Observation of an Ion-Beam-Driven Instability in a Magnetized Plasma

We have observed, by CO₂-laser-light scattering, large-amplitude electron-density fluctuations with kλ_D~0.1 and a narrow angular spread in k--> about the direction normal to a transient shocklike distance. This shock propagates radially inwards in a cylindrical plasma [n_e=(1-6)×10¹³ cm^-3, T_e<~1 eV] having an axial magnetic bias field of 170 G. The observed fluctuations are seen in a narrow region in front of the shock where a stream of reflected ions passes through the background plasma, and are thought to be driven by this stream.