Alcator C-Mod Weekly Highlights Feb 23, 2004 Research operations continued at Alcator C-Mod last week. Three run days were scheduled and completed. Experiments were carried out in support of the transport and RF physics topical science programs. Progress on the lower hybrid project continued. This week will be a maintenance week, with no plasma operation planned. The C-Mod Program Advisory Committee will be meeting at MIT Monday through Wednesday. We have now completed 40 research run days of the planned total of 76 for 2004 (19 weeks of single shift, 4 days/week operation). Operations ----------- Regular maintenance was carried out on Tuesday last week, following the Monday holiday. Three run days were accomplished, with a total of 62 plasma discharges produced with a startup reliability of over 80%. Some run time was lost due to computer problems involving the new data acquisition server. This issue will be addressed during this week's maintenance period. Wednesday's run began MP#380 to evaluate fast particle and minority current drive effects on the sawtooth period. In H-mode,we found that the sawtooth period was more strongly affected by the RF at 0.8 MA versus 1.0 MA. As in L-mode the period of the sawteeth is lengthened for co-current (+90deg) phasing and shortened for ctr current phasing (-90). Three monster sawteeth were obtained, with a C-Mod record 80 msec sawtooth period achieved on one shot. During the course of this experiment, inner-wall limted H-modes were obtained with a threshold power of ~4.5MW at Bt=5.2T, nebar=1.6e20/m^3, Ip=1MA. These experiments contribute to the C-Mod Joule milestone SC6-1b, "Compare energy confinement, H-mode thresholds, and divertor particle dynamics in single-null, double null and inner-wall limited discharges." Thursday's run was concerned with MP#385 from the transport group. This experiment is to extend studies which correlate core and SOL flows with the L/H threshold. Specifically, the MP addresses the effects of parameters, including density and x-point location, which have been observed to be important in the threshold scaling. The toroidal field scaling will be addressed in a future run day using a different ICRF frequency. This experiment also contributes to milestone SC6-1b. Two experiments on Friday continued our investigation of the L-H transition. The first half of the run was devoted to continuation of MP#375 on the role of magnetic balance and associated changes in rotation in determining the L-H threshold, with concentration on near-DN and upper null (opposite to ion grad-B drift direction) configurations. This experiment also contributes to the JOULE milestone SC6-1b. The second half-day completed experiments for MP#330A, which attempted to use ICRF heating with the resonance positioned close to the plasma edge in order to influence the edge electric field and thereby influence the H-mode transition. Previous experiments on this MP had been limited in available RF power. On Friday power steps up to 5MW were reliably applied for a range of plasma conditions. Collaborations and Joint Experiments ------------------------------------- Experiments were carried out at JFT-2M in support of an ITPA Joint Experiment to compare the C-Mod EDA and JFT-2M HRS regimes, which exhibit similar edge relaxation and particle transport behavior. A C-Mod run day had previously been carried out to develop a compatible equilibrium for this experiment, and additional C-Mod experiments will be scheduled after the JFT-2M results are assessed. Lower Hybrid Project -------------------- We continued checking wiring and cable connections on the forward waveguide last week. The fixture needed to install the launcher in the vacuum test chamber is nearly complete, with installation expected this week. We continued work on the new phase and amplitude control computer. New control software has been installed, and we are trying to work out some reliability issues with the vendor. Testing models have been developed so that hardware test of the vector modulators can begin. CPLD programming has been completed and bench tests for the serial fiber optic links (SFOL) have begun. All XMTR and RCVR boards have been assembled. Anti-alias filters have been set to the proper bandwidth. An improved PLC program for the klystron control system was implemented last week. Improved scaling of the klystron analog signals should result in easier calibrations and better status displays. We continued to prepare for a 24-window braze test. The 24-guide prototype has been prepared to accept the windows, and has been shipped to the vendor for copper plating. We hope to complete this braze this week. ICRF ---- All four ICRF transmitters were operational in support of plasma experiments this week. Up to 5.2 MW power for a pulse length of 0.5 sec was achieved. Preparations are being made for converting FMIT #3 and FMIT #4 to 50 MHZ operation during the maintenance week. Operation at 50 MHz will allow D-He3 heating experiments to be conducted at 5 rather than 8 tesla. This will permit direct comparisons of He3 and H minority heating with the same target plasma conditions, alternately using 50 MHz and 80 MHz RF, with 4 MW of source power available at each frequency. Diagnostic Neutral Beam ----------------------- The final component arrived last week for the manufacture of a new DNB liquid helium fill tube. A solid model has been completed and the final review will take place this week. Even without the cryopump, the beam has run sufficiently well with just the turbopump to support the initial operation of the Compact Neutral Particle Analyzer (CNPA), as well as MSE, CXRS, and BES measurements during plasma operation. Travel and Visitors -------------------- Ron Bravenec (UT-FRC) continued his visit. This week he was primarily concerned with measurement of the width of the main energy component of the beam independently of the other beam components. With this measurement, the description of the beam will be essentially complete in so far as interpretation of previous diagnostic measurements is concerned. Stewart Zweben (PPPL) was at MIT 2/20 to perform an experiment on edge ICRF heating on C-Mod. This experiment attempts to modify the edge electric field through fast ion loss and therefore affect and possibly control edge turbulence. Bob Ellis (PPPL) was at MIT 2/18 for discussions on the Lower Hybrid launcher lifting fixture. Earl Marmar and Amanda Hubbard participated in the meeting of the FESAC sub-panel on priorities in the fusion program in San Diego, Thursday and Friday last week.