A multichannel diagnostic for measuring low amplitude, long wavelength (k⊥ρi<0.5) density fluctuations along the outer half of the plasma has been installed on TFTR. It is based on observing fluctuations in the Hα fluorescence of a neutral heating beam due to collisional excitation from the plasma and impurity ions. Both radial and poloidal correlation lengths as short as 2-3 cm can be determined, with the spatial resolution limited primarily by the width and geometry of the three neutral beam sources. Optical fibers transmit the light from a 20-cm-diam vacuum window, reentrant mirror, and lens assembly to 16 interference filter/photomultiplier combinations located outside the radiation area. Initially, the fibers comprise a fixed 55-channel radial array and readily movable 10-channel vertical arrays which can be positioned at 27 radial locations. The filters are designed to accept the Doppler-shifted H α emission from primary energy component of the neutral beam, and reject background lines and unshifted edge Hα. The measurable fluctuation amplitude (@xa S/N=1) is limited to 0.5% over a 100 kHz bandwidth by the photon noise associated with the dc level of the beam emission. The contribution of impurity collisions to the total beam fluorescence will be determined directly by measuring impurity density fluctuations using charge-exchange recombination emission from the n=8-7 CVI line at 5292 Å.
