| Summary: | 碩士 === 國立成功大學 === 太空與電漿科學研究所 === 105 === As shown by Landau in a framework of linear theory, it seems considerably difficult to excite undamped electrostatic (ES) waves without suffering from phase-mixing even in collisionless plasmas. The solution of the Vlasov-Poisson equations obtained by Bernstein, Greene and Kruskal in 1957, which is so called BGK modes, is an answer to this question in the framework of the nonlinear theory. In fact, BGK modes have been observed in various circumstances such as in space, numerical simulation and laboratory plasmas. This master thesis describes our attempt at the generation of BGK modes with well-controlled techniques in laboratory plasmas.
We conducted two kinds of experiments to generate high amplitude and steady-state electrostatic waves in the linear magnetized plasma device, MPX. The first experiment is the generation of phase-space electron holes by utilization of the autoresonance mechanism. Autoresonance is a nonlinear phenomenon that an oscillator self-adjusts its amplitude for achieving phase locking with an external exciter [W. Bertsch et al., Phys. Rev. Lett., Vol.91, 265003 (2003)]. In our case, the oscillator corresponds to ions or electrons in a magnetic mirror, and the exciter corresponds to an external potential source, which aims at the excitation of longitudinal electrostatic waves in the plasma with bucket-electron holes. The oscillating external potential was applied at one end of the MPX plasma; responses (space potential) of the plasma were monitored by emissive probes with high temporal resolution in the downstream region along the field lines. A large amplitude pulsed response having a negative potential was observed during the application of the frequency-chirped drive. During the pulse propagation, we observed small phase-space electron holes with the use of a fast voltage scanning Langmuir probe. However, the hole size we excited was not large enough to prevent the wave decay. One possible reason is that electron confinement time was shorter than time duration of the external drive chirp, i.e., the time necessary for production of bucket-electron holes.
The second experiment is production of steady-state shock in MPX plasmas with the application of diverging magnetic fields. We discovered a strong steady–state strong shock (∆φ_s〉 T_e/e ) having a triple-layer (TL) of charge density, where ∆φ_s, Te and e are the potential depth, the electron temperature and elementary charge, respectively. We obtained the phase space diagram of the electrons and ions from the Langmuir probe and Mach probe measurements. The TL was accompanied a density jump by approximately on order. The electron phase space diagram obtained from LP measurement with the Druyvesteyn method indicates that trapped electron islands exist behind the potential dip of TL, which is consistent with a BGK picture. The diverging magnetic fields may be a key to produce the TL, which inferred from experimental results of the varied field profile of MPX plasma.
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