Cyclotron Instabilities in an Anisotropic Plasma

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In the present review we shall generalize and systematize results that have been obtained in the investigation of the stability of a plasma in a magnetic field when the velocity distribution is not isotropic. The first work in this field is that of Harris [1], who showed that a plasma with an anisotropic electron velocity distribution is unstable against the excitation of electron cyclotron waves. Later, attention was directed to the stability problem in a plasma with an anisotropic ion velocity distribution (Timofeev [2], Kitsenko and Stepanov [3], Dnestrovskii, Kostomarov, and Pistunovich [4], Dory, Guest, and Harris [5], Hall, Heckrotte, and Kammash [6], Post and Rosenbluth [7], and others). A plasma in an adiabatic trap must necessarily have an anisotropic velocity distribution since the effective confinement of plasma in such a system is possible only when the energy associated with the ion motion along the magnetic field is much smaller than the transverse energy. Under these conditions, the theory indicates the possibility of excitation of oscillations with frequencies close to the ion-cyclotron frequency, and this theoretical result has been verified many times in experiments in which noise is observed at the cyclotron frequencies. Since the intensity of this noise is many orders of magnitude greater than the thermal level, it is justifiably associated with the excitation of unstable waves [8, 9].