EXPERIMENTAL-STUDY OF THE COLLISIONAL PARALLEL VELOCITY SHEAR INSTABILITY

Simultaneous density and electric field fluctuation spectra, associate d with velocity shear (both in the transverse to the magnetic field io n flow velocity and in the parallel ion flow velocity), have been obse rved by the DE 2 spacecraft in the F region [Basu et al., 1984]. Basu and Coppi [1988, 1989] argued that the subkilometer scale irregulariti es are due to collisional electrostatic modes excited in a partially i onized plasma in the presence of shear in the parallel ion flow. In th is paper we describe results of a laboratory investigation of the effe cts of neutral-particle collisions on the parallel velocity shear inst ability, performed using a double-ended Q machine. The critical value of the shear dv(0)/dx, the derivative of the field-aligned ion flow wi th respect to the transverse (to B) coordinate, is determined and comp ared with theoretical predictions.