Gain and bandwidth of the TM-mode gyrotron amplifiers

Abstract

This work discusses the feasibility of gyrotron amplifiers using TM modes by investigating their dispersion relations starting with the relativistic Vlasov equation. For an input signal carrying a real frequency ω, the complex wave number kz solved from the dispersion relation tells us the growth rate and the bandwidth of an amplifier. Three modes (TE01, TM11, and TM01) operating at the fundamental cyclotron harmonic (s = 1) with the same cutoff frequency will be discussed. The results show that the TM11- and TM01-mode gyrotrons require much higher beam current than the TE01-mode gyrotron traveling-wave tube (gyro-TWT) amplifier. The TM modes are unsuitable for the gyro-TWT amplifier as discovered by other authors. The gyrotron backward-wave (gyro-BW) amplifier is also examined. The dispersion relations indicate that the TM modes are as good as the TE modes. The gain and bandwidth of the TM11-mode gyrotron backward-wave amplifier (gyro-BWA) are studied by changing either the magnet field or the beam voltage. The TM11 mode is suitable and has a decent bandwidth for the operation of the gyro-BW amplifier.