Measurement of the energy distribution function in a space simulation plasma

The Space Chamber of the Institute of Space and Aeronautical Science, University of Tokyo, aims to simulate a low-temperature unbounded ionospheric plasma with a low plasma density by providing two plasma sources of backward diffusion type at both ends of it. Although a relatively quiescent equilibrium plasma should be obtained, it deviates sometimes from the ideal case because oscillations appear under certain conditions. Experiments were performed to see how these oscillations are reflected on the energy distribution function of electrons. At low pressures, non-Maxwellian distributions having two groups of electrons were observed, which diffused in the velocity space. Furthermore, the plasma was not isotropic but showed a drifting characteristic. Possible reason for this might be the drift instability. Three types of micro-instabilities inherent to the diffusion plasma are pointed out. These results will be suggestive in designing a Vlasov plasma in laboratories. (auth.)