Single-mode operation of a Bragg free-electron maser oscillator.

Detailed studies of the spectral characteristics and spatial mode structure of a single-mode, high power Raman Free-Electron Maser (FEM) oscillator operating with a Bragg resonator are reported. The FEM oscillator generated approximately 1 MW of microwave power in a single axial mode at 27.47 GHz, with an electron beam energy of 320 keV and a transmitted current of 30 A, yielding an efficiency of 10.3%, in good agreement with simulations. These results indicate that a Raman free electron maser can operate at high power and efficiency with stable single-mode output in the long pulse (equilibrium) regime. 1 Free-Electron Lasers (FELs) or Masers (FEMs), as they should be termed for the microwave region of the spectrum, are tunable sources of intense coherent electromagnetic radiation [1,2]. Some of their most remarkable properties include tunability [3], wide band-width [4], high gain [5], and the large phase shift that the resonant beam-wave interaction induces on the amplified electromagnetic wave [6]. Potential applications of FEM amplifiers and oscillators are numerous, and include their use for radar and plasma heating. Operation of the FEM oscillators in a single mode at high efficiency is of paramount importance for these applications; thus the avoidance of mode competition effects, which are known to be prevalent in FEM and FEL oscillators [7], becomes a primary concern. This paper reports on the successful single mode operation of a high power FEM oscillator. Mode competition results are discussed in the context of multi-mode oscillator theory [7] and previous experiments [8]. Several past experiments on FEM oscillators have been reported. Pioneering work at Columbia [9] and NRL [10] showed the possibility of generating high power microwave oscillations by providing feedback to the growing electromagnetic waves amplified by the FEL interaction. More recently, FEM oscillators employing Bragg resonators have been described [11,12]. These previous experiments were either at low power [11] or at short pulse and wide bandwidth [12]. The present experiments [13] are at high power, and, in addition, a careful study of the spectral properties of the FEM radiation has been carried out. In particular, the detailed spatial (transverse and axial) and temporal (mode competition) [7] structure of the operating mode has been studied experimentally. Most previous experiments with free electron masers operating in the Raman regime have operated with short pulses. Those experiments have either run with pulses of 10 to 100 ns duration or have had voltage and current variations …