Lasing in a wide 2400 Å - 6900 Å spectral range (from visible to ultraviolet) was reached in an optical klystron OK-4 installed on the VEPP-3 storage ring, OK-4 is a first FEL operating in UV.

Following the first storage ring Free Electron Laser (FEL) acheived between 1983 and 1986 in the visible on ACO, new experiments are under developpment on the new ring Super-ACO for extending the radiation from the visible to the UV and VUV ranges. For FEL experiments, Super-ACO is operated at 600MeV, with two stabilized opposite bunches, separated by 120ns. The permanent magnet optical klystron consists in two undulators of 10 periods of 13cm each, and a 0.5m long dispersive section in the middle, with a maximum deflection parameter K value of 5.75. The 18m long optical cavity and the mirrors were set-up at the end of January. A specific "low emittance" optics provides 2% of gain at 600MeV. The laser oscillation was acheived on Super-ACO in February at 633nm with 2Å of bandwidth. Tunability results from the undulators gap change. The FEL remains down to 3mA per bunch. Except from the microtemporal structure due to the storage Ring, the macrotemporal structure seems to be continuous, showing the high stability of the positron beam.

The first part of this paper recalls the experiments of "Coherent Harmonic Generation" that have been performed with the "Optical Klystron" installed on the ACO storage ring at Orsay. Coherent radiation in the VUV is produced, inside the Optical Klystron, by the particle beam when it is "bunched" by an incident laser field. In the second part, we present the sytem that is operating on the new positron storage ring SuperACO. It is constitued by a mode-lock Q-switch doubled Nd-Yag laser (200MW/532nm at 10Hz) focused in the optical klystron, and by a measurement device (ultra-vacuum monochromator). The tunability of the wavelength is also planned by using a dye laser. The first experiments will start in spring 1989.

The CLIO collaboration (Collaboration pour un Laser Infrarouge a Orsay) started in 1986, in order to build an infrared laser at LURE (Orsay). It includes the design and the construction of a new FEL dedicated RF linear accelerator, to make a tunable laser in a broad wavelength range (typically 2- 20 μm) with a peak power in the MW range and an average power of a few tens of watts. In order to obtain enough gain at low wavelength (λ≈2 μm) together with a good energy extraction at long wavelength, a two component undulator has been designed and built. It is made of two identical 1=0.96 m long undulators with a computer controlled and automatically adjustable magnetic gap, the second undulator being possibly tapered. The undulator period was chosen as λ0=4 cm. Preliminary calculations indicated that the regime in which a half length constant gap undulator is followed by a tapered one provides the best energy extraction. To adjust easily the FEL wavelength, the non hybrid technology was chosen. With a judicious choice of a few electron energies the spectral region from 2 to 20 μm can be spanned by varying the undulator gap between 10 and 30 mm.

A program to construct a test superconducting electron linear accelerator (LISA) is in progress at Frascati INFN National Laboratories. The electron beam will be used to realize a FEL in the infrared region 11 - 18 μm in collaboration with a group of ENEA-CRE Frascati. The characteristics of the electron beam, the undulator and the expected performances of the FEL are described. Some future developments - energy doubling, energy recovery, third harmonic operation - are briefly presented. Such improvements aim to realize a high power, high efficiency FEL exploiting the features of a SC linac.

Improvements in the operational characteristics of the Los Alamos free-electron laser have produced experimental results in reasonable agreement with theory and simulation. These experiments include suppressing sidebands by dispersive elements and cavity-length detuning, achieving higher extraction efficiencies, demonstrating deceleration of electrons in the bucket of a tapered wiggler, using a prebuncher at high power, lasing with a highly tapered wiggler, determining harmonic content, and observing harmonic lasing. The results of these experiments will be described.

To reach high-peak powers and high efficiencies with a Free Electron Laser (FEL), the following components have been designed and will be described : i) a photo-injector using an RF gun cavity at 144 MHz to produce electron bunches, ii) a 433 MHz 3-cell cavity powered by a new 6 MW peak power and 200 μs pulse duration klystron, iii) a tapered hybrid wiggler using permanent magnets and pulsed coils for online fine tuning . By integrating these three main components (photo-injector, accelerating cavities, tapered wiggler coupled to an optical cavity), an experiment will be performed in the 20 μm range. Simulation codes have also been developed to follow electron bunches from the cathode down through the injector, the accelerator and the transport system. The physical and technical parameters of the wiggler have been optimized as well to maintain a strong interaction strength between electrons and photons.

Outline is presented of activities on FEL in Japan. In the Compton regime, two projects with storage rings are going on. At ETL, studies on three major elements (accelerator, FEL device, optical cavity) at about 585nm are carried out with a TOK and a 220-MeV beam. At UVSOR of Inst. for Molecul. Sci., studies are being made at 488nm with a 500-MeV beam. Gain measurements with external lasers are being tried at both laboratories. A spontaneous emis-sion has been observed with an RF linac under a collaboration of The Univ. of Tokyo and Jpn. Atom. Energy Res. Inst. (JAERI). In the Raman regime, two programs are in progress. Studies with an induction linac, and an electromagnetic wiggler using a CO2 laser are going on at ILE of Osaka Univ. At The Inst. of Phys. and Chem. Res., a cold REB has been developed, and FEL experiments are being carried out with it. Many other FEL projects are proposed. A 25-MeV superconducting linac for FEL is under construction at JAERI. A 35-MeV double-sided microtron for FEL study is near completion at Nihon Univ. Two RF linacs of The Univ. of Tokyo and The ISIR of Osaka Univ. are being modified for FEL experiments. Con-struction of an RF linac is proposed at Tokyo Inst. of Tech. An X-band single-stage FEL is under construction at Nat. Lab. for High Energy Phys. A circular FEL is in preparation at The Inst. of Space and Astronaut. Sci. An undulator has been constructed at Mitsubishi Electric Co., and the magnetic field has been measured. A Smith-Purcell type FEL is studied at Tohoku Univ. A high-gain FEL at a by-pass section of a ring being proposed at Kyushu Univ. is in the stage of design work.

ELFA (Electron Laser Facility for Acceleration) has both a fundamental and a technological novel goal: i) the fundamental goal is to test with short bunches the existence of three different high-gain regimes at the heart of FEL physics: the already observed steady-state regime and the two novel regimes of cooperative synchrotron radiation, i.e., weak and strong superradiance. ii) the technological goal consists in exploring the possibilities of matching the advanced technologies of high-gain FEL's and of superconductive acceleration. The experimental apparatus - photocathode injector, accelerating structure with 352 MHz superconducting cavities, wiggler - is discussed with emphasis on the problems related to the very high current, I ≈ 400 A. Alternative normal-conducting acceleration schemes at 1.3 GHz and 425 MHz are also discussed. The applications of ELFA should range from high-gradient particle acceleration to plasma heating and condensed matter physics.

A near-infrared Free-Electron-Laser is under construction at the newly erected superconducting 130 MeV electron accelerator. The first part of the accelerator went into operation last summer at the Institute of Nuclear Physics of the Technische Hochschule Darmstadt. Utilizing the electron beam tunable between 35 and 50 MeV and a hybrid undulator with K=1 and λ = 3.2 cm it becomes possible to build a cw-laser which will emit in a wavelength region from 5.24 to 2.57 μm. Presently the room temperature injection of the linac is converted into a high current injection, since peak currents of 2.7 A are required for the FEL experiment. A micropulse repetition rate of 10 MHz (300th subharmonic of the accelerator frequency) that matches the length of the optical cavity (15 m), together with the low emittance (en = 2 π mm mrad) and the low energy spread (ΔE ≤ 13 keV) required in the injector allows to transport a charge of about 10 pC per microbunch which is sufficient for the planned FEL experiment. As an undulator a hybrid system consisting of 80 periods of permanent magnet material and tunable poles of high permeable material is chosen. Simulations predicting the performance of the FEL exhibit a peak output power of 300 to 500 kW in the wavelength region considered. The scientific program deals with the development of the FEL, its components and a possible future use of the FEL radiation, which will be unique with respect to the features region of wavelength, tunability, pulse time structure and cw-mode.

A free-electron laser (FEL) user facility is being constructed at the National Institute of Standards and Technology (NIST) in collaboration with the Naval Research Laboratory. The FEL, which will be operated as an oscillator, will be driven by the electron beam of the racetrack microtron (RTM) that is nearing completion. Variation of the electron kinetic energy from 17 MeV to 185 MeV will permit the FEL wavelength to be tuned from 200 nm to 10 pm. Performance will be enhanced by the high brightness, low energy spread, and continuous-pulse nature of the RTM electron beam. We are designing a new injector to increase the peak current of the RTM. A 3.6-m undulator is under construction, and the 9-m optical cavity is under design. The FEL will emit a continuous train of 3-ps pulses at 66 MHz with an average power of 10-200 W, depending on the wavelength, and a peak power of up to several hundred kW. An experimental area is being prepared with up to five stations for research using the FEL beam. Initial operation is scheduled for 1991.

PALADIN is a single pass, free electron laser amplifier located at the Lawrence Livermore National Laboratory. This FEL is designed to run at 10.6 μm. The 1-kA, 45-MeV electron beam is provided by the Advanced Test Accelerator. The wiggler is 25 m long with an 8 cm period. The input optical signal to the amplifier is provided by a conventional CO2 laser, which can produce a peak input power of either 18 kW or 3.6 MW. We have demonstrated 31 dB of gain with the 18-kW input and 12.9 dB of gain for the 3.6-MW input, producing over 70 MW of optical power. Using the 18-kW input, the gain saturated at about 12 m into the wiggler; with the 3.6-MW input, the gain saturated at about 8 m. Modeling results are shown.

Problems relevant to a continuous wave Free Electron Laser (FEL) in the centimeter - millimeter region are investigated. The ideas are applied to the FEL experiment in progress at the Legnaro (Padova) INFN laboratory. The accelerator characteristics and laser parameters are briefly discussed. The laser could sweep the centimeter - millimeter region until 2.5 mm with a power around 15 kW.

An experimental and theoretical research program on the ubitron/FEL is in progress at the Naval Research Laboratory. The theoretical program includes the nonlinear analysis of two specific configurations: a helical wiggler/axial guide field in combination with a cylindrical waveguide, and a planar wiggler in combination with a rectangular waveguide. The analysis includes the description of the injection of the beam into the wiggler, wiggler inhomogenieties, efficiency enhancement by means of tapered wigglers, beam thermal effects, and harmonic interactions. The planar wiggler model includes parabolic pole faces for enhanced focussing of the beam. Operation of a 13-18 GHz ubitron/FEL with a helical wiggler/axial guide field configuration is reported. The experiment employs a 190-250 keV electron beam with curents up to 100 A. Small signal gains as high as 17-19 dB have been observed, and are in reasonable agreement with the simulations. The initial design work on a third harmonic experiment has begun based upon a planar wiggler/rectangular waveguide configuration. Because the effect of a tapered wiggler has been shown to reduce the influence of beam thermal spread, issues relevant to the design of a tapered wiggler harmonic experiment are discussed.