M. Arbel

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.

Free electron maser oscillations near waveguide cutoff

Abstract In waveguide-based FEMs there are two possible frequency radiation bands corresponding to the two intersections of the beam line with the waveguide dispersion curve. The low-frequency intersection point occurring near the waveguide cutoff frequency was studied by us experimentally. Special modifications of the existing TAU FEM facility were made in order to investigate FEM radiation near cutoff. Some rectangular waveguide components, used for the radiation coupling from the resonator to the detection system, were replaced by components based on a double-ridged waveguide having a lower cutoff frequency and a wider frequency band. FEM operation in the vicinity of the waveguide cutoff frequency was studied in free-running oscillator configuration using a unique experimental setup enabling time-dependent spectral measurements within a single radiation pulse. The observed FEM radiation was of good spectral purity without any significant mode competition.

Performance improvement of FEMs by prebunching of the electron beam

Abstract FEM performance enhancement achieved by use of r.f. prebunching of the e-beam in the FEM developed at TAU was investigated theoretically and experimentally. For FEM operation as an oscillator, use of e-beam prebunching enables stable, coherent, high output power throughout the r.f. output pulse at any selected oscillator eigenfrequency for which the net gain is above unity. Prebunching enables faster r.f. output power buildup. An eigenfrequency of maximum efficiency and power output can be selected by e-beam prebunching at or near that eigenfrequency. FEM efficiency is thus, considerably improved. By contrast, FEM operation without prebunching leads to saturation in the highest gain mode, giving a lower efficiency. Frequency “hopping” of the FEM r.f. output between various eigenfrequencies is also attainable via prebunching. FEM operation as a high gain amplifier between the premodulator input and the FEM output is reported for our FEM operating with an e-beam current of only 0.6 A. High FEM gain, broad bandwidth, high power operation possibilities at millimeter waves are also described.

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.

Experimental investigation of mode build-up and mode competition process in a prebunched free-electron maser oscillator

A unique experimental set up at Tel-Aviv University (TAU) enables the study, observation and quantitative measurements of the mode competition process in a Free-Electron Maser (FEM) oscillator. The experimental results provide data on the mode build-up from noise level to steady-state saturation and on mode competition. The experimental results are recorded with the aid of a fast digital oscilloscope and are analyzed to obtain the Fourier components by use of a computer program, which we developed. The Fourier analysis shows clearly the build-up and competition of the longitudinal modes (eigenfrequencies) excited in the oscillator. We studied mode competition for two cases (1) free-running oscillator (no beam prebunching) (2) oscillator with prebunching of the e-beam. We also compared the experimental results with the results of the multi frequency simulation code MALT1D3, and found good agreement. ( 1998 Elsevier Science B.V. All rights reserved.

Visualization and simulation of electron beam transport along a FEL planar wiggler

Abstract We have developed and employed a special apparatus for viewing and investigating the motion and behavior of an e-beam inside a planar wiggler. The total current I(z), the current density J(x, y, z), the beam position and cross-section shape were investigated along an evacuated waveguide cavity located on the axis of the wiggler of the TAU 70 KeV FEM. The beam cross-section image could be viewed on the screen through a window at the end of the evacuated waveguide using a CCD camera equipped with a zoom lens. An exact 3D simulation code (PTIMF) developed in our group for simulation of electron trajectories inside and along the wiggler, was used to calculate trajectories of electrons in the wiggler region under conditions similar to those prevailing in the experiment. The correlation of experimental results with those predicted from simulations was good. It is presented and discussed in this paper.

Enhanced super-radiant emission of FEM near waveguide-cutoff and near zero-slippage conditions

We report on super-radiance obtained from the TAU FEM just above waveguide cutoff and near grazing intersection. Grazing intersection (or ‘‘Zero Slippage’’) is defined as the point at which the two synchronous frequencies merge to one frequency. In this case, the radiated power frequency can be tuned over a very wide band by change of the premodulation frequency. Near the lower synchronous frequency, the super-radiance power is much greater and the spectral width is much narrower than those at the higher synchronous frequency. The super-radiance emission near cutoff (lower synchronous frequency) and near to the upper synchronous frequency was measured and compared to those predicted by an analytical model for a wide range of frequencies. r 2002 Published by Elsevier Science B.V.

Prebunched-beam free electron maser

The development status of a prebunched FEM is described. We are developing a 70 KeV FEM to allow high gain wideband operation and to enable variation of the degree of prebunching. We intend to investigate its operation as an amplifier and as an oscillator. Effects of prebunching, frequency variation, linear and nonlinear effects, will be investigated. The prebuncher consists of a Pierce e-gun followed by a beam modulating section. The prebunched beam is accelerated to 70 KeV and injected into a planar wiggler containing a waveguide. The results obtained to date will be presented. These include: characterization of the e-gun, e-beam transport to and through the wiggler, use of field modifying permanent magnets near the entrance and along the wiggler to obtain good e-beam transport through the wiggler, waveguide selection and characterization.

Space Charge Effects in a Prebunched Beam Free Electron Maser (FEM)

Publisher Summary This chapter describes space charge effects in a prebunched beam free-electron maser (FEM). An electron beam prebunched at a frequency near to the synchronous free-electron laser frequency and passing through a magnetic undulator emits coherent synchrotron undulator radiation at the bunching frequency. In the collective regime, the spectral properties of the emitted super radiant power depend on the electron beam current density. For a low-density electron beam, the monochromatic super-radiant emission power is maximal at a bunching frequency equal to the electron-beam synchronism frequency, independent of the beam current. However, for a dense electron beam, the space charge effects become dominant and synchronism takes place with either the slow or the fast space-charge (Langmuire) wave, which propagate on the electron beam with different phase velocities. Consequently, radiated power maxima can occur at two well-separated frequencies.

Enhancement of FEM radiation by prebunching of the e-beam (stimulated super-radiance)

An electron beam (e-beam) prebunched at the synchronous FEM frequency and traversing through a waveguide, located coaxially with a magnetic undulator, emits coherent radiation at the bunching frequency. Introduction of both a premodulated e-beam and a radio-frequency (r.f.) signal at the same frequency at the input of the waveguide can lead to more efficient interaction, and thus more power can be extracted from the electron beam. In order to achieve this, the density modulation of the electron beam should be at an appropriate phase with respect to the r.f. signal. We report a first experimental demonstration of the influence of the phase difference between the r.f. input signal and the fundamental component of the density modulation of the e-beam on the radiated power in a Free-Electron Maser (FEM). Our experimental system allows control of the current density modulation, of the r.f. input power level, in the undulator region and of the phase between that r.f. input and the modulation of the e-beam. A comparison between measured radiation power with that predicted by theory for various phase differences, current density modulation, and r.f. signal levels, was made. Good correlation was obtained. r 2001 Elsevier Science B.V. All rights reserved.