Strong low-frequency turbulence observed in presheaths

A noise whose frequency is from 0 to ωpi/10, where ωpi is the ion plasma frequency, is observed in presheaths in both sides of a plane stainless-steel mesh grid immersed in a uniform and unmagnetized plasma. To excite the noise, a negative dc voltage lower than -20 V is applied to the grid. The frequency spectrum of the noise is expressed with the Lorenz distribution a/(f2+fo2), where f is the frequency and a and fo are constants. The noise is found to propagate away from the grid with the speed of one-fifth of the ion-sound velocity. The noise is considered to be excited in the presheaths by counter-streaming ion-beams whose velocity are slower than the ion-acoustic velocity. The kinetic dispersion relation is solved numerically and results are compared with experimental findings