Alcator C-Mod Weekly Highlights Mar 8, 2004 Research operations resumed at Alcator C-Mod last week. Five run days were scheduled and three were completed. Testing of the ICRF system at 50MHz revealed a limitation in the DC breaks in the coaxial transmission line at low frequencies. Progress continues to be made on the Lower Hybrid system. Plasma operations are scheduled to continue this week. Four run days are planned. Operations ----------- Alcator C-Mod operated for three full days last week. A total of 44 plasma discharges were obtained with a startup reliability of over 90%. An additional 34 machine cycles with magnetic fields were produced during a calibration run for the MSE diagnostic on Thursday. Plasma operations on Monday and for three shots on Tuesday were aimed at commissioning the J-port ICRF system for 50MHz operation. These efforts were unscuccessful, and the remainder of Tuesday and Wednesday were devoted to investigating the cause, as detailed in the ICRF section below. Operation resumed on Thursday with a "beam-into-gas with fields" run for characterization and calibration of the MSE diagnostic, as described under the DNB section. Friday's run was in support of MP#357 "Low frequency Active MHD Spectroscopy". A new audio frequency amplifier was employed to drive 40-60 A into each of the two Active MHD antennas under a variety of plasma conditions, including operation near the density limit and at low q. Data from these experiments are being analyzed. Several shots were again lost last week to a malfunction of the data acquisition server computer. Measures taken earlier to identify the source of these crashes indicate that the RAID disk controllers in the new servers are misbehaving under conditions of high data access, such as occurs during the shot cycle. An interim fix which transfers some of the load from these disks to external drives is being implemented. ICRF Systems ------------- Monday's run was devoted to commissioning the J-port ICRF antenna with the FMIT#3 and 4 transmitters tuned to a frequency of 50MHz. Both transmitters had operated successfully into dummy loads at up to 1.5MW each, but operation into plasma was characterized by high levels of RF leakage and bad matching to the antenna, resulting in high circulating power and arcing. The FMIT#1 and 2 transmitters (D- and E-port dipole antennas) were operated at the normal frequencies of 80 and 80.5 MHz, without any difficulty. Retuning of FMIT#3 transmitter on Monday evening to avoid a possible oscillation, and disconnection of several possible sources of RF leakage from the tokamak, did not improve the behavior on Tuesday morning, and Tuesday's run was terminated after three plasma shots to allow a more complete assessment. Investigations on Tuesday and Wednesday indicated that the commercial coaxial dc breaks in the 9 inch transmission line were unsuitable for use at 50 MHz. A design for a satisfactory replacement has been developed, in consultation with PPPL, and will be implemented. FMIT#3 and 4 are being retuned to 70MHz, and the corresponding matching network is being installed at J-port, in preparation for this week's operation. The scheduled experimental program employing 70MHz ICRF, originally planned to begin later this month, will be brought forward. Lower Hybrid System ------------------- Several 3-window prototypes were brazed at our vendor mid last week to test a variety of gaps, braze quantity, and copper plating thicknesses. Excellent results were obtained and information from these tests were used to braze a 24-window prototype that will be carefully analyzed when it returns from the vendor early this week. One of the 3-window prototypes was installed on our high power test stand and tested to 100 kW for 0.5 s. This power level is approximately 5 times what we expect during operation on C-Mod. Careful visual and microscopic analysis of the prototype indicated no arc marks or damage of any kind. We continue to work on amplitude and control hardware and software, and completion of the circulator acceptance tests. A klystron with a low water flow rate was repaired last week and tested to full rated power for 0.5 s. This klystron was previously designated as a spare to be used in limited pulse length operation. Diagnostics ------------ Thursday's run was devoted to calibration of the MSE diagnostic. Previous experiments had shown a surprisingly large effect of varying the temperature of the optical passband filters on the measured polarization fraction and the inferred angle of the magnetic field. The calibration activity was designed to clarify the reasons for this behavior. MSE measurements were performed at several different filter temperatures as the vertical field was scanned through its available range. Initial results confirm the temperature sensitivity observed previously in plasmas, and show that the effect is strongest at the plasma edge. Improved measurements of the MSE spectrum were also obtained. Travel and Visitors ------------------- Miklos Porkolab attended the VLT PAC meeting at UCSD on March 2,3. He was substituting for Jeff Freidberg on the PAC, who could not attend. Gerd Schilling (PPPL) was at MIT for the week to participate in ICRF experiments.