Steepening and self-refraction of pressure-driven low frequency turbulence

Summary form only given. Observations of growth and nonlinear saturation of highly oblique ion-acoustic waves near the lower hybrid frequency are reported. The waves are driven unstable by the electron diamagnetic drift in a linear, pulsed discharge plasma column (1-m diam., 2-m length, n_e≃10¹² cm^-3 , kT_e≃2 eV) immersed in a weak axial DC magnetic field (B_o=10 G). The ions (Ar) are essentially unmagnetized. while the electrons undergo a diamagnetic drift due to radial pressure gradients. When the diamagnetic drift exceeds the sound speed, low-frequency fluctuations grow in the region of steep density gradients. The frequency is below but near the lower hybrid frequency, the waves propagate at ν_phase≃kT_e/m_i ¹²≃2×10⁵ cm/s<ν_dia initially along the azimuthal diamagnetic drift. As they grow to large amplitudes, the wave fronts steepen into electrostatic shocks, and the direction of wave propagation bends toward the radial direction. While steepening is understood as a nonlinear feature of sound waves, the new self-refraction effect is thought to arise from the diamagnetic drift, which is modified by the large amplitude wave itself. The wave refracts so as to reduce the local drift velocity whereby it saturates