High X-ray Conversion Efficiency Research Articles

Multi-keV x-ray radiator from titanium cylindrical cavity at the Shenguang-III prototype laser facility

Great efforts have been made to create a bright K-shell source using the thin metal cylindrical cavities in the past few decades. Several metal materials such as titanium (Ti), iron, and so on have been tested for high x-ray conversion efficiency mainly at the OMEGA and NIF laser facilities. Recently, x-ray sources in Ti K-shell transition energy range were investigated at the Shenguang-III prototype laser facility with ∼5 kJ laser energy. The experiments were aimed to reproduce the previous ones at OMEGA, but with an extraordinarily small volume of cylindrical cavity and detailed characterizations of the x-ray source. The cavities were 800 μm inner diameter, 800 μm length, and 30 μm thick plastic tubes supporting 1 μm thick Ti. Seven laser beams were focused to 200 μm diameter. The combination of the small cavity volume and the focused laser spots is intended to improve the electron temperature with limited laser energy since the electron temperature is a key issue for high x-ray conversion efficiency. Thomson scattering was adopted to experimentally probe the electron temperatures at special time and space zones as well as the average temperature obtained from the Ti K-shell spectrum. The evolutions of the electron temperature and density are predicted by the radiation hydrodynamic simulation. A top view and two photon energy bands of x-ray source images provide a way to directly observe the plasma movement toward the cavity axis and distinguish the different emission mechanisms between the Ti K-shell and lower energy x rays. Six Higher-energy x-ray detectors located at different angles were used to record the Ti K-shell x-ray emission and demonstrate its isotropic feature. The characteristics of the x-ray radiate intensity including the time evolution, the angular distribution, and the total yields for both the photon energy regions above and below 4 keV are compared between the cavity and planar targets. Obviously, different behaviors were found between the two photon energy regions and the two types of targets. The x-ray conversion efficiency of the Ti cylinder was determined to be ∼4% and ∼21% in 4π sr in the Ti K-shell (4–7 keV) and <4 keV range, respectively. The Ti K-shell conversion efficiency obtained in the present experiments is between the ones driven by 13.5 and 4.5 kJ laser energy at OMEGA.

Investigation of high X-ray conversion efficiency Kr filled gas sources at the National Ignition Facility

We report on the performance of high x-ray fluence Kr K-shell sources that are being developed for high energy density experiments. These targets are 4.1 mm in diameter 4.4 mm tall hollow epoxy tubes having a 40 μm thick wall holding 1.5 atm of Kr gas. For these shots, the National Ignition Facility laser delivered a nominally constant total energy of ≈750 kJ of 351 nm (3ω) light at the three power levels [e.g., ≈120 (low), ≈145 (medium), and ≈210 TW (high)]. The Kr K-shell (Ephoton = 8–20 keV) x-ray radiant intensity and radiant energy (kJ/sr) of these sources were found to increase as a function of laser power but began to plateau at the highest laser power. The Kr K-shell radiant energy increased from ≈1 kJ/sr at ≈120 TW to ≈2 kJ/sr at ≈210 TW. Radiation hydrodynamics simulations predict radiant energies to be always higher than these measurements. The increase in K-shell emission is attributed to its strong dependence on the electron temperature. Electron temperature distributions were inferred from measured Heα and Lyα line emission through the use of a genetic algorithm and Scram modeling. The inferred temperatures from the experiment are 20% to 30% higher than those predicted from modeling.

Investigation of the cylindrical vacuum hohlraum energy in the first implosion experiment at the SGIII laser facility

The cylindrical vacuum hohlraum energy at the SGIII laser facility [X. T. He and W. Y. Zhang, Eur. Phys. J. D 44, 227 (2007) and W. Zheng et al., High Power Laser Sci. Eng. 4, e21 (2016)] is investigated for the first time. The hohlraum size and the laser energy are intermediate between the Nova and NIF typical hohlraum experiments. It is found that the SGIII hohlraum exhibits an x-ray conversion efficiency of about 85%, which is more close to that of the NIF hohlraum. The LARED simulations of the SGIII hohlraum underestimate about 15% of the radiation flux measured from the laser entrance hole, while the capsule radiation drive inferred from the x-ray bangtime is roughly consistent with the experiments. The underestimation of the SGIII hohlraum radiation flux is mainly caused by the more enclosed laser entrance hole in the LARED simulation. The comparison between the SGIII and NIF hohlraum simulations by LARED indicates that the LARED generally underestimates the measured radiation flux by 15% for the high x-ray conversion efficiency hohlraums, while it can roughly predict the capsule radiation drive inside the hohlraum.

Bright x-ray stainless steel K-shell source development at the National Ignition Facility

High x-ray conversion efficiency (XRCE) K-shell sources are being developed for high energy density experiments for use as backlighters and for the testing of materials exposed to high x-ray fluxes and fluences. Recently, sources with high XRCE in the K-shell x-ray energy range of iron and nickel were investigated at the National Ignition Facility (NIF). The x-ray conversion efficiency in the 5–9 keV spectral range was determined to be 6.8% ± 0.3%. These targets were 4.1 mm diameter, 4 mm tall hollow epoxy tubes having a 50 μm thick wall supporting a tube of 3 to 3.5 μm thick stainless steel. The NIF laser deposited ∼460 kJ of 3ω light into the target in a 140 TW, 3.3 ns square pulse. The absolute x-ray emission of the source was measured by two calibrated Dante x-ray spectrometers. Time resolved images filtered for the Fe K-shell were recorded to follow the heating of the target. Time integrated high-resolution spectra were recorded in the K-shell range.

The X-ray properties of the energetic pulsar PSR J1838���0655

We present and interpret several new X-ray features of the X-ray pulsar PSR J1838−0655. The X-ray data are obtained from the archival data of Chandra, RXTE and Suzaku. We combine all these X-ray data and fit the spectra with different models. We find that the joint spectra are difficult to fit with a single power law; a broken power-law model with a break at around 6.5 keV can improve the fit significantly. The photon index changes from Γ= 1.0 (below 6.5 keV) to 1.5 (above 6.5 keV); this indicates a softer spectral behaviour at hard X-rays. The X-ray flux at 2–20 keV is found to be 1.6 × 10−11 erg cm−2 s−1. The conversion efficiency from the spin-down luminosity is ∼0.9 per cent at 0.8–10 keV, which is much higher than that (∼10−3 to 10−4 per cent) of the pulsars that show similar timing properties. We discuss non-thermal radiation mechanisms for the observed high X-ray conversion efficiency and find that emission from the magnetosphere of a greatly inclined rotator is the most favourable interpretation for the conversion rate and the pulse profiles at X-ray bands. A line feature close to 6.65 keV is also detected in the spectra of Suzaku/X-ray imaging spectrometer; it might be the Kα emission of highly ionized Fe surrounding the pulsar.

High Kα x-ray conversion efficiency from extended source gas jet targets irradiated by ultra short laser pulses

The absolute laser conversion efficiency to Kα-like inner shell x-rays (integrated from Kα to Kβ) is observed to be an order of magnitude higher in argon gas jets than in solid targets due to enhanced emission from higher ionization stages following ultrashort pulse laser irradiation. Particle-in-cell and spectral simulations indicate that these observations are consistent with Kα emission from a warm Ar plasma subject to hot electron inner-shell ionization. These results demonstrate that gas jet targets are bright, high conversion efficiency, high repetition rate, debris-free multi-keV x-ray sources for spectrally resolved scattering and backlighting of rapidly evolving dense matter.

Underdense radiation sources: Moving towards longer wavelengths

Underdense radiation sources have been developed to provide efficient laboratory multi-keV radiation sources for radiography and radiation hardening studies. In these plasmas laser absorption by inverse bremsstrahlung leads to high x-ray conversion efficiency because of efficient ionization of the low density aerogel or gas targets. Now we performing experiments in the soft x-ray energy regime where the atomic physics models are much more complicated. In recent experiments at the NIKE laser, we have irradiated a Ti-doped SiO 2 aerogel with up to 1650J of 248 nm wavelength light. The absolute Ti L-shell emission in the 200-800 eV range is measured with a diagnostic that uses a transmission grating coupled to Si photodiodes. We will give an overview of the temporally-resolved absolutely calibrated spectra obtained over a range of conditions. Eventually we hope to extend our studies to x-ray production in the EUV range.

Soft X-ray emission from noble gas clusters excited by an ultrashort KrF laser pulse

X-ray conversion efficiency from laser energy was measured for various noble-gas clusters excited by an ultrashort KrF laser pulse. For Xe clusters, the conversion efficiency from laser energy into soft X-rays with wavelengths from 5 to 20 nm was estimated to be 1.1 %/sr, which was comparable to that obtained for solid targets. This high X-ray conversion efficiency was due to the large absorption fraction of the laser light by Xe clusters.

Investigation of energy transfer in plane laser-irradiated targets with high X-ray conversion efficiency

Results of experimental and computational investigations devoted to energy transfer mechanisms and X-ray conversion efficiency in laser-produced plasma are presented and discussed. The layers of different thicknesses and diameters deposited on the plane mylar substrate were irradiated by the focused beam of Nd:glass laser. Spectrally, temporally, and spatially resolved measurements of soft X-ray emission have been carried out at power densities of 1013-1014 W/cm2. The conditions of “re-emission” zone formation have been established. Radiative heat conductivity is shown to be the important energy transfer mechanism in the experimental conditions under investigation.

Efficient keV x-ray emission from plasmas generated with 4 ps KrF excimer laser pulses

A high x-ray conversion efficiency is observed from plasmas produced with 4 ps pulses at 249 nm using commercial KrF lasers. Copper plasmas convert 11% of laser energy into hν∼1.2 keV photons, carbon plasmas 6.8% into ‘‘water window’’ photons (0.28 keV<hν<0.53 keV) and aluminum plasmas 0.4% into hν∼1.6 keV photons.

Materials For Laser-Plasma X-Ray Source Targets Applicable To Lithography

AbstractIn recent years, several X-ray lithography experiments involving laser plasma sources have been reported. The high X-ray conversion efficiency (η – 10%) in the keV range observed in various laboratories, using high laser intensities (I > 5 × 1012 W cm−2), has already made these sources an attractive alternative to the electron storage rings for X-ray lithography in proximity printing (XRL). In addition, X-rayproduction in the region around 130 Å has also been investigated at intensities of about 1011 W cm−2 for applications to X-ray Projection Lithography (XRPL). Conversion efficiency up to 1% into a 3 Å bandwidth has been demonstrated. In principle, a large variety of X-ray spectra can be obtained from a laser plasma source depending on the laser-target parameters. However, high conversion efficiencies, in a given spectral range, can only be achieved under specific plasma conditions (target atomic number, plasma temperature, plasma volume and spatio-temporal expansion...). This leads to some restrictions in possible target materials and irradiation conditions (laser wavelength, laser intensity, pulse duration). In this paper, we will discuss this physical aspect basing our analysis on both the theoretical and experimental studies of the X-ray spectra in different X-ray energy ranges and for various target atomic numbers. In addition, we will examine issues related to the laser plasma X-ray source design for both XRL and XRPL applications.

High X-ray conversion efficiency with target irradiation by a frequency tripled Nd : Glass laser

High conversion efficiency of laser energy into X-rays from a laser irradiated target is of great interest for a variety of dynamical (pulsed) studies, e.g.: radiography of laser-imploded targets, structure determination by diffraction and absorption fine-structure, and X-ray laser pumping. We report here on a frequency tripled Nd : glass laser used to irradiate targets of various materials at ∼5 x 10 14 W/ cm 2. We find conversion efficiencies of between 1% and 0.1% (with respect to the incident laser energy) for individual X-ray lines between 1.8 and 7.8 keV. These efficiencies are more than an order of magnitude higher than whose achieved with 1.06 μm lasers.

The efficient identification of relativistic particles by transition radiation

A system of transition-radiation detectors has been constructed and exposed to beams of electrons and pions in the energy range 3–15 GeV at SLAC. Transition radiation was generated in a variety of stacks of mylar foils (radiators), and background tests have been performed with solid targets replacing the radiators. The radiation intensity was detected with 7 multiwire proportional chambers and with high- Z scintillators [Ba(Pb)SO 4, CaWO 4]. To obtain a high X-ray conversion efficiency, the proportional chambers had a thickness of 4.5 cm and were filled with xenon-CO 2 or krypton-CO 2 mixtures. The transition-radition yield has been measured, and its dependence on the particle energy and on the radiator configuration has been investigated. While the raw data demonstrate a good separation between electron- and pion-induced signals, a more detailed analysis of the data has been performed and will be discussed. The results of this analysis show that a very efficient identification of individual particles is possible. Typically, a detection efficiency for electrons above 90%, combined with a pion-electron discrimination ratio of 10 −3, has been achieved. Some conclusions with respect to the design of a practical detector for relativistic particles will be drawn.