Y. M. Yakover

Efficiency enhancement of free electron Maser oscillator by mode selection with a prebunched electron beam

We present a method for enhancing the efficiency of a Free Electron Laser Maser oscillator by locking it to a preferred resonator mode. This is done by prebunching of the e beam before injection into the wiggler. In a free running oscillator, the longitudinal mode that dominates the mode competition process during the oscillation buildup period is usually the highest gain mode. However, this mode does not extract the highest energy from the e beam. Lower eigenfrequency modes would provide a higher efficiency if they could dominate the mode competition process. By prebunching the e beam at a frequency near any one of the longitudinal eigenfrequencies of the resonator (having a gain>1), we can make that mode dominant at saturation. The eigenfrequency for which the maximum efficiency is obtained is the lowest eigenfrequency of the resonator for which the net small signal gain is greater than 1. Employing an experimental setup of a prebunched beam Free Electron Maser, we demonstrated efficiency enhancement of...

Radiation measurements in the new tandem accelerator FEL

The Israeli tandem electrostatic accelerator FEL (EA-FEL), which is based on an electrostatic Van der Graaff accelerator was relocated to Ariel 3 years ago, and has now returned to operation under a new configuration. In the present FEL, the millimeter-wave radiation generated in the resonator is separated from the electron beam by means of a perforated Talbot effect reflector. A quasi-optic delivery system transmits the out-coupled power through a window in the pressurized gas accelerator tank into the measurement room (in the previous configuration, radiation was transmitted through the accelerator tubes with 40 dB attenuation). This makes it possible to transmit useful power out of the accelerator and into the user laboratories. After re-configuring the FEL electron gun and the e-beam transport optics and installing a two stage depressed collector, the e-beam current was raised to 2 A. This recently enabled us to measure both spontaneous and stimulated emissions of radiation in the newly configured FEL for the first time. The radiation at the W-band was measured and characterized. The results match the predictions of our earlier theoretical modeling and calculations.

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.

Simulation of predicted performance and interpretation of radiation measurements on the Israeli tandem free-electron laser

Abstract The experimental results of radiated power and its spectral parameters as measured on the Israeli electrostatic accelerator tandem FEL (EA-FEL) are interpreted and numerically simulated. Two complementary simulation codes, FEL3D and MALT1D show good agreement with experimental results. The FEL3D code (a non-linear three-dimensional ‘amplifier’ code, based on a coupled-mode approach), was employed for calculation of the small-signal gain, extraction efficiency and saturation power. This code (FEL3D) allows one to simulate FEL operation of multiple transverse modes at a single frequency taking into account space-charge effects. Multi-frequency simulation of the mode competition process was carried out using the MALT1D code based on a space–time multi-longitudinal mode simulation of a FEL oscillator.

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.

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

Study of radiation build-up and spectral evolution in the Israeli electrostatic accelerator free-electron laser oscillator

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

Israeli tandem FEL: first-lasing results and future plans

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.

High Spectral Coherence in Long-Pulse and Continuous Free-Electron Laser: Measurements and Theoretical Limitations

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.