The current-driven electrostatic ion-cyclotron instability in a $Q$ machine operated in the low-density regime ${(\frac{{\ensuremath{\omega}}_{\mathrm{pe}}}{{\ensuremath{\omega}}_{\mathrm{ce}}})}^{2}\ensuremath{\ll}1$ is stabilized by externally generated rf electrostatic fields in the lower-hybrid range. Stabilization is due to a resonant ponderomotive force, which reduces the instability frequency, thus increasing the ion-cyclotron damping. The experimental results are in good agreement with an analysis which describes the effect of rf power on the instability frequency shift and growth rate.

We show that the principal observational features of power line radiation in the magnetosphere can be described as a van der Pol oscillation subject to incomplete entrainment by an externally injected signal. The nonlinear mechanism is referred to as "periodic pulling" and has been observed in laboratory low‐temperature plasma experiments.

Frequency-jump effects associated with nonlinear mode competition are investigated in an oscillator configuration consisting of a passive linear resonance system coupled to an active nonlinear resonance system. These effects give rise to a hysteresis pattern whose height and width can be related to system parameters such as the resonance frequencies, dissipation, coupling coefficient, etc. It is noted that these effects offer a novel means of determining these parameters in cases in which conventional techniques may not be desirable or as advantageous. The analysis provides a qualitative explanation of empirical observations in a recent nuclear magnetic resonance experiment [R.S. Timsit and J.M. Daniels, Rev. Sci. Instrum. 47, 953 (1976)]. The results also apply to other nonlinear resonance systems such as lasers, microwave generators, and electronic oscillators.

Q-machine experiments are described in which discrete resonant modes characteristic of the finite length and radius of a plasma column in which (ωpe/ωce)2<<1 are excited by an inductive coupler in the lower-hybrid range of frequencies. The experimental results are in agreement with an analysis in which ion effects and electron thermal effects are taken into account. Inductive coupling is found to give high-efficiency excitation of single modes without resonance cones. The dependence of mode properties on plasma parameters and geometric parameters is examined in the linear regime and the relation of these experiments to plasma heating and to the suppression of drift instabilities by lower-hybrid fields is discussed.

A convection instability characteristic of plasmas in an inhomogeneous azimuthal magnetic field is treated in the linear stage and in nonlinear saturation. The analysis is done in such a way that collisional and collisionless limits can be taken, and these limits are displayed along with the more general intermediate result. The instability, known previously in the literature in its collision-dominated form, is shown to be a ’’flute’’ instability with collisional modifications to the growth rate. The nonlinear saturation is analyzed by examining a finite amplitude restoring-force term in the differential equation that describes the instability. This term is due to the fact that the instability convects plasma into striations of the plasma column surface, modifying the density gradient driving force. The effects of finite ion gyroradius are displayed, and applications of this study to convection cells in a thermal plasma and to exploding wire plasmas are discussed.

Stabilization of drift waves by external rf fields at frequencies near the lower-hybrid frequency has been observed experimentally in a $Q$ machine in the collisionless regime with ${(\frac{{\ensuremath{\omega}}_{\mathrm{pe}}}{{\ensuremath{\omega}}_{\mathrm{ce}}})}^{2}\ensuremath{\ll}1$. Stabilization is due to a resonant ponderomotive force which increases the drift frequency, thus enhancing the electron Landau damping. The observations are in general accord with analyses in the literature when finite-geometry effects appropriate to the present experiment are taken into account.

The IEEE publications listed are now available or will be soon. Single copies may be ordered by using the order form on page 110 of this issue. When ordering, please be certain to include the volume number, issue number, month, and year and the title of the publication. Copies of individual articles are not available.

The unstable modes on a low-density, cold-electron-beam-plasma system are treated as an ensemble of Van der Pol oscillators. This model predicts the observed amplitude limiting of unstable waves at the neighboring frequencies of a launched wave that traps the beam electrons.

An oscilloscope display is described that can be used for signal-to-noise enhancement, presentation of spectral data in compact form, and X-Y data plotting. The display also functions as an optical signal averager and is useful in the detection of signals whose frequencies are not known a priori and signals whose frequencies drift in time. Results obtained in simulation tests and in experimental research on plasma waves and instabilities are described.