High-frequency instabilities arising in electron beam-plasma systems have been investigated in the frequency range 0.4 ωce ≤ ω ≤ 4ωce. Two types of electron beams (cold and helical) and two types of magnetic fields (uniform and mirror field) have been used. The plasma was created from hydrogen gas by the beam-induced r.f. fields. In the uniform magnetic field, the spectrum of the instabilities is in good agreement with a simple linear theory taking into account the finite transverse dimensions of the system. This is true even in the turbulent state of the plasma (ωpe/ωce > 1), in contradistinction to earlier experiments, if the fluctuations of plasma density are taken into account. The plasma has a tendency to exist at discrete values of density corresponding to the excitation of electron cyclotron frequency harmonics. The instabilities have a finite relaxation time (about 100 oscillation periods) and a finite relaxation length (about 5 wavelengths) due to non-linear effects. In the case of a mirror field, the instabilities are excited in regions where the field is least inhomogeneous (top of the mirrors and bottom of the containment zone). The heating of electrons occurs at the slope of the mirror. The significance of these results for the heating of electrons in a mirror machine is discussed.
Parametric instabilities and turbulent heating of a plasma in the field of a fast magneto-acoustic wave