Alcator C-Mod Weekly Highlights December 2, 2002 Preparations for invessel work continued last week. Work on ICRF and Lower Hybrid systems moved forward. Analysis of data from the last campaign continues. Operations ---------- The entry way tent for invessel access was setup up last week and tools and equipment prepared for work. However, the radiation levels from limiters in the machine remained too high for manned access until this Monday. Invessel pictures from the first survey may be found at http://www.psfc.mit.edu/cmod/operations/EngImages/CmodImages.asp?ref=/INVESSEL/2002/1st_Survey_12_03_2002/ The GA long pulse ICRF dummy load installation has been completed. We are now in the process of preparing the electrolyte solution for the load. We continue installation of the Lower Hybrid crowbar wiring. Data acquisition equipment is being mounted in equipment racks. Preparations are being made for high power tests of the klystrons and HVPS. The location of the high resistance to ground of the TF coil is still being investigated. The cryostat boot near the TF bus connection at A-Horizontal has been opened up for better access. Physics ------- Analysis of sawtooth propagation in the visible continuum (Z-meter) profiles for ITB discharges shows a clear barrier in the particle transient-transport at the location of the ITB foot. The profiles of time-to-peak after the sawtooth crash are very reminiscent of those seen in the electron heat pulses (S. Wukitch et al., APS-DPP 2002: www.psfc.mit.edu/cmod/sciprogram/APS/APS2002/wukitch.pdf) So far, the particle pulse analysis has been carried out for high field ITB discharges (B=6T) with combined on- and off-axis ICRF heating. The next step in the analysis will be to do direct comparisons of the particle and heat pulse evolution on the same discharges. The time resolution for the particle measurements was 1 msec, which is just fast enough to see the effect. In future experiments, the acquisition rate will be increased to give time resolution of 250 micro-seconds. Clear evidence of locked modes on C-Mod has now been found by taking the difference of poloidal field pick-up coil signals on the wall and the outboard limiters from opposite sides of the tokamak toroidally. This technique eliminates most of the equilibrium n=0 changes and leaves predominantly n=1 field perturbations. As we went to higher plasma currents, more pronounced effects of locked modes on confinement were observed as well as a stripe of particle and heat flux on the shelf of the divertor in the plasma video images. By integrating the difference of coil signals on the AB and GH limiters around the time when the sawteeth became small and rapid, and there was a drop in the stored energy, an odd n poloidal field perturbation was found ranging from 60 - 150 Gauss at the outboard limiter. This perturbation is a surprisingly large amplitude locked mode compared with those seen previously on C-Mod prior to density limit disruptions which usually reach amplitudes of only 50 Gauss after 10 to 20 ms. The large amplitude locked modes in these recent high current discharges also lead to disruptions, but after several hundred ms and there is no evidence of a mode slowing down and locking on the fast sampled dB/dt signals. Martha Redi, PPPL, visited C-Mod at MIT this week on 11/26 and 11/27. She met with Catherine Fiore for discussion of the ITB off-axis RF H-mode ITB trigger. Earlier work done with GS2 suggests that the trigger for ITB formation is high Te, lowering the strongest driving force for ITG at the trigger time, the ion temperature gradient mode driving force, (grad Te)/Te. Whether this is also the case for ITB in the ohmic H-mode will be examined. Next week gyrokinetic calculations for full profile plasma with GYRO for CMOD will be begun, through a nationally orchestrated videoconference course given by Jeff Candy at GA. MSE Diagnostic -------------- Based on discussions with Gerritt Kramer and Fred Levinton, Steve Scott, PPPL has implemented a new analysis algorithm for the Motional Stark Effect diagnostic that numerically simulates the operation of a lock-in amplifier. Previously, the MSE analysis simply performed a fast-Fourier transform on the time-domain MSE signals to isolate the frequency components that correspond to the drive waveforms of the two photoelastic modulators (PEMs). This technique was effective in discriminating against signals at other than the frequencies of interest, but it didn't impose the additional constraint that the MSE signals must also be in phase with the PEM drive. The new technique discriminates signals both on the basis of frequency and phase. The new analysis based on the numerical lock-in algorithm significantly improves the MSE diagnostic's effective signal-to-noise ratio, particularly at high plasma density where there is a large background noise source arising from continuum radiation. The scatter in the inferred polarization angles as a function of time, or equivalently the scatter across similarly-prepared shots, is decreased by more than a factor of five in some channels. This technique should enable the MSE diagnostic to measure the q(r) profile to the center of higher density plasmas than previously possible.