Initial Results from Beam-Emission Spectroscopy on Alcator C-Mod*

An eight-channel beam-emission spectroscopy (BES) system has been installed on C-Mod to measure density fluctuations. The optical system, shared with a motional Stark-effect (MSE) diagnostic,1 views the radially incoming beam2 from a point near the midplane inside the vacuum vessel and near the outer wall. To date, we have measured radial profiles of absolute brightness from the beam Hα emission and from background bremsstrahlung and Dα. These are compared to a BES simulation that uses the actual viewing geometry, the optical bandpass filter characteristics, beam parameters, and C-Mod density and temperature profiles as input. First measurements of density fluctuations inferred from fluctuations in the beam emission will also be presented. 1 N. L. Bretz et al., poster UP1.108. 2 E. C. Eisner et al., poster UP1.107. BES Status u In-vessel optics (with shutter), relay optics, and fiber holder installed. MSE polarizer and PEMs attenuate 2/3 of light. u Eight-channel filter spectrometer complete: optics aligned, filter characteristics measured, detector electronics functional (not cryo-cooled). u Nine optical fiber bundles run from fiber holder to filter spectrometer. u Fibers and filter spectrometer absolutely calibrated. u CAMAC data acquisition system complete and integrated into MDSplus database structure. u Radial profiles of bremsstrahlung, ambient Dα and Hα, and beam emission measured and compared to predictions of a BES simulation code. u Beam cross section has been measured and used to verify beam focussing (perveance). u Scalings of beam emission with plasma density and neutral pressure have been used to determine that re-ionization may be attenuating beam. u Estimates of signal-to-noise of autopower spectra are very low at present beam performance and throughput of machine optics.