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 −
cosΔϕ, where Δϕ 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.
© 1982 Optical Society of America
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