EXCITATION OF THE MODIFIED SIMON-HOH INSTABILITY IN AN ELECTRON-BEAM PRODUCED PLASMA

An intermediate frequency (f(ci) < f < f(ce)) electrostatic instabilit y has been observed in an electron beam produced, cylindrical plasma c olumn. This instability has been identified as a new instability, the modified Simon-Hoh instability (MSHI), which has an instability mechan ism similar to the Simon-Hoh instability (SHI). This instability can o ccur in a cylindrical collisionless plasma if a radial dc electric fie ld exists and if this radial dc electric field and the radial density gradient are in the same direction. The origin of the dc electric fiel d is found to be the difference between the ion and the electron radia l density profiles. In such a plasma if the ions are essentially unmag netized but the electrons are magnetized, a velocity difference in the theta direction can arise because of the finite ion Larmor radius eff ect. This leads to a space charge separation between the electron and ion density perturbations in the theta direction. The consequent pertu rbed azimuthal electric field E(theta1) and the enhancement of the den sity perturbation by the E(theta1) X B0 velocity occur in the same man ner as in the SHI. The instability frequency is decided by the ion azi muthal drift velocity. This new instability has been investigated thro ugh experiments and theory.