Alcator C-Mod Weekly Highlights August 17, 2001 Disassembly of Alcator C-Mod continued this week. Equipment on and around the machine was removed. The next major operation will be the removal of the cryostat. Work continued on the LH MIE Project and ICRF upgrades. Operations ---------- Diagnostic equipment on the east side of the cell diagnostic stand has been removed in preparation for installation of the new mezzanine. The mezzanine will be used to support the LH klystrons and associated equipment. Power distribution transformers and ICRF transmission lines have also been relocated. All diagnostic and support equipment mounted on the igloo has been removed, allowing the upper igloo covers and 1st and 2nd igloo tiers to be removed from the cell. The cryostat is now in full view for the first time in nearly three years. Disassembly of the cryostat will begin next week. Physics ------- Analysis of the total particle inventory and main-chamber fueling for a series helium discharges has been performed. In these discharges, a constant plasma density was maintained after the helium gas puff was turned off, as one would expect for a 100% recycling gas species. The measured amount of helium injected into the vessel roughly agrees with total inventory of helium ions in the plasma and helium neutrals in the main chamber and divertor. The agreement was found to persist over the full set of discharge parameters. These results suggest that the present array of neutral pressure measurements roughly captures the full inventory of neutrals in the vessel. Also during these discharges, a set of 3 outer divertor flappers was repetitively opened and closed at 100 msec intervals. From the resulting rate of plasma and neutral density rise in the main-chamber, the transient neutral flow rate through a single flapper when it was initially opened appears to be about 3 Torr-l/s. This flow rate is about an order of magnitude lower than that expected from the vacuum conductance and the pressure under the flapper at the time of opening. These results suggest that plasma-neutral and/or plasma-wall interaction at or above the flapper opening may limit the effective neutral conductance. Further experiments have been done using the li-pellet ablation trail technique to measure internal magnetic field pitch angles. The pellet position is monitored using a downward imaging photodiode array located at the same toroidal location as the pellets (C port), which gives the radial position as a function of time. The images of the Li II ablation trails are taken with the PSI-3 ultrafast camera, in collaboration with PPPL and Princeton Scientific Instruments, which provides 12 sequential 2-d snapshots (64 x 64 pixels each) with time resolution as fast as 0.5 microseconds. For the pellet images, 30 microsecond imaging was used, which yields a spatial resolution of about 2 cm in the major radius direction. Several techniques are being investigated to measure the angle of the ablation trail, which is aligned with the local total magnetic field, and to evaluate the uncertainties. One technique, which appears to be very promising, involves taking the 2-d Fourier transform of the image, and then measuring the angular properties in k-space. A typical set of images from one discharge, along with an inferred pitch angle profile, can be seen at http://www.psfc.mit.edu/people/marmar/li_theta_1010731004.pdf Typical error bars range from +/-0.15 degrees to +/-0.4 degrees. These measurements can be used to constrain the EFIT reconstructions and thus yield important q-profile information. Lower Hybrid MIE Project ------------------------ We continue assembling the TPS PC boards. PLC development also continues. Modification of drawings for the rear panels of the EMI/RFI equipment rack have been released to the machine shop. Site modifications for the project are moving forward. The contractor for the water cooling system has been on site several weeks now running the cooling lines. The concrete slab that will support the high voltage power supply will be poured next week after several weeks of prep work. ICRF Systems ------------ The E and J-Port antennas have been removed from the vessel. The new J-Port strip lines are in excellent condition (no indication of arcing on the strip lines, vacuum feedthroughs, or vacuum vessel). Changes made to J-Port during the last up-to-air were all very successful. However, some arc marks were discovered at the ground bridge of the J-Port straps where the voltage exceeded 15 kV/cm in an E||B location. The bolts holding down some of the plasma facing boron nitride tiles also showed some arc marks. These locations are now being carefully modelled and changes will be made during the machine inspection period to eliminate these effects. Modelling of transmission line components suggests that we could use a double stub system to match the plasma load to the transmitters. This design could allow us to test real time matching using fast ferrite stub tuners (from AFT) during plasma operation. Improvements in the matching network operation could also be made and are under investigation. Work continues on development of the new phase detectors. The latest prototype is together and under test. The design of new equipment racks and the layout of the associated equipment continues. Inner Divertor -------------- The vendor for the rear backing plates is still on schedule for an early September delivery of all plates. We continued work developing a more compact stud welding system. DNB --- The DNB did not operate due to disassembly of C-Mod. Analysis of the diagnostic calibration data taken during the previous week continued. Travel and Visits ----------------- Stewart Zweben visited from PPPL this week to work on the fast imaging camera.