Alcator C-Mod Weekly Highlights April 28, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 13.1 weeks Operations ---------- Research operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 96 plasma discharges were produced with a reliability of 85%. The runs supported experiments in the Lower Hybrid and ICRF Physics research areas. Overnight boronizations were carried out prior to the runs on Tuesday and Wednesday. During operation on Thursday, significant increases in radiated power were observed, leading to increased disruptivity. Spectroscopic analysis identified higher than normal levels of iron and chromium in the discharges. Operation with outer gaps in excess of 2.5cm were found to allow operation with near-normal radiative power fractions. From various diagnostic measurements, including camera images, we inferred that a stainless steel object was protruding slightly into the normal plasma volume, probably in the vicinity of A-port. Further investigation carried out during the maintenance day on Monday revealed that the A-port scanning probe was inadvertently inserted; the probe has now been withdrawn to its normal rest position. Plasma operations are planned to continue this week. Operation Details ----------------- The run on Tuesday was the first of this campaign to utilize 50 MHz ICRF, using the four-strap J-port antenna. The principal experiment was MP#549, "MCCD effect on sawtooth period with energetic particles". A toroidal field ramp was used to identify the mode conversion layer location and make initial observations of the MCCD effect on the sawtooth period. Subsequently a fixed Bt was used to compare the effect with different antenna phasing. Phase contrast imaging (PCI) measurement showed that at Bt=5.1 T, using a 150msec He3 puff, the MC waves were peaked at R ~ 65 cm, very close to the q=1 surface for these plasmas. More detailed analysis will determine whether the current driven was outside or inside the rational surface. The observations in L-mode, using only the 50MHz system, showed that the sawtooth periods were the longest in co-CD antenna phase, and shortest in counter-CD antenna phase, indicating that local MCCD affects the sawtooth period, consistent with prior C-Mod observations. Results obtained by combining the 50MHz MCCD with minority proton heating at 80MHz from the D- and E-port antennas were more complex. Early in the RF pulse similar changes in sawtooth period were observed, while later in the pulse, when a fast particle population was established, the effect of MCCD was less apparent. On several shots with MCCD significant co-current toroidal rotation, up to 100 km/sec, was observed. This velocity is more than a factor of two larger than the typical intrinsic rotation for the observed plasma current and stored energy and may be attributable to mode conversion flow drive. This unexpected aspect of the experiment was pursued further on Friday. Wednesday's experiment was devoted to MP#536 "Evaluation of FWEH, FWCD, and MCCD". Clear evidence of fast wave electron heating at 50 MHz was obtained for the first time in C-Mod. The target plasma was a 1MA upper single null discharge with Bt=5.2T and electron temperature Te~4.5 keV. At lower temperatures the heating was weak, suggesting poor wave damping. Impurity production with the RF was higher for the fast wave than for minority proton heating using 80 MHz from the D- and E-port antennas. The experiment on Thursday was in support of MP#551 "Penetration of accessible LH waves", which is intended to investigate the observed decrease of non-thermal electron generation with increasing target density in LHCD experiments under conditions which are expected to have good wave accessibility. The planned experiment could not be completed, due to the effects of the enhanced iron radiation described above. Attempts to couple the LH waves with the large outer gaps required to avoid the high radiated power fractions were not successful over the range of target density attempted, and only very limited data were obtained. During Thursday's run, high resolution spectra were acquired in the spectral region used by the MSE diagnostic for current profile measurements. One MSE view was made available for the measurement. The purpose of these measurements is to look for evidence of impurity spectra which may affect the MSE measurements. On Friday, the observations of RF-induced rotation reported during Tuesday's experiment (MP#549) were investigated further. Similar plasmas to those employed on Tuesday were used, while varying B-field, He-3 levels, and RF waveforms. The rotation was observed to increase by ~35 km/sec with 3MW at f=50MHz, phased in the counter-current drive direction in L-mode, with only 10kJ increase in stored energy. In H-mode, velocity changes of 50km/sec were observed with a 14kJ increase in stored energy. The effect is strongest for counter-CD phasing. Central minority heating at 80MHz did not result in changes in rotation stronger than the typically observed delta_W/Ip scaling. The effect was strongest with ~15% He3 for mode conversion, while higher He3 concentrations resulted in no excess change in rotation. The results were similar for Bt=5.1 and 5.3T, except that higher delta_W was observed at 5.3T (MC layer nearly on-axis). No effect was observed when the MC layer was moved far off-axis (Bt=4.8T). Diagnostics ----------- The Beam Emission Spectroscopy (BES) diagnostic was modified for beam attenuation measurements. This new capability may improve the accuracy of measurements of boron density using CXRS data. The wavelength control for the BES diagnostic was re-calibrated using standard lamps so that the required spectrum can be accurately selected. The fiber optics were reconfigured to provide views along the trajectory of the beam from the edge to the core. Data were acquired during plasma operation this week. Steve Scott (PPPL) held a conference call with Jinseok Ko (MIT), and Steve Allen, Scott Lerner, Mike Mikowski, and Chris Holcomb of LLNL to discuss possible optical designs for an in-situ, post-shot calibration system for MSE. Scott Lerner has identified a promising design for a curved mirror that would properly simulate the light pattern for the two spatial channels at the edge of the MSE field-of-view. Travel and Visitors -------------------- John McLean of D-Tacq Systems began a visit to work on the upgrade to the Lower Hybrid Coupler Protection System (CPS). Progress was made in testing the first three of the boards to be used in the system. Six more boards were shipped and will be installed soon for more testing while John is here. Bill Beck, David Gwinn, Jim Irby, Rui Vieira, and Alex Zhukovsky participated remotely from MIT in the NCSX Cryostat and Cryo System Peer Review on April 23. Joe Snipes participated in the Confinement Database and Modeling ITPA meeting at ORNL last week. He presented the latest C-Mod results on the dependence of the H-mode threshold low density limit on plasma current and toroidal field. Russ Doerner visited from UCSD to give a seminar entitled 'Issues Associated with Codeposition in ITER; What to Expect'. Part of the visit was to discuss ongoing collaborations on hydrogenic retention (both at C-Mod and at PISCES). Topics included the current understanding of C-Mod results as well as the testing of C-Mod tile samples at PISCES. Collaborations on developing a monitor of molybdenum erosion through use of a mixture of copper in molybdenum was also discussed. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly