Dark-ground illumination as a quantitative diagnostic for plasma density

Abstract

Radial electron density profiles of a toroidal belt pinch plasma have been obtained by a single measurement. Collimated ruby laser light, incident on the plasma, is focused to a diffraction-limited spot (100 microm). The technique, a variation of the dark-ground microscope, involves masking the center of the plasma diffraction pattern with a thin wire. Undiffracted light is blocked by a thin wire, whereas light diffracted by the plasma is spread beyond the wire and onto a photoplate. The resulting interference generates a high-contrast fringe pattern whose intensity varies as 1 - cosDeltaø, where Deltaø is the phase shift induced by the plasma. The fringes are recorded on Polaroid-type 46L transparency film. Using this technique, radial density profiles of the plasma produced in the Columbia Torus I belt pinch have been measured. The plasma minor cross section is elliptical with 2a ~ 2 cm, 2b ~ 30 cm, and (n(0))~3x 10(16)/cm(3). Average densities as low as 2 x 10(15)/cm(3) have been measured.