High Average Power Free-electron Research Articles

Optical design of the time-resolved ARPES beamline of the new material spectroscopy experimental station for the update of CAEP THz-FEL facility

Abstract The Chinese Academy of Engineering Physics Terahertz Free Electron Laser Facility (CAEP THz FEL, CTFEL) is the only high-average power free electron laser terahertz source based on superconducting accelerators in China. The update of the CTFEL is now undergoing and will expand the frequency range from 0.1-4.2 THz to 0.1-125 THz. Two experimental stations for material spectroscopy and biomedicine will be built. A high harmonic generation (HHG) lightsource based beamline at the material spectroscopy experimental station for time-resolved angle-resolved photoemission spectroscopy (ARPES) research will be constructed and the optical design is presented. The HHG lightsource covers the extreme ultraviolet (XUV) photon energy range of 20-50 eV. A Czerny-Turner monochromator with two plane gratings worked in conical diffraction configuration is employed for maintaining the transmission efficiency and preserving the pulse time duration. The calculated beamline transmission efficiency is better than 5% in the whole photon energy range. To our knowledge, this is the first time in China to combine THz-infrared FEL with HHG light source, and this experimental station will be a powerful and effective instrument and give new research opportunities in the future for users doing research on dynamic evolution of excited electron band structure of material surface.

Design Considerations for Compact High-Average Power Free-Electron Lasers/Masers at Terahertz Frequencies

Interest is increasing in high-power terahertz (THz) sources of radiation. The terminology is fluid, but researchers in the field typically refer to frequencies ranging from about 300 GHz to 10 THz as THz radiation. In this article, we present a description of design considerations for a compact, high-average power free-electron laser. At present, THz radiation is generated by a variety of mechanisms, including laser-based sources and electron-beam-based sources. We provide a short description of current THz source technology to give background against which to compare the present concept; however, this should not be considered a comprehensive discussion of such technologies.

Design study of an improved laser wire system for electron beam measurement

The laser wire (LW) method has been demonstrated to be an effective non-interceptive technique for measuring transverse profile and emittance of electron beams in colliders, storage rings and dumping rings. In this paper, we present an improved design of high repetition LW system for high average power free electron lasers (HAP FELs) and energy recovery linacs (ERLs). This improved LW utilizes the excess power of the photocathode drive laser, thus making itself much cheaper and simpler. The system main parameters are optimized with numerical calculations and Monte Carlo simulations, indicating that resolutions would be better than 100 μm and scanning time less than 1 minute. Status of the experiment preparation is also presented.

High average power free-electron lasers

Ever since the first free-electron laser (FEL) was both con- ceived and demonstrated in the 1970s at Stanford, it has attracted much attention as a promising foundation for a high average power light source. Since its light is produced from free electrons, an FEL can be designed to lase at virtually any wavelength. Furthermore, since its light is generated from a relativistic beam that is continually replenished, its gain medium cannot be damaged by any conventional means. Accord- ingly, as there have been several ambitious attempts to design and construct high average power FELs over the past several decades, we chronicle some of these efforts. Additionally, we provide an overview of FEL technology and theory, discuss some of the obstacles that remain in the construction of a high-power FEL, and provide a road map toward a megawatt-class device. © 2012 Society of Photo-Optical Instrumentation Engineers

Wall-Plug Efficiency and Beam Dynamics in Free-Electron Lasers Using Energy Recovery Linacs

In a high average power free-electron laser (FEL) the wall-plug efficiency is of critical importance in determining the size, complexity, and cost of the overall system. The wall-plug efficiency for the FEL oscillator and amplifier (uniform and tapered wiggler) is strongly dependent on the energy recovery process. A theoretical model for electron beam dynamics in the energy recovery linac is derived and applied to the acceleration and deceleration of nano-Coulomb electron bunches for a tapered FEL amplifier. For the tapered amplifier, the spent electron beam exiting the wiggler consists of trapped and untrapped electrons. Decelerating these two populations using different phases of the radio-frequency wave in the recovery process enhances wall-plug efficiency. For the parameters considered here, the wall-plug efficiency for the tapered amplifier can be ~10% using this approach.

A CW normal-conductive RF gun for free electron laser and energy recovery linac applications

Currently proposed energy recovery linac and high average power free electron laser projects require electron beam sources that can generate up to ∼ 1 nC bunch charges with less than 1 mm mrad normalized emittance at high repetition rates (greater than ∼ 1 MHz ). Proposed sources are based around either high voltage DC or microwave RF guns, each with its particular set of technological limits and system complications. We propose an approach for a gun fully based on mature RF and mechanical technology that greatly diminishes many of such complications. The concepts for such a source as well as the present RF and mechanical design are described. Simulations that demonstrate the beam quality preservation and transport capability of an injector scheme based on such a gun are also presented.

Simulation of a high-average power free-electron laser oscillator

In this paper, we compare the 10 kW-Upgrade experiment at the Thomas Jefferson National Accelerator Facility in Newport News, VA, with numerical simulations using the MEDUSA code. MEDUSA is a three-dimensional FEL simulation code that is capable of treating both amplifiers and oscillators in both the steady-state and time-dependent regimes. MEDUSA employs a Gaussian modal expansion, and treats oscillators by decomposing the modal representation at the exit of the wiggler into the vacuum Gaussian modes of the resonator and then analytically determining the propagation of these vacuum resonator modes through the resonator back to the entrance of the wiggler in synchronism with the next electron bunch. The bunch length in the experiment is of the order of 380--420 fsec FWHM. The experiment operates at a wavelength of about 1.6 microns and the wiggler is 30 periods in length; hence, the slippage time is about 160 fsec. Because of this, slippage is important, and must be included in the simulation. The observed single pass gain is 65%--75% and, given the experimental uncertainties, this is in good agreement with the simulation. Multipass simulations including the cavity detuning yield an output power of 12.4 kW, which is also in good agreement with the experiment.

Review of various approaches to address high currents in SRF electron linacs

The combination of high-brightness electron sources and high-current SRF Energy Recovery Linacs (ERL) leads to a new emerging technology: high-power, high-brightness electron beams. This technology enables extremely high average power free-electron lasers, a new generation of extreme brightness light sources, electron coolers of high-energy hadron storage rings, polarized electron–hadron colliders of very high luminosity, compact Thomson scattering X-ray sources, terahertz radiation generators and much more. What is typical for many of these applications is the need for very high current, defined here as over 100 mA average current, and high brightness, which is charge dependent, but needs to be in the emittance range of between submicron up to perhaps 50 μm, usually the lower – the better. Suffice it to say that while there are a number of projects aiming at this level of performance, none is anywhere near it. This work will review the problems associated with the achievement of such performance and the various approaches taken in a number of laboratories around the world to address the issues.

Pulsed laser deposition with a high average power free electron laser: Benefits of subpicosecond pulses with high repetition rate

We have conducted experiments exploring pulsed laser deposition of thin films using the high average power Thomas Jefferson National Accelerator Facility Free Electron Laser. The combination of parameters of this laser, including subpicosecond pulses, high average power, high repetition rate, and tunability, makes it a unique tool for the study of the effects of laser characteristics on thin-film quality. When compared to ablation and deposition with an ultrafast, high energy per pulse, low repetition rate laser (amplified Ti:sapphire), we find that the lower energy per pulse with high repetition rate of the free electron laser leads to very different plasma emission and produces films with high quality with the potential of very high deposition rates. This is demonstrated in the optical spectroscopy of plasma emission from Ti and the growth of Ni80Fe20 thin films.

Research on DC-RF superconducting photocathode injector for high average power FELs

To obtain high average current electron beams for a high average power Free Electron Laser (FEL), a DC-RF superconducting injector is designed. It consists of a DC extraction gap, a 1+ 1 2 superconducting cavity and a coaxial input system. The DC gap, which takes the form of a Pierce configuration, is connected to the 1+ 1 2 superconducting cavity. The photocathode is attached to the negative electrode of the DC gap. The anode forms the bottom of the 1 2 cavity. Simulations are made to model the beam dynamics of the electron beams extracted by the DC gap and accelerated by the superconducting cavity. High quality electron beams with emittance lower than 3 π-mm-mrad can be obtained. The optimization of experiments with the DC gap, as well as the design of experiments with the coaxial coupler have all been completed. An optimized 1+ 1 2 superconducting cavity is in the process of being studied and manufactured.

Design of energy-recovery transport for the JAERI FEL driven by a superconducting linac

A high-average power free-electron laser driven by a superconducting linac has been developed in Japan Atomic Energy Research Institute (JAERI), and stable laser output over 0.1 kW in infrared region is now available. For further increasing of FEL output power, installing energy-recovery transport has been planned. The lattice design for the energy-recovery transport is discussed in the present paper. It is found that a recirculation transport, which fulfills the requirements for energy acceptance and isochronicity, can be realized by adding another triple-bend arc to the existing beam line.

Current status of the superconducting RF linac driver for the JAERI free electron laser

Abstract A prototype for a quasi-cw, and 1 kW high-average power free electron laser (FEL) driven by a 15 MeV superconducting rf linac has been developed, and constructed at Tokai, JAERI. Cryogenic stand-by loss ( 7 MV/m and Q ∼ 2 × 10+9) of four JAERI superconducting accelerator modules have been successfully demonstrated. Beam test and commissioning of the JAERI superconducting rf linac have been successfully performed to get electron beams ranging from 10 to 23 MeV and the fully transmitted of ∼ 16 A peak current around 17 MeV. The measured energy resolution of a pre-accelerated beam is about 3% of FWHM, and that of a fully-accelerated beam about 0.8% or less. The measured micropulse width is adjustable in the range from 10 to 60 ps. FEL opticals and beam transport elements around the undulator are now under commissioning. Spontaneous emissions in the wavelength of 20 to 80 μm or longer have been observed by using the Ge(Cu) and other detectors, a home-made fast current amplifier, and a 24 m long light transporting system.

JAERI quasi-cw, and high-average power free electron laser driven by a superconducting rf linac

We have developed and constructed a prototype for a quasi-cw, and high-average power free electron laser driven by a superconducting rf linac at the Japan Atomic Energy Research Institute (JAERI), Tokai. We have successfully demonstrated expected performances of the JAERI 250 keV electron gun and bunching system, and completed an extension to an old 5.5 MV electrostatic accelerator building as an FEL accelerator vault in the 1991 Japanese fiscal year. All solid state rf amplifiers, liquid He refrigerators, a hybrid undulator and vacuum pumping stations are ready for use now. The first superconducting accelerator module is now under preparation for a cold rf test, and the other three modules are now under assembly. A description and the latest results of the JAERI superconducting rf linac FEL will be discussed, and reported in detail.

The high average power free electron laser program

Abstract The potential advantages of the FEL for military applications (tunability, short-wavelength operation, efficiency, and high power) have been the goals of the Joint U.S. Army/SDIO, and Defense Research Project Agency (DARPA) programs since 1976. From the outset of the program, it was clear that funding constraints would mandate the development and demonstration of these attributes in separate but closely coupled subscale technology efforts prior to integration in a full scale weapons prototype. These efforts have so far successfully demonstrated the tunability, short wavelength generation, and high extraction efficiency of FELs. Among the most important attributes and technologies that have been developed to date are validated design codes, high quality electron beams for short wavelength operation, ring resonators, tapered wigglers, optical gain guiding, cryogenic accelerator components, energy recovery, and critical high average power components such as CW433 MHz klystrons, photoinjectors, and accelerating cavities. These technologies will be integrated into the 10 μm. 100 kW average power laser experiment (APLE), which will be built at the Boeing facility in Seattle, and which can be upgraded to operate at higher powers and shorter wavelengths. The accomplishments of the program will be described, and the paths to a high power, cryogenic FEL will be outlined.

Progress in accelerator technology for strategic defense

Progress in accelerator technology has made possible many applications in strategic defense. The feasibility of developing high brightness neutral particle beams has been demonstrated with a low energy pulsed test stand, and through individual component development. Free electron lasers (FELs) employing high peak power electron beams from induction linear accelerators are now a promising approach to ground based lasers. Advances in radio-frequency linear electron accelerators suggest applications for high average power free electron lasers for space or ground basing. The invention of laser guided electron beams using ion focusing has led to new recirculating accelerator concepts, beam transport with high efficiency, and novel applications for intense beams combining the best features of lasers and particle beams.