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.
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.
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.
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.
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