Alcator C-Mod Weekly Highlights August 25, 2003 We continued work on lower hybrid and ICRF systems this week. The alternator inspection continues. Physics ------- Data from a June experiment focusing on fast dynamics at the L-H transition are being analyzed. In this experiment, RF power was turned on quickly at varying power levels ranging from the threshold (1.4 MW) to over 5 MW. Heating was off axis (80 MHz, 6.1 T) to minimize sawtooth heat pulses. Systematically, a small, fast jump is seen on the edge temperature and its gradient, followed by a slower, apparently diffusive response. Fitting edge channels of the FRCECE diagnostic, we find a characteristic time of ~0.6 ms and delta Te of ~30 eV. These values did not change significantly with input power. At higher powers, the transition occurs close to the RF turn on, when dW/dt is large. After correcting for the dW/dt term and other terms in the power balance, we find the net power across the LCFS varied by only about 50% at the transition, indicating that it occurs quickly once the instantaneous conducted flux reaches the threshold value. In low power shots, a long period of 'dithering' L-H-L cycles is observed. These observations are being compared to modeling by Benjamin Carreras of ORNL. Edge Thomson Scattering profiles of ne and Te before the L-H transition are also being compared to theoretical thresholds. Analysis of data from the deuterium DNB run last month continues. The goal of this run was to investigate the slowing down and/or loss of fast beam particles by looking at their signature on the DD fusion neutron signal. This analysis is quite relevant to RF heating on C-Mod, since these beam deuterons presumably undergo the same physics as the H-minority tail generated by ICRF. Making the simplifying assumption that the deuteron slowing down can be represented as a single exponential decay time, tau_s, allows for the determination of tau_s by fitting to the temporal behavior of the 2.45 MeV neutron signal. Since the slowing down times are comparable to the timescale for variations in the beam current, the fit must actually use a convolution integral. The principal findings are that a single tau_s does indeed fit the neutron rise, fall, and flattop behavior for each shot, and that the slowing down times agree with classical values in magnitude and scaling. Further analysis of absolute neutron rate is continuing, in order to put further limits on the prompt loss fraction. More detailed analysis will be done with the TRANSP code. Operations ---------- We continued work on new amplifiers for the PCI (Phase Contrast Interferometer) diagnostic. Work also proceeded on a new high power amplifier for the MHD antenna. Work on the alternator inspection has moved from the stator to the field rotor. UT inspections of the bearings have been completed successfully. UT inspections of the rotor are in progress. ICRF Systems ------------ Modifications needed for operation of the J-Port antenna at 50 MHz continued. We also continued inspections of rf transmission line, phase shifter, and stub tuner components in preparation for the next run campaign. The AFT fast-ferrite-tuner prototype arrived this week.This system had been on loan from GA to ASDEX-UPGRADE, and its return was facilitated by T.V. GEORGE, OFES. It will be used in ICRF experiments at C-Mod, allowing the match between the transmitter and the antenna to be adjusted automatically on a millisecond timescale. Lower Hybrid Systems -------------------- PPPL made good progress this week reworking the Forward Waveguide [FWG] plates. Polishing of the copper plating on the FWG plates is now complete. One stacked assembly has been re-assembled and satisfactory rf losses were achieved during low power tests. The second stacked assembly will be completed next week. We plan to begin high power tests at MIT on one of the stacks in about one week. Prototype testing and analysis [by MIT and PPPL] continues on the titanium couplers. Emphasis by PPPL this week was placed on developing an acceptable method of removing the ceramic bricks brazed in the titanium couplers. Two removal methods were being developed: chemical etching/fluid flow and thermal/pressure. However, the chemical etching/fluid flow method has proved to be very successful, and is the most likely method to be chosen for use on the actual couplers. A purchase order was placed at PPPL for a full set of new metalized ceramic bricks. Delivery is expected in late September. The initial high power tests of the prototype circulator were completed this week. Full 250 kW, 5 second long pulses were achieved into a properly matched load. Under these conditions the circulator showed very low insertion loss and better than specification return power to the klystron. A plate was then used to cover the output port and simulate the short circuit conditions that might occur during a waveguide arc or when the coupler is badly matched to the plasma. The circulator performed well for 5 s at power levels of up to 125 kW under these conditions. Progressively shorter pulse lengths were obtained as the power was increased up to the specification value of 250 kW. In all cases the circulator provided excellent protection for the klystron. Travel and Visits ----------------- On 8/19 Bob Childs visited PPPL to inspect the forward wave guide assembly and gather engineering info. Wolfgang Weiser, AFT, was here this week to witness the high power circulator tests.