D. Oepts

The Free-Electron-Laser user facility FELIX

Abstract The Free Electron Laser for Infrared eXperiments FELIX presents to its users a versatile source of radiation in the infrared and far-infrared spectral regions. Presently, the wavelength range of operation extends from 5 to 110 μm (2000−90 cm−1). The wavelength is continuously tunable over an octave in a few minutes. The output normally consists of macropulses of 5–10 μs duration, formed by a train of micropulses of a few ps length. Average power in the macropulses is of order 10 kW, peak power in the micropulses is in the MW range. The temporal and spectral characteristics of the micropulses can be controlled by varying the synchronism between the electron pulses and the optical pulses circulating in the laser cavity. Transform-limited pulse lengths in the range 2–20 ps can be generated. Long-range coherence has been induced by phase-locking successive micropulses, and narrow-band, essentially single-mode, radiation has been selected from the output.

Broadband tunability of a far‐infrared free‐electron laser

A unique property of the free‐electron laser (FEL) is its capability to be tuned continuously over a wide spectral range. This is a major difference with all other high‐power lasers. However, the tunability of first‐generation FELs used to be quite poor (typically 10% or less), due to constraints imposed by the accelerator and the undulator. The free electron laser for infrared experiments (FELIX) uses an undulator with an adjustable gap, which permits wavelength scans over an octave in typically 2 min without the need for any readjustment of the electron beam. Results obtained in operation of the long‐wavelength FEL of the FELIX facility are presented. These involve measurements of the spectral range covered (16–110 μm), the output power, and the influence of the cavity desynchronism. The results are compared with numerical simulations.

Short-pulse effects in a free-electron laser

The Free-Electron Laser for Infrared eXperiments (FELIX) offers a unique combination of short electron bunches and long wavelengths, i.e. a slippage parameter /spl mu//sub c/ ranging up to 10. As a consequence, pronounced short-pulse effects can be observed. In this paper the experimental observation of two of these effects is discussed, namely the occurrence of limit-cycle oscillations and the feasibility of tuning of the micropulse duration. The stable limit-cycle oscillation of the macropulse power is due to a modulation of the optical micropulse shape. This is a consequence of a combination of high optical power and short pulses. The former causes synchrotron oscillations of the electrons and the effect is, therefore, closely related to spiking phenomena. The short-pulse nature of FELIX ensures that the oscillations do not evolve into the chaotic behavior normally associated with spiking and the sideband instability. Experimental results are compared with numerical simulations. >

Consequences of Short Electron-Beam Pulses in the Felix Project

Abstract We discuss the consequences of short micropulses on the output of infrared and far-infrared free electron lasers with special reference to the FELIX project which operates with 3 ps long electron pulses.

Free-electron laser operation and self-amplified spontaneous emission using a step-tapered undulator

Abstract We present an experimental and theoretical evaluation of a new method of enhancing the efficiency and gain of the free-electron laser (FEL) and observations of self-amplified spontaneous emission at start-up of the step-tapered FEL. The stepped undulator is divided into two uniform sections of different deflection strengths, the upstream K 1 and the downstream K 2 , and a step of Δ K = K 2 − K 1 ≈ 0.03 with K 1 K 2 for mid-infrared operation.

Limit cycle behaviour in FELIX

Abstract The free electron laser for infrared experiments (FELIX) operates at wavelengths up to λ = 110 μ m. A radio-frequency linear accelerator is used to produce electron micropulses with a duration of about 3 ps. With N = 38 undulator periods, this puts FELIX well into the regime where the slippage length, Nλ , exceeds the electron micropulse length, and prominent short pulse effects are expected. One of these effects, stable limit cycle oscillations of the pulse energy, has not been detected experimentally before. Such oscillations occur when the saturated optical pulses move away from the electron pulses, due to the changing balance between lethargy and desynchronism, while new subpulses grow periodically. In FELIX, limit cycle behaviour is clearly demonstrated. The observations are in agreement with numerical simulations of the pulse propagation, and the oscillation period is given by a simple formula containing the slippage length and the desynchronism between optical and electron pulses. We also show how lethargic behaviour can be used to reduce the optical bandwidth of the FEL and to store optical energy in the optical cavity without saturation limiting the energy stored.

Enhancement of FEL efficiency by using short electron pulses

In this paper we show that for an intermediate gain free electron laser oscillator, the efficiency is dramatically enhanced by operating with electron pulses shorter than the slippage distance. We discuss both the consequences of operating at longer wavelengths where lethargy effects are important and the implications of working with short electron pulses when inhomogeneous broadening is important. The discussion is supported by a one-dimensional multiparticle simulation that takes into account both saturation and short pulse effects.

Status of the Dutch Free-Electron Laser for Infrared Experiments

Abstract We review the status of the Dutch Free Electron Laser for Infrared eXperiments (FELIX), with which radiation in the range between 3 μm and 3 mm will be generated. Among our research objectives are (i) rapid tunability and (ii) mode reduction by means of an intracavity etalon. The first stage of the project deals with generation of radiation with a wavelength between 8 and 80 μm. The undulator of the former UK-FEL project will be used in this stage. The design of the accelerator, with which 70-A, 3-ps bunches are accelerated to a maximum energy of 45 MeV, is presented. It consists of a triode, a 4-MeV buncher, and two travelling-wave linac structures. Gain calculations leading to the best choice for the number of undulator periods are discussed.

A far-infrared broadband (8.5–37 μm) autocorrelator with sub-picosecond time resolution based on cadmium telluride

A background-free autocorrelator has been developed for measuring the duration of far-infrared laser pulses in the spectral range from 8.5 to 37 μm by using an 840-μm-long wedged cadmium telluride crystal as the second-harmonic generator. Typical intensity second-harmonic autocorrelation traces are given for the wavelengths of 19.7 and 37 μm, indicating FWHM pulse duration of 0.90 and 1.5 ps respectively. An interferometric autocorrelation trace at 18.2 μm has been measured for the first time, and the distortion of autocorrelation traces due to the absorption and re-emission of ambient water vapor is shown at 28 μm.

Optimization of the FELIX accelerator with respect to laser performance

Abstract In this paper we discuss the performance of the FELIX accelerator in relation to the laser performance. Over the past year, a number of improvements have been made to the accelerator, both to the hardware and to the way in which it was operated, that have resulted in a reduction of the time needed to reach saturation from 9 to 3 μs. Energy spread and stability, both short and long term, and operational flexibility, an important issue for a user facility, are addressed. Surprisingly, “best” FEL performance is not obtained at the same operation point that gives the smallest energy spread, which suggests that the electron bunch length is not fixed. Evidence is presented for the conjecture that the non-isochronicity of the bend plays a major role. Measurements of enhanced spontaneous emission and of coherence between successive optical micropulses, indicating a spatial structure in the electron microbunches on the scale of an optical wavelength, are also discussed.