抄録
The helical instability and oscillations in extremely short positive columns immersed in longitudinal magnetic field are experimentally investigated. It was found that several higher order modes of helical instabilities begin to grow in turn with azimuthal mode number m coexisting with helical m=1 mode as the magnetic field increases above a critical field Bc for the m=1 mode. With further increase of magnetic field nonlinear properties of the instabilities become predominant, resulting in the appearance of higher harmonics of the m=1 mode Oscillation frequency of the helix first increases with increasing magnetic field and finally decreases passing through a maximum in accordance with the nonlinear theory of Holter and Johnson. In a sufficiently short positive column, the plasma is always stable for all modes of instabilities independent of the magnetic field being in good agreement with short column theory of Timofeev.
