ELECTRON-ION HYBRID INSTABILITIES DRIVEN BY VELOCITY SHEAR IN A MAGNETIZED PLASMA

The stability of a magnetized plasma is investigated in which a sheared electron flow channel is present. The flow's peak velocity and shear scale length are denoted by V and L, respectively. If the velocity channel is perpendicular to the confining magnetic field and L less-than-or-equal-to rho(i) (rho(i) is the ion Larmor radius) an electrostatic instability develops whose frequency is on the order of the lower hybrid frequency. For V/(OMEGA(e)L) > 0.02 (OMEGA(e) denotes the electron cyclotron frequency), the peak growth rate is on the order of the lower hybrid frequency when k(parallel-to) = 0 (in here, k(parallel-to) is the wave number along the magnetic field). For V/(OMEGA(e)L) > 0.1 and k(parallel-to) = 0, the spectrum peaks when k(y)L approximately 1, where ky is the wave number in the direction of the flow velocity. For this mode it is shown that (i) a net cross-field current is not required for the onset of instability and (ii) the growth rate is not reduced by a velocity profile with no net flow (spatially averaged). Hence it is concluded that velocity shear is the only source of free energy. Further, it is shown that density gradients do not stabilize this mode. It follows that the mode presented in this work cannot be identified with the well-known modified two-stream instability. If the velocity channel is parallel to the confining magnetic field and the plasma is weakly magnetized, an instability driven by velocity shear is shown to exist, provided that V/(omega(pe)L) > 0.32, where omega(pe) is the electron plasma frequency. It is shown that a net plasma current is not required in order for this instability to be excited.