Alcator C-MOD Weekly Highlights May 1, 1995 Plasma operations on Alcator C-MOD resumed last week after a one-week scheduled maintenance period. Following power system checkout, several plasma shots were run on Monday to verify that all systems were operational and to evaluate machine cleanliness after the vent the previous week. Four physics run days were scheduled and completed. A detailed study of x-point characteristics (MP#109) was carried out on Tuesday and Wednesday. For these experiments the x-point was positioned somewhat higher than the standard location to provide good viewing by the various spectroscopic diagnostics. The x-point was observed in three regimes on each shot: low-density sheath-limited divertor, high-density detached divertor, and with RF to re-attach the divertor at high density. Goals of the experiments included determination of what impurities are responsible for the radiation coming from the X-pt region; determination of the reason for the location of the peak in emissivity near the X-pt; obtaining profiles of electron density from below the strike points to above the X-pt using stark-broadened Balmer lines and the views of the K and A bottom arrays; documentation of the presence or absence of molecular hydrogen lines in the visible and/or in the VUV; measurement of the optical thickness of the n=3 to n=1 line (Lyman_beta) in H for views through or below the X-pt. The Chromex spectrograph covered the spectrum from 400 nm to 1300 nm for 14 different spatial locations (5 looking at the outer divertor, 8 looking at the inner divertor and 1 from K-top viewing the whole floor of the machine). The McPherson view was scanned across the x-point for 2 different wavelength settings (C-V & C-VI in one case, C-III & LyBeta & O-VI for the other case). After completing these scans, the x-point radial location was scanned from 54 to 57 cm with diagnostic settings held fixed to see how the x-point location affects the radiation pattern. Radiation stays at the x-point through detachment for the case of the x-point far from the inner divertor; with the x-point near the inner divertor the radiation peak is at smaller major radius than the x-point before detachment and shifts inside the x-point after detachment. Deuterium Paschen series lines were investigated. These lines are Stark-broadened (width approximately 30 A) and should allow density measurement. C-VI and C-V emission indicated heating at the x-point during RF. C-III, Ly-Beta and O-VI emissions at the x-point are strongly affected by the H-mode. This could be due to profile or density effects. The Mo-I emission is primarily found at the outer divertor (not the inner divertor) and then only at low densities. The CD band emission in the divertor was measured and spatial profiles were obtained. PEP mode studies were carried out on Thursday, using the Li pellet injector. Attempts to optimize pellet/RF timing indicate that the RF should be turned on just before the time of maximum xray peakedness. A scan of target density indicated that a value of about nebar=1.6e20/m3 gave optimum results. At lower currents (600kA) it was found that the PEP mode duration and the post pellet particle confinement time doubled. However, the peak neutron rate was less than at the higher currents. Ramping the current down from 800kA to 600kA immediately following pellet injection resulted in a PEP mode with a neutron rate characteristic of the 800kA shots with the longer duration typical of the 600kA case. Images of the pellet ablation pattern ("cigar" pictures) were obtained for analysis of the poloidal field angle. Double pellet injections were used on several shots to determine whether the current profile differs significantly between PEP and normal discharges. The EFIT code has been modified to accept the cigar angle data as inputs in determining equilibrium reconstructions. An arc in the E-port transmission line limited the power levels available during the latter part of the PEP mode run on Thursday. This line was disassembled Thursday night and the arc site determined to be at a Teflon insulator. Repairs and clean-up were carried out over the weekend, and the system is expected to be back online for this week's runs. On Friday an experiment (MP#111) was carried out to explore a spectroscopic edge/divertor electron density diagnostic for C-Mod and future tokamaks such as ITER. The Ni- and Co-like charge states of Pd, Mo, and Nb exist near the edge of the plasma. The ratio of electric quadrupole transitions (E2)to the electric dipole transitions (E1) in these ions have a strong density dependence in the n_e ~ 1E20 - 1E21 m-3 range. The laser blow-off impurity injector was used to introduce these elements into a series of reproducible shots which had a density ramp going from nebar ~0.9e20 to 2.3E20/m3. The line ratios were measured successfully using the Johns Hopkins Molybdenum Monitor diagnostic. The amount of the injected impurity was monitored using the 2pi bolometer and the McPherson spectrograph. HIREX observed lines from Mg-, Na-, Ne-, and F-like Nb, as well as lines from Ne-like Pd. The McPherson measured Mg- and Na-like lines from each of the 3 elements. Looking at the X-pt region the McPherson observed the Ni-like Nb spectrum. Preliminary results indicate that the line ratios in Mo agree with the theoretical predictions at lower densities. The preliminary result for Nb is that the ratios are in qualitative agreement with theory. Mark May from Johns Hopkins was Session Leader for these experiments; Kevin Fournier from JHU was also at C-MOD participating in this run. Rob Pinsker from DIII-D is beginning a one-week visit, during which he will be collaborating with the C-MOD ICRF group. Ben Welch is visiting from the University of Maryland. Dr. Mike Finkenthal is visiting from Johns Hopkins this week for discussions concerning future collaborations on divertor diagnostics. Chris Brunkhorst has arrived from PPPL for an extended visit under the auspices of the C-MOD/PPPL RF collaboration. He will be helping with the engineering on the ICRF system. Martin Greenwald attended the ESNET Steering Committee meeting in Austin. The meeting's stress was on ESnet future services and international connectivity.