High resolution laser induced fluorescence Doppler velocimetry utilizing saturated absorption spectroscopy.

A high resolution laser induced fluorescence (LIF) system has been developed to measure the flow velocity field of neutral particles in an electron-cyclotron-resonance argon plasma. The flow velocity has been determined by the Doppler shift of the LIF spectrum, which is proportional to the velocity distribution function. Very high accuracy in velocity determination has been achieved by installing a saturated absorption spectroscopy unit into the LIF system, where the absolute value and scale of laser wavelength are determined by using the Lamb dip and the fringes of a Fabry-Perot interferometer. The minimum detectable flow velocity of a newly developed LIF system is +/-2 m/s, and this performance remains unchanged in a long-time experiment. From the radial measurements of LIF spectra of argon metastable atoms, it is found that there exists an inward flow of neutral particles associated with neutral depletion.

Spontaneous formation of a plasma hole in a rotating magnetized plasma: a giant burgers vortex in a compressible fluid.

Spontaneous formation of a cylindrical density cavity, or "plasma hole," has been observed in a rotating magnetized plasma. Density of the plasma hole is one-tenth of that of ambient plasma and is bounded by a steep transition layer of the order of several ion Larmor radii. The flow velocity field associated with the plasma hole is experimentally determined, exhibiting a monopole vortical structure. It is found that the vorticity distribution is localized near the center of the hole and is identified as a Burgers vortex. This is the first experimental observation of a Burgers vortex in a plasma.