Laser-plasma X-ray Source Research Articles

Generation of photoionized plasmas in the laboratory of relevance to accretion-powered x-ray sources using keV line radiation

We describe laboratory experiments to generate x-ray photoionized plasmas of relevance to accretion-powered x-ray sources such as neutron star binaries and quasars, with significant improvements over previous work. A key quantity is referenced, namely the photoionization parameter, defined as ξ=4πF/ne where F is the x-ray flux and ne the electron density. This is normally meaningful in an astrophysical steady-state context, but is also commonly used in the literature as a figure of merit for laboratory experiments that are, of necessity, time-dependent. We demonstrate emission-weighted values of ξ>50 erg-cm s−1 using laser-plasma x-ray sources, with higher results at the centre of the plasma which are in the regime of interest for several astrophysical scenarios. Comparisons of laboratory experiments with astrophysical codes are always limited, principally by the many orders of magnitude differences in time and spatial scales, but also other plasma parameters. However useful checks on performance can often be made for a limited range of parameters. For example, we show that our use of a keV line source, rather than the quasi-blackbody radiation fields normally employed in such experiments, has allowed the generation of the ratio of inner-shell to outer-shell photoionization expected from a blackbody source with ∼keV spectral temperature. We compare calculations from our in-house plasma modelling code with those from Cloudy and find moderately good agreement for the time evolution of both electron temperature and average ionisation. However, a comparison of code predictions for a K-β argon X-ray spectrum with experimental data reveals that our Cloudy simulation overestimates the intensities of more highly ionised argon species. This is not totally surprising as the Cloudy model was generated for a single set of plasma conditions, while the experimental data are spatially integrated.

Spatial and temporal evolution of laser plasma produced using a double-stream gas puff target

Laser plasma produced using a double-stream gas puff target is an intense source of soft x-ray (SXR) and extreme ultraviolet (EUV) radiation, however, without the harmful emission of debris associated with a solid target. Debris-free laser plasma x-ray and EUV sources have been applied in many various applications, including metrology, imaging in a nanoscale, tomography, processing materials, emission and absorption spectroscopy, laboratory astrophysics and astrochemistry, radiobiology, and radiochemistry. In this work, the results of the experimental and theoretical studies on the spatial and temporal evolution of laser plasma produced as a result of irradiation of an argon/helium gas puff target with laser pulses of 1.3 or 6 ns time duration generated with an Nd:YAG laser system are presented. Imaging and spectral measurements of SXR emission from the plasma, created in the double-stream gas puff target, have been performed with the use of an x-ray streak camera. The analysis of the results of spectral measurements, supported by numerical simulations of plasma x-ray emission, allowed the estimation of the plasma electron temperature and its changes over time. Experimental data were compared with the results of theoretical studies performed using a computer model of plasma hydrodynamics. It was shown that plasma expansion is fast enough to reduce the plasma density in the laser focus area during the laser–plasma interaction.

ELI Gammatron Beamline: A Dawn of Ultrafast Hard X-ray Science

The realization of compact X-ray sources is one of the most intriguing applications of laser-plasma based electron acceleration. These sources based on the oscillation of short micron-sized bunches of relativistic electrons provide femtosecond X-ray pulses that are collimated, bright, and partially coherent. The state-of-the-art laser plasma X-ray sources can provide photon flux of over 1011 photons/shot. The photon flux can further be enhanced with the availability of high repetition rate, high-power lasers, providing capacities complementary to the large scale facilities such as synchrotrons and X-ray free-electron lasers. Even though the optimization of such sources has been underway for the last two decades, their applications in material and biological sciences are still emerging, which entail the necessity of a user-oriented X-ray beamlines. Based on this concept, a high-power-laser-based user-oriented X-ray source is being developed at ELI Beamlines. This article reports on the ELI Gammatron beamline and presents an overview of the research accessible with the ultrashort hard X-ray pulses at the ELI Gammatron beamline.

Nanometer-Resolution Imaging of Living Cells Using Soft X-ray Contact Microscopy

Soft X-ray microscopy is a powerful technique for imaging cells with nanometer resolution in their native state without chemical fixation, staining, or sectioning. The studies performed in several laboratories have demonstrated the potential of applying this technique for imaging the internal structures of intact cells. However, it is currently used mainly on synchrotrons with restricted access. Moreover, the operation of these instruments and the associated sample-preparation protocols require interdisciplinary and highly specialized personnel, limiting their wide application in practice. This is why soft X-ray microscopy is not commonly used in biological laboratories as an imaging tool. Thus, a laboratory-based and user-friendly soft X-ray contact microscope would facilitate the work of biologists. A compact, desk-top laboratory setup for soft X-ray contact microscopy (SXCM) based on a laser-plasma soft X-ray source, which can be used in any biological laboratory, together with several applications for biological imaging, are described. Moreover, the perspectives of the correlation of SXCM with other super-resolution imaging techniques based on the current literature are discussed.

L-Shell X-Ray Conversion Yields for Laser-Irradiated Tin and Silver Foils

We have employed the VULCAN laser facility to generate a laser plasma X-ray source for use in photoionization experiments. A nanosecond laser pulse with an intensity of order 1015 Wcm−2 was used to irradiate thin Ag or Sn foil targets coated onto a parylene substrate, and the L-shell emission in the 3.3–4.4 keV range was recorded for both the laser-irradiated and nonirradiated sides. Both the experimental and simulation results show higher laser to X-ray conversion yields for Ag compared with Sn, with our simulations indicating yields approximately a factor of two higher than those found in the experiments. Although detailed angular data were not available experimentally, the simulations indicate that the emission is quite isotropic on the laser-irradiated side but shows close to a cosine variation on the nonirradiated side of the target as seen experimentally in the previous work.

Simulation of subpicosecond laser-plasma X-ray radiation source

The issue of optimizing laser-plasma X-ray sources is still relevant. In this context, numerical simulation methods are very effective. A series of 2d PIC calculations using the EPOCH code were carried out. The laser pulse duration was 0.7 ps, the peak intensity was 3· 1020 W/cm2, and silicon foil between 2 and 5 μm thick was used as a target. The conversion efficiency into the continuous X-ray spectrum was calculated. The results of comparison of numerical calculations with experimental measurements showed their satisfactory agreement. Keywords: laser plasma, petawatt laser, X-ray radiation, PIC simulation.

Моделирование рентгеновского излучения субпикосекундного лазерно-плазменного рентгеновского источника

The issue of optimizing laser-plasma X-ray sources is still relevant. In this context, numerical simulation methods are very effective. A series of 2d PIC calculations using the EPOCH code were carried out. The laser pulse duration was 0.7 ps, the peak intensity was 3x1e20 W/cm2, and silicon foil between 2 and 5 µm thick was used as a target. The conversion efficiency into the continuous X-ray spectrum was calculated. The results of comparison of numerical calculations with experimental measurements showed their satisfactory agreement.

Shape of flyers driven by metal-electric explosion: experimental method (Part I)

To study the internal mechanism of the metal-electric explosion process, it is necessary to obtain internal information regarding the flow field by performing transmission imaging of the electric explosion plasma. In this study, a laser plasma X-ray source is used as the transient X-ray source. Flyer dynamic X-ray imaging technology of laser plasma X-ray sources is realized by designing and developing flyer-driving devices, laser plasma X-ray devices, and timing synchronization modules. This technology has the advantages of producing images having high-brightness and high temporal and spatial resolutions, and it can adapt to the transmission imaging of high-speed micro flyers and their accompanying plasma. The experimental results show that a clear image of a high-speed flyer and the internal state information of the plasma can be obtained by using this technology. The spatial resolution is 8 μm/pixel, and the temporal resolution is 1 ns. This study presents an experimental method for further studies on the evolution process of plasma and flyers.

Betatron-type laser-plasma x-ray sources generated in multi-electron gas targets

Betatron-type laser-plasma x-rays are recorded simultaneously with their corresponding relativistic electron spectra in a laser wakefield acceleration scheme. The role of the multi-electron gas target in the betatron-type x-ray efficient generation is experimentally examined. A proof of principle experimental study shows that by using a multi-electron gas target and appropriately adjusting the pumping laser intensity an increase in betatron-type x-rays efficiency could be achieved. This is attributed to sophisticated control of the type of the electron injection inside the plasma bubble related to the tunneling ionization process occurring after the laser pulse peak. This method depends primarily on the gas target charge state chosen for attaining the ionization injection scheme and could be extended to a wide range of relativistic laser intensities.

Validation of a laser driven plasma X-ray microfocus source for high resolution radiography imaging.

Hard X-ray radiation with high brightness and high fluxes is nowadays available on the fourth generation of synchrotrons and X-FELs, but the large size and complexity of these sources makes its use difficult for widespread applications. New table top X-ray sources driven by ultrashort high power lasers offer a compelling route to expand the availability of hard X-ray sources. They can be used for advanced imaging techniques, due to its small source size and spatial coherence. We present in this paper the validation of a compact laser-driven X-ray microfocus source for high-resolution radiography imaging. This novel device was built at the Laser Laboratory for Acceleration and Applications (L2A2) at the University of Santiago de Compostela. This paper describes the laser-plasma X-ray source with improved stability and characterize some of its properties. We demonstrate the high-contrast and resolution of the images obtained with this source by using masks with well known geometries, and detailed analysis by using the modulation transfer function. Finally, we discuss the properties of this source in comparison to other compact microfocus X-ray sources.

Coded apertures with scatter and partial attenuation for high-energy high-resolution imaging

Laser-plasma x-ray sources have garnered interest from various communities due to their ability to generate high photon-energies from a small source size. The passive imaging of high energy x-rays and neutrons is also a useful diagnostic in laser-driven fusion capsules as well as laboratory astrophysics experiments which aim to study small samples of transient electron-positron plasmas.Here we study a coded aperture with scatter and partial attenuation included, which we call a ‘CASPA’, and compare them to the more common method of pinhole imaging. As well as discussing the well-known increased throughput of coded apertures, we also show that the decoding algorithm relaxes the need for a thick substrate. We simulate a 511 keV x-ray source through ray-tracing and Geant4 simulations to show how incomplete attenuation of the source by the mask may be less detrimental to imaging using a CASPA than to using a standard pinhole system.

Pulsed laser-plasma soft X-ray source as a compact tool for X-ray absorption spectroscopy of metal oxides

Laser-plasma short-wavelength source based on double-stream gas-puff target and emitting soft X-rays (SXR) is demonstrated as a compact tool suitable for X-ray absorption spectroscopy (XAS) of metal oxides. The source, optimized for emission in the spectral range between 1.5 and 5 nm from a krypton/helium target appears to be useful for the near edge X-ray absorption spectroscopy (NEXAFS) close to the Ti L-edge and Mo M-edge, as well as the K-edge of oxygen. High photon flux from the source enabled the simultaneous acquisition of the reference and transmission spectra using a flat-field grating spectrometer. The measurements conducted in the transmission mode revealed the spectral features near the absorption edges (Ti-L2,3 and Mo-M5) in metal oxides.

Efficient absorption of laser light by nano-porous materials with well-controlled structure

The absorption of high-power nanosecond laser pulses in a porous matter is investigated through theoretical and numerical calculations. The effects of structural properties of the porous target such as size of pores and thickness of solid elements on the laser absorption are investigated. The time and space-dependent absorption coefficient of the laser created partially homogenized plasma is used in a plasma hydrodynamic code to reproduce the laser absorption and plasma formation processes in a porous matter. It is observed that the structural characteristics of the porous material can be optimized to significantly increase the laser energy absorption. For porous targets with pore sizes in the range 30–60 nm a value around 1000 nm could be desirable for the wall thickness to increase the laser absorption efficiency to more than 90%. The results can be advantageous in production of efficient laser absorber targets which are desirable in advanced applications such as inertial confinement fusion and laser-plasma x-ray sources.

Chemical Dosimetry in the “Water Window”: Ferric Ions and Hydroxyl Radicals Produced by Intense Soft X Rays

A standard Fricke dosimeter was used to measure the absorbed dose via the oxidation yields of Fe3+ ions in an aqueous environment induced by soft X rays within the "water window" spectral range. We also exploited the property of a neutral solution containing terephthalic acid as a tool for selective detection of OH radicals. Both dosimetric systems were irradiated using the experimental pulsed laser-plasma soft X-ray source as well as conventional 1.25-MeV gamma rays. Radiation chemical yields of Fe3+ ions and OH radicals were determined to be (5.13 ± 0.94) × 10-1 µmol·J-1 (4.95 ± 0.91 100eV-1) and (2.33 ± 0.35) × 10-2 µmol·J-1 (0.23 ± 0.03 100eV-1), respectively. Measurements were supported by Monte Carlo simulations to estimate the linear energy transfer of the water window radiation. The simulation results are in good agreement with expected linear energy transfer of ions inducing the same Fe3+ ion and OH radical radiation chemical yield.

Overcoming the dephasing limit in multiple-pulse laser wakefield acceleration

The electric field in laser-driven plasma wakefield acceleration is orders of magnitude higher than conventional radio-frequency cavities, but the energy gain is limited by dephasing between the ultra-relativistic electron bunch and the wakefield, which travels at the laser group velocity. We present a way to overcome this limit within a single plasma stage. The amplitude of the wakefield behind a train of laser pulses can be controlled in-flight by modulating the density profile. This creates a succession of resonant laser-plasma accelerator sections and non-resonant drift sections, within which the wakefield disappears and the electrons rephase. A two-dimensional particle-in-cell simulation with four 2.5TW laser pulses produces a 50MeV electron energy gain, four times that obtained from a uniform plasma. Although laser red-shift prevents operation in the blowout regime, the technique offers increased energy gain for accelerators limited to the linear regime by the available laser power. This is particularly relevant for laser-plasma x-ray sources capable of operating at high repetition rates, which are highly sought after.

Time-resolved diffraction with an optimized short pulse laser plasma X-ray source.

We present a setup for time-resolved X-ray diffraction based on a short pulse, laser-driven plasma X-ray source. The employed modular design provides high flexibility to adapt the setup to the specific requirements (e.g., X-ray optics and sample environment) of particular applications. The configuration discussed here has been optimized toward high angular/momentum resolution and uses Kα-radiation (4.51 keV) from a Ti wire-target in combination with a toroidally bent crystal for collection, monochromatization, and focusing of the emitted radiation. Ti-Kα1 photons per pulse with relative bandwidth are delivered to the sample at a repetition rate of 10 Hz. This allows for the high dynamic range (104) measurements of transient changes in the rocking curves of materials as for example induced by laser-triggered strain waves.

Improved stability of a compact vacuum-free laser-plasma X-ray source

We report the development of a stable high-average power X-ray source generated by the interaction of ultrashort laser pulses (35 fs, 1 mJ, 1 kHz) with a solid target in air. The achieved source stability, which is essential for the applications foreseen for these laser-driven plasma accelerators, is due to the combination of precise positioning of the target on focus and the development of a fast rotating target system able to ensure the refreshment of the material at every shot while minimizing positioning errors with respect to the focal spot. This vacuum-free laser-plasma X-ray source provides an average dose rate of 1.5 Sv/h at 30 cm and a repeatability better than 93% during more than 36 min of continuous operation per target.

EXAFS of titanium LIII edge using a compact laboratory system based on a laser-plasma soft X-ray source

We present results of applying a compact laboratory system based on a laser-plasma soft X-ray (SXR) source to the extended X-ray absorption fine structure (EXAFS) spectroscopy of titanium in the vicinity of the LIII absorption edge. The source, operating with a plasma produced as a result of irradiation a krypton/helium double-stream gas-puff target, was optimized for an intense emission in the spectral range between 200 and 700 eV. A broad SXR spectrum and high photon flux are indispensable in the EXAFS studies. The experimental setup assured the simultaneous acquisition of both the reference and the absorption spectra. A grazing-incidence flat-field spectrometer was used to record the spectra. The absorption spectra of a thin (200-nm thick) titanium sample revealed the features in the EXAFS region allowing for quite accurate determination of the radial distances between the atoms. Results are in good agreement with the output of numerical modeling based on photoelectron wave function scattering and with the data reported for using a synchrotron source. This confirms the suitability of the source for using it in the standard EXAFS method.

Abnormal spectral distortion of a silicon sensor-based single photon counting charge coupled device (PIXIS-XB: 1300R) in detecting laser plasma x-ray source of 20–100 keV

Single photon counting using a charge-coupled device (CCD) is a conventional method of measuring the x-ray production of laser plasma. But the spectrum is seriously distorted when measuring a short pulse laser plasma x-ray source. Here we explore this abnormal spectral distortion using a silicon sensor-based CCD (PIXIS-XB: 1300R). We found that the spectral distortion is caused by the continuous spectrum of Compton scattering electrons and the K-α and K-β characteristic lines of Ag elements excited by the harder x-rays of >25 keV. We calibrated the detection efficiency of the incident x-ray, the excitation efficiency of the Ag characteristic lines and the Compton scattering electrons spectrum in the x-ray energy range from 20 keV to 100 keV. We also improve the algorithm of spectrum reconstruction to loosen the exposure level limit markedly. Our effort extends the detecting range of silicon sensor-based CCDs from 0.5–30 keV to 0.5–80 keV. The present analysis can also be used in the spectral extraction of GaAs and CdTe high-Z sensor-based CCDs.

A “water window” tomography based on a laser-plasma double-stream gas-puff target soft X-ray source

A desktop laser-plasma double-stream gas-puff target soft X-ray (SXR) source, operating in the so-called “water window” spectral range (λ = 2.3–4.4 nm). was successfully employed to acquire 3D tomographic images in transmission mode, using a Fresnel zone-plate microscope. The microscope, operating at the He-like nitrogen spectral line, 2.88-nm wavelength, allows acquiring 3D SXR volumetric reconstructions with a resolution of a few microns. The development of a compact system offers the possibility to obtain 3D reconstructed images in relatively short time and in a laboratory environment, without the involvement of large “photon facilities”, such as synchrotrons or free electron lasers, however, with the modest volume resolution at this point. The possibility to obtain three-dimensional images in the SXR wavelength range using a compact, laser-plasma based system may be useful for material and life sciences, where it is required to visualize small features of the samples in 3D that are not visible with a single 2D image. Details about the equipment and the setup constructions as well as some imaged samples will be presented and discussed.