The electron cyclotron instability of a beam-plasma system in a multiple magnetic mirror

A 2 keV, 10 mA electron beam creates its own Helium plasma (nsub(e) approx. = 109 cm-3, Tsub(e) approx. = 4 eV) in a cylindrical inter action chamber (L = 75 cm, phi = 8 cm, Psub (He) approx. = 1.3 x 10-3 torr) in a multiple magnetic mirror (Bsub(av) approx. Bsub(o) G, periodicity of 22 cm). The frequency spectrum reveals the excitation of the electron cyclotron instability with a central frequency which depends almost exclusively on the average magnetic field and with an amplitudeb varying with the mirror ratio as well as with the average plasma density, These experimental results are interpreted in terms of the analysis of the instalbility theoretical spectrum, constructed from the space-time evolution, in the WKB approximation, of waves with frequencies belonging to the unstable band. (author)