Alcator C-Mod Weekly Highlights March 17, 2008 FY2008 weeks of research operations: Target: 15 weeks Completed: 8.4 weeks Operations ---------- Research operations continued at Alcator C-Mod last week, with four run days scheduled and completed. A total of 91 plasma discharges were produced with a reliability of 74%. The runs supported experiments by the Transport Physics Group , H-mode Integrated Scenarios and Rotation Thrusts, and the Diagnostics Group. In addition, beam-into-gas calibration (without fields) of the MSE system was carried out during the Monday maintenance day. Overnight boronization was performed prior to the plasma run on Thursday. A total of 3 hours of run time were lost last week due to delays arising from power system repairs and magnet cooling issues. Plasma operations are planned to continue this week. Operation Details ----------------- The principal experiment on Tuesday was MP#529, "Toroidal field dependence of the low density limit to Ohmic H-mode", which comprises part of ITPA Joint Experiment CDB-11. The issue addressed by these experiments is the behavior of the L-H threshold power at low density, which is important for extrapolation to the proposed ITER H-mode scenario. The C-Mod experiments were run with toroidal fields down to 2.2 T, in order to match the operating range of facilities like DIII-D as part of the ITPA joint experiment. An attempt to go to densities as low as nebar = 0.25e20/m^3 did not achieve H-mode, but Ohmic H-modes were obtained down to nebar = 0.34e20/m^3, much lower than previously found at higher toroidal fields. To achieve H-mode at that low a density required 2.3 times the H-mode threshold scaling power given by Pth = 1.42 nebar^0.58 B0^.82 a^.81 R^1.0 . It should be noted that DIII-D, ASDEX-Upgrade, JET, and JT-60U all find a low density limit around nebar = 0.25e20/m^3, so these new data from C-Mod now approach the low density limit found on these other devices by operating at similarly low toroidal field. The previous low density limit found on C-Mod was between 0.8 - 1.0 x 10^20/m^3 at 5.4 T. These data indicate that the low density limit decreases approximately linearly with toroidal field, independent of machine size. If this is indeed the case, this would make the H-mode difficult to achieve on ITER at its prescribed target density of 0.5e20/m^3 without having more than twice the threshold scaling of input power, which would require perhaps greater than 80 MW of input power (assuming the above threshold power scaling) in deuterium. More recent H-mode threshold scalings predict even higher threshold powers. The total auxiliary heating power presently envisioned for ITER is 73 MW. About two hours of run time on Tuesday were devoted to obtaining Thomson scattering density calibration data using the ECE cut-off method. In these discharges, carried out at fields near 4 T, the density is ramped during the discharge allowing the cutoff density to be determined for ECE channels on the low-field side. These data permit calibration coefficients of the core Thomson scattering channels to be calculated. The last hour of Tuesday's run was used to begin the Ohmic H-mode experiment MP#493, which was continued on Thursday (see below). Wednesday's run was devoted to MP#523, "Effect of Lower Hybrid Current Drive on Rotation profile". This experiment continued the documentation of the unexpected modification to the toroidal rotation profile observed during LHCD. The emphasis was on determining the response at different LH phasing. Data were obtained at phasing of 60 and 75 degrees, and at fields of 5.4 and 6.2 T with net LH power up to 1MW coupled to the plasma. The goal of Thursday's experiment (MP#493 Ohmic L-H threshold experiments) was to investigate in detail the plasma conditions near the last-closed flux surface (pressure gradients, collisionality, plasma flows...) that are associated with ohmic L-H transitions. H-mode transitions were obtained by ramping down the toroidal field. The experiment was carried out following an overnight boronization, resulting in H-modes that quickly evolved into EDA behavior, with associated internal transport barrier (ITB) formation. Initially 0.8 MA lower null discharges were employed, and probes were scanned to document the edge shear layer. Next a series of matched upper null discharges were run, which surprisingly also exhibted L-H transitions at similar parameters. The EDA and ITB characteristics were found to be very sensitive to radiated power, such that even a minimal argon puff normally employed for diagnostic purposes had to be curtailed to avoid Elm-free behavior, with no subsequent development of an ITB. Large amplitude quasi-coherent mode (QCM) signals were seen on the scanning probes. The phase velocities of the QCM were found to be very similar to the broadband turbulence prior to the H-mode. Ohmic H-modes were also studied at 0.6 MA in LSN and USN. Again, an ohmic L-H transition (EDA followed by ITB) was seen in USN at about the same time as in LSN. Ohmic H-mode transitions were also obtained at 1.2MA and 5.4T (no field ramp required) in the LSN configuration; there was insufficient time to investigate the corresponding USN case at this current. In addition to the scanning probe data, CXRS data were also obtained using the NINJA gas puff system (cameras and fast diodes) and the DNB systems. Friday's run was devoted to MP#537 "Momentum impulse from rapid SSEP changes", which is intended to investigate the effect of the "edge event" generated when the plasma is quickly moved from LSN to USN, or vice versa. Recent results imply there is a propagation of momentum in the core region. The measurements in the pedestal region show some indication that this edge event might not propagate through the pedestal but rather cause a re- arrangement of flows there. The run was hampered by poor startup reliability for unknown reasons. The run was successful at incorporating the DNB CXRS and the Mach probes into the sweeping ssep experiment. The flows were seen to change at the first sweep, from lower to upper null, and generally recover their original velocity at the reverse transition. Formation of a marfe was observed to suppress the flow modification during the return to LSN. Useful flow data were obtained for a single set of plasma parameters; the remainder of this experiment will have to be rescheduled. Data System ----------- On Monday of last week we experienced a disk storage problem which caused data corruption of the online storage of C-Mod data archives for the years 2007 and 2008. All of this data has been restored from offline backup tape storage and is once again online and available for access. ICRF Systems ------------- The replacement FPA tube was successfully installed in the FMIT#2 transmitter (E-port antenna). During testing into a dummy load it was determined that the grid bias supply regulator was defective, probably as a consequence of the failure of the original FPA tube. The regulator has been removed and is under repair. Long-pulse Diagnostic Neutral Beam System ----------------------------------------- The DNB operated successfully into plasma on Wednesday, Thursday, and Friday. After a LHe fill on Thursday morning, the beam operated at full performance parameters (50 kV, 7 amps, 1.5 s w/modulation, and 75-80% full-energy fraction). Travel and Visitors -------------------- Earl Marmar attended and participated in a Fusion Facilities Coordinating Committee meeting on Monday, March 10 at OFES in Germantown, MD. Earl and Miklos Porkolab attended and made presentations at the OFES FY2010 Budget Planning meeting, March 11-12, in Gaithersburg, MD. The C-Mod presentation can be seen at URL http://www.psfc.mit.edu/research/alcator/program/work_proposals.html. Martin Greenwald traveled to Washington, D.C. for the OFES budget meetings and to attend a two-day workshop on FES network requirements. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly