M. Kozlova

Aberration-free laser beam in the soft x-ray range.

By seeding an optical-field-ionized population-inverted plasma amplifier with the 25th harmonic of an IR laser, we have achieved what we believe to be the first aberration-free laser beam in the soft x-ray spectral range. This laser emits within a cone of 1.34 mrad(1/e(2)) at a repetition rate of 10 Hz at a central wavelength of 32.8 nm. The beam exhibits a circular profile and wavefront distortions as low as lambda/17. A theoretical analysis of these results shows that this high beam quality is due to spatial filtering of the seed beam by the plasma amplifier aperture.

Demonstration of a spatial filtering amplifier for high-order harmonics.

We report what is to our knowledge the first demonstration of spatial filtering of a high-order harmonic beam into a soft-x-ray laser plasma amplifier at 32.8 nm. After amplification the seed energy is enhanced by a factor of 50, and the beam profile of the amplified beam exhibits an Airy-like shape due to the spatial filtering by the optical field ionized plasma. Moreover, the transverse coherence of the spatially filtered amplified beam is strongly enhanced, resulting in the generation of a peak coherent power of 0.9 x 10(5) to 1.8 x 10(5) W.

Temporal Coherence and Spectral Linewidth of Neon-Like XUV Lasers Pumped in the Quasi-steady State Regime

We report recent experimental progress in the characterization of the temporal coherence and related spectral linewidth of plasma-based soft X-ray lasers (SXRL). New measurements were carried out with two types of SXRLs pumped in the quasi-steady state (QSS) regime, in a capillary-discharge plasma and in a laser-produced plasma. We describe the main results obtained from both experiments and compare them to dedicated numerical simulations. We discuss the results in the context of the possibility to achieve XUV lasers with pulse duration below 1 picosecond.

Stable femtosecond X-rays with tunable polarization from a laser-driven accelerator

Technology based on high-peak-power lasers has the potential to provide compact and intense radiation sources for a wide range of innovative applications. In particular, electrons that are accelerated in the wakefield of an intense laser pulse oscillate around the propagation axis and emit X-rays. This betatron source, which essentially reproduces the principle of a synchrotron at the millimeter scale, provides bright radiation with femtosecond duration and high spatial coherence. However, despite its unique features, the usability of the betatron source has been constrained by its poor control and stability. In this article, we demonstrate the reliable production of X-ray beams with tunable polarization. Using ionization-induced injection in a gas mixture, the orbits of the relativistic electrons emitting the radiation are reproducible and controlled. We observe that both the signal and beam profile fluctuations are significantly reduced and that the beam pointing varies by less than a tenth of the beam divergence. The polarization ratio reaches 80%, and the polarization axis can easily be rotated. We anticipate a broad impact of the source, as its unprecedented performance opens the way for new applications.

Characterization of Zn X-Ray Laser at PALS Centre, Its Applications in Dense Plasma Probing and Astrophysics

This report presents the results from experiments at PALS Centre using a Zn X-ray laser with the pulse length of 0.15 ns and the wavelength of 21.2 nm, working in single or double pass regime with the output energy of 0.4 mJ or 4 mJ per pulse, respectively. The current X-ray laser was experimentally examined to obtain its temporal coherence and spectral width using a path-difference interferometer. The double pass regime shows that QSS plasma based source-amplifier is promising for “short” fs soft X-ray pulses. The X-ray laser is commonly used for user’s experiments. Its advantages can be shown in applications such as probing of dense plasmas (up to 2.5×1024 cm−3) or single shot experiments (4×1014 photons/pulse). The simple technique based on Talbot effect was used to obtain the gradients of electron densities of line plasmas produced under conditions corresponding to XRL’s amplifiers operating in TCE and QSS regime. To investigate radiative shock wave in laboratory is challenging in aspects of the optimization of experimental parameters. Due to the high electron density (1022 cm−3) produced in the gas medium propagated by the shock wave, the velocity of the shock wave, and the absorption losses on optical elements, it is necessary to use the energetic single shot probe.

Laser-driven Short-wavelength Sources at PALS and ELI Beamlines

Research on development of short-wavelength sources, namely high-order harmonic generation, plasma-based X-ray lasers, X-rays from relativistic electrons will be presented. Prospects on sources for user-based research at ELI Beamlines will be given.

Spectral and Coherence Properties of the Ne-like Zn X-Ray Laser at PALS

We present a refined measurement of the temporal coherence of the zinc X-ray laser generated at PALS, using a wavefront-division interferometer. The corresponding bandwidth of the lasing line is shown (i) to be broader than those of other types of X-ray lasers, previously characterized with the same instrument, and (ii) is compatible with the amplification of subpicosecond pulses. Similar measurements were also performed for shorter amplification lengths and the small-signal gain was carefully characterized.

Spectral Properties of Collisional Xuv Lasers for the Amplification of Femtosecond Pulses

We discuss the role of the spectral properties of XUV lasers in the amplification of femtosecond pulses in two different regimes. We present an overview of our recent spectral characterization of the four types of collisionally-pumped systems, also including other measurements from the literature. This is used to assess the potential of the different types of XUV lasers to amplify femtosecond pulses, either in the adiabatic or in the dynamic regime.

Laser-Driven Plasma-Based Incoherent X-Ray Sources at PALS and ELI Beamlines

In this paper, we report on development of incoherent secondary X-ray sources at the PALS Research Center and discuss the plan for the ELI Beamlines project. One of the approaches, how to generate ultrashort pulses of incoherent X-ray radiation, is based on the interaction of femtosecond laser pulses with underdense plasma. This method, known as laser wakefield electron acceleration (LWFA ), can produce up to GeV electron beams emitting radiation in the collimated beam with a femtosecond pulse duration. This approach was theoretically and experimentally examined at the PALS Center. The parameters of the PALS Ti:Sapphire laser interaction were studied by extensive particle-in-cell (PIC) simulations with radiation postprocessors in order to evaluate the capabilities of our system in this field. The compressed air, and a mixture of helium and argon were used as accelerating medium. The accelerator was operated in the bubble regime with forced self-injection and resulted in the generation of stable relativistic electron beams with the energy up to 80 meV, hence, the betatron X-ray radiation with critical energy in the keV range was generated. The extensions of this method to the ELI Beamlines facility will enable to generate much higher X-ray energies from 10 keV up to 1 meV with 10 Hz repetition rate. Such source is suitable for various applications like phase contrast imaging or X-ray absorption experiments in a single shot.

A compact ultra-intense plasma-based EUV laser with circular polarization capability

We demonstrated an original technique which allowed us to achieve ultrashort pulse duration EUV lasing by increasing the plasma density. As demonstrated using a lower density amplifier, the emission can be made fully circularly polarized.