Relaxation of azimuthal flow pattern from ring to bell shape through two-dimensional turbulence triggered by diocotron instability

• Published in: Journal of the Physical Society of Japan 2006-10, Vol.75 (10), p.104502
• Main Authors: KAWAI, Yosuke, KIWAMOTO, Yasuhito, ITO, Kiyokazu, SANPEI, Akio, SOGA, Yukihiro, AOKI, Jun, ITOH, Kimitaka
• Format: Article
• Language: English
• Subjects: Exact sciences and technology; Physics; Physics of gases, plasmas and electric discharges; Physics of plasmas and electric discharges; Plasma turbulence; Waves, oscillations, and instabilities in plasmas and intense beams
• ISSN: 0031-9015; 1347-4073
• DOI: 10.1143/jpsj.75.104502

Experimentally observed dynamics of vortex patches, as generated by the diocotron instability in a magnetized pure electron plasma, is examined in terms of the power spectra of the distributions of particles and flux, and the energy spectrum. The global pattern of the azimuthal-flux in the initial ring distribution is torn into mesoscopic blobs with local circulation at the cost of the reduction of the energy in the radial shear of the azimuthal flow. In a turbulent stage characterized by mergers between the blobs, the energy spectrum broadens upward in the wave-number space while the dominant mode shifts downward in such a way as is consistent with a cascade model of the turbulence theory. The rapid relaxation is accompanied by the reduction of enstrophy and enters a slow process whereby a symmetric pattern of the azimuthal flow emerges in a convex profile. Until the asymptotic state, the energy and the angular momentum of the whole plasma system remain conserved but the radial shear is substantially reduced. The spectra in the final state concentrate at symmetric modes with a slight contribution from the orbiting motion of the plasma column. From these experimental results, a picture of the turbulent relaxation involving the vortex dynamics covering wide length scales is given in the wave-number space, which has previously not been pursued with regard to pure electron plasmas.