Excitation of the modified Simon–Hoh instability in an electron beam produced plasma

An intermediate frequency (fci≤f≤fce) electrostatic instability has been observed in an electron beam produced, cylindrical plasma column. This instability has been identified as a new instability, the modified Simon–Hoh instability (MSHI), which has an instability mechanism similar to the Simon–Hoh instability (SHI). This instability can occur in a cylindrical collisionless plasma if a radial dc electric field exists and if this radial dc electric field and the radial density gradient are in the same direction. The origin of the dc electric field is found to be the difference between the ion and the electron radial density profiles. In such a plasma if the ions are essentially unmagnetized but the electrons are magnetized, a velocity difference in the θ direction can arise because of the finite ion Larmor radius effect. This leads to a space charge separation between the electron and ion density perturbations in the θ direction. The consequent perturbed azimuthal electric field Eθ1 and the enhancement of the density perturbation by the Eθ1×B0 velocity occur in the same manner as in the SHI. The instability frequency is decided by the ion azimuthal drift velocity. This new instability has been investigated through experiments and theory.