A.P. Sergeev

High efficiency FEL-oscillator with Bragg resonator operated in the reversed guide field regime

Abstract The narrow-band high-efficiency FEL-oscillator with Bragg resonator operated in the reversed guide field regime was realized on the basis of linear induction accelerator formed 1 MeV, 200 A, 200 ns electron beam. At a frequency of31 GHz radiation with a power of 23 MW and an efficiency of 19% was registrated. The measured frequency bandwidth was less than the zone of effective reflection of the Bragg mirrors. The narrow width of the spectrum was encouraged by the selective properties of the Bragg resonator in combination with the high quality of the helical electron beam formed in the reversed guide field regime.

Investigation of a microwave FEL with a reversed guide field

Abstract This paper presents the investigation of a FEL in the 8-mm wavelength range. The principal features of a FEL with a reversed guide field are discussed in comparison with a nonreversed guide-field configuration. The experiments on a FEL with a reversed guide field are described. The obtained experimental data are compared with theoretical predictions.

Broadband instability in free electron lasers

Abstract Experimental and theoretical investigation of some instabilities developing in the relativistic electron beam in the periodical magnetostatic pump is presented. It is shown, in particular, that under cyclotron resonance condition the generation of the electromagnetic waves is possible in a broadband regime. This broadband regime is due to development of cyclotron mode broadband instability not investigated earlier.

FEL-oscillator for feeding of high-gradient accelerating structure

Millimeter-wave FEL-oscillator is one of a possible source of RF power for accelerating structures of future linear colliders with high energy gain. JINR-IAP collaboration develops a high-efficiency single-mode FEL-oscillator with a Bragg resonator and reversed guide field. Output power of 48 MW at the frequency of 30.6 GHz with spectrum width 0.25% was registered in recent experiments using induction linac LIU-3000 (0.8 MeV, 200 A, 200 ns). In this paper we discuss technical solutions for providing of: frequency stability with accuracy better than 0.3%; precise frequency coincidence of the oscillator and the accelerating structure; RF power transportation from the oscillator to the structure.

New scheme of two beam accelerator driver based on linear induction accelerator

The two beam accelerator (TEA) drivers based on a linear induction accelerator were discussed previously. In these schemes the driver electron beam moves through alternating discrete row of microwave generators (free electron lasers (FEL), relativistic klystrons, etc.) and reaccelerator sections. The microwave power is totally extracted from the driver after every generation section. The continuous microwave power extraction along the whole driver length was designed in CLIC. A new scheme of TEA driver based on a linear induction accelerator is suggested. The scheme is quite uniform and has the following characteristic properties: a) the electron beam bunching occurs at a rather low initial energy; b) the bunched beam is accelerated in the accompanying of the microwave that provides the steady longitudinal beam bunching along the whole driver; c) there is no total microwave power extraction anywhere in the driver; d) a waveguide is used along the driver.

Starting mode regime in an FEM oscillator with a Bragg resonator

In an FEM oscillator with a Bragg resonator excitation of an additional starting mode can be exploited as a source of an initial signal for the operating mode. In such a regime the starting mode has a rather small detuning from synchronism resulting in a high temporal gain but a rather small electron efficiency. With the starting mode growth the situation becomes favorable for excitation of the operating mode. Afterwards due to a non-linear competition mechanism the operating mode suppresses the starting mode and a single-mode regime of oscillations is realized with a rather high efficiency.

Precise frequency tuning in the millimeter-wave FEL-oscillator with the Bragg resonator

Abstract A FEL-oscillator with the traditional Bragg resonator consisting of the waveguide with corrugated ends (mirrors) allows to obtain single-mode generation with high efficiency. However, the output radiation frequency cannot be fixed more precisely than the mirrors reflection band, typically at 2–3%. There exists another scheme of the Bragg resonator without the smooth tube between mirrors. Depending on the corrugation phase shift between the mirrors this resonator has a single high-quality mode inside the reflection band or two modes near the ends of the reflection band. Variation of the corrugation phase shift from 0 to 2π tunes the mode frequency from higher to lower limit of the reflection band. Bragg resonators of this type were designed both for high-efficiency FEL-oscillator and for precise radiation frequency meter. Experimental investigation of the tunable FEL-oscillator is now in progress.

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.

Development of frequency-tunable FEM-oscillator with Bragg resonator for testing high-gradient accelerating structures

This paper is devoted to progress in the JINR-IAP FEM experiment based on a 1 MeV/200 A/250 ns LINAC. This FEM generates 30 GHz pulses of 150 - 200 ns pulse duration and 20 - 30 MW power with an efficiency on the level of 20

Efficient schemes of millimeter wave FEM-oscillator and amplifier

25%. The high efficiency together with high stability of single-mode single-frequency operation is provided by the use of a reversed guide magnetic field and a novel Bragg resonator with a step of phase of corrugation. At the present stage of the experiments precise tuning of the oscillation frequency was performed by mechanical variation of the value of the phase shift in the Bragg resonator. As a result, the FEM was tuned in the frequency range of 6% with a precision of about 0.1%. Both the spectrum width and the dependence of the frequency tuning value on the corrugation phase shift are in good agreement with the results of simulations. The project to use this high power FEM source for studying surface heating effects in 30 GHz accelerating structures is discussed. A special cavity has been designed to enhance magnetic fields at the cavity surface. The cavity will be studied with regard to 200 - 500/spl deg/C surface pulsed heating stress of the cavity as a consequence of 10/sup 6/ RF-pulses, which are planned to be produced at a repetition rate of 1 Hz.