Free-electron Maser Research Articles

Short-wave sectioned free-electron masers with Bragg resonators based on the traveling and quasi-critical wave coupling

We consider the possibility of implementation of a free-electron maser with a two-mirror resonator composed of modified and conventional Bragg mirrors, operated in the short-wave part of the millimeter-wave range. The use of a modified Bragg mirror based on the traveling and quasicritical wave coupling at the input of the interaction space permits the transverse-index selection of modes. Amplification of the synchronous co-propagating wave by an electron beam is reached mainly in the regular part of the resonator. Even slight reflections from the conventional output Bragg cavity, which directly couples the co- and counter-propagating traveling waves, turn out to be sufficient for generation of self-excited oscillations. It is shown that the new scheme of a free electron maser ensures the oscillation frequency stabilization with respect to the electron-energy variation. With the optimal choice of the parameters, the oscillation frequency is close to the cutoff frequency of a quasi-critical wave excited in the modified Bragg structure.

Free-electron masers based on planar Bragg waveguides

A new scheme of the free-electron maser (FEM) with a ribbon electron beam is proposed in which the radiation is channeled in an open planar Bragg waveguide structure of two parallel plates with corrugated inner surfaces. The direction of corrugation is parallel to the radiation group velocity vector. The electron beam propagates in the waveguide at a large angle to its axis. The electron translation velocity is parallel to the direction of propagation of one of the partial waves that form the waveguide mode, which makes the maximum Doppler up-shift of the radiation frequency possible. A nonlinear analysis shows that the proposed FEM scheme provides a high gain (up to 30 dB). Using the Bragg waveguide, it is possible to ensure effective mode filtration, thus controlling the spatial structure of the output radiation.

THE ENGINEERING AND CONSTRUCTION OF A PRE-BUNCHED FREE ELECTRON MASER

We are developing prototype free electron maser (FEM) that is compact, tuneable and efficient for potential industrial use. Therefore we define the characteristics for the construction of a novel X-band rectangular waveguide pre-bunched free electron maser (PFEM). Our device operates at 10 GHz and employs two rectangular waveguide cavities (one for velocity modulation and the other for energy extraction). The electron beam used in this experiment is produced by thermionic electron gun which can operate at 3 kV and up to 5 mA. The resonant cavity consists of a thin gap section of height 1.5 mm which reduces the beam energy required for beam wave interaction. The prototype design, engineering and construction process are reported in this paper.

Generation of spatially coherent radiation in free-electron masers with two-dimensional distributed feedback

The results of theoretical and experimental studies on the generation of spatially coherent electromagnetic radiation in a planar free-electron maser with two-dimensional distributed feedback are reported. A two-dimensional Bragg structure is used at the initial part of the interaction space to ensure the transverse synchronization of the radiation. The possibility of the narrowband generation in the 75-GHz frequency band is demonstrated experimentally for a sheet kiloampere electron beam whose width is 20 times larger than the wavelength.

Free-electron maser based on a cavity with two- and one-dimensional distributed feedback

The study of a coaxial free-electron maser (FEM) based on two-dimensional (2D) and one-dimensional (1D) distributed feedback, driven by a 70mm diameter, annular electron beam is presented. A new cavity formed with 2D and 1D periodic lattices, positioned at the input and output of the interaction region, respectively, was used. It has been demonstrated that 2D distributed feedback in the input mirror allowed 8mm radiation emitted from different parts of the electron beam to be synchronized. The FEM operating in the 35.9–38.9GHz frequency region generated 65MW, 150ns duration millimeter wave pulses which contained ∼10J of energy in the pulse.

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.

PROTOTYPE DESIGN OF COMPACT AND TUNEABLE X-BAND PRE-BUNCHED FREE ELECTRON MASER

At the University of Liverpool, we are developing prototype free electron maser (FEM) that are compact, powerful and efficient for potential industrial applications. The design, set-up and results of a novel X-band rectangular waveguide pre-bunched free electron maser (PFEM) are presented in this paper. Our initial device operates at 10 GHz and employs two rectangular waveguide cavities (one for velocity modulation and the other for energy extraction). The electron beam used in this experiment is produced by thermionic electron gun which operates at 3 kV and up to 50 µA. The nominal beam diameter is 1 mm passing across the X-band cavity resonators. The resonant cavity consists of a thin gap section of height 1.5 mm which reduces the beam energy required for beam wave interaction. The results, progress so far and the scope of work for the next couple of months are reported.

Experimental and theoretical studies of a coaxial free-electron maser based on two-dimensional distributed feedback

The first operation of a coaxial free-electron maser (FEM) based on two-dimensional (2D) distributed feedback has been recently observed. Analytical and numerical modeling, as well as measurements, of microwave radiation generated by a FEM with a cavity defined by coaxial structures with a 2D periodic perturbation on the inner surfaces of the outer conductor were carried out. The two-mirror cavity was formed with two 2D periodic structures separated by a central smooth section of coaxial waveguide. The FEM was driven by a large diameter (7 cm), high-current (500 A), annular electron beam with electron energy of 475 keV. Studies of the FEM operation have been conducted. It has been demonstrated that by tuning the amplitude of the undulator or guide magnetic field, modes associated with the different band gaps of the 2D structures were excited. The Ka-band FEM generated 15 MW of radiation with a 6% conversion efficiency, in good agreement with theory.

Linear theory of the electron beam-wave-plasma interactions in a magnetized plasma waveguide

The theoretical study on the electron beam-wave interactions in a plasma waveguide immersed in a finite magnetic field is given in the article, in which both the plasma and the electron beam are considered as special media. Making use of the constitutive transformation and the Lorentz transformation in the four-dimensional space, the permittivity tensor of the stationary magnetized plasma, the permittivity tensor, the permeability tensor, and the chiral tensor of the electron beam in the rest (laboratory) frame are acquired. Therefore, two coupled wave equations for the magnetized plasma and electron beam have been obtained and the dispersion relations are then achieved by solving these coupled equations together with the boundary conditions including the surface current density due to the ripple of the plasma/beam. As an example of the applications of this approach, the beam-wave interactions in a practical plasma Cherenkov maser have been studied and the numerical calculations have been carried out in detail. It has been found that the present approach is more accurate and can provide clearer mode information for the electron beam-wave interactions in a magnetized plasma waveguide. This approach can be exploited in a number of electron beam-wave interaction systems including some kinds of free electron devices, plasma filled Cherenkov radiated free electron lasers and masers.

Theory of a planar free-electron maser with transverse electromagnetic flux circulation in a 2D Bragg mirror

A planar free-electron maser with a resonator consisting of a 2D entrance Bragg mirror and a 1D exit Bragg mirror is theoretically studied in the framework of a nonstationary 2D model. In such a configuration, the 2D Bragg mirror provides synchronization of the radiation emitted by a wide (compared with the wavelength) ribbon-shaped electron beam. The transverse electromagnetic energy fluxes arising in this mirror are closed through an additional coupling waveguide, which provides a single-mode single-frequency masing regime insensitive to variation of the beam parameters over wide limits.

Mechanism of free electron maser self-excitation using coupled propagating and trapped modes

A new scheme of feedback in a free-electron maser (FEM), which is based on the mutual scattering of propagating and quasi-cutoff (trapped) modes on a periodic Bragg structure, is considered. The trapped mode makes possible self-excitation of the system, while the energy exchange in a stationary generation regime is almost entirely determined by the interaction with a synchronous propagating wave. Using the proposed method, it is possible to improve mode selectivity with respect to the transverse index and accordingly decrease the working wavelength.

Feasibility of using a free-electron maser with a Bragg resonator for testing high-Q resonant structures

The operation of a pulsed free-electron maser with a Bragg resonator and resonant termination is simulated in terms of nonstationary equations. It is shown that the termination partially loses its reflection properties and accumulates microwave energy when the eigenfrequency of the terminating resonator coincides with the frequency of the maser transition radiation. The difference between these frequencies produces considerable reflections and in the long run quenches masing. The masing stability depends on the lag of the reflected signal and channel losses.

Experimental Study of Coaxial Free-Electron Maser Based on Two-Dimensional Distributed Feedback

The first experimental study of a coaxial free-electron maser (FEM) based on two-dimensional (2D) distributed feedback is presented. A new type of cavity formed with coaxial 2D surface photonic band gap structures was used. The FEM was driven by a large diameter (7 cm), high-current (500 A), annular electron beam of energy 475 keV. By tuning the amplitude of the undulator or guide magnetic field, modes associated with the different band gaps of the 2D structures were excited. The -band coaxial FEM generated 15 MW of radiation with a 6% conversion efficiency, in excellent agreement with theory.

Time-dependent simulation of free-electron laser amplifiers and oscillators

Time-dependent free-electron laser simulations use a variety of techniques. Particle-in-cell codes have been used to simulate free-electron masers; however, this is not feasible at short wavelengths. Most simulations use a slowly varying envelope approximation in both in $z$ and $t$, where the particles and fields are advanced in $z$ using the same process as in steady-state simulations and then the time derivative describing slippage is applied. We describe the inclusion of this technique in the non--wiggler-averaged code MEDUSA, which is then applied to study temporal behavior in amplifiers and oscillators.

Improving selectivity of free electron maser with 1D Bragg resonator using coupling of propagating and trapped waves

(Received 31 December 2004; published 28 April 2005)A novel scheme of the Bragg free-electron maser (free-electron laser) [FEM (FEL)] based on thecoupling of an amplified propagating wave with a quasicutoff mode trapped inside the cavity isconsidered. The cutoff mode is essential for FEM self-excitation while energy extraction from theelectron beam in the steady-state oscillation regime is almost completely determined by the propagatingmode, synchronous to the beam. The main advantage of the discussed scheme over the traditional schemeof Bragg FEM is improving selectivity over the transverse index. Based on the proposed feedback schemethe advance of the JINR-IAP FEM to the shorter wavelength is discussed.

Study on the high frequency structure of FEM amplifier

The compact free electron maser (FEM), with the advantages of small size and low cost, is important for realistic applications in medicine and industry. In order to utilize the planar wiggler to the best advantage and simultaneously to reduce the necessary electron beam voltage and the size of FEM device, we propose to utilize an elliptical waveguide as the high frequency structure of planar wiggler FEM. A set of universal operating equations suitable for elliptical waveguide and rectangular waveguide FEMs is derived by using the nonlinear theory. The characteristics of the FEMs are numerically analyzed, comparatively.

Design and construction of a table top microwave free electron maser for industrial applications

This paper reports the development of a reasonably low-cost free electron maser (FEM) as a source of tuneable, focused microwave power for industrial applications. In this paper the principle of operation and general system set-up will be presented. To date the FEM is operating at a power level that has allowed us to perform experiments with low-pressure plasmas in UV lamps.

Possible ways of improvement of FEM oscillator with Bragg resonator

Two methods of efficiency enhancement of free-electron maser (FEM) oscillator with the Bragg resonator having a shift of corrugation phase were studied. The first one is a “starting mode” regime using an additional high-frequency mode located in the vicinity of the main mode. The Bragg resonator having a shift of corrugation phase seems to represent the most attractive oscillator configuration. Another possibility is a special variation of corrugation depth of Bragg reflectors (mirror profiling) providing the possibility of optimizing the longitudinal distribution of the RF field amplitude inside the interaction space and depressing of parasitic side modes of the resonator. Optimization of RF power extraction from FEM, based on Talbot effect, has also been considered. Perspectives of future FEM development are discussed including schemes of pulse power compression and submillimeter wave generation.

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.

Regime of non-resonant trapping in a Bragg-cavity FEM oscillator

A method for realization of the regime of non-resonant trapping of electrons in a free electron maser (FEM)–oscillator is proposed and theoretically studied. A possibility to achieve a high (over 50%) electron efficiency and a very weak sensitivity to the electron-beam quality is demonstrated for a moderately relativistic mm-wavelength FEM.