A theoretical investigation of the plasma klystron amplifier

Abstract

The plasma klystron amplifier concept is investigated. The plasma klystron consists of a relativistic annular electron beam propagating through a space between a plasma column and drift tube wall. The annular electron beam, premodulated at the first cavity in the klystron, enters the space. An axial electric field accompanied by the modulated beam excites space charge waves in the plasma column, which acts like an inductive medium amplifying the electrostatic waves. The physical mechanism of self‐amplification of the electrostatic waves is the two‐stream instability, which is investigated in detail. Instability criteria indicate that the plasma should be opaque to sustain instability. A theoretical model to describe current modulation of the annular electron beam propagating through the space is developed. The electron beam current is modulated by the amplifying electric field fed by the plasma column. Saturation distance of the current modulation is a weakly dependent function of the initial energy modulation at the first cavity. Thus, a relatively low‐power microwave input may well excite the two‐stream instability and deliver a highly modulated beam to the extraction cavity. Due to a relatively large growth rate of the instability, required interaction length of the beam and plasma column is short for most applications.