Osamu Fukumasa

Computer simulation of nonlinear interaction between a cold beam and a weakly collisional plasma

The spatial growth of the instability and the nonlinear interaction between a small cold beam and a warm background plasma are examined by means of particle simulation. Up to the first maximum in the amplitude oscillation of the wave, quantitative measurements confirm the predictions based on the single wave model: the magnitude of the growth rate, monochromaticity of the unstable mode, the maximum wave amplitude, and the phase space orbits of the beam electrons. After the first maximum of the wave amplitude, the spatial dependence of the wave amplitude cannot be explained by the single wave model predictions and the wave power is anomalously overdamped by a factor of 0.1 smaller than the predicted value in the first minimum. The extended single wave model equations suggest that this anomalous phenomenon is caused by weak collisions of the order of ν/ωp∼10−3 within the background plasma.

Numerical study on the destruction of amplitude oscillations in a small cold beam-a weakly collisional plasma system

The destruction mechanism of the amplitude oscillation in the system of a cold beam and a weakly collisional plasma is examined numerically. The excitation of the neighboring waves depends not only on the beam strength s=((nb nu b2)/(6np nu t2))1/3 but also on the beam velocity nu b/ nu t. Even if s is large, the system becomes stable for the excitation of the neighboring waves with large nu b/ nu t. Consequently, collisional effects become more important for the destruction process because either of the collisional and the neighboring wave effects competes with another in playing a decisive role for the destruction of amplitude oscillations.

Suppression of electron waves in relation to the deformation of the electron beam distribution function

Abstract The change of the electron beam distribution function due to the wave excited by the beam density modulation is observed, in relation to the suppression of electron waves in a beam-plasma system.

Experimental study on excitation and suppression of a multimode beam-plasma instability in a bounded system

Four or five longitudinal electron waves of axial beam modes are excited simultaneously in a bounded electron beam-plasma system. They are accompanied with an ion-acoustic wave of an axial mode under appropriate plasma conditions. This ion-acoustic wave can not be excited directly by beam-plasma interaction, where vb/vte>or=3. When a beam density is modulated by an external r.f. signal whose frequency is close to any one of frequencies of electron waves excited spontaneously, all of the electron waves and the ion-acoustic wave are suppressed simultaneously. It is verified experimentally that quenching of the ion-acoustic wave is not brought about directly by the beam modulation, but by the damping of the electron wave which constitutes an energy source of the ion-acoustic wave.

Velocity space diffusion of ions in controlled turbulence

The velocity space diffusion of ions due to ion wave turbulence is investigated experimentally using an ion wave echo. Experiments are so arranged that the streaming perturbation due to the first excited wave propagates through the localized region where the ion wave turbulence is excited, to interact with the second excited wave. The resultant second order ion wave echo damps in comparison with the case of no turbulence. The variation of the echo amplitude as the power of the spectrum of the turbulence is changed is observed, which is in quantitative agreement with the theoretical prediction. It is also revealed that the turbulent waves are far more effective than the monochromatic waves in increasing the effective collisions.