Instability of decelerating electron flow injected into a planar diode

The instability of a decelerating electron flow injected into a planar diode has been studied using the results of experiments and the exact solution of a system of inhomogeneous hydrodynamic equations formulated in the first-order approximation of the theory of stability. A dispersion equation is obtained that relates the frequencies and increments (decrements) of the arising electromagnetic oscillations to the electron flux parameters and the diode characteristics. A solution of this dispersion equation shows that a diode, in which the decelerating electron flow is propagating, features a previously undescribed instability of oscillations in the microwave range. This instability takes place in cases where no potential minimum is formed in the diode in a stationary state. The development of the instability of oscillations with theoretically predicted frequencies and increments has been confirmed in experiments.