Propagation of surface waves and instability wave growth in an ion beam-plasma system

The system of a slowly diverging low density He + ion beam (energy 20 keV), traversing a stationary plasma column (n e approximately=10 7 cm -3 , T e <0.35 eV) created by the beam itself through collisional ionization is studied. No magnetic field is present in the experiment. Within the limits of the detector sensitivity, no spontaneous instabilities are observed. The propagation characteristics of waves excited from the outside are investigated with an interferometer technique. The existence of propagation in the surface wave mode on the beam plasma column is observed. From the dispersion in and opposite to the ion beam direction, the absence of plasma drift and a value for the plasma density can be determined. Spatial growth of waves modulated on the ion beam is found in the interaction with the surface wave mode on the plasma column, when the modulation frequency omega satisfies the condition omega < omega pe / square root 2. A quantitative agreement of the experimental growth rates, with the theoretically expected values for this interaction, is observed.