High-frequency oscillations of an electron beam generated by a ferroelectric cathode with anomalous plasma resistance caused by ion-acoustic instability

Spectroscopic measurements are reported of the plasma formed inside a cathode having a ferroelectric source incorporated in it. The measurements were performed during generation of a high-frequency modulated electron beam in a planar diode with the above cathode. It was found that there is a spatially periodic structure in the plasma density and electric field that vary in the range of 5×10¹³-3×10¹⁴ cm^-3 and 0-1 kV/cm, respectively. Plasma electron temperature decreases sharply from 8 eV in the vicinity of the ferroelectric surface to 2 eV in the bulk of the plasma. A qualitative model is suggested, which is based on fast periodic appearance of anomalous plasma resistance. The latter is supposed to be due to generation of ion-acoustic instability, which is controlled by the ratio between the velocities of the current carrying electrons and the ion sound.