Storage Ring Free Electron Laser Research Articles

The OK-5/Duke storage ring VUV FEL with variable polarization

The OK-5/Duke storage ring free electron laser (FEL) project was started in 1998. Presently, the components of the OK-5 FEL and the new South straight section are in the final stage of manufacturing. This paper describes the design and the main features of the OK-5/Duke storage ring FEL. The basic concepts and main compromises made in the design process are presented. Plans for the OK-5 FEL commissioning are discussed.

Time structure of the OK-4/Duke storage ring FEL

In this paper, we present results of experimental and theoretical studies of macro- and micro-temporal of dynamics of the OK-4/Duke storage ring FEL (SR FEL) and electron beams. The experimental part of these studies utilized the Hamamatsu C5680 dual-sweep streak-camera with 1.2 ps resolution. We use both numerical and analytical tools for theoretical analysis of the FEL and e-beam distributions without any pre-imposed limitations. Our experimental results are in good agreement with the theoretical predictions.

Helical wigglers for the OK-5 storage ring VUV FEL at Duke

In this paper, we present the design and parameters of electromagnetic wigglers with controllable polarization for the VUV OK-5/Duke storage ring FEL. The OK-5 FEL, the first distributed optical klystron, is comprised of four wigglers and three matching sections with individually controlled quadrupoles and bunchers. The geometry and the relative strength of horizontal and vertical fields determine the polarization of the radiation from the OK-5 wigglers. We compare the predicted and measured quality of the wiggler fields.

Operation of the OK-4/Duke storage ring FEL below 200 nm

For a number of years the wavelength of 200 nm was a psychological barrier for FEL oscillators. The progress towards short wavelength was marginal since the OK-4/VEPP-3 storage ring FEL lased at 240 nm in 1988. After 10 years, in 1998, the OK-4/Duke FEL and the NIJI-IV FEL group moved the limit to 217 and 212 nm, respectively. Improvements of the OK-4/Duke storage ring FEL gain above 10% and the use of custom manufactured mirror coatings brought the success in August 1999. The OK-4 FEL lased in the range from 193.7 to 209.8 nm using electron energies from 500 to 800 MeV. In this paper, we present the description of the OK-4/Duke FEL up-grades and the lasing results below 200 nm obtained in August and October of 1999.

Towards the Fourier limit on the super-ACO Storage Ring FEL

Systematic studies on the Free Electron Laser (FEL) line and micropulse have been performed on the Super-ACO storage ring FEL with a monochromator and a double-sweep streak camera under various conditions of operation (detuning, “CW” and Q-switched mode). From these data, it appears that the FEL is usually operated very close to the Fourier limit.

Performance of the OK-4/Duke storage ring FEL

In this paper, we report measured parameters of the OK-4 FEL driven by the Duke storage ring. The OK-4 FEL was being operated continuously for 2 yr in the broad wavelength range for user applications utilising spontaneous and coherent XUV and UV radiation as well as Compton back-scattered γ-rays in the range of 2–58 MeV. During this time, the OK-4 FEL lased in the range from 193.7 to about 730 nm using five sets of mirrors and electron beam energies from 240 to 800 MeV. Our predictions for the OK-4 FEL are compared with measured performance, both in the CW and in the giant pulse mode. We discuss our future plans for the OK-4 FEL operation as well as the construction and commissioning of the OK-5 FEL with helical wigglers.

Local energy exchange in a storage-ring free-electron laser

In this paper a theoretical model is presented, which is based on a pass to pass analysis of the localized interaction between a short laser pulse with a wider electron distribution. It can be applied to a large class of physical phenomena and, in particular, to the case of a storage-ring free-electron laser (FEL). Numerical results are confirmed by experimental measurements done on the ACO and Super-ACO FELs.

The European UV/VUV storage ring FEL at ELETTRA: first operation and future prospects

A European project to develop the first storage ring free-electron laser on a third-generation synchrotron radiation facility is presented, including a description of the main features, initial performance at 350 and 220 nm and future prospects.

Gamma rays produced by inverse Compton scattering in the Super-ACO storage ring free electron laser

The inverse Compton scattering between positron bunches and the free electron laser (FEL) of the Super-ACO storage ring generates a collimated and tunable gamma-ray beam. The use of the FEL instead of an external conventional laser, automatically provides the transverse alignment and synchronisation of the electron and optical beams. The Super-ACO FEL is operated in the 300– 430 nm range at 800 MeV with a high repetition rate (8 MHz) and an average transmitted power of 300 mW at 350 nm . High energy gamma-rays of 35 MeV are produced with a rate of 1.5×10 7 photons/ s and good monochromaticity (about 10%).

The super-ACO free electron laser source in the UV and its applications

Free electron lasers (FELs) are new sources of tuneable coherent radiation, based on the interaction of a relativistic electron beam with a permanent magnetic field. The Super-ACO FEL operates in the UV (down to 300 nm ) at 800 MeV , the nominal energy of the electron beam, with a high repetition rate (8 MHz) . It presents a high average extracted power (up to 300 mW ), short pulses (15–50 ps FWHM) and small bandwidth (3×10 −2 nm) . Taking advantage of these characteristics, we demonstrated for the first time the possibility of using a storage ring FEL as a coherent source of radiation for scientific applications. In particular, the tuneable Super-ACO FEL source, combined with synchrotron radiation covering the X-ray to infra-red range, is a unique tool for the time-dependent studies of excited states. Such analysis benefits from the natural synchronisation of both sources at a high repetition rate, their mutual tunability, high intensity and coherence. Several experimental set-ups are now under operation.

Super ACO FEL oscillation at 300 nm

Some recent improvements, involving both the optical cavity mirrors and the positron beam dynamics in the storage ring, have allowed us to achieve a laser oscillation at 300 nm on the Super ACO Storage Ring FEL. The Super ACO storage ring is operated at 800 MeV which is the nominal energy for the usual synchrotron radiation users, and the highest energy for a storage ring FEL. The lasing at 300 nm could be kept during 2 h per injection, with a stored current ranging between 30 and 60 mA. The FEL characteristics are presented here. The longitudinal stability and the FEL optics behaviour are also discussed.

Gamma rays produced by intra-cavity inverse Compton scattering of a storage ring free-electron laser

A collimated tunable gamma-ray beam has been generated by inverse Compton scattering of an additional positron bunch with the Super-ACO storage ring free-electron laser (FEL) positioned inside the optical resonator in the UV range (345-355 nm). This specific novel set-up ensures a natural transverse overlap and synchronization. The tunable gamma-ray beam produced covers the energy range from a few MeV up to 35 MeV, the maximum energy achievable with a FEL. The number of scattered gamma rays is relatively large (5000 000 photons/s at 800 MeV and 2000 000 photons/s at 700 MeV, i.e. the greatest value for a FEL). Furthermore, a significant number of high-energy gamma rays have also been produced by scattering of the spontaneous radiation stored in the optical cavity. The scattering is performed from a positron bunch outside the undulator, demonstrating for the first time the possibility of optimizing the yield and/or the spectrum by alteration of the stored beam without alteration of the FEL.

Storage ring free electron lasers and saw-tooth instability

Abstract We show that Free Electron Lasers (FEL) operating with storage rings may counteract beam instabilities of the Saw Tooth (STI) type. We use a model based on a set of equations that couple those describing the FEL evolution to those accounting for the STI dynamics. The analysis provides a clear picture of the FEL-STI mutual feedback and clarifies the mechanisms of the instability inhibition. The reliability of the results is supported by a comparison with fully numerical codes.

Inter-dependence of the electron beam excitations with the free electron laser stability on the super-ACO storage ring

Storage ring free electron lasers have a complex dynamics as compared to the LINAC driven FEL sources since both the laser and the recirculating electron beam behaviours are involved. Electron beam perturbations can strongly affect the FEL operation (start-up, stability) whereas the FEL can stabilize beam instabilities. Experimental analysis together with simulations are reported here. Improvements of the Super-ACO FEL for users is discussed, and consequences are given in terms of electron beam tolerances for a source development for users.

Influences of electron beam properties on spontaneous radiation from an optical klystron

Spectrum of the radiation from an optical klystron is very sensitive to the beam energy spread which is an important factor for the storage ring free electron laser. In order to make use of the spectrum for a measurement of the relative energy spread, we have investigated influences of the transverse beam property on the spectrum using a helical optical klystron on the UVSOR storage ring. From analysis of the experimental results, it has been found that the most important effect comes from inhomogeneous magnetic fields and a horizontal beam size.

Storage ring free electron laser dynamics, with the inclusion of bunch lengthening and energy spread increasing effects

Storage ring free electron laser dynamics, with the inclusion of bunch lengthening and energy spread increasing effects

OK-4/Duke storage ring FEL lasing in the deep-UV

In this paper we present the result of the deep-UV lasing with the OK-4/Duke storage ring FEL. The OK-4 FEL was initially commissioned at Duke Free Electron Laboratory in November 1996 followed by a yearlong shutdown. After re-establishing the storage ring operation in early 1998, the OK-4 FEL lasing was demonstrated in the wavelength range of 226–256 nm in April 1998. The OK-4 FEL successfully lased from 217 nm to 235 nm in August 1998. A number of lasing modes with electron energies ranging from 300 to 750 MeV have been established to accommodate application researches. Starting September 1998, the coherent deep-UV radiation is used for applications in surgery, cell biology, surface physics, and nuclear physics.

Storage ring free electron laser and longitudinal instabilities experiments

There is a strong interdependence between the behaviour of Storage Ring Free Electron Lasers (FEL) and the stability of the longitudinal beam from which the FEL is generated. This paper presents the experimental analysis of these phenomena: influence of the longitudinal electron bunch distribution on the FEL growth, operation and saturation, resulting performances in terms of FEL temporal coherence; and inversely, the FEL-induced effect on the longitudinal beam, involving saturation processes. The long-term FEL stability is also discussed, considering the modification of the FEL equilibrium state and perturbations. The knowledge on the FEL longitudinal dynamics is a quite relevant issue for the development of Storage Ring FELs as coherent powerful sources for users.

Storage ring free electron laser amplifiers and microwave instability

This work is aimed at investigating the storage ring free electron laser longitudinal dynamics. In particular we analyze the effects of the free electron laser interaction on the turbulent anomalous bunch lengthening due to the microwave instability. It is shown that the free electron laser interaction can counteract the microwave instability by simultaneously heating the e-beam, and by modifying the machine characteristic damping times.

Storage ring free electron laser dynamics, with the inclusion of bunch lengthening and energy spread increasing effects

We develop a rate equation model of the Storage Ring Free Electron Laser dynamics, which includes bunch lengthening and energy spread increasing effects, due to the combined mechanism of potential well distortion and collective random excitations. The model is based on the ordinary storage ring Free Electron Laser equations, coupled to Haissinski-type equations, accounting for the previously quoted effects. The obtained results are shown to provide a reliable tool, yielding a transparent understanding of the interplay between Free Electron Laser and e-beam induced effects.