Alcator C-Mod Weekly Highlights July 9, 2001 Plasma operations continued at Alcator C-Mod last week. Three run days were scheduled, but only one-and-a-half were completed, due to a problem with switchgear in the RF power system on Tuesday, and a broken in-vessel instrumentation cable which necessitated an up-to-helium vent on Thursday-Friday. Eighteen plasma shots were produced with a startup reliability of 75%. Plasma operations are scheduled to resume this week. Physics and Operations ---------------------- Tuesday's run was intended to carry out MP#246, to measure profiles of toroidal rotation in high-power ICRF-heated EDA H-mode plasmas. The start of the run was delayed by nearly six hours because of a failure of a mechanical component in the switchgear providing AC to the FMIT#4 ICRF transmitter, and by an overtemperature indication in an alternator bearing, subsequently traced to faulty instrumentation. Once these problems were remedied, a few useful shots for this MP were obtained, but the MP was not completed. The run scheduled for Thursday was in support of MP#291A, which aims to determine whether the spontaneous internal transport barrier (ITB) formation observed with off-axis ICRF heating can also be produced when the ICRF resonance is on the low-field (outboard) side of the axis. A previous attempt to generate an ITB with a low-field-side resonance was unsuccessful, but there was speculation that the available RF power at that time may have been insufficient. The experiment began with a repetition of the conditions under which an ITB was produced with high-field-side ICRF resonance, and the phenomenon was again observed. However, the shots were plagued by disruptions and impurity injections. After a number of such disruptive discharges, the problem was identified using the wide-angle TV view: a broken cable was moving into the plasma near the outer divertor, resulting in the impurity injections and disruptions. The cable was identified as being one of a set connected to the new B-dot probes on the J-port antenna. The vessel was back-filled with helium, and a "clean" vent carried out on Friday. The broken cable, and two others in the same set, were removed. Two additional cables, and a remaining broken-off segment of a third, which could not be removed, were secured and left in place. The vessel was pumped down on Friday afternoon and a bake to 120C carried out over the weekend. Glow cleaning was begun on Sunday. Operation will resume this week with clean-up and recovery discharges. We anticipate that several days of such operation will be required, after which physics experiments will be resumed. Piggyback experiments last week continued to exploit the capabilities of the new ultra-fast Princeton Scientific Instruments camera. We are using it to make two important, but quite different, measurements. First, it is being used to measure the radial profile of the poloidal magnetic field, where the changing tilt of the "cigar-shaped" Li^+ ablation cloud is imaged as the pellet traverses the plasma. (The Li^+ ablation cloud is constrained to be parallel to the local magnetic field.) Based on the images, a preliminary profile of the field pitch angle has been contructed, and we are now attempting to measure the pellet position more accurately. The second measurement we have undertaken with this camera is to follow the evolution of turbulence found in the SOL. We have found previously that the edge turbulence is "bursty", that is, occurs with a higher incidence of large density regions than would by expected from a normal distribution. These bursts have a lifetime of ~10-50 microseconds. With the camera we are able to observe the growth, movement, and disappearance of the emission from the high-density clumps. To date we have done this using a radial view where they appear as toroidal filaments. We find that at least some of the filaments move poloidally with a speed of ~200 m/s. We will soon move the view so that we can resolve both poloidal and radial motion. ICRF System ----------- With the exception of the mechanical failure in the FMIT#4 switchgear which contributed to the delay of the run on Tuesday, all four ICRF systems functioned well during physics experiments last week. Total power levels up to 3.5MW were provided. All antennas heated well, and excellent EDA H-mode discharges were produced. Following the discovery of the broken coax line on Thursday, it was decided that, in view of the short amount of time remaining for this experimental campaign, the frequency changeover from 78MHz to 70MHz for the FMIT#3 and 4 transmitters and the transmission line to the J-port antenna should begin immediately. This decision was taken to provide the maximum opportunity to carry out the planned experiments which require the lower frequency heating component, particularly double barrier experiments with combined on- and off-axis heating. Re-plumbing of the J-port transmission line for 70MHz is substantially completed. Re-tuning of the transmitters has begun, and will continue early this week, with the assistance of a PPPL engineer. Diagnostic Neutral Beam System ------------------------------- A beam-into-gas experiment was completed early in the week. The gas pressure in the C-Mod vessel was scanned to 1 mT, a pressure predicted to induce at least 60% reionization of the beam. The experiment will be interpreted for diagnostic calibration and as a reionization benchmark. The beam was instrumented to look for the cause of short arcs, one of the nuisance problems which does not prevent beam operation, but does result in loss of data. Inner Divertor Fabrication Project ---------------------------------- All 12 Inconel C-plates are now in-house. Work is proceeding on the mock-up, and on developing installation procedures. Lower Hybrid MIE Project ------------------------ The Transmitter equipment rack High-Voltage Control sub-chassis wiring has been completed. Work continues on the Transmitter Protection System (TPS) rack wiring. PLC I/O rack modules have been ordered in preparation for HVPS acceptance tests in October/November. Equipment to make the User Interface Control Station was specified and will soon be ordered. This will also be needed for the HVPS tests. Work began on the User Interface Control Station screen display software. Visitors and Travel ------------------- Howard Yuh attended the PPPS-ICOPS meeting in Las Vegas from June 20-22 to present an overview oral on the C-Mod EDA mode focusing on the modelling efforts and progress made with the new C-Mod Beowulf parallel Linux computer cluster.