Alcator C-Mod Weekly Highlights July 27, 1998 Engineering: A quarterly review of the C-Mod program took place on July 23. The presentations were made via a 3-way video-conferencing link among MIT, PPPL and OFES. Presentations were given on the TF magnet repair (D. Gwinn), the DNB and Texas diagnostics (W. Rowan), the ICRF antenna and PPPL diagnostics (R. Wilson), other internal upgrades and modifications (R. Boivin), the Ideas Forum and FY99 operations plans (S. Wolfe), recent physics analysis (I. Hutchinson, M. Greenwald, and P. Bonoli), and budget (I. Hutchinson). Replacement of feltmetal on the TF magnet arms began last week. Four inner fingers on the Bottom F-Right bundle have been repaired. Fixtures required to balance the heat load on the two outside fingers are being reworked for increased mechanical stiffness. Based on a careful visual inspection, the quality of all the joints is excellent. We are still evaluating non-destructive ultrasonic techniques for final inspection. Two inner fingers on the Bottom A-Right bundle have also been repaired. We have noticed a pressure non-uniformity related to the coil spacer plates and pressure bladders. These components are being reworked to apply a more uniform pressure to the solder joints; production soldering, using two 10 hours shifts per day, will commence once these issues have been resolved. Electroforming of the finger on the central column was performed over the weekend. The process was started Friday night at 7:00 pm and continued until about 4:30 am Monday, for a total of about 58 hours. The new copper appears to be well bonded to the original copper, and after cleanup, exact thicknesses will be determined. If the new piece is thick enough, the process will be complete. Otherwise, one additional short session of electroforming will be required. Non-uniformity of silver-plating on feltmetal has resulted in our planning to reprocess 20 sheets to remove the silver in a vacuum oven, after which they will need to be replated. Tests on 2 full size sample pieces had yielded excellent results; however, the process for the larger batch was not performed to the same specifications. We are investigating alternate suppliers of silver-plating, and are considering setting up to do the job in-house, as well. We do not anticipate that this will have a significant impact on the schedule. We expect delivery of additional virgin feltmetal sheets from the manufacturer by the end of July. Rogowski windings for the full-scale TF test joint have been successfully tested and calibrated. The extremely restricted space in the TF sliding joint required a novel Rogowski design. This instrumentation will allow us to continuously monitor the current flowing through each of the four feltmetal pads in the sliding finger joint during full-current (220 kA) test pulses. The high voltage standoffs for the DNB accelerator grids were successfully tested to 58 kV. Minor breakdown difficulties, which had been seen at lower voltage, were due to water line routing, and these were easily remedied by rerouting the lines. Operation of the beam now awaits completion of the controls for the accelerator supply. While the detailed design of the controls was going forward, we started to enhance the existing beam diagnostics with a spectroscopic system for measurement of beam components and with an infrared camera for additional beam profile measurements. The infrared camera is being tested at UT-FRC, the spectrometer and Camac data acquisition equipment are at the PSFC, and assembly is underway. Manufacture of fiber optic control links continued. In the ICRF area, work continues on FMIT source number 4. We have received the teflon tubing required to repair the arc damage to the sockets. The 2274 vacuum tube that was experiencing some problems has been sent to Eimac for evaluation. The LOGBOOK and EDGE databases have been loaded into an SQLSERVER database under Windows/NT and testing of this database and the interface to it from our workstations has begun. Initial tests indicate the performance should be 2-10 times better than the RDB implementation under OpenVMS. A testing and migration plan is being developed. Physics: Paul Bonoli gave a presentation during the July 23 Quarterly Review of a collaborative effort between C-Mod and Professor C.S. Chang (Courant Institute) to understand the toroidal rotation measurements in C-Mod in terms of a theory developed by Professor Chang. The key points of his theory are that when the horizontal orbit shifts of fast minority ions (produced by ICRH) are not negligible compared to the Doppler broadened minority resonance, there can be an inward radial drift of the fast minority ions. The resulting charge separation creates a large radial electric field which can generate poloidal and toroidal plasma flows. The poloidal flow is found to be heavily damped so that the radial electric field (E_r) is balanced mostly by (V_phi X B) to satisfy E_r + (V X B)_r = 0. During EDA discharges with normalized beta above about 1.2, an apparently new type of ELM behavior is observed on C-Mod. These discharges have substantial edge pedestals and are well above the threshold for type III ELMs. The observed "high beta" ELMs exhibit a large signal on H-alpha (often bipolar, with the H-alpha level dipping well below the equilibrium EDA level), are quite short, perhaps 100 microseconds or less, seem to be localized within the transport barrier, and do not lead to any significant loss of particles or energy. Particle fluxes can, however, be seen by the divertor probes. The perturbation on the ion saturation current is intermittent, suggesting that the particle flux can be interrupted temporarily. This observation is consistent with the bipolar H-alpha signal and the observation of ECE signals which are either refracted or cutoff by high density regions that develop in the pedestal during these events. Otherwise similar high beta ELMfree discharges do not develop this type of ELM activity. Travel and Visits: Dave Johnson, from PPPL, visited MIT last week to perform calibrations of the x-point Thomson system. Professor Yuichi Takase, from the University of Tokyo, visited the Alcator C-Mod Group last week. He collaborated with Paul Bonoli on code development problems related to the MHD equilibrium and current drive code ACCOME. (The development of this code is part of an official joint effort between MIT and JAERI.) We discussed problems related to solution convergence in the ACCOME equilibrium solver (SELENE). It was discovered that the convergence properties of high beta equilibria with nonmonotonic current density profiles could be greatly improved by starting with a well-converged equilibrium at lower beta and low current drive power and then increasing the off-axis current drive power in small steps. Professor Takase will be using ACCOME to study high harmonic ICRF fast wave current drive in spherical tokamak configurations. Steve Wukitch returned from Garching, Germany at the end of the week. During his 3 week visit, he participated in ICRF experiments on Asdex-UG. Most of the experiments focused on measuring the power deposition profiles from mode converted ion Bernstein waves using a power modulation technique. Successful measurements of the deposition profile were made, and the data will be compared with simulations using the TORIC code. Poloidal flow measurements were also attempted; the data have yet to be analyzed. Piggyback experiments were performed, looking at 2nd harmonic damping on deuterium beam ions and preliminary analysis indicates the presence of a deuterium tail; detailed analysis of these results is ongoing. Miklos Porkolab participated in the MFE/ICF program leaders' meeting at General Atomics, San Diego, on July 20,21. This is the first time that the leaders of the two communities have met to discuss improved communication in the common quest to harness fusion energy. Agreement was reached for a joint workshop, to be held at PPPL in mid-September, at which information on technical issues will be shared.