To ensure high selectivity of excitation of the operating high-cyclotron-harmonic waves in gyrotrons, as well as to provide an increase in the efficiency of high-harmonic gyrotrons by decreasing Ohmic heating, we propose using resonators of special shapes. In this work, we describe axially-symmetrical cavities with one or more axial grooves with mode selective properties, as well as quasi-axially-symmetrical cavities with small azimuthal irregularities. The use of cavities of these types can be the way of achieving a stable single-mode gyrotron operation at frequencies of about 1 THz with a sufficiently high (several kW) output radiation power level.
The 30 keV / 0.7 A CW gyrotron is developed for the spectroscopy applications. Recently, selective operation at the second (0.267 THz) and at the third (0.394 THz) cyclotron harmonics was achieved. Special quasi-regular cavities are designed to achieve the fourth-harmonic operation at frequencies of up to 0.65 THz. The pulsed 80-100 keV / 0.7-1.0 A gyrotron is aimed to provide high-power pulses of radiation at the third cyclotron harmonic at frequencies close to 1 THz. Now we study possibilities to increase the peak level of the output power up to the level of several kW in order to use this gyrotron in plasma applications.
Novel schemes of the trapping regime in the free-electron devices were studied. Proof-of-principle experiments on implementation of “non-resonant” trapping were performed in Ka-band and a high-efficiency ultra-wideband freeelectron maser-amplifier has been demonstrated. For shorter-wavelength free-electron lasers (FELs), we describe two different ways to use the regime of so-called “multi-stage” trapping: a narrow-band FEL-amplifier and a FEL operating in multi-frequency SASE regime. The advantages of the proposed regimes for increasing the FEL efficiency and for decreasing the sensitivity to the spread in parameters of the feeding electron beams are demonstrated.
Powerful coherent terahertz radiation from short photo-injector electron bunches is required for many applications including mastering terahertz and x-ray frequency ranges. If the effective length of the electron bunch is shorter than the wavelength of the radiated wave, then the coherent spontaneous emission process does not need a special electron bunching and starts immediately. However, the initial coherence of radiation is rapidly broken due to increase in the bunch phase size, in particular, because of the strong Coulomb repulsion of particles inside a dense bunch. Stabilization of the bunch lengths or even compression of dense electron bunches in terahertz electron sources may be provided by various methods described briefly in this work.
A natural problem arising in the case of realization of a THz electron maser with a high-current relativistic electron beam is the use of an oversized cavity when it is difficult to provide selective excitation of a definite transverse mode. Our idea is to give up working on a fixed transverse mode, and to use excitation of a supermode formed by a fixed set of several transverse modes of an oversized waveguide. We propose to use the Talbot effect to create an oversized microwave system that provides a high Q-factor for this supermode. We present a design of a Free-Electron Maser fed by a 10 MeV / 2 kA / 200 ns electron beam and based on excitation of a Talbot-type supermode at a frequency close to 2 THz. The presentation includes results of our multi-frequency multi-mode simulations of the electron-wave interaction during the spatio-temporal process of formation and amplification of the supermode in an oversized microwave system. The calculated efficiency of this FEM at the level of 5-10% corresponds to the GW level of the output power.
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