S. N. Sedykh

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.

Alignment of the optical feedback system of VUV regenerative FEL amplifier at the TESLA test facility at DESY

In this paper, we describe optical feedback system of VUV Regenerative FEL Amplifier (RAFEL) at the TESLA test facility at DESY. The aim of the RAFEL experiment is to construct fully coherent, tunable VUV radiation source by means of applyingnarrow-band optical feedback in the VUV SASE FEL operatingcurrently at DESY. One of the problem of the realization of the RAFEL is severe requirements for the angular stability of the optical elements (about few microradians). This problem has been solved by means of installation of active alignment system with reference laser. Another problem is alignment of optical elements separated by 65 m within complicated experimental conditions connected with aperture limitations (down to 6 mmÞ: This problem has been solved in two steps. Preliminary alignment with an accuracy of about 80 mrad has been performed with laser alignment system and OTR screens used at the TTF accelerator for electron beam diagnostics. Final alignment has been performed with VUV SASE FEL radiation. Measured feedback coefficient is about 1 percent and is in agreement with the designed value. r 2002 Elsevier Science B.V. All rights reserved. PACS: 41.60.Cr; 52.75.M; 42.62.Cf

Progress in development of powerful sub-mm Bragg FEM based on moderately relativistic electron beam

A project to generate powerful sub-mm pulses in JINR-IAP FEM-oscillator driven by 0.8 MeV / 200 A linac is in progress. Present paper describes recent design and key elements of the oscillator.

High-power broadband 30-GHz FEM amplifier operated in the grazing incident regime

A high-power broadband free electron maser (FEM) amplifier has been realized in collaboration between JINR (Dubna) and IAP RAS (Nizhny Novgorod) based on the linac LIU-3000 (0.8 MeV/200 A/250 ns). To achieve a wide frequency amplification band, the regime characterized by grazing of the dispersion curves of the beam electrons to the operating wave (the so-called grazing regime) was studied in the FEM. According to the simulations, such an operating regime allows an instant amplification band of up to 5%–7% with output power at the level ∼20 MW and a gain of about 30–35 dB to be obtained in the Ka band. The proof-of-principle experiments examined this scheme in the 30-GHz frequency range and confirmed the simulations. As a result, the predicted power level with design parameters has been achieved, and fast frequency tuning within the feeding magnetron band ±0.5 GHz has been demonstrated.

Sub-millimeter Bragg FEM based on moderately relativistic electron beam: project and first experiments

Sub-mm Bragg FEM (free-electron maser) is developed based on 0.8 MeV / 200 A linac. Present paper describes recent design and key elements of the oscillator. Results of the first experiments, which proved operability of novel FEM scheme, are described.

Powerful free-electron masers with novel Bragg resonators

High-efficiency Ka-band FEM-oscillator was elaborated during the last few years in collaboration between JINR (Dubna) and IAP RAS (N.Novgorod). The induction linac LIU-3000 (JINR), which generates a 0.8 MeV / 200 A / 250 ns electron beam with a repetition rate of 1 Hz, drives the FEM-oscillator. Transverse velocity in the magnetically guided beam is pumped in a helical wiggler of 6 cm period. The main advantages of the JINR-IAP FEM is the use of a reversed guide magnetic field, which provides high-quality beam formation in the tapered wiggler section with a low sensitivity to the initial beam spread, alongside with Bragg resonator having a step of phase of corrugation [2], which possesses high electrodynamical mode selection. As a result, stabile single-mode operation with high electron efficiency was achieved in the FEM. At the present stage the FEM generates 20 MW / 200 ns pulses at 30 GHz with the spectrum width of 6 - 7 MHz, which is close to the theoretical limit (Fig.2). Stability of the FEM radiation parameters (frequency, output power, pulse shape, etc.) was demonstrated over a sequence of ~ 105 pulses.

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.

Mode splitting effect in FEMs with oversized Bragg resonators

Splitting of the fundamental mode in an oversized Bragg resonator with a step of the corrugation phase, which operates over the feedback loop involving the waveguide waves of different transverse structures, was found to be the result of mutual influence of the neighboring zones of the Bragg scattering. Theoretical description of this effect was developed within the framework of the advanced (four-wave) coupled-wave approach. It is shown that mode splitting reduces the selective properties, restricts the output power, and decreases the stability of the narrow-band operating regime in the free-electron maser (FEM) oscillators based on such resonators. The results of the theoretical analysis were confirmed by 3D simulations and “cold” microwave tests. Experimental data on Bragg resonators with different parameters in a 30-GHz FEM are presented. The possibility of reducing the mode splitting by profiling the corrugation parameters is shown. The use of the mode splitting effect for the output power enhancemen...

Powerful short wavelength fems operated at harmonics of bounce frequency: Recent results and prospects

Summary form only given. High-efficiency narrow-band 30-GHz Bragg FEM-oscillator was elaborated during the last few years in JINR-IAP collaboration based on 0.8 MeV / 200 A / 250 ns LINAC LIU-3000 (JINR). An attractive solution to prospect FEM into short wave-lengths keeping beam energy and geometry of electron-optical system is operation at harmonics of bounce-frequency. Two FEM schemes exploiting this idea are under development at LIU-3000.