The beam dynamics of bright electron beams produced by Photo- Injectors brought up to evidence, during the last few years, a number of new aspects concerning the physics of intense relativistic charged particle beams which are far from thermal equilibrium. The theoretical understanding of these issues has become quite mature only recently, well after the first measurements on such beams were done at several laboratories which are active in the development of Photo-Injectors. This has been due mainly to the fact that the traditional formalism based on rms description of the beam distribution, via the envelope equation, fails to explain the coherent collective plasma motion which dominates the behavior of these beams: in this paper a complete description based on a generalized model will be presented, able to describe both the collective (laminar) regime, which dominates the first part of the injector, and the transition into the thermal regime, which dominates the dynamics into the following booster Linac. The impact of such a model on the performances in the generation of high brightness electron beams is finally discussed.
There is considerable interest in using RF photocathode guns and injectors in a variety of applications for electron sources. Unfortunately, the short operating time of the cathode and the complicated laser needed to drive the cathode limits these devices to the research laboratory. However, recent developments in cathode research indicate the possibility of a robust, long-lived cathode with high sensitivity at visible wavelengths. The combination of coating with a protective layer and operating at an elevated temperature could significantly increase the lifetime of cesium-antimony cathodes. This paper reviews the cathode quantum efficiencies and lifetimes observed in RF guns, discusses the drive laser issues and describes recent advances in both metal and cesium-based cathodes.
The Gun Test Facility (GTF) at the Stanford Linear Accelerator Center (SLAC) was created to develop an appropriate injector for the proposed Linac Coherent Light Source (LCLS) at SLAC. The LCLS design requires the injector to produce a beam with at least 1 nC of charge in a 10 ps or shorter pulse with no greater than 1 (pi) mm-mrad normalized rms emittance. The first photoinjector under study at the GTF is a 1.6 cell S-band symmetrized gun with an emittance compensation solenoid. Emittance measurements, reported here, were made as function of the transverse laser pulse shape and the Gaussian longitudinal laser pulse length. The lowest achieved emittance to data with 1 nC of charge is 5.6 (pi) mm-mrad and was obtained with both a Gaussian longitudinal and transverse pulse shape with 5 ps FWHM and 2.4 mm FWHM respectively. The measurement is in agreement with a PARMELA simulation using measured beam parameters. There are indications that the accelerator settings used in the results presented here were not optimal. Simulations indicate that a normalized emittance meeting the LCLS requirement can be obtained using appropriately shaped transverse and temporal laser/electron beam pulses. Work has begun on producing temporal flat top laser pulses which combined with transverse clipping of the laser is expected to lower the emittance to approximately 1 (pi) mm-mrad for 1 nC beams with optimal accelerator settings.
An overview of particle and photon beam bunch length measurements is presented in the context of free-electron laser (FEL) challenges. Particle-beam peak current is a critical factor in obtaining adequate FEL gain for both oscillators and self-amplified spontaneous emission (SASE) devices. Since measurement of charge is a standard measurement, the bunch length becomes the key issue for ultrashort bunches. Both time-domain and frequency-domain techniques are presented in the context of using electromagnetic radiation over eight orders of magnitude in wavelength. In addition, the measurement of microbunching in a micropulse is addressed.
Ultrashort micropulse characterization of IR-FEL is essential to both FEL development and applications such as terahertz spectroscopy and coherent excitation. Due to wavelength limitation of nonlinear medium, existing techniques of pulse shape measurement are not suitable to the Far IR-FEL of above 100 micron. We present the technique of interferometric cross- correlation in the Fourier space to directly obtain the phase and amplitude information of mid-infrared short optical pulse. By introducing a grating pair as disperser, the interferometric cross-correlation for different spectral components of spectrally expanded pulse is recorded by the 2-D array detector as function of relative delay and spectrum. The relative phases in the pulse spectrum are retrieved from the 2-D interferogram, and the spectrum amplitude is taken from the square root of the power spectrum, the pulse shape can be accurately reconstructed by fast Fourier transform of the complex spectrum field. Without nonlinearity, this technique is very likely to be extended to the Far IR-FEL.
The JAERI superconducting rf linac based FEL has successfully been lased to produce a 0.1 kW or higher light output in quasi continuous wave operation. As our current program goal is the 1 kW or higher-class light output in the average, brief history and current status towards the goal are reported, and discussed here in detail. As our next program goal is the 100 kW or higher-class light output in average, strategical and tactical paths towards the next goal are discussed here as long and short termed ones in the JAERI superconducting rf linac based FEL.
We are reporting on the current design status of the 200 kW average power FEL at a 0.84 micron wavelength for power beaming of satellites. The project includes a cw RF photocathode gun injector, a cw linear accelerator, achromatic beam transport lines and an FEL amplifier. Problems that are specific to our design are considered.
Construction of a single-pass free-electron laser (FEL) based on the self-amplified spontaneous emission (SASE) mode of operation is nearing completion at the Advanced Photon Source (APS) with initial experiments imminent. The APS SASE FEL is a proof-of-principle fourth-generation light source. As of January 1999 the undulator hall, end-station building, necessary transfer lines, electron and optical diagnostics, injectors, and initial undulators have been constructed and, with the exception of the undulators, installed. All preliminary code development and simulations have also been completed. The undulator hall is now ready to accept first beam for characterization of the output radiation. It is the project goal to push towards full FEL saturation, initially in the visible, but ultimately to UV and VUV, wavelengths.
Over the last few years, there has been a growing interest in self-amplified spontaneous emission (SASE) free-electron lasers (FELs) as a means for achieving a fourth-generation light source. In order to correctly and easily simulate the many configurations that have been suggested, such as multi- segmented wigglers and the method of high-gain harmonic generation, we have developed a robust three-dimensional code. The specifics of the code, the comparison to the linear theory as well as future plans will be presented.
The design status and R&D plan of a 1.5 angstrom SASE-FEL at SLAC, called the Linac Coherent Light Source (LCLS), are described. The LCLS utilizes one third of the SLAC linac for the acceleration of electrons to about 15 GeV. The FEL radiation is produced in a long undulator and is directed to an experimental area for its utilization. The LCLS is designed to produce 300 fsec long radiation pulses at the wavelength of 1.5 angstrom with 9 GW peak power. This radiation has much higher brightness and coherence, as well as shorter pulses, than present third generation sources. It is shown that such leap in performance is now within reach, and is made possible by the advances in the physics and technology of photo- injectors, linear accelerators, insertion devices and free- electron lasers.
The concept of Free Electron Lasers (FEL) based on the principal of Self-Amplified Spontaneous Emission (SASE) was established more than a decade ago. The pace of R&D efforts towards using the concept for Fourth Generation Radiation Facilities has been picking up as SASE experiments at optical and infrared wavelengths are being conducted and SASE projects at x-ray wavelengths are under construction or are expecting funding in the foreseeable future. Computer simulation codes are essential tools for a meaningful design of an FEL project. During the 1980s a number of codes had been written, suitable for simulating the SASE process. These codes have been used for the designs of the Linac Coherent Light Source (LCLS) and TESLA Test Facility (TTF) projects as well as for a number of long-wavelengths experiments. Based on experience gained, existing codes are being improved and new codes are under development to study additional aspects of the FEL design. This paper compares SASE FEL codes that are presently available by focusing on aspects such as time dependence, separated function focusing, field errors, undulator module separations, wakefield treatment, computer platforms code availability, post-processors and others.
The electron beam in a scanning electron microscope (SEM) and a diffraction grating placed in the focal regime of the SEM have been used to generate coherent tunable radiation in the FIR region of the spectrum. A brief survey of the basic theory governing the operation of the device and a summary of recent experimental results are presented.
After having obtained spontaneous emission of the CAEP FIR- FEL, we have made some improvements for the electron beam. A new thermionic cathode 1 + 1/2 cavity is used to get better emitting performance than the old one at the same condition, and a new beam transport line is designed for better transport efficiency. We are continuing to perform further experiments. The machine performance and some new results will be presented in this paper.
Sustained/long duration X-ray output has been demonstrated emanating from the monochromatic X-ray beamline of the Vanderbilt Free-Electron Laser. Tunable, pulsed monochromatic X-rays ranging in energy from 14 - 18 keV are produced by inverse Compton scattering created by the counter propagation of the FEL e-beam and its own infrared beam. These beams are focused and optimized at an interaction zone between the linac and the wiggler where they are brought into coalignment. The X-rays produced exit the beamline through a beryllium window and are directed onto mosaic crystals which divert the beam to an imaging laboratory on the floor above the vault. The initial application of these X-rays is directed toward human imaging, specifically for the diagnosis of breast diseases including cancer. The characteristics of the X-rays are such that they can be used in standard geometry monochromatic imaging, CT like images of the breast using a rotating mosaic crystal 'optic,' time-of-flight imaging and phase contrast images. Eventual extension to other portions of the body, cell biology and material sciences are already anticipated.
We propose a method to improve the quality of a hole-coupled resonator for a free-electron laser oscillator. A mesh or grating with a reflectivity of 50 - 70%, which is quite easy to be produced, can be used to cover the hole. Both the extraction and diffraction losses can be reduced effectively and the optical cavity Q value can be improved to obtain higher net gain which allows the FEL oscillator to be operated in worse conditions. These advantages and properties are confirmed by numerical simulations using a three-dimension code with the parameters of a compact far-infrared FEL oscillator built in China Academy of Engineering Physics (CAEP). Furthermore, it can be used as an electron transparent mirror to make the FEL device more compact and less expensive.
Beaming of laser energy through the atmosphere is a means of supplying electrical power to orbiting satellites. By using a new ground based free electron laser developed by Lawrence Berkeley National Laboratory many times the amount of electrical power can be generated by the satellite from the same area of array now used for solar power. There is currently a shortage of power in space, and the demand is rising exponentially. Satellites which appear from earth to remain fixed at one point in the sky are said to be in geosynchronous orbit (GEO). They are used to relay long distance telephone communications, television, e-mail, credit card checks from the local gas pump and a myriad of other applications. In addition to the shortage in power there is also a shortage of available bandwidth. In response to that problem a higher frequency band, the Ka band, is being opened for satellites and fifty companies in the United States are planning to launch satellites which use it. Unfortunately rainfade is a serious problem at this frequency and ten times the usual power is sometimes needed to overcome the effects of rain. Laser power beaming is an answer to this problem. Key elements are a powerful 200 kW free electron laser and a fully compensated 11-meter diameter adaptive optics projection telescope. Remarkable progress has been made on both these ambitious objectives in the last two years.
The results of the first year of a Gas Research Institute funded research program to study laser-rock-fluid interaction will be presented. The overall purpose of this research is to determine the feasibility, costs, benefits, and the environmental impact of using laser technology to drill and complete oil and gas wells. When drilling and completing petroleum wells, many rock types (sandstone, limestone, dolomite, granite, shale, salt, concrete) and fluids (fresh water, salt water, oil, hydrocarbon gas, drilling fluids) must be penetrated by the laser. The Free-Electron Laser (FEL) technology is attractive because of the ability to tune the laser to different wavelengths. Laser energy absorbed by rocks is related to the wavelength of the laser source. The mechanisms of rock destruction (spalling, melting and vaporization) are therefore a function of the wavelength. The ability to transmit laser energy over long distances (up to 5000 m or 15,000 ft) is also a function of wavelength. Results of tests conducted at the U.S. Air Force and the U.S. Army's high power laser facilities are presented. The challenges ahead to advance a fundamental change in the methods currently used to drill and complete petroleum wells are discussed.
Free Electron Lasers have capabilities in several regions of the electromagnetic spectrum that are not found in conventional lasers. Among these capabilities are power and tunability in the short-wavelength region of the ultraviolet and soft x-rays and in the mid- and far-infrared region. By using the intra-cavity inverse, Compton scattering high intensity hard x-rays and gamma rays can be generated. For the first time tunable, bright sources of light at each extreme of the spectrum are available for biomedical science. Application from clinical use of gamma and hard x-rays to surgery and chemical imaging of cellular and sub cellular components are possible.
An intense pico second FEL induces a surface ablation on optical materials, which are used for focusing and sharing of the FEL power in application researches. The FELs at the FELI are available in 40 to approximately 0.2 micrometer wavelength region, and therefore various kinds of optical materials are used for output windows and coupling mirrors such as ZnSe, SiO2 coated with Ta2O5 or HfO2 in the facility. In addition, the FEL macropulse is continued for approximately 20 microseconds of several MW micropulse train. In high power applications the focused FEL power density is over several GW/cm2. When the power density of the FEL is exceeded over the threshold of the optical materials, the luminescence emits from the material surface. We found the ablation threshold of ZnSe and SiO2 due to both 9.2 micrometer FEL and 1.054 micrometer Nd:YLF laser irradiation in pico-second duration. In case of ZnSe material we have observed that the luminescence is synchronized with 22.3 MHz FEL micropulse, and that the ablation thresholds vary of the square root of the laser duration scaling law in 3 - 12 ps duration.
This paper presents the results of simulations for the photoinjector Radio Frequency (RF) cavities of the 1 kW visible FEL currently under construction at the FEL Boeing laboratory in Seattle (WA). The simulations were performed using PARMELA, a particle 'pusher' code developed at Los Alamos National Laboratory, and the pic code ARGUS developed at S.A.I.C. The results indicate that, for the system under consideration, at low charge (less than 5.0 nC) the agreement between the codes is good. The space charge effects are incompletely accounted by PARMELA at charge (greater than 5.0 nC). The results of this work clearly establish the region of applicability of the PARMELA code for photo-injector cavities of the 1 kW visible FEL.