MODE CONVERSION OF PARALLEL-FLOW-SHEAR DRIVEN LOW-FREQUENCY INSTABILITIES BY THE REVERSAL OF SHEAR SIGN IN A PLASMA

Parallel flow shears in magnetized plasmas are experimentally found to give rise to several types of low-frequency instabilities. When the sign of shear is positive, the fluctuation propagating to the azimuthal direction is observed around the region of the parallel flow shear. When the shear sign is negative, on the other hand, the axial wave number of the observed fluctuation becomes relatively large in comparison with the case of the positive shear sign. These instabilities are considered to be related to the Kelvin-Helmholtz instability driven by the shear and the ion-acoustic-like instability modified by the shear, respectively.