The cavity optics within high power free-electron lasers based on energy-recovering accelerators are subjected to
extreme conditions associated with illumination from a broad spectrum of radiation, often at high irradiances. This is
especially true for the output coupler, where absorption of radiation by both the mirror substrate and coating places
significant design restrictions to properly manage heat load and prevent mirror distortion. Besides the fundamental lasing
wavelength, the mirrors are irradiated with light at harmonics of the fundamental, THz radiation generated by the
bending magnets downstream of the wiggler, and x-rays produced when the electron beam strikes accelerator diagnostic
components (e.g., wire scanners and view screens) or from inadvertent beam loss. The optics must reside within high
vacuum at ~ 10<sup>-8</sup> Torr and this requirement introduces its own set of complications. This talk discusses the performance
of numerous high reflector and output coupler optics assemblies and provides a detailed list of lessons learned gleaned
from years of experience operating the Upgrade IR FEL, a 10 kW-class, sub-ps laser with output wavelength from 1 to 6
The IR Upgrade FEL at the Thomas Jefferson National Accelerator Facility (JLab) was used to make measurements of the absorption in laser cavity mirrors, both high reflectors and outcouplers. Measurements were made at 10, 6, and 3 um, by determining the temperature rise of the cooling water of the FEL cavity mirrors while operating at high average power, and by using a laser vacuum calorimeter and interpreting the data using the ISO 11551 standard.
Lasing at high power in FELs has been achieved so far only with a near-concentric resonator . Though this design can scale up to quite high power, it is ultimately limited by the mirror steering stability as the resonator design approaches con-centricity. This constraint may be avoided by using a near-confocal resonator operated in a ring configuration. It is found that, if a small amount of gain focusing is present, the near-confocal resonator eigenmodes are modified such that the lowest order mode collapses around the electron beam and is large in the return (non-focusing) direction. This eigenmode is stable and is relatively insensitive to changes in the mirror radii of curvature and the strength of the electron beam focusing. This paper will present the theory of this new concept.
Mirror heating in a high power FEL can alter the optical mode and affect the gain of the laser. This can lead to a large reduction of the laser power from ideal values. Measurements of the power and mode size in the Jefferson Lab IR Demo laser have shown clear evidence of mirror distortion at high average power loading. The measurements and comparisons with modeling will be presented. Both steady state and transient analyses and measurements are considered.
A versatile free electron laser (FEL) user facility has recently come on line at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) providing high average (kilowatt-level) power laser light in the infrared. A planned upgrade of the FEL in this facility will extend the wavelength range through the visible to the deep UV and provide the photobiology community with a unique light source for a variety of studies. Planned and potential applications of this FEL include: IR studies of energy flow in biomolecules, IR and visible imaging of biomedical systems, IR and visible studies of photodynamic effects and UV and near visible studies of DNA photodamage.
The performance of laser pulses in the sub-picosecond range for materials processing is substantially enhanced over similar fluences delivered in longer pulses. Recent advances in the development of solid state lasers have progressed significantly toward the higher average powers potentially useful for many applications. Nonetheless, prospects remain distant for multi-kilowatt sub-picosecond solid state systems such as would be required for industrial scale surface processing of metals and polymers. We present operation results from the world's first kilowatt scale ultra-fast materials processing laser. A Free Electron Laser (FEL) called the IR Demo is operational as a User Facility at Thomas Jefferson National Accelerator Facility in Newport News, Virginia, USA. In its initial operation at high average power it is capable of wavelengths in the 2 to 6 micron range and can produce approximately 0.7 ps pulses in a continuous train at approximately 75 MHz. This pulse length has been shown to be nearly optimal for deposition of energy in materials at the surface. Upgrades in the near future will extend operation beyond 10 kW CW average power in the near IR and kilowatt levels of power at wavelengths from 0.3 to 60 microns. This paper will cover the design and performance of this groundbreaking laser and operational aspects of the User Facility.
Jefferson Lab's IR Demo FEL Facility includes an associated 600 m<SUP>2</SUP> user facility containing six separate laboratory areas. In the summer of 1999 we began delivery of beam int two of these labs as part of our commissioning of the FEL optical transport and laser safety systems. The high average power capability in the mid-IR, along with an ultrafast high PRF temporal structure makes this laser a unique source for both applied and basic research. While commissioning, we conducted several test, primarily of laser-materials interactions that take advantage of the unique characteristics of this FEL. An overview of the FEL facility and its current performance, along with a synopsis of current and future experiments, will be presented.
The Thomas Jefferson National Accelerator Facility (formerly known as CEBAF) has embarked on the construction of a 1 kW free-electron laser operating initially at 3 microns that is designed for laser-material interaction experiments and to explore the feasibility of scaling the system in power and wavelength for industrial and Navy defense applications. The accelerator system for this IR demo includes a 10 MeV photocathode-based injector, a 32 MeV CEBAF-style superconducting radio-frequency linac, and single-pass transport which accelerates the beam from injector to wiggler, followed by energy-recovery deceleration to a dump. The electron and optical beam time structure in the design consists of a train of picosecond pulses at 37.425 MHz pulse repetition rate. The initial optical configuration is a conventional near-concentric resonator with transmissive outcoupling. Future upgrades of the system will increase the power and shorten the operating wavelength, and utilize a more advanced resonator system capable of scaling to high powers. The optical system of the laser has been modeled using the GLAD<SUP>R</SUP> code by using a Beer's-law region to mimic the FEL interaction. Effects such as mirror heating have been calculated and compared with analytical treatments. The magnitude of the distortion for several materials and wavelengths has been estimated. The advantages as well as the limitations of this approach are discussed.
The Laser Processing Consortium, a collaboration of industries, universities, and the Continuous Electron Accelerator Facility in Newport News, Virginia, has proposed building a demonstration industrial processing laser for surface treatment and micro-machining The laser is a free-electron laser with average power output exceeding 1 kW in the ultraviolet. The design calls for a novel driver accelerator that recovers most of the energy of the exhaust electron beam to produce laser light with good wall-lug efficiency. The laser and accelerator design use technologies that are scalable to much higher power. We will describe the critical design issues in the laser such as the stability, power handling, and losses of the optical resonator, and the quality, power, and reliability of the electron beam. We will also describe the calculated laser performance. Finally progress to date on accelerator development and resonator modeling will be reported.
The Continuous Electron Beam Accelerator Facility (CEBAF) is a prime example of the progress that SRF technology has made. CEBAF was designed to operate at 1500 MHz due to the needs of nuclear physics users and the relative maturity of the Cornell cavity design on which it is based. CEBAF will be operational in 1994 and is designed for 4 GeV of energy at 200 microamps of CW average current. The specified emittance (< 1 nm at 1 GeV) and energy spread (< 10<SUP>-4</SUP>) are extremely tight although the design peak current is too low to give significant gain in an FEL. The CEBAF cavities have a specified gradient of 5 MeV/m but delivered gradients have been significantly higher, recently exceeding an average of 8.5 MeV/m for approximately 100 production cavities. The CEBAF injector has been operated at energies up to 85 MeV and has met all designs specifications in over 3000 hours of around-the-clock operation. Two FELs utilizing a high charge injector have been designed. The CW nature of the beam results in high average powers (order 1 kW) from the IR FEL in the region from 4 to 20 microns and from the UV FEL in the 0.15 to 0.25 micron region. Tunable radiation at this power level places extreme demands on the optical systems. The approaches developed to resolve the optics issues will be examined and progress in implementation and testing is presented.
A design for a hole outcoupling experiment for the Mark III FEL at Duke University is presented. The theoretical results of a matrix formulation, comprised of a gain matrix describing the FEL gain interaction and a loss matrix describing the mode mixing and losses at the cavity end mirrors, are applied to determine useful hole sizes and to predict the optical outcoupling percentage, the optical intracavity mode structure, and the diffractive losses.