Alcator C-Mod Weekly Highlights Dec. 11, 2000 Plasma operations continued last week at Alcator C-Mod. Five run days were scheduled and completed. Toroidal field and current were back in the normal orientation, i.e. with the ion grad-B drift direction toward the lower divertor. All runs were in support of the evaluation of the J-port antenna operating in the four-strap configuration. A total of 80 plasmas were produced with a startup reliability of over 90%. ICRF Systems ------------- We began operating J-port antenna as a four strap in heating phase ([0,pi,0,pi]) to test if the RF-plasma interaction observed in October was the a result of a failed decoupler stub or associated directly with the antenna. To briefly summarize the earlier observations, an RF-plasma interaction was observed to occur on the antenna protection box and Faraday screen. This interaction would become disruptive to the plasma at power levels above 2MW. After conditioning, a similar effect was observed during last week's experiments. For [0,pi,0,pi] phasing, the power level at which the plasma disrupted was ~2.5 MW. For other phases, the limit decreased: for [0,.85pi,0,.85pi] and [0,1.15pi,0,1.15pi] the limit was ~1.8 MW and for [0,0,0,0] the limit was 0.8MW. In addition, the hot spot location on the antenna side limiter and Faraday screen changed with plasma current, consistent with the field-line pitch. This suggested that there is an electric field developing along a field line connecting the corner of the antenna box, side plate and Faraday screen. One possible explanation is that an asymmetry in the RF flux coupled in the area enclosed by this field line interacts with the plasma density resulting in excessive heat load to the antenna structure. Further analysis is continuing, and near-term modifications to the antenna based on these results are under consideration. An additional aspect of the 4-strap operation was that significant time was required to recondition the antenna so it would operate without max faults in the first 100 msec on a daily basis. There was evidence of a problem between the shorted end of the antenna and the voltage probes. After conditioning the antenna in vacuum to 100 kW, the antenna began operating routinely into plasma. Some effort was spent attempting to eliminate RF interference in several poloidal field coil power supplies. Under some conditions, transient interference was observed to compromise control and lead to disruptions. Further dummy load tests suggest that FMIT#3 has significant RF leakage. During test shots we were able to reduce this interference by adding additional RF shielding, but these measures were less effective during machine operation. Additional RF filtering (ferrites) in the PF power supply control circuitry appeared to suppress the RF pickup. The J-port antenna is now being returned to a 2-strap configuration, and transmitter #4 is being re-tuned to 70MHz. Experiments using the new antenna/transmitter configuration will begin this week. Diagnostic Neutral Beam System ------------------------------- The DNB was operated into plasma at 47 kV with up to 5 A. Signals were observed on the MSE and BES diagnostics. We are putting together the information that we need to compare these two signals to a detailed description of the beam component mix. This information will allow us to benchmark the calculations and to verify the performance of the beam. The DNB component measurements still show some water in the beam. A baking system was completed and operated for the first time. RGA measurements indicate a reduction in the ambient water in the beamline. We will continue baking through the first couple of days this week. The fueling system was also baked, checked for leaks, and minor repairs completed. Lower Hybrid Project -------------------- Modification of the first 4-klystron RF "cart" continued last week. The electrical design for the cart was completed, and parts ordered for wiring. The set-up lab area was cleared in preparation for the lower hybrid space change scheduled to begin next week. Electrical design of transmitter instrumentation and control continued. Modelling of water loads for the lower hybrid coupler continued. A meeting to discuss the layout of water cooling and power systems for the lower hybrid system was held last week with MIT engineers. A review of data acquisition plans for the LHCD was held. The plans for Compact PCI based data acquisition for LHCD, Magnetics, ECE, and the LHCD experiment were discussed. A decision was reached to pursue CPCI based data acquisition for these applications. The basic building block of the proposed solution would be a CPCI digitizer with 32 channels, 200 kHz simultaneous sampling, 16 bit, 1 MSample / channel of memory.