise is still under investigation. Physics ------- Stewart Zweben has begun to analyze the motion of the plasma edge turbulence in high speed C-Mod videos taken with the Princeton Scientific Instruments cameras. He has also made a new movie showing 15 of the best shots from the 2001 run. The movie can be seen at: http://www.pppl.gov/~szweben/C-Modvideo/cmod_video.html Wednesday's run was primarily devoted to MP#331, an initial study of error-field effects using the non-axisymmetric control coils (A-coils). Locked modes were induced in a 1MA, 2e20/m^3, 5.4T discharge using a "quadrupole" configuration of four control coils, which produces the maximum ratio of 2/1 to 1/1 perturbation at the q=2 surface of about 0.8. The threshold amplitude for production of a locked mode in this configuration was determined to be 0.4e-3<|B_r21|<0.55e-3 tesla, uncorrected for any background non-axisymmetry arising from PF coil misalignment. The same A-coil configuration operating with the opposite polarity was successfully used to suppress a "naturally" occurring locked mode in a low-density 1MA discharge. Additional experiments with different helical field configurations and toroidal phase were not observed to produce locked modes in the 2e20/m^3 target discharge. Detailed analysis of these results is underway. ICRF Systems ------------- Conditioning of the antennas, particularly J-port, into plasma discharges continued last week. The main purpose was to increase the power handling under L-mode conditions. The conditioning was slowed by a problem with J3 stub tuner that was rectified on Friday. Digitizer problems also slowed the conditioning process. Nevertheless, by end of Friday J-port was operating reliably at 2 MW and had short pulses up 2.5 MW. A control problem with D-port limited its availability. The directional coupler at the transmitter output, DC0, appears to intermittently under report the actual power. The DC0 is used in the power feedback, causing the output power to exceed the intended demand. During testing of this system, a rf switch was observed to arc at high power. The arcing was eliminated after re-seating the switch. DNB Diagnostic Systems ---------------------- The diagnostic neutral beam continues to perform well, firing on nearly all C-Mod shot cycles for the Tuesday-Thursday runs (and also between shots) at 49-50 kV and 4.0-4.7 A. It is now realized that the primary source of pitch angle variation in the MSE data is actually due to a windowing apodization issue in the analysis methods, and not to variation in the measured signals. Improvements in the analysis methods are being investigated to avoid this issue, which should yield much less variation in the pitch angle results. Lower Hybrid System ------------------- Testing of the 12th klystron was completed last week. The first of the external circulator waveguide assemblies is being readied for installation. Two additional klystrons, which will be used as spares for the initial phase of LH operation, have been shipped out for refurbishment and an upgrade to the cathode performance. A meeting was held at MIT Tuesday-Wednesday to discuss the Lower Hybrid launcher status; participants were Stefano Bernabei, Rich Hawryluk, Joel Hosea, Doug Loesser, Gerd Schilling, and Randy Wilson from PPPL and Bill Beck, Bob Childs, Monty Grimes, Amanda Hubbard, Ian Hutchinson, Jim Irby, Earl Marmar, Ron Parker, Miklos Porkolab, Rui Vieira, and Steve Wukitch from MIT. A list of observed issues was generated, and tentative schedules for addressing these and conduct of pre-installation testing was developed. An engineer from OPAL-RT was onsite all last week consulting on interface issues for the LH Control System and training C-Mod staff on the OPAL-RT system. Programming modifications to allow direct communication between the OPAL-RT system and Mdsplus were carried out and tested successfully. Travel and Visitors ------------------- Stefano Bernabei, Rich Hawryluk, Joel Hosea, Doug Loesser, Gerd Schilling, and Randy Wilson were at MIT 5/6-7 for discussions on the Lower Hybrid launcher. Miklos Porkolab, Martin Greenwald and Joe Snipes participated in the US ITER forum at the University of Maryland, May 8-9. Greenwald presented a proposal for US leadership in data system software for ITER, and Snipes presented a proposal for an Active MHD Spectroscopy system.