Alcator C-Mod Weekly Highlights Sep. 21, 1999 Plasma operations continued at Alcator C-Mod last week. Four runs were scheduled, but Friday's run was re-scheduled due to hurricane Floyd. A total of 66 plasmas were produced with a reliability of 97%. A fresh boronization was carried out on Monday. A total of 114 psi of diborane was used for an average deposition of 1667 Angstroms. Boronization was followed by 4.25 hours of helium ECDC at 5e-5 Torr. Tuesday's run was devoted to a survey of impurity concentrations during EDA H-modes (MP#241). The concentrations were measured using passive x-ray and VUV spectroscopy. This run was carried out immediately after a boronization and the plasmas were extremely clean relative to pre-boronization levels. Z_eff was 1.2-1.25. The E and D antennas were used for auxiliary heating. RF powers varied from 0.9 to 2.2 MW. EDA H-modes were routinely achieved; the H-factor was typically 1.6. During the EDA H-mode phase, the radiated power as measured by the 2pi foil bolometer typically increased from 400 to 950 kW, while the power as measured by the 2pi diode bolometer increased from 50 to 250 kW. (This was for input powers of ~2.6 MW.) Ar was puffed for diagnostic purposes on all but two shots, and was found to account for ~20% of the radiated power increase during H-mode. If we take the foil and diode bolometers at face value, then ~300 kW (55%) of the power loss brought about by EDA is from neutrals, while 80-100 kW (20%) is from Ar. The rest (100-150 kW) is probably accounted for by B and F. This requires further quantitative analysis. The following impurities were measured: Ar (x-ray and VUV) B (H-like and Li-like) F (recombination on fully stripped and Li-like) C (Li-like) O (Li-like) Mo (Na-like and Ne-like) Fe (Na-like) Wednesday's run was in support of MP 239, (Hubbard, Carreras et al, "Critical Exponents for L-H transitions and Pedestal Widths"). The basic plan was to measure the input power dependence of the edge pedestal parameters, particularly the Te pedestal, but also density pedestal. The main technique was to use RF power ramps, of various peak levels and durations, to measure the response of pedestal parameters as a function of net input power. In addition, shots with constant power (or long steps) and Bt ramps were employed to measure the Te pedestal width in more detail. The nominal conditions were 5.4 T, 800 kA and nebar~1.5e20/m3. Some shots were also produced to test the part of the Carreras/Diamond model which predicts a dependence of pedestal widths (esp ne) on the ionization rate. This involved power ramps at both higher and lower ne. Under the nominal conditions, steady EDA with constant Prad was obtained even at the lowest power. On a pair of shots, we ramped the power down. Strong hysteresis was found; EDA H-modes were maintained down to P_rf~150 kW. Temperature pedestals got very small at the low powers (~ 200 eV vs ~ 400 with high power). The widths increased at high power, but the dependence was less clear at low P. In the second half of the run, the target density was varied. The highest density shot, ne~3e20/m3, stayed in L-mode at 1.9 MW. A series of shots with ne~2.4e20 (at the L-H transition) and 3-3.5e20 in EDA were studied. A power rampdown shot was included in this sequence, again showing big hysteresis. Te pedestals were measured at 2.0 and 1.0 MW. They were rather cold, and the width was not notably different than at lower ne. The visible bremsstrahlung emissivity widths did seem wider. The final sequence of shots had progressively lower starting ne, and power ramps. Two of these started as ELM-free, and then went to EDA at higher power providing a comparison of the two modes, including fluctuations, widths etc. The low and high ne transitions should provide interesting data on neutral effects etc, once the Lyman alpha data are analysed. Thursday's run was in support of MP#237, "Current dependence of toroidal rotation". The purpose of this run was to determine the scaling of the rotation velocity with plasma current, for otherwise constant target plasmas, for H-mode discharges. The range of currents achieved was from 0.8 down to .4 MA, however there was only one good shot at .4 MA due to difficulty coupling 2 MW of ICRF power. The run was terminated at 3PM due to concerns over the approaching storm; completion of these experiments was re-scheduled for this week. Physics and Analysis -------------------- Toroidal rotation measurements in the plasma periphery (Rtan = 87.69 cm) using ambient boron impurity emission are now routinely made. A boron line at 2821.68 angstroms is measured in 4th order with a 1 m spectrometer and an array detector. Data are collected on virtually every shot and compiled into a database. Preliminary analyses indicate that the ions are rotating in the co-current direction at roughly 10-25 km/sec. Entry into H-mode correlates with the toroidal velocity increasing and exit from H-mode correlates with the velocity decreasing. David Winslow (U. Texas) continued analysis of divertor probe fluctuation data, looking at the relationship between floating potential fluctuations and swept probe biasing. In addition, preliminary experiments using a driven tile probe system were performed. Computer systems: ----------------- Live streaming video and audio from the control room is now available during C-Mod operations. The URL is http://www.psfc.mit.edu/cmod/control_room_rv1.html . ICRF Systems: ------------- Post-boronization conditioning of D and E-port antennas went well. EDA H-modes were readily obtained with moderate power. A maximum power of 2.5 MW was obtained and at present the power is limited by the screen power dissipation. This limit arises because the 2274 plate impedance is high at 80 MHz and the anode voltage approaches the screen voltage during a cycle resulting in high screen current. After implementing a cavity model based on work by C. Brunkhorst (PPPL), FMIT#2 was retuned to lower its plate impedance. The trend predicted by the model was verified in testing into dummy load, and high power was obtained with lower screen current (about 400 kW). Testing into plasmas will begin next week. After disassembling and inspecting FMIT#3 coax for arcing damage (none found), the line was reassembled, pumped down, purged and backfilled with insulating gas. A few shots were taken into plasma at power levels of 0.5 MW per transmitter at the end of the week with no indication of problems. A concept for the J-port antenna coupling loop modifications to reverse alternate current strap phasing has been worked out, and circuit dimensioning and fine tuning is in progress. Travel and Visitors: -------------------- Bill Dorland visited the PSFC on Wednesday through Friday. He worked with Martin Greenwald in making initial comparisons between experimental data and recent simulations of ETG turbulence. He also consulted with David Mikkelsen on runs of his (Dorland's) linear gyro-kinetic stability code GS2.