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We present measurements of a new method of generating harmonic radiation in a free-electron laser oscillator using a two-section undulator. To produce coherent harmonic radiation the undulator is arranged so that the resonance frequency of the downstream undulator section matches an harmonic of the upstream undulator. Both the fundamental and the harmonic optical fields evolve in the same optical cavity and are coupled out with different extraction efficiency, through a hole in one of the cavity mirrors. We present measurements that show that the optical power at the second and third harmonic can be enhanced by more than an order of magnitude. We examine the dependence of the harmonic power on the optical cavity desynchronism and the cavity roundtrip number to evaluate the influence of the fundamental power on the evolution of the harmonic. We compare our measurements with predictions of a multi-electron numerical model to evaluate the bunching at the harmonic frequency. This fundamental/harmonic mode of operation of the FEL may have useful applications in the production of coherent X-ray and VUV radiation, a spectral range where high reflectivity optical cavity mirrors are difficult or impossible to manufacture.

The user-facility FELIX employs two FELs together covering the spectral range from 6.5 to 110 μm. Adjustment of the undulator strength permits wavelength tuning over a factor of two within two minutes while continuously providing several kilowatts of output power. As FELIX combines short electron bunches with long wavelengths, short-pulse effects play an important role. A spectral analysis and intensity correlation measurements show that the micropulses are virtually transform limited. At small cavity desynchronisation, the duration can be much smaller than the electron-bunch duration (3 ps). Indications of the formation of subpulses within the micropulse have been observed.

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.

We report the use of FELIX (free electron laser for infrared experiments) to study photon drag in p-silicon over the wavelength range 25-80-mu-m. Previous 'spot' measurements using a pulsed water vapour laser had indicated high response at 28 and 33-mu-m associated with transitions within the valence band. These earlier results have been confirmed and the main futures of the extended photon drag spectrum are also explainable by reference to the valence band structure. A practical outcome of the experiment is the design of a subnanosecond response time detector to observe the 1GHz frequency micropulse output from FELIX.

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.

We present the first experimental demonstration of phase locking between successive optical micro-pulses in a free-electron laser in which the electron pulses are separated by a fraction of the optical-cavity round-trip time. A Fox-Smith interferometer arrangement has been used to induce a high degree of interpulse phase coherence. Surprisingly, a substantial amount of coherence has also been observed without the phase-locking arrangement. A tentative explanation of this phenomenon is given.

Measurements of the dispersive and absorptive properties of the free-electron-laser gain medium carried out on the United Kingdom Free Electron Laser Scotland are presented. We show how measurements of gain can be used, both with and without the optical cavity, to establish the correct cavity length for synchronism and determine important physical parameters. We present results of optical injection experiments.

An undulator free-electron-laser experiment (U-FEL) is under way at the ENEA-Frascati Center. The U-FEL employs a 20 MeV microtron as electron-beam source and operates in the middle infrared region around 30 μm. Recent results on the spontaneous-emission measurements are discussed.

The simulation of injection gain measurements in a free electron laser with a radiofrequency electron source is presented using the nonhomogeneous low-gain small-signal pulse propagation equation. Modifications for high gain are also presented. The tuning of the optical cavity length is investigated.

We present measurements of the temporal and spectral properties of radiation produced from the step-tapered undulator infrared free-electron lasers (FELs), CLIO in France and FELIX in the Netherlands. Using a two section undulator with independently adjustable deflection parameters, K, the FEL will operate either with an enhanced efficiency and improved spectral properties (with a small positive Delta K step) or simultaneously at two frequencies (for large Delta K). Using a dispersion-free hole output coupler the maximum wavelength difference delta lambda/lambda, has now been extended to more than delta lambda/lambda approximate to 0.6. We also present measurements that show that the FEL FELIX will produce significant power simultaneously at two wavelengths by coherent spontaneous emission when the wavelength is long and the electron bunch is short. The efficiency, spectral and temporal properties have been measured. We show that at the maximum efficiency, Delta K approximate to 0.02, the optical pulses generated are smooth and close to Fourier transform limited. By adjusting Delta K the optical pulse duration can be varied by a factor of 2 or more and sidebands due to synchrotron oscillations can be suppressed.

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. >

In the free electron laser for infrared experiments (FELIX), a radio-frequency linear accelerator is used to deliver electron micropulses with a duration of about 3 ps. The micropulses are repeated at a frequency of 1 GHz. The length of the optical cavity is 6 m, so that 40 independent optical micropulses are simultaneously circulating in the cavity. Almost complete coherence has been induced between the multiple pulses in FELIX by means of an intracavity Fox-Smith interferometer. A large fraction of the cavity modes is suppressed in this way, and the remaining lines have a relative width on the order of 1:10 6 . The stability is in the same range. The gain and saturated power of the laser are almost unaffected. A single mode can in principle be selected from the output beam. This makes the resulting radiation useful for high-resolution spectroscopy applications. Coherence between successive micropulses has also been observed, quite unexpectedly, without the Fox-Smith arrangement.

The FELIX (free electron laser for infrared experiments) project involves the construction and operation of a rapidly tunable FEL in the spectral region between 3 and 300 μm. The spectral region between 8 and (at least) 80 μm is addressed in the first stage of the project. Nearly all components of FELIX are now available. In this article we review the design of the main components, discuss the status of the project, and present results of some performance tests.

The aim of the UK FEL project is to study the characteristics of a single-pass FEL over a wide operating range, using a four-section 5 m wiggler yielding a maximum K value of 2.7. The laser is driven by the 165 MeV Kelvin Laboratory linac. It has a design operating wavelength range of 2–20 μm, although higher harmonics are also under investigation. We report direct measurement of optical gain using a cw CO2 laser between 9 and 11 μm. Simultaneous spectra are obtained of both gain and spontaneous emission as a function of linac energy or wiggler field as the tuning parameter. The linewidth obtained is close to that expected for homogeneous broadening, and the gain is consistent with that predicted theoretically for the current available. Strong enhancement of both spontaneous emission and measured gain is obtained in the presence of a high-Q FEL cavity, as expected by simple considerations of optical confinement. These results imply that despite the shortness of our macropulse (∼ 2.5 μm), it will be possible to diagnose buildup to oscillation from an injected signal.

The current status of the UK FEL program is reviewed at the end of its second year. A description of the optical and electron beam diagnostics is given, and a preliminary study of spontaneous emission at harmonic frequencies is reported.

We describe the data acquisition and optical diagnostic systems of the UK free-electron laser. Measurements of optical gain are reported and the first attempt at oscillation with laser injection is described.

A brief summary is given of the collaborative project between ENEA and Heriot-Watt University on the Italian ENEA free-electron laser (FEL). The importance of optical diagnostics is discussed, and an outline of the physical process of gain in a FEL is given, with special reference to phase sensitive coherent gain measurement.

Measurements of the dispersive and absorptive properties of the free-electron-laser gain medium carried out on the United Kingdom Free Electron Laser Scotland are presented. We show how measurements of gain can be used, both with and without the optical cavity, to establish the correct cavity length for synchronism and determine important physical parameters. We present results of optical injection experiments.