Alcator C-Mod Weekly Highlights May 15, 2000 Operations ----------- Plasma Operations continued at Alcator C-Mod last week. Four run days were scheduled and completed. A total of 79 plasma shots were produced, with a startup reliability of 90%. Last week was the first official week of the physics phase of the current campaign. Runs were dedicated to experiments in Core Transport, Impurity Control, and Neutral Effects. One run was also devoted to continuing development and commissioning of the ICRF system. A fresh boronization is being performed on Monday. Physics operations are scheduled to continue this week. Physics and Diagnostics ----------------------- One run day last week was dedicated to a determination of the profiles of the impurity diffusion coefficient and convection velocity during P-mode, induced by lithium pellet injection (MP#256). Complete radial brightness profiles for He-like argon lines were obtained. Density and temperature profiles were obtained from Thomson scattering. The radial scan was done at 1 MA plasma current, with the prospect of moving the location of the P-mode peaking out in radius. However, the peaking occured inside of 6.5 cm in minor radius, the same location that was seen for 0.8 MA previously. Most shots had 2 pellets, for a comparison of Ohmic P-mode with ICRF PEP mode. This should allow for a complete characterization of the impurity transport coefficient profiles in conjunction with modeling from MIST. The effects of main chamber recycling on H-mode threshold and confinement were investigated (MP #262) in a systematic scan of the inner gap, with the outer gap held constant at a relatively large value of approximately 20mm. The gap to the inner wall was varied from 0 (marginally limited) to over 25mm. Plasma current, target density, and overall plasma shape were held constant. The dominant x-point was at the bottom, and the upper, secondary x-point was outside the vessel. During the scan from 26mm to 3mm we observed no change in divertor gas pressure, outboard midplane gas pressure, divertor radiation, or Zeff. However, the gas pressure in the upper "divertor" region changed by a factor of about 5 and the H-mode power threshold increased by ~40%; no H-mode was obtained for the marginally-limited cases. Detailed analysis of these data is in progress. The purpose of Friday's run (MP#225) was to isolate the relative contribution of Mo sources from the inner wall, outer limiter/antennas, and divertor to the core Mo levels. A new feature of these experiments was the use of emissive probes to directly measure the plasma potential during RF operation. the potentials measured by the probes (one on each side of the AB limiter) were found to correlate with operation of the antennas connected to them by field lines. In the case of the A side of the limiter this means the J4 antenna, for the B-side of the limiter this means the D-antenna. Potentials in the range of 50-200V were observed whan the corresponding antenna was energized. The measured potentials verify that sheath rectification does occur, and that the potentials are substantial. The results on the comparison of antenna sources indicate that the Mo source rate and effect on the core by the J antennas are considerably reduced from last run period. However, the source rate from the J antennas and the effect on the core (n_mo) were typically a factor of 2-3 larger than with the D-antenna for equivalent launched power. Results from the optical diagnostic investigating fluctuations in the C-Mod SOL show no obvious differences in the SOL fluctuation characteristics during EDA H-mode and L-mode plasmas. Frequency spectra at radial locations rho=4, 8, and 11 mm are similar, as are the autocorrelation functions of the intensity fluctuations. In addition, radial profiles of intensity fluctuations have been measured using this diagnostic by scanning the separatrix dynamically during ohmic, L-mode discharges. Inside of rho=20 mm the fluctuation level is 10 to 20%, rising slightly at the smaller rho's. Outside of rho=20 mm, it increases rapidly to ~60%. Since the dependence of the line intensity has a less-than-linear dependence upon n_e, this intensity fluctuation profile implies that the normalized density fluctuation profile decreases (with increasing rho) from 40-80% at rho's < 10 mm, is ~20-30% for 1020 mm. These observations show an opposite trend from the results in which an "effective diffusion coeficient" is derived from determinations of the SOL particle fluxes, gradients, and source functions. The trend in D_eff is one that decreases to quite low values as rho decreases from 10 mm to 0mm (the separatrix). ICRF Systems ------------ The ICRF system was run in support of physics experiments this week, as noted above. D-port and J-port antennas were used to provide up to 1.5MW and 2.5MW, respectively. An arc in an elbow located below the phase shifter in the E-port external transmission line prevented use of the E-port antenna this week. This component has now been replaced. DNB Systems ------------ The DNB operated into C-Mod plasmas 4 days last week, although two days were cut short due to HV overheating problems. A perveance scan at 39kV was completed, voltage regulation of filament supplies was tested with and without CMOD shots, and a boron CXRS line at 7619A was checked. The BES signals were compared for low voltage (34kV & 32kV), close-to-perveance beams and high-voltage (45kV), far-from-perveance but higher current beams. Data analysis is ongoing. Instrumentation was set up to monitor substation voltage at the alternator exciter supply input. A survey of available power sources for the new DNB service was made and plans for both boosting the filament voltage and adding different service are being made. Travel, Visitors, etc. ------------------------ The regular Quarterly Review of the Alcator C-Mod Program was held on 11 May, by video conferencing. Bill Rowan (U. Texas) presented a Status Report on the Diagnostic Neutral Beam; Jim Irby reported on other aspects of the Machine and Facility Status; Ron Parker and Montgomery Grimes presented a Lower Hybrid Project Status Report; Amanda Hubbard and Catherine Fiore summarized some Recent Physics Results; and Steve Wolfe presented plans for the Physics Schedule for the current campaign. Rostom Dagazian, Warren Marton, Curt Bolton, and Erol Oktay participated from the Office of Fusion Energy Sciences. Martin Greenwald spent two days at ORNL participating in a review of the fueling and heating technology programs. Ron Bravenec (UTx) was at C-Mod last week primarily to learn how to operate the DNB. This is in keeping with the commmittment of the Texas group to take on primary responsibility for routine operation of the beam. BES signals from the beam emission were also measured and found to be small. This result is believed to be due to two principal reasons: i) the deuterium beam does not provide enough Doppler shift to separate the beam emission from the background D-alpha emission, and ii) the beam is presently off perveance and therefore not well focussed. David Winslow (UTx) installed the probe head on the UT Turbulence Probe and started baking the line. A small leak was discovered in a vent valve which will be repaired.