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 µm). 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 - cosΔø, where Δø 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))~3× 1016/cm3. Average densities as low as 2 × 1015/cm3 have been measured.
S. F. Paul, "Dark-ground illumination as a quantitative diagnostic for plasma density," Appl. Opt. 21, 2531-2537 (1982)