Alcator C-Mod Weekly Highlights June 23, 2003 Plasma operations continued on Alcator C-Mod last week. Four run days were scheduled and completed. Experiments included MiniProposals from the Transport, MHD, and Divertor/Edge research topics. This week will be a maintenance week. C-Mod research operations are scheduled to resume next week. Operations ---------- A fresh boronization was carried out over Monday night, prior to the week's operation. A total of 97 tokamak plasma discharges were produced last week in support of four principal experiments. Startup reliability was over 80% for the week. All tokamak systems performed well. A newly calibrated control relay was installed in the TF#2 breaker on Thursday, replacing a unit which was suspected of causing spurious trips. The new unit is performing nominally, with no faults observed. In a variation from our standard practice, overnight electron cyclotron discharge cleaning (ECDC) was not carried out prior to runs on Wednesday and Thursday. Perhaps in consequence, startup reliability during Thursday's run was poor, with a large fraction of fizzles. ECDC was carried out over Thursday night. The air conditioning in the cell and cooling water source for equipment in the diagnostic labs was offline on Thursday and Friday due to an unannounced service interruption by MIT Physical Plant. Operation of some data acquisition systems and diagnostics was impacted by the outage. Physics ------- During a 4.5 T run day two weeks ago, several FRCECE (electron cyclotron emission radiometer) channels experienced cutoff as the plasma density increased. During the brief time interval while the ECE signals were transitioning from above cutoff to below cutoff, broadband high-frequency fluctuations (delta f ~ 50 kHz, peak f ~ 135 kHz) were observed on a set of four 1 MHz digitizer channels located within 5cm of the plasma center on the high-field side. We speculate that these may be density fluctuations, as they only appeared very near cutoff, when the FRCECE system is sensitive to changes in density due to refractive effects. On Wednesday, an experimental study (MP#339) of mode locking at lower plasma current was conducted, using the new A-coils to apply non-axisymmetric perturbations. In low density plasmas near the threshold for locking by the intrinsic error field, locked modes were induced by applied fields at a variety of mode phases, mapping out the regions of locking and unlocking. These data established the phase and magnitude of the equivalent intrinsic error field for these plasmas: a (2,1) amplitude of about 0.35 mT (about Bt/10000) and also the threshold field for locking the mode. Prior scaling fits of the magnitude of the threshold field predict that C-Mod should require approximately a factor of 10 higher value for locking. Therefore, our experiments, which observe locking at much lower values than predicted, contradict these earlier scalings and consequently improve prospects for avoiding locked modes in ITER, because they disprove the previous strong inverse scaling of threshold with size. Thursday's run was in support of MP#335: "Edge plasma flows in upper versus lower x-point discharges". The primary purpose of these experiments is to investigate the sensitivity of the magnitude and direction of the parallel plasma flows (routinely observed in the edge plasma) to the magnetic geometry, in particular upper, lower and double-null configurations. Results were limited by poor startup reliability on this day. Nevertheless, strong parallel plasma flows were detected on the inner wall probe (ISP), always directed toward the inner divertor strike point (down for SNL, up for SNU), while the outer midplane probe (ASP) detected little or no flow at nebar~1.7 (SNL or SNU) and 1.4e20/m^3 (SNU) and a flow in the co-current direction for nebar~1.0e20 (SNU). The latter co-current flow for low density has been seen previously in SNL discharges, suggesting that the mechanism responsible for it is independent of x-point location. The Chromex spectrometer saw some significant doppler shifts in the HeII line (He was puffed from NINJA on inner and outer midplane) from both inner and outer fiber views, corresponding to Mach numbers in the ~.2 range. This should allow us to get an independent estimate of the local flow velocities. Blobs seen on the PSI camera appeared to drift downward in SNL discharges but upward in some SNU discharges, suggesting that the flow fields (across B and/or parallel to B) had changed in a way that influences the blob motion. ICRF Systems ------------- The ICRF system operated nominally last week, providing heating power in support of experiments as required. Lower Hybrid System ------------------- At PPPL, the forward waveguide stack has been disassembled and is being prepped for replating. It is hoped that 25 of the 50 plates will be through the plating process by early July, after which they will be baked at MIT to assure the quality of the plating. At MIT, similations of the coupler during high power testing are underway. Initial results indicate steps will have to be taken to reduce the effects of the undriven (passive) guides during the tests. Work continues on the lower hybrid active control system. Boards are being designed that will allow automatic testing and calibration of this complex system. Travel and Visitors ------------------- Stew Zweben (PPPL) was at MIT 6/16-20 for edge turbulence measurements with the Gas Puff Imaging diagnostic. Good edge turbulence imaging data was obtained on about 50 C-Mod shots. These images showed poloidal and radial turbulence propagation with increased "blob" intermittency nearer to the outer wall. Data was also taken during the up/down separatrix experiment and the fast H-L transition experiment. Manfred Bitter and Ken Hill came 6/18-20 and made significant progress on investigating the problems with the imaging X-ray crystal spectrometer data processing electronics. A complete checkout of all electronic modules with help from Willy Burke (MIT) and review of computer settings with Korea resulted in proper acquisition of signals from an external X-ray source. Argon spectra are now being obtained from C-Mod plasmas. Detector and/or electronics issues encountered at high count rates are being investigated. Gerd Schilling was at MIT 6/17-20 to participate in ICRF operation. He also learned the procedures and computer settings for accessing the new C-Mod LINUX cluster remotely. Douglas Loesser, PPPL, was at MIT on Monday. He delivered two stainless steel forward waveguide plates to a local vendor for stripping and copper plating. After being returned from the vendor, the plates were baked in our oven over the weekend to verify the quality of the plating. Monty Grimes was on travel last week to San Antonio, TX, to inspect the waveguide needed for the high power launcher tests. He also traveled to San Jose, CA, to visit the vendor conducting the refurbishment and upgrades to two of our klystrons.