Laser phase errors in seeded free electron lasers

Ratner, Daniel; Fry, Alan; Stupakov, Gennady; White, William E.

doi:10.1103/physrevstab.15.030702

Cited by 58 publications

(47 citation statements)

References 19 publications

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“…The FEL pulses (time duration ∼70 fs full-width-half-maximum (FWHM) [15], pulse energy ∼0.7 µJ) were focused on a 30 × 15 µm 2 spot size and spatially overlapped with the optical pump beam. The latter was a Ti:sapphire laser pulse frequency up-converted to the third harmonic (wavelength 260 nm, pulse duration 100 fs FWHM, pulse energy 45 µJ, spot size 70 × 100 µm 2 ) and synchronized with the FEL pulse with a jitter less than 10 fs [16].…”

Section: Resultsmentioning

confidence: 99%

Transient EUV Reflectivity Measurements of Carbon upon Ultrafast Laser Heating

et al. 2017

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Time resolved extreme ultraviolet (EUV) transient reflectivity measurements on non-equilibrium amorphous carbon (a-C) have been carried out by combining optical and free electron laser (FEL) sources. The EUV probing was specifically sensitive to lattice dynamics, since the EUV reflectivity is essentially unaffected by the photo-excited surface plasma. Data have been interpreted in terms of the dynamics of an expanding surface, i.e., a density gradient rapidly forming along the normal surface. This allowed us to determine the characteristic time (τ 1 ps) for hydrodynamic expansion in photo-excited a-C. This finding suggests an extremely narrow time window during which the system can be assumed to be in the isochoric regime, a situation that may complicate the study of photo-induced metastable phases of carbon. Data also showed a weak dependence on the probing EUV wavelength, which was used to estimate the electronic temperature (T e ≈ 0.8 eV) of the excited sample. This experimental finding compares fairly well with the results of calculations, while a comparison of our data and calculations with previous transient optical reflectivity measurements highlights the complementarities between optical and EUV probing.

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Section: Resultsmentioning

confidence: 99%

Transient EUV Reflectivity Measurements of Carbon upon Ultrafast Laser Heating

et al. 2017

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“…The XUV pulse duration T FEL was deduced from its dependence on the harmonic number n h in a seeded FEL, which is given by T FEL ¼ T seed Â (n h ) À 1/3 (ref. 33), and using for present experiment the measured seed pulse duration T seed ¼ 210 fs and the 10th harmonics. The average pulse energy was 80 ± 10 mJ with a repetition rate of 10 Hz.…”

Section: Methodsmentioning

confidence: 99%

Determining the polarization state of an extreme ultraviolet free-electron laser beam using atomic circular dichroism

et al. 2014

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Ultrafast extreme ultraviolet and X-ray free-electron lasers are set to revolutionize many domains such as bio-photonics and materials science, in a manner similar to optical lasers over the past two decades. Although their number will grow steadily over the coming decade, their complete characterization remains an elusive goal. This represents a significant barrier to their wider adoption and hence to the full realization of their potential in modern photon sciences. Although a great deal of progress has been made on temporal characterization and wavefront measurements at ultrahigh extreme ultraviolet and X-ray intensities, only few, if any progress on accurately measuring other key parameters such as the state of polarization has emerged. Here we show that by combining ultra-short extreme ultraviolet free electron laser pulses from FERMI with near-infrared laser pulses, we can accurately measure the polarization state of a free electron laser beam in an elegant, non-invasive and straightforward manner using circular dichroism.

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“…As seen in Eq. (2), when p ≈ λ 2 =λ X ≫ 1 the bunching produced by EEHG is highly sensitive to the instantaneous fields of the second laser [9]. Variations in the phase of the input laser are multiplied by a factor p in the bunching phase.…”

Section: Discussionmentioning

confidence: 99%

Stable, coherent free-electron laser pulses using echo-enabled harmonic generation

Penn

2014

Phys. Rev. ST Accel. Beams

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Beam lines using echo-enabled harmonic generation can be designed to have extremely low sensitivity to energy chirps in the electron beam, as shown through theory and detailed simulations. These designs would allow stable and coherent radiation to be produced even when using electron beams with a large amount of shot-to-shot jitter in the longitudinal profile. DOI: 10.1103/PhysRevSTAB.17.110707 PACS numbers: 41.60.Cr, 42.65.Ky I. MOTIVATIONA major motivation for seeding a free-electron laser (FEL) with external lasers is to achieve stable output with coherent pulses having a time-bandwidth product close to the transform limit. While the quality of the seed laser itself is important, any nonuniformity in the electron bunch can also lead to variations in both the power and phase of the output pulse. Gradients in the local average (slice) electron energy are particularly deleterious to output mode quality, especially when a high harmonic of the initial seed laser is desired. Harmonic generation typically requires a dispersive element such as a chicane which shifts different parts of the beam by a distance proportional to the local energy offset. An initially coherent signal can become highly degraded in quality after a combination of longitudinal dispersion and harmonic upshifting.Echo-enabled harmonic generation (EEHG) [1] uses phase mixing to combine the energy modulations from two input lasers with wavelengths λ 1 and λ 2 , producing bunching at a wavelength λ X . The corresponding wave numbers k ≡ 2π=λ are related by k X ¼ pk 2 − mk 1 , where p and m are integers. Sign conventions are chosen so p and m are typically positive. A chicane is placed after each modulating undulator with dispersion R 1 and R 2 . The convention used here is that drifts and simple chicanes have dispersion R 56 > 0.The EEHG scheme is inherently less sensitive than high gain harmonic generation (HGHG) [2] to energy chirps in the electron beam [3,4]. Through careful adjustment of the parameters for EEHG, this sensitivity will be further reduced to the point where slice energy variations comparable to the FEL bandwidth itself introduce only modest phase errors. Following this prescription allows even an electron beam with large energy chirps that fluctuate from shot to shot to generate stable and coherent radiation. The technique of coupling multiple inputs through phase mixing is used in other fields such as magnetic resonance and plasma physics. The added flexibility from allowing for reduced coupling could be useful in a wide range of systems to enhance stability or coherence. II. ANALYSISIn the HGHG scheme, microbunches experience a longitudinal displacement of roughly Δz ≃ R 56η as they pass through a chicane, whereη is the relative offset of the average electron energy in the microbunch from the nominal energy. These displacements change the phase of the complex bunching parameter, defined asb ≡ hexpð−ihk 1 zÞi at the harmonic h of an input laser with wavelength λ 1 . The resulting phase shift is given byThe quantity ΔΨ will also b...

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Laser phase errors in seeded free electron lasers

Cited by 58 publications

References 19 publications

Transient EUV Reflectivity Measurements of Carbon upon Ultrafast Laser Heating

Transient EUV Reflectivity Measurements of Carbon upon Ultrafast Laser Heating

Determining the polarization state of an extreme ultraviolet free-electron laser beam using atomic circular dichroism

Stable, coherent free-electron laser pulses using echo-enabled harmonic generation

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