Alcator C-Mod Weekly Highlights March 22, 2004 Research operations continued on Alcator C-Mod last week. A total of four run days were completed. This brings our total research runtime in FY04 to 49.5 days (12.4 weeks), which is to be compared with the 18 weeks called for in Milestone SC7-6a. Progress also continued on the Lower Hybrid project. Plasma operations are scheduled to continue this week. Operations ---------- Five run days were scheduled last week, and four were completed. Monday and Tuesday were limited to 1/2 day each as a result of computer and network outages. A fresh boronization was carried out on Wednesday evening. A total of 86 plasma discharges were produced last week with a startup reliability of over 90%. Experiments last week continued our investigations of Internal Transport Barriers, and also supported experiments on locked mode effects and transport transitions. The run time on Monday and Tuesday was devoted to a continuation of MP#341, "Power absorption inside ITB's". These experiments were intended to extend the results obtained on 3/9/2004, in which we were successful in heating inside the core barrier with up to 1.5MW of on-axis power. However, wall conditions had degraded over the intervening week, and the ITB's formed in these runs were much more subject to high radiation and impurity accumulation, leading to back transitions. Under these conditions, the ITB's were found to be suppressed with much lower central heating powers than was observed previously, indicating that wall conditions are a key feature in permitting ITB's to be maintained at high central power. Wednesday's run was devoted to MP#386, a study of the impact of intrinsic rotation on the locked mode threshold in upper and lower single null discharges. Experiments were carried out at 1MA,5.4T and 600kA,4.1T using the non-axisymmetric control coils to induce locked modes at high density and suppress them at lower density in both upper and lower null ohmic discharges. At 1MA and high density (nebar~1.5e20/m^3) the intrinsic core rotation is observed to be more negative (counter-current) in upper null than in lower null discharges, while at lower nebar~8e19/m^3 the rotation is more positive in the upper null cases. At the time of mode locking the core rotation was close to zero in both cases. Further detailed analysis will be needed to see if there is a definite result on the threshold error field that can be attributed to velocity differences. Following a fresh boronization on Wednesday night, the run on Thursday began in support of MP#324A, "ITB Formation with Two-Frequency ICRF in Optimized Shear Plasmas with Ip Ramp". An important aspect of this experiment, which follows up on previous work, was the observation of Alfven cascades (AC's) which occur during the current profile evolution. Conditions under which cascades were obtained were established and good data obtained on both PCI and magnetic diagnostics. Low n (n=1, 2) Alfven Cascades were observed. There appears to be a threshold for obtaining the AC's of nearly 2.5 MW of ICRF and at that power level one to three cascades are visible on the magnetic pick-up coil signals or on the PCI diagnostic. The k_R measured by PCI was about 1 cm^-1. About halfway into Thursday's run, loss of the filament in the FMIT#4 driver tube prevented further operation at 70MHz. Following a couple of hours of continued single-frequency studies, the remainder of the day was given over to completion of work on MP#300, on possible hysteresis effects in ITB formation during slow scans of the toroidal field (resonance location). This experiment established that the threshold field at which the ITB occurs or is lost is quite sharp with no observable hysteresis. For the conditions studied the transition point was 4.75 +/- 0.1 T. The run on Friday was in support of MP#367A, which aims to determine whether transitions in transport and turbulence near integral q values, such as have been observed in stellarators, also occur in tokamaks. Slow ramps of plasma current at fixed field were employed to scan the edge q in limiter discharges down to qa=2.6, with fluctuations observed using the PCI and reflectometry diagnostics. No changes in the turbulence spectrum or transport were observed, in contrast to the behavior of stellarator high-k turbulence observations on W7-AS. ICRF Systems ------------ The available power was reduced on Thursday as a result of a driver tube socket problem in FMIT#4 which caused the filament to appear to be open-circuited; when the tube was removed the filament was found to be intact and the socket connections are now under repair. On Friday, the filament power supply on the FMIT#1 final power amplifier failed as a result of a mechanical problem. This system is also under repair and both transmitters are expected to be back in service this week. Lower Hybrid Project -------------------- A problem with the vendor's furnace delayed completion of the 3-window prototype braze for two days last week. However, the braze was completed on Friday and is now under evaluation. The leak in the forward waveguide was tracked down to a metal seal at the back end of the assembly. With help from Dave Miller, PPPL, the fwg rear flange was disassembled and the seal removed. A manufacturing defect in the seal was identified, the vendor consulted, and new seals will soon be on order. Reassembly has begun with seals we had in-house. We continue to develop launcher PLC control software and test routines for verifying proper operation of phase and amplitude control loop components. Travel and Visitors ------------------- John Rice, Paul Bonoli, and Joe Snipes were at the ITPA meetings for the Transport, Steady-State Operation and Energetic Particles, and Confinement Database groups in Naka, Japan from 8 - 11 March 2004. John Rice is presently collaborating on experiments at LHD. Martha Redi (PPPL) was at C-Mod 3/15-19 and worked on analysis of the off-axis RF ITB experiment. Discussions were held with Steve Wukitch, Martin Greenwald, Nils Basse, and others. Dave Miller (PPPL) was at MIT 3/18-19 assisting in investigating the source of a vacuum leak in the Lower Hybrid launcher's Forward Waveguide. A quarterly televideo review was held on March 11. Presentations were made by Earl Marmar and Jim Irby. The viewgraphs can be found at http://www.psfc.mit.edu/cmod/sciprogram/quarterly_1040311.pdf In attendance were E. Marmar, J. Irby, S. Wukitch, M. Porkolab at MIT, R. Dagazian, E. Oktay at DoE OFES, G. Schilling, D. Meade at PPPL, and W. Rowan at U. Texas. On Monday, March 15, E. Marmar and R. Parker attended a meeting of the Fusion Facilities Coordinating Committee at DoE OFES in Germantown. The main topic of the meeting was a discussion of research coordination among C-Mod, DIII-D and NSTX. On March 16 and 17, E. Marmar and M. Porkolab attended the OFES Budget Planning Meeting in Gaithersburg, MD. Earl Marmar gave a presentation on C-Mod results, plans and budgets; the viewgraphs can be found at http://www.psfc.mit.edu/cmod/sciprogram/C-Mod_Budget2005.pdf Miklos Porkolab gave a presentation on MIT Institutional Issues