Evidence for microbunching "sidebands" in a saturated free-electron laser using coherent optical transition radiation

We report the first measurements of z-dependent coherent optical transition radiation (COTR) due to electron-beam microbunching at high gains ( >10(4)) including saturation of a self-amplified spontaneous emission free-electron laser (FEL). In these experiments the fundamental wavelength was near 530 nm, and the COTR spectra exhibit the transition from simple spectra to complex spectra ( 5% spectral width) after saturation. The COTR intensity growth and angular distribution data are reported as well as the evidence for transverse spectral dependencies and an "effective" core of the beam being involved in microbunching.     

Maximum entropy principle and coherent harmonic generation using a single-pass free-electron laser

Single-pass free-electron lasers constitute an example of systems with long-range interactions. The light-particle interplay leading to the power growth and successive relaxation towards a quasi-stationary state is governed by the Vlasov equation. A maximum entropy principle inspired to Lynden-Bell's theory of “violent relaxation" for the Vlasov equation can be invoked to analytically characterize the behaviour of the saturated system. In particular, we here concentrate on the case of coherent harmonic generation obtained from an externally seeded free-electron laser and provide a simple strategy to predict the laser intensity as well as the final electron-beam energy distribution.     

Autocorrelation measurements of free-electron laser radiation using a two-photon QWIP

The two-photon QWIP comprises three equidistant subbands, namely two bound states localized in the quantum well and an extended state in the continuum. This device is very promising for quadratic autocorrelation measurements of pulsed mid-infrared lasers due to its resonantly enhanced optical nonlinearity and sub-ps time resolution. We report on interferometric autocorrelation measurements of ps optical pulses from a free-electron laser (FEL). The intense FEL radiation further allows us to study the saturation properties of two-photon QWIPs at liquid nitrogen temperature and their detection properties at 300 K. The device is well suited for standard diagnostics of the FEL pulse shape via interferometric autocorrelation.     

Fully coherent x-ray pulses from a regenerative-amplifier free-electron laser

We propose and analyze a regenerative-amplifier free-electron laser (FEL) to produce fully coherent, hard x-ray pulses. The method makes use of narrow-bandwidth Bragg crystals to form an x-ray feedback loop around a relatively short undulator. Self-amplified spontaneous emission (SASE) from the leading electron bunch in a bunch train is spectrally filtered by the Bragg reflectors and is brought back to the beginning of the undulator to interact repeatedly with subsequent bunches in the bunch train. The FEL interaction with these short bunches regeneratively amplifies the radiation intensity and broadens its spectrum, allowing for effective transmission of the x rays outside the crystal bandwidth. The spectral brightness of these x-ray pulses is about 2 to 3 orders of magnitude higher than that from a single-pass SASE FEL.     

Soft-x-ray laser interferometry of a pinch discharge using a tabletop laser

We have used a tabletop soft-x-ray laser and a wave-front division interferometer to probe the plasma of a pinch discharge. A very compact capillary discharge-pumped Ne-like Ar laser emitting at 46.9 nm was combined with a wave division interferometer based on Lloyd's mirror and Sc-Si multilayer-coated optics to map the electron density in the cathode region of the discharge. This demonstration of the use of tabletop soft-x-ray laser in plasma interferometry could lead to the widespread use of these lasers in the diagnostics of dense plasmas.     

Plasma-aided radiation guiding in a free-electron laser

The introduction of a plasma in a free-electron laser (FEL) helps radiation guiding via nonlinear refraction. At high-radiation power density, when oscillatory electron velocity is comparable to the electron thermal velocity, the radiation pushes plasma radially out, forming a depleted plasma duct and guiding the radiation. The radius of the self-trapped laser is ~c/ω_po, where ω_po is the unperturbed plasma frequency and c is the velocity of light in vacuum     

Spatially coherent radiation from a coaxial free-electron laser with a resonator composed of one-dimensional and two-dimensional Bragg mirrors

It is shown that the application of a resonator composed of one-dimensional and two-dimensional coaxial Bragg mirrors provides a spatially coherent radiation from a hollow electron beam with a transverse size several orders of magnitude larger than the wavelength. The two-dimensional Bragg mirror placed at the cathodic end of the resonator synchronizes the radiation across the hollow electron beam. The standard one-dimensional Bragg mirror placed at the collector end closes the feedback loop and reduces ohmic losses to as low as 5–10% of the lasing power.     

Nonlinear evolutionary processes in a free-electron laser generator of diffracted radiation

Nonlinear evolutionary processes with two control parameters, one of which is related to the electrodynamic structure (positive feedback) and the other is related to the constant electric field applied to the electron flux, are studied in a free-electron laser, which is a diffracted-radiation oscillator. To this end, first, the linear spectral problem for an open-cavity resonator is investigated and the dispersion relation near the Morse critical point is established. Then the nonlinear evolutionary equation with two control parameters is constructed. Analysis of the latter makes it possible to determine the properties of the parametric dependence of the bifurcation and structural stability, which are determined by small variations of the control parameter (tuning of the cavity). This explains the operating efficiency of a diffracted-radiation oscillator in the millimeter and submillimeter wavelength ranges. Zh. éksp. Teor. Fiz. 111, 2243–2262 (June 1997)     

Generation of spatially coherent radiation in free-electron masers with two-dimensional distributed feedback

The results of theoretical and experimental studies on the generation of spatially coherent electromagnetic radiation in a planar free-electron maser with two-dimensional distributed feedback are reported. A two-dimensional Bragg structure is used at the initial part of the interaction space to ensure the transverse synchronization of the radiation. The possibility of the narrowband generation in the 75-GHz frequency band is demonstrated experimentally for a sheet kiloampere electron beam whose width is 20 times larger than the wavelength.     

Continuously tunable, high-power, single-mode radiation from a short-pulse free-electron laser

This paper gives the first demonstration of high-power, continuously tunable, narrowband radiation that is produced by means of a free-electron laser (FEL) in the far-infrared region of the electromagnetic spectrum. A Fox-Smith intracavity étalon was used to induce phase coherence between the 40 optical micropulses that were circulating in the laser cavity. The corresponding phase-locked spectrum consisted of a comb of discrete frequencies separated by 1 GHz. A pair of external Fabry-Pérot étalons was used to filter out a single line from this spectrum. The power in the selected narrow line at 69 microm wavelength was equal to 250 mW during the macropulse of the laser. The spectral width of the selected line is as small as that of a single cavity mode, i.e., a fraction of 25 MHz, in single macropulses of the laser. The average bandwidth of 25 MHz is determined by mode hopping of the phase-locked FEL. The selected frequency hops over 25 MHz between the extrema of this band. The influence of partially coherent spontaneous emission and mode hopping on the final linewidth was studied. The narrow-linewidth radiation was scanned in frequency over 1 GHz. We show that the possibilities to scan over smaller or larger frequency intervals are unlimited.     

A new powerful source for coherent VUV radiation: Demonstration of exponential growth and saturation at the TTF free-electron laser

We present experimental evidence that the free-electron laser at the TESLA Test Facility has reached the maximum power gain of 10⁷ in the vacuum ultraviolet (VUV) region at wavelengths between 80 and 120 nm. At saturation the FEL emits short pulses with GW peak power and a high degree of transverse coherence. The radiation pulse length can be adjusted between 30 fs and 100 fs. Radiation spectra and fluctuation properties agree with the theory of high gain, single-pass free-electron lasers starting from shot noise. Received 26 April 2002 Published online 28 June 2002     

Proposal for intense attosecond radiation from an x-ray free-electron laser

We propose the use of an ultrarelativistic electron beam interacting with a few-cycle, intense laser pulse and an intense pulse of the coherent x rays to produce a multi-MW intensity, x-ray pulses approximately 100 attoseconds in duration. Because of a naturally occurring frequency chirp, these pulses can be further temporally compressed.