Mechanism of operational frequency tunability of the relativistic klystron amplifier

It is found that frequency tunability of the modulated current is related to periodicity of the Bessel's function of first kind.Ｒesponding to a same numerical values of the Bessel's function, there may be several numerical values of argument of the Bessel's function．Ｔhe frequency of the modulated current depends on the distance between the medium cavity and extract cavity, and the travel velocity of the electron beam, and so on,which are included in the argument of the Bessel's function. The theoretical estimations agree very well with the experimental results. The radiation frequency tunability of the relativistic klystron amplifier still depends on design of the extract cavity．