Plasma Phasography

Abstract

The phase-contrast microscope of Zernike may be scaled in size and combined with ``giant pulse'' ruby-laser illumination to make possible the observation of macroscopic moving tenuous objects, such as jets, shock waves, and plasmas. The resulting instrument, referred to as the laser phase contrast (LPC) system, has performance similar to that of an optical interferometer operated in the single-fringe mode. The operation of the LPC system is treated theoretically and conditions for optimum imaging are derived. The ``phase plate'' of Zernike is modified into a ``phase mirror'' for use with focused laser illumination. Quite different mirrors are required for the full range of interesting plasmas accessible to the LPC system. The experimental model is described. Phasograms of weak and strong shocks in air and argon down to 0.5 mm Hg pressure are shown. Rayleigh-Taylor instability behind the shock is observed. Gas jets of various refractivities are also used as test objects. Phasograms of vacuum sparks producing jets of ablated electrode material are shown. Many objects studied are invisible to schlieren photography using the same illumination and optics.