Alcator C-Mod Weekly Highlights Oct 28, 2002 Plasma operations continued at Alcator C-Mod last week, with four run days scheduled and completed. All of the experiments were in support of the Advanced Tokamak task. Progress also continued on the Lower Hybrid Fabrication Project. Physics Operation is scheduled to continue this week. Operations ----------- Physics operation was carried out Tuesday through Friday of last week, with a total of 104 plasma discharges produced and a startup reliability of 88%. All tokamak subsystems continued to perform nominally. A fresh boronization was carried out on Wednesday night to provide optimal wall conditions for Thursday's experiment. The runs on Tuesday, Wednesday, and Friday were in support of MP#324A, which deals with modification of the initial current rise phase to produce optimized shear discharges. Both on- and off-axis ICRF heating were applied during the current rise phase, starting as early as 50msec after plasma initiation. For one aspect of these experiments we developed a plasma startup scenario at BT=4.5 tesla, as opposed to our standard 5.3T startup. This scenario was used to provide on-axis heating at 70MHz, along with high-field side off-axis heating at 80MHz, both applied during the early current ramp. Experiments were also carried out at 5.3T, using the 80MHz for on-axis heating, and the 70MHz resonant on the low-field side. Central temperatures up to 5keV were obtained in sawtooth-free plasmas. The onset of sawtooth activity was delayed to over 0.3sec. During Friday's experiments, we succeeded in producing H-modes prior to the onset of sawteeth. Data on the q-profile were obtained with MSE as well as by the Li pellet ablation cloud imaging technique. Toroidal rotation profiles in the startup phase were also obtained. Thursday's run completed the MP#317 experiments on the two frequency double barrier (ITB in EDA H-mode) discharges using the 70MHz low-field side resonance minority ICRF to generate the ITB. The day's experiments were principally concerned with using the 80MHz ICRF to heat inside the core barrier and arrest the rise of the central density. It was found that 600kW of central ICRF heating was sufficient to produce essentially steady-state central density and to heat inside the barrier. Physics ------- Data from the new inner wall scanning probe (ISP) and the scanning probe on the outer midplane (ASP) has been analyzed for a recent run where the magnetic flux balance between upper and lower x-point locations was varied (see MP#323,"Scoping Experiments for Upper Divertor Cryopump Operation in Unbalanced Double-Null H-mode Discharges"). Electron pressure profiles in the scrape-off layer (SOL) recorded by the ISP and ASP are found to agree when the flux surface mapping of the upper x-point (SSEP) was located beyond the main-chamber limiter location. As SSEP was varied towards a double-null magnetic configuration, a sharp drop-off in the density profile was seen in the ISP data. The break-point in the density profile was found to correspond to the location where magnetic flux surfaces change topology from 'common' surfaces (circulating the core plasma) to a 'private' surfaces (being restricted to a region on the high field side of the torus). These observations are consistent with recent conclusions drawn from D-alpha camera measurements which indicated a sharp decay of the density on the high field side of the secondary separatrix flux surface. The data indicate that cross-field transport into the private, high-field magnetic flux zone is greatly reduced relative to that in the common flux zone. This result is consistent with the drive for SOL turbulence being dominated by magnetic curvature effects. In addition, normalized ion saturation current levels in the common flux zone were seen to be reduced by a factor of 5 on the high field side relative to the low field side. These data suggest that while the turbulence drive is dominated by magnetic curvature effects, the fluctuations exhibit a clear ballooning character, ruling out a simple flute-like description for the transport physics in the far SOL. These probe observations also have important implications for experiments in C-Mod. When operating in a balanced double-null configuration, the SOL on the high field side is very thin (few mm). This should allow the plasma to be efficiently fueled by gas puffed through an existing inner wall capillary tube on the midplane. In addition, the thin inner SOL indicates that particle fluxes to the divertors predominately flow in the outer divertor legs in double-null discharges. A cryopump designed to operate in near double-null discharges should therefore be aimed at pumping the particle flux on the outer divertor leg. DNB Systems ------------ The beam operated in support of experiments during the last three operational days of the week. Modifications to improve gas flow to the arc and thus maintain a constant accelerator current limited operation on Tuesday. The BES system successfully observed the quasi-coherent (QC) mode during the EDA portions of the discharges during Thursday's run. The mean frequency of the mode (~ 90 kHz) matches that observed by the PCI diagnostic. The mode is apparent only on channels at a single radius (radial channel spacing was ~1.35 cm) and the BES signal typically much larger during the DNB pulse than before or after. Cross-correlations between channels indicate the mode has a mean poloidal wave number of ~2 cm^-1. When the spectra are calculated using a time window equal to the beam length (50 ms), the mode actually appears to be composed of three or more distinct modes (also confirmed by PCI data). This may be due to the mode jumping from one discreet frequency to another during the time window. Further analysis is continuing. During the current ramp-up experiments we acquired MSE profiles in discharges heated with RF for comparision with ohmic reference plasmas. The intent is to document the q-profile in discharges that may have a reverse-shear character. Analysis is in progress. ICRF System ----------- The feeds to the four-strap J-port antenna were successfully reconfigured to allow the antenna to operate as a two-strap dipole driven at 70MHz by Transmitter #3 alone, during repairs to FMIT#4. All three antennas operated well during plasma operations last week, with up to 4MW total (at two frequencies) being coupled during the current rise phase in support of the MP#324A experiment. In preparation for the experiments planned for the next two weeks, Transmitter #3 is now being re-tuned to operate at a frequency of 78MHz. Lower Hybrid MIE Project ------------------------- A discussion of the status of the Lower Hybrid project was held 10/22. Present at PPPL were Joel Hosea, Greg Pitonak (DoE), Randy Wilson, and Skip Schoen. Present via teleconference at MIT were Ron Parker, Jim Irby, and Gerd Schilling. At MIT, work began on modification of the water cooling system to install pressure relief valves. Work is also continuing on the waveguide support structure. At PPPL, the engineers are developing a detailed assembly procedure for the launcher, and producing the associated drawings. Budgetary restrictions have resulted in a decision to limit the initial number of RF probes on the system to 10, rather than the 96 originally planned. Travel and Visitors ------------------- Amanda Hubbard attended the meeting of the pedestal ITPA working group in Garching, Oct. 21-23. She also had some useful discussions on 10/24 with Lorne Horton, Wolfgang Suttrop and Gabriella Saibene regarding results of, and plans for, ASDEX-Upgrade and JET pedestal collaborations. Joe Snipes participated in the H-mode ITPA working group meeting at Cadarache, on Monday-Wednesday. He then went to JET to work on q-control experiments using LHCD. Bob Granetz participated in the MHD ITPA working group meeting in Garching. Paul Bonoli participated in the ITPA Steady State and Energetic Particle working group meeting at Caderache. John Rice participated in the ITPA Internal Transport Barrier Meeting, also in Caderache. Bruce Lipschultz participated in the ITPA Divertor Working Group meeting in Lausanne.