R.J. Bakker

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. >

1 GHz modulation of a high-current electron gun

Abstract Measurements on a triode electron gun operated with grid modulation at 1 GHz are presented in this article. The gun has a miniaturized cathode—grid s

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.

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.

Free electron laser induced two‐photon absorption in Hg1−xCdxTe

The powerful output of FELIX, the recently built Rijnhuizen free‐electron laser, is used for the first frequency dependent study of nonlinear optical excitation of Hg1−xCdxTe in the far‐infrared spectral region. Two‐photon interband absorption has been investigated as a function of power and wavelength from 20 to 40 μm. This nonlinear optical absorption is used to perform autocorrelation experiments yielding the length of the micropulses of the free‐electron laser on a femtosecond time scale. The predicted dependence of the micropulse length on the synchronization between optical and electron pulses in the laser cavity is observed.

Bunching of an intense electron beam extracted from a triode gun modulated at 1 GHz

We present measurements of the bunch length and emittance of a high-current electron beam, which is extracted from a triode modulated at 1 GHz and subsequently compressed by means of velocity modulation in a prebuncher cavity. The prebuncher is detuned by about 1 MHz in order to ensure that the total field, which is the sum of the field from an external generator and the beam-induced field, has the correct phase and amplitude. The bunch length is measured by means of a capacitive pick-up probe and a 20 GHz sampling oscilloscope. A bunch length of 40 ps, at a charge of 220 pC, has been obtained at the end of the drift space behind the prebuncher, which has a length of 200 mm. The normalized emittance is 36-pi mm mrad.

Agility of FELIX regarding wavelength and micropulse shape

Abstract 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.

A Low-Energy-Spread Rf Accelerator for a Far-Infrared Free-Electron Laser

A high electron current and a small energy spread are essential for the operation of a free electron laser (FEL). In this paper we discuss the design and performance of the accelerator for FELIX, the free electron laser for infrared experiments. The system consists of a thermionic gun, a prebuncher, a buncher and two standard commercial linac sections. The gun is operated with a pulse duration of 280 ps and a bunch charge of 200 pC. After compression to 35 ps by the prebuncher, the bunches are accelerated to 4 MeV in the buncher and simultaneously compressed to 6 ps. The principle of the method is that the order of the electrons is conserved in the buncher, so that the resulting more or less linear energy-phase relationship along each bunch can be compensated effectively against space charge forces and the accelerating field gradient in the linacs, via an appropriate choice of the phase of the rf wave. Behind the linacs an rms energy spread of 0.30% has been measured.

Update on FELIX

Abstract 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.