A Kadomtsev–Petviashivili equation is derived for a two-dimensional ion-acoustic soliton propagating in a collisionless weakly relativistic plasma containing the finite temperature ions. This equation is solved in a stationary frame to obtain expressions for the phase velocity, amplitude, width, peak ion density, peak pressure, and energy of the soliton. It is shown that both the relativistic effect and the ion temperature greatly influence the phase velocity, amplitude, and the width. The soliton behavior is described in detail, and a comparison is made between the present results and the previous theories.

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Wave excitation and fast ion generation in a double plasma device are studied experimentally. Results show how a compressive density perturbation, propagating as a wave, is formed in the target plasma when a positive potential pulse is externally applied. Further, it is shown that fast ions are generated in a narrow plasma region close to the sheath near the grid because of reflection by the wave front during propagation of the wave.

When a gridded energy analyzer is used for energy analysis of plasma particles, it is experimentally found that the characteristics of the analyzer can be heavily affected by the space charge of particles accumulating behind its entrance grid. In this experiment, using a large model analyzer, the current profiles of accumulating ions in the analyzer and the effects of the space charges on ion energy distributions are directly measured in detail. Finally, problems related to the ion accumulation and the space charge effects in a gridded energy analyzer are generally discussed on the basis of the above experimental results.

The limitation in amplitude of ion-acoustic solitons or laminar shocks excitable in a double plasma device is clarified to result from the effect of pulsed fast beam ions, generated by the pulse externally applied for wave excitation. Beam ions faster than the soliton, which suppress the growth of the proto-soliton amplitude at an early stage of wave excitation, are always generated in the case of the double plasma ͑DP͒-excitation, when a pulse high enough is applied so as to excite a large amplitude soliton. Controlling the generation of such fast beam ions by adjusting the rise time of the applied pulse, ion-acoustic solitons or laminar shocks of high Mach numbers up to M Ӎ1.4 are observed to be formed even in a plasma with a finite ion temperature such as T i ӍT e /20.