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Externally imposed resonant magnetic islands may be used to slow down the growth rate of the tearing mode via Rutherford’s mechanism. However, this is effective only if the harmonic number of the externally imposed islands is sufficiently high.

The nonlinear evolution of an ion‐acoustic wave packet is studied. Experimentally, it is found that (i) nonlinear phase modulation develops in the wave packet; (ii) the phase modulation, together with the dispersion effect, causes expansion and breaking of the wave packet; (iii) the ions trapped in the troughs of the wave potential introduce self‐phase modulation; and (iv) the ion‐acoustic wave is stable with respect to the modulational instability. Computer simulations have reproduced the experimental results. The physical picture and the model equation describing the wave evolution are discussed.

A poloidal quadrupole field is applied to a tokamak discharge in order to study the magnetic field‐line reconnection. Elongation of the poloidal cross‐section of the plasma is observed when the quadrupole field strength is below a critical value. As the quadrupole field is increased above this critical field strength, reconnection occurs and the magnetic axis changes from being the center of elliptic magnetic surfaces (i.e., an ’’O’’ point) to the center of symmetry of hyperbolic surfaces (an ’’X’’ point). The reverse process is also observed.

The detailed identification of two instabilities driven by an ion beam injected parallel to the confining magnetic field, a resonant ion cyclotron instability, and a nonresonant ion cyclotron drift wave driven by the density gradient of the beam, is reported. Experimental results are in good agreement with numerical calculations based on the kinetic Vlasov equations. Quasi‐linear velocity space diffusion of the beam ions by wave‐particle interaction is observed, demonstrating enhanced energy transfer. The relevancy of these results to tokamak neutral beam injection heating is discussed.

Small particles in plasmas can form a coulomb lattice. The conditions for solidification in a laboratory plasma are discussed.

The steepening and breaking of the velocity‐modulated ion beam in phase space is observed experimentally. Depending on the range of velocity perturbation amplitude, three different types of beam evolution are identified. A simple theory which takes into account the ion‐temperature effect explains the observed behavior of the beam.

The response of electrons in a collisionless plasma to a fast‐rising voltage step is investigated experimentally and theoretically. At small amplitudes the step generates a wave train, the leading edge of which propagates along the column at the long‐wavelength electron plasma wave velocity. At large step amplitudes a fast burst of electrons is observed, with kinetic energy roughly proportional to the step amplitude, followed by a nonlinear electron plasma wave signal which shows solitonlike behavior.

Growth of two types of sideband waves has been observed experimentally when a relatively large‐amplitude ion‐acoustic wave propagates in a collisionless plasma having the electron‐to‐ion temperature ratio Te/Ti≈20. The observed frequency spectrum and growth rate, as well as the frequency and amplitude ranges of the large‐amplitude wave for which the sideband wave growth is observed are in good agreement with the theory of trapped particle instabilities. The sidebands extract the energy from the large‐amplitude wave and cause it to be heavily damped.

Low‐frequency electrostatic waves which propagate across the magnetic field are studied experimentally. The observed dispersion relation agrees with that of the ion cyclotron harmonic wave. Strong damping of the wave is observed.

A hot electron plasma is produced in a magnetic‐mirror field by high‐power microwave discharges. When an instability is triggered in a hot electron plasma, it is observed that the instability is accompanied by a burst of energetic electrons escaping from the magnetic mirror, a fast rising x‐ray signal, a sudden increase of light intensity, and an intense burst of microwaves composed of harmonics of 2.1 GHz. This frequency cannot be explained in terms of the electron cyclotron frequencies associated with the magnetic field intensity within the plasma chamber.

The perturbed part of the ion distribution function is observed experimentally when an ion wave or ion wave echo is excited in a plasma. In the case of the wave the perturbed distribution is confirmed to survive, as predicted by Landau, after the wave damps away. In the case of the ion wave echo the features of the oscillations of the distribution function in velocity space are in agreement with the prediction of the perturbation theory carried to the second order. However the second‐order perturbed distribution function, which is responsible for the formation of the echo, is observed to undergo a strong reduction in amplitude after the appearance of the echo electric field.

Ion‐acoustic solitary wave (solitons) have been studied experimentally by employing a double‐plasma device. The solitary waves are found to be produced from both a single compressional pulse and a continuous wave. A rarefaction pulse also produces solitons if the pulse width is sufficiently wide. A theory based on the Schrodinger equation accounts for the number of solitons. Recurrence to the original state is observed when a continuous wave is launched. A simple wave‐wave coupling analysis for the recurrence of the original state is given.

Theory and experiments of spatial ion‐wave echoes are reported. In the collisionless approximation, the calculated line shape of the second‐order echo is found to be a universal function of Δx/δ, the distance from the expected position of the echo divided by the Landau damping length of the ion‐wave constituting the echo. Higher‐order echoes predicted by Gould are observed up to the fourth order with respect to the external fields at the predicted positions with the predicted dependence of their amplitude on the amplitude of the primary waves. In addition, new types of higher‐order echoes which result from the electric fields of other echoes are predicted and observed. Measurements of the peak amplitude of the second‐order echo as a function of the separation l between the two exciters indicates that the memory of the past excitation retained in the distribution function damps exponentially with the attenuation length proportional to the wavelength of the excited wave. The effect of ion‐neutral collisions...