A. Gover

Free electron maser experiment with a prebunched beam

An experimental project aimed at demonstrating Free Electron Maser (FEM) operation with prebunching is under way at Tel-Aviv university. The FEM utilizes a 1.0 A prebunched electron beam obtained from a microwave tube. The electron beam is bunched at 4.87 GHz and is subsequently accelerated to 70 keV. The bunched beam is injected into a planar wiggler (B, = 300 G, A, = 4.4 cm) constructed in a Halbach configuration with 17 periods. The wiggler utilizes a new scheme for horizontal focusing based on the use of two long permanent magnets at the sides of the wiggler. We plan to study FEM gain enhancement and radiation features due to the prebunched (superradiant) mode of operation. In an oscillator configuration the experiment should be enable study of seed injection by prebunching. Simulation of FEL operation shows an expected gain of approximately 100% and an rf output power of 5 kW. In this paper we review the design of the main parts of the experimental set-up, and present recent analytical, numerical, and experimental results.

A kinetic small signal gain analysis of a planar wiggler FEL, operating in the high harmonic (strong wiggler) regime

Abstract A kinetic linear analysis of a strong planar wiggler FEL operating at the fundamental or odd harmonic frequencies is presented in this paper. The gain dispersion equation in the high harmonic regime is shown to be an extension of the known general FEL equation, and converges to it in the weak pump limit. In the low gain, cold, tenuous beam regime we get the known gain expression with the Bessel function correction term. A similar term also appears in the gain expression of the high gain strong pump limit. When space charge effects are significant (Raman regime), the strong pump gain dispersion relation is more complicated than the corresponding weak pump relation. In the cold beam limit it leads to a quintic polynomial dispersion relation instead of the known cubic equation. This brings about gain curve dependences on the detuning parameter which are double humped and with a relatively high bandwidth. The higher the order of the harmonic frequency, the stronger is the effect of the axial velocity spread on the gain and the tighter are the beam acceptance parameters. The analytical model permits an arbitrary axial velocity distribution. The gain damping effect is computed, both with a simple model of a Gaussian electron velocity distribution and with a more accurate model, taking into account an analytical expression for the asymmetrical skewed velocity distribution produced by the combined finite emittance and energy spread effects.

A novel free-electron maser as a high power microwave source of sophisticated signals

A novel Free Electron Maser (FEM) developed at Tel-Aviv University and operating in C-band is described. The FEM, utilizing prebunching of its e-beam at the operating frequency, can operate as an oscillator or as an amplifier. In first experiments the FEM was operated as an oscillator without and with e-beam prebunching. Without prebunching the signal builds up out of noise in resonator modes for which the gain is highest. Mode competition and single mode evolution were observed. With prebunching the FEM oscillator could be locked to the prebunching frequency which could be at or near every eigenfrequency of the resonator for which the round trip net gain is larger than the loss. Use of the prebuncher as a signal modulator to obtain AM, FM and PM modulation is described. Frequency-agile operation can be achieved by switching of the prebuncher locking frequency. An FEM utilizing a broadband prebuncher enables smart signal modulation and frequency agility, together with a wideband interaction circuit should allow development of high power, wide band sophisticated signal FEMs.

The axial velocity spread acceptance of free electron lasers and its application to X-ray FEL design

Abstract : We present various criteria for the definition of axial velocity spread acceptance of free electron lasers (FEL), and provide analytical approximations, numerical calculations and design curves for this parameter in the low, high, and intermediate gain regimes. This parameter is used to calculate the energy spread acceptance, emittance acceptance and other e-beam quality parameters. A special emphasis is put on the design considerations of short wavelength (X-ray) FELs. For this reason, the parameters calculation in this work is limited to the tenuous beam regimes only, and parameters are calculated for intermediate and moderately high gain regimes, which correspond to the low quality factor of X-ray resonators in the present state of the art.

Interpretation of wave-profile modification measurements in high-gain raman free-electron-laser experiments

Abstract This paper presents a theoretical analysis of wave-profile modification measurements in FELs, using data that was reported recently [Phys. Rev. Lett. 59 (1987) 1177]. The results are related to a high-gain Raman free-electron laser, operating at long wavelengths (∼ 3 cm). Two possible mechanisms for wave-profile modification were examined. One is the known e-beam optical guiding effect. The other is that of the transverse electrostatic fields (TEF), excited by the bunched electron beam. The numerical results suggest that in the above-mentioned reference the measured wave-profile modification was predominantly a consequence of the TEF components. An experimental setup for verification of this analysis is proposed.

Feasibility of high gain X-ray free electron lasers

We investigate the feasibility of a tunable VUV/soft X-ray FEL operating in a dedicated storage ring. Because of poor mirror reflectivity available at these wavelengths, the FEL must operate in the intermediate high gain regime (gain = 3−5). In order to obtain this high gain, a long interaction length (27 m), a high current (270 A) and strong magnetic fields (aw > 1) are necessary. Consequently some velocity spread effects and possibly even some space charge effects must be considered. We use a one-dimensional, comprehensive small signal gain analytical model to conduct a parametric study of the FEL operating parameters and their scaling laws. The results of the analytic model are compared with results obtained with the 2-D FEL simulation code FRED, and the effect of optical guiding on the gain is estimated using an enhanced filling factor based on the observed guided mode size. We conclude that oscillation below 300 A is feasible with existing mirror technology.

EFFICIENCY ENHANCEMENT OF A PRE-BUNCHED FREE-ELECTRON MASER OSCILLATOR BY LOCKING TO A SINGLE EIGEN FREQUENCY OF THE RESONATOR

We present simulations results and experimental evidence of radiation energy extraction e

Prebunched-beam free electron maser

ciency enhancement by use of pre-bunching in a free electron maser (FEM). This enhancement is attained by locking the oscillator to operate at a distinct resonator eigen frequency (longitudinal mode) which is di!erent from the maximum-gain frequency of the free running oscillator. The mode selection is accomplished by means of pre-bunching at the desired eigen frequency. It was shown that maximum extraction e

High-efficiency nonadiabatic trapping of electrons in the ponderomotive potential wells of laser beats

ciency is obtained if the dominant resonator mode (obtained by pre-bunching at that frequency) is the one having the lowest eigen frequency under the net gain larger than unity curve (which is below the maximum gain eigen-frequency mode that would normally dominate in a free running oscillator). The numerical simulations predict that it should be possible to obtain a basic extraction e

Near-field analysis of the knife-edge technique

ciency enhancement of the oscillator of 50%. Experimental results so far have shown an improvement of about 30%. ( 1999 Elsevier Science B.V. All rights reserved.