Alcator C-Mod Weekly Highlights April 26, 2004 Research operations continued at Alcator C-Mod last week. We have now completed 67.5 research runs during the FY04 experimental campaign. Experiments were carried out in the RF Physics topical area and in support of diagnostic development and calibration. Progress also continued in the Lower Hybrid project. Plasma operations are scheduled to continue this week. Operations ---------- Two and a half days of research operations were accomplished last week. A total of 22 plasma discharges were produced, with a startup reliability of over 95%; of these, 17 plasmas were at high field (>7.8T). In addition, 34 no-plasma shots (beam into gas with toroidal and poloidal fields) were produced in support of calibration and development work on the MSE diagnostic (MP#394). The runs on Wednesday and Friday were both in support of MP#203 from the RF physics group on D(He3) heating at B~8T. The run on Wednesday had been scheduled to last 12 hours, but numerous power supply problems restricted the useful operation to about 4 hours (1/2 run day) overall. The power systems issues were mostly due to fuse loss in the toroidal field power supplies, and to IOC (instantaneous overcurrent) fault indications that did not appear to correspond to actual faults. These faults resulted in crowbarring of the TF current, and led to significant delays as both the supplies and crowbar circuitry need to be inspected and tested after each event. Fuses were replaced and additional instrumentation added to the TF supplies prior to the resumption of high field operation on Friday. No additional fuse loss or IOC events occurred during Friday's run. The RF heating experiment concluded a scan of He3 concentration over a factor of five in He3 gas puff duration. H-modes were produced with a threshold between 3.5 and 4 MW. The transition from He3 minority heating to the mode conversion regime was not clearly observed for the minority concentrations obtained, and additional work on this experiment is planned. On Thursday a no-plasma, beam-into-gas with field experiment was run in support of MP#394 for calibration of the MSE diagnostic. A series of pulses with varying magnetic field pitch angles were produced for five different values of the MSE filter temperature, which controls the optical passband of the filter and therefore the amount of the "sigma" line reaching the receiver. In addition to the MSE calibration, these shots were also used to calibrate the He3 pulsed gas system for use in the RF experiments on Friday, and for tests of the prototype digital plasma control system software. In addition to the TF power supply faults, we also lost some time on Wednesday when one of the 13.8kV circuit breakers failed and had to be replaced with a spare unit. This breaker has now been repaired and is available as a spare. ICRF System ----------- The ICRF system performed nominally for the plasma runs on Wednesday and Friday, with all four transmitters operational. Work is proceeding on new DC breaks for the J-port transmission lines; these are being prepared for use in conjunction with 50 MHz operation, for which the existing commercial DC breaks were found to be unsatisfactory. Lower Hybrid System ------------------- PPPL and MIT continued work on the launcher assembly last week. The forward waveguide and rear waveguide were mated with the rf gasket in place, and the new support plate was installed. Thermocouples have been attached to the rear waveguide. This assembly is now ready for high power testing. Final design details of a load adapter for the coupler end of the forward waveguide have been worked out. The adapter is being fabricated at MIT using some PPPL components and will be completed in time for high power tests this week. Measurements were taken, and plans were made, on how to complete installation of the rf probes. Following tests of the probes this week, all twelve probes will be made ready for installation by PPPL. We continued testing of the circulators last week. We now have 9 out of 12 high power circulators qualified for operation and tests have begun on a 10th. Two circulators will either have to be returned to the vendor or the vendor will visit MIT to work with us on the required rework. Work continued on the phase and amplitude control system. Nonlinear behavior in the feedback system has been traced to the vector modulator that is used to generate the drive signals to the klystrons. We continued to work with our vendor to improve software drivers used in the control system. Travel and Visitors ------------------- Nevell Greenough, Dave Miller, Doug Loesser, and Joe Frangipani, all from PPPL, were at MIT on 4/20 and 4/21 to work on the lower hybrid launcher assembly. John Rice presented a paper at the 14th Topical Conference on Atomic Processes in Plasmas in Santa Fe, NM. Two MIT graduate students, Vincent Tang and John Liptac, presented papers at the 15th Topical Conference on High-Temperature Plasma Diagnostics in San Diego. Bill Rowan, Ron Bravenec, and Perry Phillips (UT-FRC) also attended this conference and presented papers reporting on experiments at C-Mod. Matt Sampsell (UT-FRC) was at C-Mod last week working with the diagnostic neutral beam. In addition to operating the beam in support of the week's experiments, Matt also took data to evaluate a new diagnostic technique for beam neutralization.