Laser-produced Plasma Soft X-ray Research Articles

Charge-separated spectra of suprathermal highly charged bismuth ions in a dual laser-produced plasma soft x-ray source.

We investigated the charge-separated spectra of highly charged suprathermal bismuth (Bi) ions from a dual laser-produced plasma soft x-ray source developed for soft x-ray microscopy. The charge distribution of these suprathermal ions emitted from a solid planar Bi target was measured by an electrostatic energy analyzer. The maximum ionic charge state was observed to be Z = 17 and to possess a maximum energy of about 200 keV. This evaluation provides important information essential for the development of debris mitigation schemes in a soft x-ray microscope.

Laser-produced plasma soft x-ray source based on an aerosol target

Laser produced plasma (LPP) soft x-ray and extreme ultraviolet sources utilize various types of targets. Some of them are based on gaseous targets. The most important disadvantage of such targets is the very limited number of elements that can be used in the gaseous form under normal conditions, including chemical compounds in the gaseous state. In this paper, the authors propose a new type of target, constituting an extension of the gaseous target, namely aerosol target, based on solutions of solid chemical compounds in liquids. Unlike previous attempts to produce aerosol targets from overheated liquids, in our system, the target is created by injection of a mist, preformed in a dedicated cell, into the vacuum chamber. Such targets have many advantages: the emission spectra can be adjusted by choosing proper chemical compounds, the aerosol target is elongated, hence, the laser focus can be positioned at a relatively long distance from the nozzle outlet, its average density can be higher compared to gaseous targets. In this work, such a target and its usefulness for the LPP soft x-ray source was demonstrated. The target was investigated by the soft x-ray backlighting method. Its density distribution at the position of the laser focus was determined. The emission spectra from the soft x-ray range, corresponding to multiple charged ions, originating from the solvent and the diluted salt were measured. Based on numerical simulations of the spectra, an ionic composition of the resulting plasmas was determined.

Oxygen K-shell photoabsorption spectra of photoionized CO2 plasmas

We report oxygen K-shell photoabsorption measurements in CO2 plasmas, using a table-top laser produced plasma soft x-ray (SXR) and extreme ultraviolet source. Significant differences are observed between the photoabsorption spectra of the neutral CO2 molecule and photoionized CO2 plasmas, as new spectral features emerge in the case of the latter. These features are attributed to photoexcitation in O, O+, O2+ species identified with the aid of calculations performed with the Cowan suite of atomic structure codes. Such signatures of atomic ions in the photoionized plasma point to the fragmentation of the parent molecule upon absorption of an SXR photon.

NEXAFS at nitrogen K-edge and titanium L-edge using a laser-plasma soft x-ray source based on a double-stream gas puff target

We present a possibility of reaching higher energy absorption edges of organic materials, beyond the carbon K-edge, in the near edge X-ray absorption fine structure (NEXAFS) spectroscopy technique using a compact laser-produced plasma soft X-ray (SXR) source based on a double-stream gas puff target. The source was optimized for emission in the SXR spectral range from 1.5 to 5 nm wavelength using the krypton/helium target. The emission spectrum of the source and the absorption spectrum of the investigated sample were measured simultaneously by means of a grazing incidence spectrometer, equipped with a single, large aperture diffractive element. Based on both spectra, the optical density was computed for the silicon nitride membranes in a transmission mode, to reveal the NEXAFS features near the nitrogen K-edge. Moreover, due to spectral narrowing of the SXR emission by the use of titanium filter, reaching the titanium L-edge was also possible. Multiple SXR pulse data were compared to single SXR pulse (single-shot) data as well as to the numerical simulations. In this paper, the detailed information about the source, spectroscopy system, and the results of NEXAFS measurements is presented and discussed.

Selection of target elements for laser-produced plasma soft x-ray sources.

We demonstrated the upper limitation to the number of shots, i.e., target lifetime, together with the number of photons emitted in the water-window soft x-ray spectral region from a number of targets used as sources in this spectral region, for multi-shot irradiation at the same position on the target surface. The spectra involved result from unresolved transition arrays originating from n=3-n=4 transitions in medium-Z element plasmas and from n=4-n=4 transitions originating in high-Z plasmas. The output flux was maintained for the highest number of shots in the case of the high melting point element molybdenum, and the total output in the water window was 7.7×1013 photons/sr at a laser power density of 1.2×1014 W/cm2.

Development of a compact laser-produced plasma soft X-ray source for radiobiology experiments

A desk-top laser-produced plasma (LPP) source of soft X-rays (SXR) has been developed for radiobiology research. The source is based on a double-stream gas puff target, irradiated with the focused beam of a commercial Nd:YAG laser. The source has been optimized to get a maximum photon emission from LPP in the X-ray “water window” spectral wavelength range from 2.3nm (i.e., an absorption edge of oxygen) to 4.4nm (i.e., an absorption edge of carbon) (280–540eV in photon energy units) by using argon gas-puff target and spectral filtering by free-standing thin foils. The present source delivers nanosecond pulses of soft X-rays at a fluence of about 4.2×103photons/μm2/pulse on a sample placed inside the vacuum chamber. In this paper, the source design, radiation output characterization measurements and initial irradiation experiments are described. The source can be useful in addressing observations related to biomolecular, cellular and organisms’ sensitivity to pulsed radiation in the “water window”, where carbon atoms absorb X-rays more strongly than the oxygen, mostly present in water. The combination of the SXR source and the radiobiology irradiation layout, reported in this article, make possible a systematic investigation of relationships between direct and indirect action of ionizing radiation, an increase of a local dose in carbon-rich compartments of the cell (e.g., lipid membranes), an experimental estimation of a particular role of the Auger effect (in particular in carbon atoms) in the damage to biological systems, and the study of ionization/excitation-density (LET – Linear Energy Transfer) and dose-rate effects in radiobiology.

Low temperature photoionized Ne plasmas induced by laser-plasma EUV sources

AbstractIn this work, two laser-produced plasma (LPP) sources – extreme ultraviolet (EUV) and a LPP soft X-ray (SXR) source were used to create Ne photoionized plasmas. A radiation beam was focused onto a gas stream, injected into a vacuum chamber synchronously with the radiation pulse. EUV radiation spanned a wide spectral range with pronounced maximum centered at λ≈11 nm, while in case of the SXR source spectral maximum was at λ≈1.4 nm. Emission spectra of photoionized plasmas created this way were measured in a wide spectral range λ = 10–100 nm. The dominating spectral lines originated from singly charged ions (Ne II) and neutral atoms (Ne I). For the highest radiation fluence, spectral lines originating from Ne III and even Ne IV species were detected. Differences between the experimental spectra, obtained for all irradiation conditions, were analyzed. They were attributed either to different fluence or spectral distribution of driving photons.

Soft x-ray characterization of a silicon p–n photodiode using a laser produced plasma source (abstract)

Silicon p–n diodes have proved to be excellent soft x-ray detectors due to their high sensitivity, nominally flat response, and long term stability. Advances in fabrication techniques have overcome many of the limitations of older silicon diodes by minimizing the thickness of the surface dead layer which would otherwise absorb low energy x-ray photons. Silicon photodiodes with extremely thin (80 Å) surface dead layers are now available.1 One of these diodes has been characterized for spectral sensitivity at x-ray photon energies of 163 eV and 1.4 keV using a laser produced plasma soft x-ray source. Measurements have also been made to characterize the impulse response using fourth harmonic laser light from a short pulse (80 ps full width half maximum) NdYAG laser.

Soft x-ray reflectometry with a laser-produced plasma source

A compact soft x-ray reflectometer usable in a small laboratory is developed for measurement of the soft x-ray reflectance of a multilayer mirror in a wide wavelength range, i.e. , including the water window (23 to 44Å). In this reflectometer, the reflectance can be measured as a function of wavelength or incidence angle. A laser-produced plasma soft x-ray source, which is small but has high brightness, makes it possible to construct the compact soft x-ray reflectometer. The soft x rays generated by the source are monochromatized with a grazing incidence reflection grating. Incidence angle dependency of the reflectance can be measured from 2 deg (almost normal incidence) to 85 deg. Reflectances of a Ni/Ti multilayer and a Mo/Si multilayer are measured, and 10.9% at a wavelength of 40.8Å and an angle of 75.1 deg and 38% at a wavelength of 135.5Å and an angle of 25.7 deg from normal incidence are obtained, respectively.

X-ray streak camera observations of laser produced plasma corona development

Combination of pinhole imaging of laser produced plasma soft X-ray emission and X-ray streak photography allows simultaneous space and time resolved study of the axial development of the high density region of the plasma created from a plane Cu target by a 6 nsec 1.06 μ laser pulse.