Alcator C-Mod Weekly Highlights March 21, 2011 FY2011 weeks of research operations Target: 15 weeks Completed: 13.2 weeks Plasma Shots: 1563 Operations ----------- Plasma operations continued last week at Alcator C-Mod. Two run days were scheduled and completed. The runs supported Miniproposals in the ICRF and Transport areas. A total of 66 plasma discharges were produced with a reliability of 97%. These experiments were all carried out with "reversed" field and current, i.e. with the ion grad-B drift upward, with lower single null equilibria. On Friday, the process of reconfiguring the bus work for normal field and current operation was begun, in preparation for this week's experiments. C-Mod operations are planned to continue this week. Operation Details ----------------- The run on Wednesday was devoted to MP#583, "Optimization of mode conversion flow drive". This experiment was designed to evaluate MCFD in I-mode plasmas, using the D(He3) MC scenario at f=50MHz and Bt~5.1 T. At 1MA plasma current, we successfully obtained toroidal core rotation as large as 90km/sec in the co-current direction with 2.5MW from the J-port antenna alone. However, additional power at 80MHz from D&E antenna did not result in increased rotation. The MCFD did not increase the rotation when the intrinsic rotation (associated with high stored energy) was already large, and the intrinsic rotation did not increase with additional heating when the MCFD was large. Experiments at 0.8MA also showed significant MCFD in I-mode; adding 80MHz power resulted in I->H transitions and reduced the rotation, probably due to increased density in the H-mode. In 1.2MA I-modes, MCFD rotation of 110km/sec was obtained; in this case the rotation decreased with additional 80MHz heating, although the plasma remained in I-mode. Thursday's run was dedicated to MP#649 "Scaling of parallel impurity transport with Mi, q95, and nu_ii". This experiment comprises part of the thesis research of an MIT graduate student. The run focused on I-mode plasmas with substantial on-axis D(He3) mode-conversion and minority heating in order to reach high rotation speeds with MCFD, as demonstrated on Wednesday. The experiment is designed to study the in/out asymmetry of Mo density on a flux surface, using laser blow-off (LBO) Mo injections. Scans of plasma current (0.7 < Ip < 1.3MA) and density (0.8 < nebar < 2.1e20/m^3) were carried out to provide data over a range of q95, collisionality, and Mach number. The data are being analyzed. Both of these runs also produced discharges in which the central electron temperature exceeded 8keV. These data are useful inputs for MP#610, "Investigation of Thomson/ECE discrepancy in high temperature discharges", contributing to ITPA Joint Expreriment DIAG-03. ICRF Systems ------------ We completed the conversion of FMIT#3 and #4 to 50MHz operation. Some issues were encountered with matching of the Intermediate Power Amplifier (IPA) to the Driver on FMIT#4, resulting in a higher than optimal VSWR of ~1.7. The tune was adjusted to arrive at satisfactory screen current and anode impedance, resulting in output power of ~1.9MW into the dummy load from FMIT#4, with somewhat lower power available during plasma operation, where the load mismatch is greater. Further operational experience is required to optimize the tuning at this frequency. The current straps for the ARRA-funded rotated four-strap antenna have been bead blasted and prepared for brazing. The removal of tooling marks on the TZM rods continues. Machining of the TZM side tiles has been completed except for the thermocouple detail required in five tiles. The design for the bridge to Z plate shorts has been completed, and a purchase order has been placed for their fabrication. Another pair of the top/bottom plates has been shipped and another set is under Q.C. inspections at the vendor. One of the bellows tubes has been polished and has passed acceptance inspection. The remaining three tubes have been sent out for polishing by the same vendor. Lower Hybrid System -------------------- The LH system was not required for any plasma operations last week. Design work on the prototype ferrite tuner stub assembly continues. New ribbon coils and thicker ferrite samples have been ordered for additional testing. Testing of phase shifters and attenuators for the LH3 control system shows excellent phase and amplitude accuracy (< ±0.2 deg, < ±0.08 dB) across a range of 360 deg and 15 dB. The phase shifter manufacturer has been contacted regarding glitches in amplitude associated with sudden large jumps in control voltage. The layout of the 4th klystron cart filament power supply and transformer placement were measured to assist with CAD modeling of the cart. Elbow arc detector chassis wiring was completed for the five chassis. Work on the chassis design for the waveguide switch interface boards was finished. Diagnostics ----------- On Friday we carried out a series of beam-into-helium gas experiments without fields. These experiments were used to evaluate the filter functions of the MSE system and to provide a relative (channel to channel) intensity calibration of the CXRS diagnostic for helium concentration and helium fast ion measurements. Target pressures from 0.1 to 6mTorr were employed. Travel and Visitors ------------------- David Green (ORNL) visited MIT on Friday to present a PSFC Seminar entitled "Predicting the response of tokamak plasmas to radio frequency waves: progress, plans and pretty pictures". While visiting the PSFC, David met with John Wright, Abhay Ram, Paul Bonoli, Orso Meneghini, Syun'ichi Shiraiwa, and Aaron Bader to discuss issues related to RF wave propagation and absorption in the ion cyclotron and lower hybrid range of frequencies. Martin Greenwald traveled to Washington to participate in a Fusion Simulation Program briefing for Ed Synakowski and OFES staff. Bruce Lipschultz traveled to the ITER site to chair a conceptual design review for the pressure gauges to be used in the divertor and main chamber of ITER. The meeting was held March 15-16. _______________________________________________ Cmod_weekly mailing list Cmod_weekly@lists.psfc.mit.edu http://lists.psfc.mit.edu/mailman/listinfo/cmod_weekly