High-power Free-electron Maser Research Articles

High-power free-electron maser operated in a two-mode frequency-multiplying regime

The frequency multiplication effects in high-power free-electron masers (FEM) with Bragg cavities were studied to provide the advance of the oscillators into short-wavelength bands. Theoretical analysis of frequency-multiplying FEMs was carried out within the framework of the averaged coupled-wave approach. Proof-of-principle experiments were performed based on a moderately relativistic induction linac LIU-3000 (JINR). As a result, an FEM multiplier operated with a megawatt power level in the 6-mm and 4-mm wavelength bands at the second and third harmonics, respectively, was realized. The possibility of using two-mode bichromatic FEMs for powering a double-frequency accelerating structure was discussed.

Modeling of a High-Power Wideband Free-Electron Maser Amplifier with an Operating Frequency of 30 GHz to be Used in Particle Acceleration Experiments

We study the possibility to develop a high-power free-electron maser amplifier with an operating frequency of 30 GHz to be used in particle acceleration experiments. Various operation regimes are considered, which are capable of ensuring a wide instantaneous amplification band with a high electron efficiency. It is shown that it is possible to achieve a gain ratio of more than 25 dB and an output power of 20 MW in a frequency band with a width of up to 20% of the operating frequency in the so-called dispersion curve contact regime by using a moderately relativistic electron beam formed by the LIU-3000 induction accelerator in an undulator with a regular winding. Application of the profiled undulator allows one to achieve a radiation power of up to 50 MW in a band having a width of more than 50% of the operating frequency in the regime of non-resonant trapping and deceleration of particles.

Effect of ripple taper on band-gap overlap in a coaxial Bragg structure operating at terahertz frequency

Based on the mode-coupling method, numerical analysis is presented to demonstrate the influence of ripple taper on band-gap overlap in a coaxial Bragg structure operating at terahertz frequency. Results show that the interval between the band-gaps of the competing mode and the desired working mode is narrowed by use of positive-taper ripples, but is expanded if negative-taper ripples are employed, and the influence of the negative-taper ripples is obviously more advantageous than the positive-taper ripples; the band-gap overlap of modes can be efficiently separated by use of negative-taper ripples. The residual side-lobes of the frequency response in a coaxial Bragg structure with ripple taper also can be effectively suppressed by employing the windowing-function technique. These peculiarities provide potential advantage in constructing a coaxial Bragg cavity with high quality factor for single higher-order-mode operation of a high-power free-electron maser in the terahertz frequency range.

High-power free-electron maser with frequency multiplication operating in a shortwave part of the millimeter wave range

The possibility of using frequency multiplication in order to obtain high-power short-wavelength radiation from a free-electron maser (FEM) with a Bragg resonator has been studied. Preliminary experiments with an LIU-3000 (JINR) linear induction accelerator demonstrate the operation of a frequency-multiplying FEM at megawatt power in the 6- and 4-mm wave bands on the second and third harmonic, respectively.

High-Current Electron Beams for High-Power Free-Electron Masers Based on Two-Dimensional Periodic Lattices

High-power gigawatt-level radiation can be generated by the interaction of an electromagnetic wave and an annular electron beam with a transverse dimension much larger than the operating wavelength. The use of such a large-circumference annular beam allows the generation of high beam currents while also maintaining low space charge and RF power densities inside the interaction region. This circumvents the problems associated with potential depression in the beam channel and RF breakdown inside the oscillator. In this paper, we present the studies of high-current magnetically confined annular electron beams and discuss their production and transportation through a coaxial beam channel which formed the interaction region of a free-electron maser (FEM). The results from numerical simulations, using the software packages KARAT and MAGIC, are compared with the experimental measurements. The operation of a FEM, driven by a high-current annular electron beam, is presented, and the tunability of the maser, inside a frequency range defined by an input 2-D Bragg mirror, is demonstrated.

Separation of band-gap overlap in a coaxial Bragg structure operating in higher-order mode at Terahertz frequency

Band-gap overlap always occurs when an overmoded coaxial Bragg structure operates in the Terahertz frequency spectrum, which may cause serious competition of undesired modes with the operating mode at the operating frequency. In the present paper it is revealed that band-gap overlap of modes can be efficiently separated by setting the phase difference between the outer- and inner-conductor corrugations to be π. Moreover, the residual side-lobes interaction of the involved modes in a coaxial Bragg structure can be entirely eliminated by applying Hamming-window distribution to both the outer-conductor and inner-rod corrugations. These peculiarities provide potential advantage in constructing a coaxial Bragg cavity with high quality factor for single higher-order-mode operation of a high-power free-electron maser in the Terahertz frequency range.

Experimental and computational studies of novel coaxial 2D Bragg structures for a high-power FEM

Two-dimensional (2D) coaxial Bragg structures have been suggested for use in high-power Free Electron Masers (FEM) to synchronize radiation from different parts of an oversized annular electron beam. In this paper, the simulations of field evolution using the three-dimensional code MAGIC are carried out and results are presented. An investigation of 2D Bragg structures obtained by corrugating the inner surface of the outer conductor of a coaxial waveguide or by lining the surface of a smooth waveguide with a dielectric material, which has a bi-periodic permittivity, has been conducted. Experimental studies of 2D Bragg structures were also undertaken and the good agreement between experimental measurements and theoretical predictions is demonstrated. Measurements of a 7 cm diameter annular electron beam produced by a high-current accelerator to be used to drive the FEM are presented and the experimental set-up discussed.

Experimental studies of two-dimensional coaxial Bragg structures for a high-power free-electron maser

The experimental studies of two-dimensional (2D) coaxial Bragg structures are presented. These structures, which realize 2D distributed feedback, have been recently proposed as a method of producing gigawatt power level spatially coherent radiation from a free-electron maser driven by a large-size relativistic electron beam of annular geometry. The experimentally obtained frequency dependence of transmission coefficients for the 2D Bragg structures are in good agreement with theoretical predictions that demonstrates the operation of the two-dimensional Bragg scattering mechanism.