Nonlinear Behaviour of High Frequency Electric Field Just before the Initiation of Beam-Plasma Discharge

The behavior of high frequency electric field nearby electron hybrid frequency, which is excited by electron beam-plasma interaction, is experimentally investigated before sudden buildup of the beam plasma discharge. The experiments are performed with a 2keV-20mA continuous or pulsed electron beam drifting in a partially evacuated region immersed in mirror magnetic field(mirror ratio of 1.4 and a peak field of lOOgauss). As the results, a frequency spread of its electric field is observed, which may be caused by nonlinear three wave interaction. In addition, a nonlinear saturation of its field intensity, which may be due to the formation of sidebands accompanied with the oscillation of the beam electrons trapped in the field potential well, is observed.