Tapered-wiggler Free-electron Laser Research Articles

Tunability of tapered free electron lasers

Free electron laser amplifiers are tunable sources under development as high power, high frequency radiation sources for magnetic fusion applications. High efficiencies can be achieved by varying the wiggler field strength and/or the wiggler period. In addition to the requirement of high efficiency, the free electron laser must be tunable for electron cyclotron heating and current drive applications in magnetic fusion devices. Although the tunability of free electron lasers is well established, the tunability of a tapered free electron laser amplifier has not been studied. In this paper we present an investigation of the tunability of a tapered wiggler free electron laser amplifier operating in the neighborhood of 94 GHz. The configuration of the free electron laser is one in which a sheet electron beam propagates through a rectangular waveguide in the presence of a planar wiggler field with tapered period. We found that the tapered free electron laser amplifier is tunable over a reasonably wide range of frequencies by small adjustments in the energy and current of the electron beam.

High-efficiency extraction of microwave radiation from a tapered-wiggler free-electron laser.

We have substantially increased the output power and extraction efficiency of a free-electron laser operating at 34.6 GHz by tapering the wiggler magnetic field. In the exponential-gain regime, the laser exhibited a measured gain of 34 dB/m. With a 50-kW input signal, the amplifier saturated in 1.3 m with a 180-MW output signal. By using a taper that brought the magnetic field at the end of the wiggler down to 45% of its initial (peak) value, we increased the output signal to 1.0 GW. This corresponds to an extraction efficiency of 34%.

Comparison of DC electric field and tapered wiggler free electron laser efficiency enhancement schemes

A constant parameter wiggler free electron laser using a dc electric field for efficiency enhancement is compared to several tapered wiggler efficiency enhancement schemes. Analytical expressions for the efficiency in the plane wave, infinitesimally small radius electron beam limit are derived and compared. Numerical simulations for a Gaussian radiation field and finite radius electron beam with an output radiation wavelength of 2 μm are presented. For finite radius electron beams, the extraction efficiency using a dc electric field is somewhat greater than or equal to that using proposed tapering schemes. While the dc field offers flexibility in efficient, tunable laser design, the required field strengths for visible radiation are in the megavolt/meter range.

The TRW/Stanford tapered wiggler oscillator

We report the operation of the first tapered wiggler free electron laser oscillator. The laser operated at 1.6 μm output with a peak power of 1.3 MW. With the wiggler taper adjusted to 0, 1 and 2%, extraction efficiencies of 0.4, 1.1 and 1.2%, respectively, were obtained.

Synchrotron-betatron resonances in free-electron lasers

The coupling between betatron and synchrotron oscillations in a tapered wiggler free-electron laser (FEL) is investigated analytically and numerically. The resonance between the oscillations, first investigated by Rosenbluth, is shown to be important only for a very limited range of parameters. In general, the resonance is not a serious detrapping mechanism for high-power FEL amplifiers.

Demonstration of large electron-beam energy extraction by a tapered-wiggler free-electron laser

Electron-beam energy spectral measurements were made on a tapered-wiggler free-electron laser amplifier. A 19-MeV electron beam from a traveling-wave linear accelerator interacted in a tapered wiggler with an intense 10.6-μm CO2 laser beam. The electron spectra show a 4% net energy loss and a 9% peak loss. Measurements of electron energy spectra, extraction efficiency as a function of electron-beam energy, and extraction efficiency as a function of optical power are presented and are consistent with theoretically predicted performance.

High-Efficiency Free-Electron Laser Results

Results obtained with a tapered-wiggler free-electron laser demonstrate the concepts proposed by Morton for enhanced efficiency. They show deceleration of electrons by as much as 7%, and extraction of more than 3% of the total electron-beam energy as laser energy when the laser is operated as an amplifier. The experiment is presently being reconfigured to examine its performance as a laser oscillator.

Mode structure of a tapered-wiggler free-electron laser stable oscillator

The transverse mode structure of a tapered-wiggler free-electron laser (FEL) oscillator operating at full saturated intensity is analyzed numerically. The unique features of the FEL geometry, and particularly the refractive property of the e -beam, which acts like a series of small focusing lenses, tend to produce a steady-state mode structure different from the TEM 00 mode of the cavity. The higher order mode content is especially evident in confocal cavities, since higher order modes constructively interfere on different round trips. Constructive addition of higher-order modes is also expected in practical FEL's operating at substantial power levels using near-concentric cavities. The presence of higher order modes has unusual effects on the intracavity intensity distribution of a confocal cavity, including different spot sizes on the front and rear mirrors and different transverse structure on forward and reverse passes. Nevertheless, the output beam quality is nearly diffraction limited for the low gain, low Fresnel number cases studied. Noticeable higher order mode content is also expected for near-concentric cavities.

Electron spectrum measurements for a tapered-wiggler free-electron laser

An experimental investigation of basic aspects of the tapered-wiggler free-electron laser (FEL) is described. The electron energy distribution resulting from the interaction is measured in a single pass amplifier configuration using a CO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> laser. This distribution is consistent with an energy loss, for some electrons, equal to the change of resonant energy from the entrance to the exit of the wiggler. A net deceleration of 2.5 percent has been achieved, compared to an expected value of 4 percent. The discrepancy stems from an oversize electron beam which results in an imperfect electron-photon overlap and increased effective electron energy spread.

HIGH-EFFICIENCY FREE-ELECTRON LASER RESULTS

Results obtained with a tapered-wiggler free-electron laser show deceleration of electrons by as much as 7%, and extraction of more than 3% of the total electron-beam energy as laser energy.

ELECTRON LASER FACILITY (ELF) AT THE LLNL ETA

A high gain mm wave tapered wiggler Free Electron Laser (FEL) amplifier has been designed and is under construction at Lawrence Livermore National Laboratory. The 5 MeV Experimental Test Accelerator (ETA) will be used as the electron source. The ELF design will be presented, along with predicted performance, at 2, 4 and 8 mm.

STATUS OF THE MSNW TAPERED-WIGGLER FEL EXPERIMENT

This paper describes an experimental investigation of basic aspects of the tapered-wiggler free-electron laser. The effect of the PEL interaction on the electron energy distribution has been measured in a simple single pass amplifier configuration, with electrons and photons making a single pass through the tapered wiggler. A peak electron energy loss of about 9 percent and a net loss of 2.5 percent have been observed.

Gain and efficiency enhancement by a multicomponent wiggler free-electron laser

A multicomponent wiggler scheme is presented as a possible solution to the problems associated with start up and saturation of a tapered wiggler free-electron laser oscillator: low small-signal gain and large shift of the output radiation frequency with input power. Optimization of the scheme is analyzed and it is shown that appropriate wiggler configurations can be found that enhance by an order of magnitude the small-signal gain and eliminate the shift in radiation frequency with input power.

Tapered-wiggler free-electron laser optimization

Recent threshold analysis of free-electron lasers (FEL's) has indicated that the use of tapered wigglers may allow substantial single-pass electron energy extraction. This has led to a current experimental verification program in which the tapered wiggler is a critical element. This paper discusses the wiggler optimization for single-pass systems in the limit of low gain and extraction. The interaction of wiggler magnet and photon beam parameters is examined to identify configurations which produce maximum electron kinetic energy extraction for a given photon power. The analysis includes both permanent and electromagnet wigglers.