Alcator C-Mod Weekly Highlights Sep. 7, 1999 Plasma operations continued at Alcator C-Mod last week. Three runs were carried out in support of four miniproposals. The principal experiments were focussed on edge and divertor issues, with continued ICRF development carried out in background mode. The tokamak continued to operate reliably, with 69 plasma shots produced at a start-up reliability in excess of 90%. Tuesday's run was dedicated to investigation of SOL transport during H-modes. Although MP#228 did not originally call for ohmic H-modes, it was recognized that ohmic H-modes provide a nice way to study H-mode transport under a variety of toroidal fields which are not compatible with ICRF heating. We were successful in getting a number of good H-modes during this run with the scanning probes recording profiles of n, Te, parallel flow, and fluctuations up to the separatrix. On a number of discharges, the scanning probes appeared to poke into a 'shear layer'-like zone where the floating potential (Vf) dropped dramatically (100 V/mm) and the fluctuations appeared to be suppressed. The 'shear layer' Vf gradient also appeared to be much stronger in the 'elm-free' phase. Observations of C+2 plume dispersal on a few selected shots showed a qualitatively similar behavior: during 'elm-free' phases there was some very strong poloidal dispersion while in EDA-like phases the poloidal dispersal was much smaller. One should keep in mind that all these 'on the fly' observations are preliminary and need to be carefully checked. Wednesday's run was concerned with MP#223 and #224 - "Recombination and Ionization in the C-Mod Divertor" and "Measurement of the plasma parameters and their scalings in recombining regions of Alcator C-Mod".The run was successful in accomplishing the following goals: 1) measuring the spatial extent and magnitude of the volume recombination over a range of densities and "degrees" of detachment 2) measuring the effect of the bypass flaps on the detachment threshold (their state had no effect on the threshold) 3) measuring the effect of the bypass flaps on the main plasma fueling rate 4) measuring the effect of N_2 impurity puffs on the detachment characteristics. These shots were all 1MA, 5.4T discharges, with densities scanned between 2 and 4.7e20/m3. Thursday's run continued studies of the effect of the bypass flaps on divertor impurity compression (MP#222). In contrast to previous work, a non-recycling impurity (N2) was used for these experiments. We operated at three nominal discharge densities (1.1, 1.6, and 3e20/m3). The N2 gas puffing position was varied from the outside midplane, to the outer divertor, to the private flux region, to the inside midplane. Discharges were run with either the flappers open or closed for the entire discharge. No major differences were observed in any discharges comparing flaps closed with flaps open. We focussed primarily on nitrogen spectroscopy, i.e. chromex (N II) in the divertor and main chamber and the MacPherson in the main chamber (li-like). Nitrogen signals were also obtained on the divertor RGA, the omegatron, and visible bremmstrahlung (which looks right at the AB limiter gas puff location). This null result was obtained despite the usual factor of 2 effect of the flapper on the divertor neutral pressure, and suggests that nitrogen does not pass through the bypass (as recycling gases do, e.g. argon); this is consistent with nitrogen being a non-recycling species. We were hoping to observe the effect of the deuteron flow that the flapper (when open) supposedly induces towards the plate....this might manifest itself as better screening when puffing N2 from the divertor.....however, no effect was observed. The most striking results of the day were related to the different screening properties at different N2 release locations. N2 released from the AB limiter was very effective at reaching the core, while releases of N2 from the outer divertor, private flux region and inner midplane were all much less effective. The inner wall result is surprising, but consistent with earlier C-Mod results which have shown preferred streaming of injected impurities at the inside midplane towards the divertor. Physics and Analysis -------------------- Results of recent absolute calibrations of core and edge Thomson scattering systems exhibit small, but non-negligible, differences from the prior calibration. The latest results from edge TS density pedestal measurements show no clear scalings of the H-mode density pedestal width with various plasma parameters, although a dependance of the maximum width on such plasma parameters as current, midplane neutral pressure and input power is observed. The relationship between pedestal height and central density in ELM-free and EDA H-mode requires further study. ICRF System ----------- We have continued to work towards having a full complement of transmitters and antennas available for plasma operation. For FMIT#1, a rebuilt high power tube has been installed and operated up to 1.8 MW for 20 msec pulses into dummy load. This power level was achieved with low oscillations. For RF power greater than this, the self-oscillations increased dramatically. In FMIT#2, the self-oscillation has been further reduced by retuning the input and output cavity. Since both transmitters oscillate, we have begun working with PPPL to understand and reduce this parasitic oscillation to an acceptable level (where filament heating is not an issue). The J-port antenna continued to clean up slowly during plasma operation. Flat top power levels of up to 1.8 MW were achieved in a piggyback mode. Plasma heating is observed in a greater fraction of the discharges and under different phasing settings. Visitors and Travel ------------------- Satarou Yamaguchi, Haruhiko Okumura, Takashi Eura and Yasuhiro Hasegawa visited from NIFS to discuss data systems collaboration between Alcator C-Mod and LHD. Martin Greenwald attended a meeting of the ESnet Steering Committee (ESSC) on which he represents the interests of the fusion energy community.