Experimental demonstration of free electron maser operation in the regime of non-resonant trapping

Abstract

The regime of nonresonant trapping aimed at improving the operation of the amplifying scheme of free electron devices has been developed and experimentally demonstrated in the Ka-band. In this regime, the wiggler is profiled in such a way that the electron beam is out of resonance with the operating wave at the beginning of the wiggler, and different electron fractions gradually get into synchronism as the wiggler's period decreases. According to the simulations, such “nonresonant” trapping provides higher efficiency, a broader amplification band, and lower sensitivity to the beam quality as compared to conventional regimes of interaction. The conclusions of the theoretical analysis are confirmed by the results of joint experiments between the Joint Institute for Nuclear Research and the Institute of Applied Physics of the Russian Academy of Sciences, in which a free-electron maser utilizing this regime was implemented: Using a set of kilowatt magnetrons with frequencies in the range from 30 to 36 GHz, an output power of up to 25–28 MW with a gain of up to 36–37 dB was demonstrated. This makes the investigated regime attractive for shorter wavelength designs of free-electron devices whose efficiency is strongly affected by the electron beam quality.

Coherent Spontaneous Emission from Short Electron Bunches: Competition between Different Transverse Waveguide Modes

“…However, if the length of the operating electron beam is much longer than the wavelength of the operating wave, the emission process is ensured as a result of bunching of the electrons in the field of the radiated wave [10,11]. In order to provide the induced characteristic of the radiation process, one should use either a feedback system in auto-oscillator schemes of electron masers (see, e.g., [12][13][14][15][16][17][18][19][20][21]) or an initial (feeding) wave signal in amplifier schemes (see, e.g., [22][23][24][25]). However, when working in sub-THz and THz frequency ranges, the first method is hampered by a number of problems (for example, the selectivity of the feedback system in an oversized microwave system, the difficulty of achieving the stationary generation regime when using short-pulse high-current electron beams, the technical difficulties of providing feedback at high frequencies).…”