抄録
A modulational instability of an ion wave in a plasma composed of electron, positive ion and negative ion has been investigated theoretically. We derive a nonlinear Schrödinger equation for ion wave and examine the signs of nonlinear and dispersion coefficients. The dispersion coefficient is negative irrespective of the wavenumber. The sign of the nonlinear coefficient changes at a critical wavenumber kc, above which the coefficient becomes negative and the ion wave is modulationally unstable. The critical wavenumber kc depends strongly on the negative ion density. At the critical density of negative ion where the nonlinear coefficient of the Korteweg-de Vries equation for ion wave vanishes, the critical wavenumber kc is reduced to zero and the wave of any wavenumber is modulationally unstable.
