Alcator C-Mod Weekly Highlights July, 31, 2006 FY2006 weeks of research operations: JOULE target: 14 weeks Completed: 16.7 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Three run days were scheduled and 3.5 days completed. One overnight boronization was carried out prior to Wednesday's run. A total of 81 plasma discharges were produced with a reliability of about 90%. Plasma experiments were carried out in support of research proposals in Lower Hybrid Current Drive Physics, Transport Physics, and MHD topics; in addition, one half-day was devoted to beam-into-gas with field experiments in support of diagnostic development activities. The run on Thursday was shortened due to difficulties with the lower hybrid data acquisition and replacement of a 13.8kV breaker to the TF power supplies. Friday's runneutrals generated from charge-exchange of beam ions on torus gas affect the MSE pitch angle measurement. A key signature of the mechanism underlying the conjecture is that there will be a range of Doppler shifts of the Stark emission, including some emission that lies on the blue side of the unshifted Halpha line, when the diagnostic neutral beam is fired into a gas-filled torus. Based on numerical calculations, this blue spectral feature was expected to be ~20% of the dominant red-shifted emission, and it was expected to have a weak dependence on toroidal field, a moderate dependence on viewing geometry of different MSE channels, and a weak dependence on pitch angle. The spectrum measured with a McPherson spectrometer confirmed the existence of a blue spectral feature, although some details of its character differ from the numerical simulations. In particular, the intensity of the blue feature was smaller than expected, and it had a stronger than expected dependence on pitch angle. The second half of Tuesday's run was devoted to a continuation of MP#444b on Alfven cascade modes during current ramp-up. The experiment employed two-frequency (70 and 80MHz) ICRF heating for on- and off-axis deposition during the current rise phase. Clear cascade activity was observed on some shots, providing additional data for determination of the scaling of the base frequency. Wednesday's experiment supported MP#428 "Separating ITB formation from the initial transient phase of EDA H-modes", which addresses the mechanism responsible for formation of Internal Transport Barriers in C-Mod discharges with off-axis ICRF heating. David Mikkelsen (PPPL) was at MIT to serve as Session Leader for this experiment. The goal was to delay the beginning of the ITB phase until after steady H-modes have been established, and look for small changes in the Te profile that are needed to form ITBs. The delay is accomplished by heating the central region for the first 150-250 msec of the RF heating pulse; central and off-axis heating are both used in this period. Removing central heating while continuing with off-axis heating is expected to slightly broaden the temperature profile shape, leading to less transport in the central region of the plasma. The experiment was successful in forming delayed ITB's using this technique. Running at 5.6 T, 0.9 MA and target nebar of 1.5e20/m^3, we created steady H-mode shots which transitioned from 100% on-axis (D+E at 80 MHz) to 100% off-axis (J at 70 MHz) heating at constant power (1-1.5 MW). The shots reached a quasi-steady state with the on-axis heating, before the frequency switchover began, about 5 tau_E times after the start of the RF. ITBs were clearly observed. ITB formation occurred at about the point where the RF power was 100% off-axis. Density peaking reached ne(0)/ne(a) = 2-3, with peak densities approaching 6e20/m^3. Initial analysis of the Te profiles showed that the gradients did relax as the power moved off-axis. Detailed analysis of the profile data is underway. The runs on Thursday and Friday were devoted to extending our results with lower hybrid current drive (MP#418 and 455). The maximum coupled LH power was increased to 900 kW absorbed, and the loop voltage was caused to reverse, at least transiently, at plasma currents up to 1MA. The reversed or zero loop-voltage condition was sustained for times approaching one current relaxation time in these experiments. Contributing to the transient behavior was a density rise during the LH pulse. The other likely reason has to do with lack of steady-state as the E-field relaxes. Non-thermal electron spatial and energy distribution information will be obtained from energy resolved hard x-ray profile and non-thermal ECE measurements. Detailed modeling and analysis of these shots will be carried out. Interestingly, higher current resulted in better LH coupling as the reflected power decreased somewhat with increased current. Also interesting was the MHD phenomena showing up on the ECE, often fairly coherent oscillations, but sometimes quiescent. The latter case seemed to correspond to the best performance. One shot was obtained with simultaneous LHCD and off-axis ICRF heating. ICRF System ------------ The transmitter#3 (D-port antenna) crowbar trigger circuit was removed for trouble-shooting and repair on Monday. The repair was completed by Monday afternoon and the transmitter was put back into service. All transmitters performed nominally during C-Mod operations last week. Long-pulse Diagnostic Neutral Beam System ------------------------------------------ The beam was run for three operational days last week, including a set of beam-into-gas experiments on Monday, dedicated beam-into-gas with fields on Tuesday morning, and beam-into-plasma diagnostic operations during experiments on Tuesday afternoon and Wednesday. Beam performance has been overall quite good at 50kV, 6A. The beam developed a cathode gas flow problem associated with a sticking solenoid plunger late in the day on Thursday; repair is expected to be straightforward. Travel and Visitors ------------------- David Mikkelsen (PPPL) visited MIT last week and served as Session Leader on Wednesday for the experiment dedicated to MP#428. Randy Wilson (PPPL) participated as session leader for LH experiments on Thursday, July 27. Ian Hutchinson and Catherine Fiore participated in the APS Division of Plasma Physics DPP-06 program sorters meeting at the American Center for Physics. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly