Solid-state Target Research Articles

Application of dense plasma focus devices and lasers in the radiation material sciences for the goals of inertial fusion beyond ignition

Specimens of materials for prospective use in chambers of nuclear fusion reactors with inertial plasma confinement, namely, W, ODS steels, Eurofer 97 steel, a number of ceramics, etc., have been irradiated by dense plasma focus devices and a laser in the Q-switched mode of operation with a wide range of parameters, including some that noticeably exceeded those expected in reactors. By means of 1-ns laser interferometry and neutron measurements, the characteristics of plasma streams and fast ion beams, as well as the dynamics of their interaction with solid-state targets, have been investigated. 3D profilometry, optical and scanning electron microscopy, atomic emission spectroscopy, X-ray elemental and structural analyses, and precise weighing of specimens before and after irradiation have provided data on the roughening threshold and the susceptibility to damage of the materials under investigation. Analysis of the results, together with numerical modeling, has revealed the important role of shock waves in the damage processes. It has been shown that a so-called integral damage factor may be used only within restricted ranges of the irradiation parameters. It has also been found that in the irradiation regime with well-developed gasdynamic motion of secondary plasma, the overall amount of radiation energy is spent preferentially either on removing large masses of cool matter from the material surface or on heating a small amount of plasma to high temperature (and, consequently, imparting to it a high velocity), depending on the power flux density and characteristics of the pulsed irradiation.

Multidimensional four-wave-mixing spectroscopy with squeezed light

A set of nonlinear spectroscopic measurements with quantum light is presented. It involves a strong pump with frequency ωpu and a weak probe at ωpr interacting with a solid state target to generate a conjugated beam ωc=2ωpu−ωpr via nondegenerate four-wave mixing. The χ(3) susceptibility can be measured by the noise spectra of the intensity difference of the squeezed beams. We discuss three spectroscopic setups based on squeezed light: first, single four-wave mixing in a single crystal; second, cascading scheme involving two crystals; third, an SU(1, 1) interferometer based on two separate four-wave mixing processes. We further investigate the microscopic noise and optical losses in all three setups. Simulations are presented for silicon-vacancy color centers in diamond.

High-resolution laboratory reflectometer for the study of x-ray optical elements in the soft and extreme ultraviolet wavelength ranges.

A high-resolution laboratory reflectometer designed for operation in the soft x-ray (SXR) and extreme ultraviolet (EUV) ranges is described. High spectral resolution, up to 0.028 nm, in a wide spectral range is achieved due to the Czerny-Turner monochromator. A laser plasma generated by irradiating a solid-state target with a focused laser beam (wavelength 1.06 µm, pulse energy 0.5 J, duration 4 ns, and pulse repetition rate 10 Hz) is used as a source of SXR and EUV radiation. The goniometer allows the study of curved optical elements with an aperture up to NA = 0.5 and a diameter of up to 500 mm. The methods providing high efficiency of the optical system and spectral resolution in a wide range of wavelengths are described in detail. The problem of taking into account high orders in the recorded spectra of a laser plasma is discussed. A comparison of the measurement results with the described reflectometer and the optics beamline at the BESSY-II synchrotron is given.

Processing and Study of Optical and Electrical Properties of (Mg, Al) Co-Doped ZnO Thin Films Prepared by RF Magnetron Sputtering for Photovoltaic Application.

In this study, high transparent thin films were prepared by radio frequency (RF) magnetron sputtering from a conventional solid state target based on ZnO:MgO:Al2O3 (10:2 wt %) material. The films were deposited on glass and silicon substrates at the different working pressures of 0.21, 0.61, 0.83 and 1 Pa, 300 °C and 250 W of power. X-ray diffraction patterns (XRD), atomic force microscopy (AFM), UV-vis absorption and Hall effect measurements were used to evaluate the structural, optical, morphological and electrical properties of thin films as a function of the working pressure. The optical properties of the films, such as the refractive index, the extinction coefficient and the band gap energy were systematically studied. The optical band gap of thin films was estimated from the calculated absorption coefficient. That parameter, ranged from 3.921 to 3.655 eV, was hardly influenced by the working pressure. On the other hand, the lowest resistivity of 8.8 × 10−2 Ω cm−1 was achieved by the sample deposited at the lowest working pressure of 0.21 Pa. This film exhibited the best optoelectronic properties. All these data revealed that the prepared thin layers would offer a good capability to be used in photovoltaic applications.

Neutron production from thermonuclear reactions in laser-generated plasmas

The production of intense neutron beams via thermonuclear reactions in laser-generated plasmas is investigated theoretically. So far, state-of-the-art neutron beams are produced via laser-induced particle acceleration leading to high-energy particle beams that subsequently interact with a secondary target. Here, we show that neutron beams of two orders of magnitude narrower bandwidth can be obtained from thermonuclear reactions in plasmas generated by Petawatt-class lasers. The intensity of such neutron beams is about one or two orders of magnitude lower than the one of the state-of-the-art laser-driven neutron beams. We study to this end the reaction 2H(d, n)3He in plasmas generated by Petawatt-class lasers interacting with D2 gas jet targets and CD2 solid-state targets. The results also show the possibility of direct measurements of reaction rates at low temperatures of astrophysical interest. In addition, the use of CD2 solid-state targets can also lead to great enhancements of the plasma screening compared to the case of D2 gas jet targets, opening new possibilities to study this so far unsolved issue in the field of astrophysics.

Novel multi-layer plastic-scintillator-based solid active proton target for inverse-kinematics experiments

We have constructed and tested a novel plastic-scintillator-based solid-state active proton target for use in nuclear spectroscopic studies with nuclear reactions induced by an ion beam in inverse kinematics. The active target system, named Stack Structure Solid organic Scintillator Active Target (S4AT), consists of five layers of plastic scintillators, each with a 1-mm thickness. To determine the reaction point in the thickness direction, we exploit the difference in the energy losses due to the beam particle and the charged reaction product(s) in the scintillator material. S4AT offers the prospect of a relatively thick target while maintaining a good energy resolution. By considering the relative energy loss between different layers, the energy loss contributed by coincident unreacted beam particles can be eliminated. Such a procedure, made possible by the multi-layer structure, is essential to eliminate the contribution from coincident unreacted beam particles, thus enabling its operation at a moderate beam intensity of up to a few Mcps. We evaluated the performance of S4AT by measuring the elastic proton–proton scattering using a 70-MeV proton beam at Cyclotron and Radioisotope Center (CYRIC), Tohoku University.

Simultaneous observation of ultrafast electron and proton beams in TNSA

The interaction of ultra-intense high-power lasers with solid-state targets has been largely studied for the past 20 years as a future compact proton and ion source. Indeed, the huge potential established on the target surface by the escaping electrons provides accelerating gradients of TV/m. This process, called target normal sheath acceleration, involves a large number of phenomena and is very difficult to study because of the picosecond scale dynamics. At the SPARC_LAB Test Facility, the high-power laser FLAME is employed in experiments with solid targets, aiming to study possible correlations between ballistic fast electrons and accelerated protons. In detail, we have installed in the interaction chamber two different diagnostics, each one devoted to characterizing one beam. The first relies on electro-optic sampling, and it has been adopted to completely characterize the ultrafast electron components. On the other hand, a time-of-flight detector, based on chemical-vapour-deposited diamond, has allowed us to retrieve the proton energy spectrum. In this work, we report preliminary studies about simultaneous temporal resolved measurements of both the first forerunner escaping electrons and the accelerated protons for different laser parameters.

X-ray-assisted nuclear excitation by electron capture in optical laser-generated plasmas

X-ray assisted nuclear excitation by electron capture (NEEC) into inner-shell atomic holes in a plasma environment generated by strong optical lasers is investigated theoretically. The considered scenario involves the interaction of a strong optical laser with a solid-state nuclear target leading to the generation of a plasma. In addition, intense x-ray radiation from an X-ray Free Electron Laser (XFEL) produces inner-shell holes in the plasma ions, into which NEEC may occur. As case study we consider the $4.85$-keV transition starting from the 2.4 MeV long-lived $^{\mathrm{93m}}$Mo isomer that can be used to release the energy stored in this metastable nuclear state. We find that the recombination into $2p_{1/2}$ inner-shell holes is most efficient in driving the nuclear transition. Already at few hundred eV plasma temperature, the generation of inner-shell holes can allow optimal conditions for NEEC, otherwise reached for steady-state plasma conditions in thermodynamical equilibrium only at few keV. The combination of x-ray and optical lasers presents two advantages: first, NEEC rates can be maximized at plasma temperatures where the photoexcitation rate remains low. Second, with mJ-class optical lasers and an XFEL repetition rate of $10$ kHz, the NEEC excitation number can reach $\sim 1$ depleted isomer per second and is competitive with scenarios recently envisaged at petawatt-class lasers.

Analysis of the Near-Surface Layers of Lithium Coatings Using Laser Induced Breakdown Spectroscopy

The paper reports results of studying the geometry of craters formed by the action of laser pulses on solid-state targets of aluminum and lithium films at a power density on the target of (1–5) × 1010 W/cm2 and variation of the number of pulses in the range of 1–150, as well as the results of ex situ layer-by-layer analysis of lithium films on quartz coatings carried out using the method laser induced breakdown spectroscopy to determine the thickness of the films.

Characterization of the relationship between neutron production and thermal load on a target material in an accelerator-based boron neutron capture therapy system employing a solid-state Li target.

An accelerator-based boron neutron capture therapy (BNCT) system that employs a solid-state Li target can achieve sufficient neutron flux derived from the 7Li(p,n) reaction. However, neutron production is complicated by the large thermal load expected on the target. The relationship between neutron production and thermal load was examined under various conditions. A target structure for neutron production consists of a Li target and a target basement. Four proton beam profiles were examined to vary the local thermal load on the target structure while maintaining a constant total thermal load. The efficiency of neutron production was evaluated with respect to the total number of protons delivered to the target structure. The target structure was also evaluated by observing its surface after certain numbers of protons were delivered. The yield of the sputtering effect was calculated via a Monte Carlo simulation to investigate whether it caused complications in neutron production. The efficiency of neutron production and the amount of damage done depended on the proton profile. A more focused proton profile resulted in greater damage. The efficiency decreased as the total number of protons delivered to the target structure increased, and the rate of decrease depended on the proton profile. The sputtering effect was not sufficiently large to be a main factor in the reduction in neutron production. The proton beam profile on the target structure was found to be important to the stable operation of the system with a solid-state Li target. The main factor in the rate of reduction in neutron production was found to be the local thermal load induced by proton irradiation of the target.

Generation of Deuterium Ions in a Vacuum Arc with Composite Gas-Saturated Cathode and in a Low-pressure Arc

Plasma containing deuterium ions is used, for example, to generate neutron flows. Neutrons are generated in nuclear reactions of interaction of accelerated ion beams extracted from deuterium plasma with a solid-state target containing heavy hydrogen isotopes – deuterium or tritium. Deuterium plasma was produced in two types of arc discharge with cold cathode: in a vacuum arc with composite cathode saturated with deuterium and in a low-pressure arc with inlet of gaseous deuterium into the discharge gap. Results of studies of the mass-to-charge fractions of the plasma of these discharges are presented, and a comparative analysis of the methods of deuterium ion generation in such discharge systems is performed.

^{93m}Mo Isomer Depletion via Beam-Based Nuclear Excitation by Electron Capture.

A recent nuclear physics experiment [C. J. Chiara etal., Nature (London) 554, 216 (2018)NATUAS0028-083610.1038/nature25483] reports the first direct observation of nuclear excitation by electron capture (NEEC) in the depletion of the ^{93m}Mo isomer. The experiment used a beam-based setup in which Mo highly charged ions with nuclei in the isomeric state ^{93m}Mo at 2.4MeV excitation energy were slowed down in a solid-state target. In this process, nuclear excitation to a higher triggering level led to isomer depletion. The reported excitation probability P_{exc}=0.01 was solely attributed to the so-far unobserved process of NEEC in lack of a different known channel of comparable efficiency. In this work, we investigate theoretically the beam-based setup and calculate excitation rates via NEEC using state-of-the-art atomic structure and ion stopping-power models. For all scenarios, our results disagree with the experimental data by approximately 9 orders of magnitude. This stands in conflict with the conclusion that NEEC was the excitation mechanism behind the observed depletion rate.

Target fabrication for laser-ion acceleration research at the Technological Laboratory of the LMU Munich

The Technological Laboratory of LMU Munich supplies various types of solid-state target for laser plasma experiments at the Centre for Advanced Laser Applications in Garching. Our main focus here is on the production of free-standing, thin foil targets, such as diamond-like-carbon foils, carbon nanotube foams (CNFs), plastic, and gold foils. The presented methods comprise cathodic arc deposition for DLC targets, chemical vapor deposition for CNFs, a droplet and spin-coating process for plastic foil production, as well as physical vapor deposition that has been optimized to provide ultrathin gold foils and tailored sacrifice layers. This paper reviews our current capabilities, which are a result of a close collaboration between target production processes and experiment, using high-power chirped pulse amplification laser systems over the past eight years.

New collective trampoline mechanism of accelerated ion-plasma sputtering

Results of the newly found anomalous accelerated plasma ion sputtering of solid-state targets are presented. The rate of sputtered particles comprises from one to several tens of atoms per one impinging ion, ion energies being in the range of tens to five hundreds electron volts. The anomalous sputtering is realized for ion current density in the plasma flow being not less than 10 mA cm−2 and threshold specific power of the ion flow from 0.7 to 2 W cm−2 depending on the target material. The qualitative explanation proposed dubbed ‘collective trampoline accelerated sputtering’. The collective nature of this sputtering regime differs from the conventional pair collision cascade one. The trampoline sputtering leads to the intense texturing of the materials with submicron characteristic structure sizes. The new sputtering method could be applied (i) in the technology of plasma ion deposition of the functional coatings to replace the widespread magnetron sputtering sources of lower productivity; (ii) in the photovoltaics to solve the problem of the so-called ‘black’ multi-crystalline silicon in order to reduce the light reflectance coefficient; (iii) in the production of silicon composite anodes for lithium ion batteries to enhance their lifetimes regarding charge/discharge cycles, for which the final positive result is largely determined by the surface structure of the silicon wafer for photovoltaics and the surface structure of the anode current collector for lithium-ion batteries.

Dependence of neutrons generated by 7Li(p,n) reaction on Li thickness under free-air condition in accelerator-based boron neutron capture therapy system employing solid-state Li target

PurposeAn accelerator-based boron neutron capture therapy (BNCT) system with a solid-state Li target is reported to have degradation of the Li target. The degradation reduces the Li thickness, which may change spectra of the generated neutrons corresponding to the Li thickness. This study aims to examine the relationship between the Li thickness and the generated neutrons and to investigate the effects of the Li thickness on the absorbed dose in BNCT. MethodThe neutron energy spectra were calculated via Monte Carlo simulation for Li thicknesses ranging from 20 to 150 μm. Using the system, the saturated radioactivity of gold induced by reactions between 197Au and the generated neutrons was evaluated with the simulation and the measurement, and those were compared. Additionally, for each Li thickness, the saturated radioactivity was compared with the number of generated neutrons. The absorbed doses delivered by 10B(n,α)7Li, 14N(n,p)14C, 1H(n, g)2H, and (n,n′) reactions in water were also calculated for each Li thickness. ResultsThe measurement and simulation indicated a reduction in the number of neutrons due to the degradation of the Li target. However, the absorbed doses were comparable for each Li thickness when the requisite number of neutrons for BNCT was delivered. Additionally, the saturated radioactivity of 198Au could be a surrogate for the number of neutrons even if the Li thickness was varied. ConclusionsNo notable effect to the absorbed dose was observed when required neutron fluence was delivered in the BNCT even if the degradation of the Li was observed.

Single-shot electrons and protons time-resolved detection from high-intensity laser–solid matter interactions at SPARC_LAB

Laser–plasma interactions have been studied in detail over the past twenty years, as they show great potential for the next generation of particle accelerators. The interaction between an ultra-intense laser and a solid-state target produces a huge amount of particles: electrons and photons (X-rays and $\unicode[STIX]{x03B3}$ -rays) at early stages of the process, with protons and ions following them. At SPARC_LAB Test Facility we have set up two diagnostic lines to perform simultaneous temporally resolved measurements on both electrons and protons.

Validity of extracting photoionization time delay from the first moment of streaking spectrogram**Project supported by the Talent Introduction Foundation of Qiannan Normal University of Nationalities, China (Grant No. qnsyrc201619), Natural Science Foundation of Guizhou Provincial Education Department for Young Talents, China (Grant No. Qian Education Contract KY[2017]339), and Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office

Photoionization time delays have been studied in many streaking experiments in which an attosecond pulse is used to ionize the atomic or solid state target in the presence of a dressing infrared laser field. Among the methods of extracting the time delay from the streaking spectrogram, the simplest one is to calculate the first moment of the spectrogram and to measure its offset relative to the vector potential of the infrared field. The first moment method has been used in many theoretical simulations and analysis of experimental data, but the meaning of this offset needs to be investigated. We simulate the spectrograms and compare the extracted time delay from the first moment with the input Wigner delay. In this study, we show that the first moment method is valid only when the group delay dispersions corresponding to both the spectral phase of the attosecond pulse and the phase of the single-photon transition dipole matrix element of the target are small. Under such circumstance, the electron wave packet behaves like a classical particle and the extracted time delay can be related to a group delay in the photoionization process. To avoid ambiguity and confusion, we also suggest that the photoionization time delay be replaced by photoionization group delay and the Wigner time delay be replaced by Wigner group delay.

Fabrication of Two-Dimensional Arrays of Fluorescent Centers in Single-Crystalline Diamond Using Particle Beam Writing

Micrometer-scale patterning was performed using the particle beam writing technique with a focused heavy-ion microbeam, allowing the creation of a unique two-dimensional distribution of fluorescent centers in single-crystalline diamond. The focused nitrogen microbeam was scanned over the target of single-crystalline diamond prepared by chemical vapor deposition to create nitrogen-vacancy (NV) centers at defined positions. Imaging using a custom-built confocal fluorescence microscopy system revealed that the desired NV distribution was generated in the target crystal with a spatial resolution similar to the beam resolution. A two-dimensional matrix barcode test pattern was successfully generated in a diamond substrate to demonstrate the encryption of information inside a solid-state target.

Aggregation-induced emission enhancement (AIEE)-active mechanofluorochromic tetraphenylethene derivative bearing a rhodamine unit

A novel tetraphenylethene derivative with a rhodamine unit was successfully synthesized via high-efficiency Suzuki coupling reaction. The highly solid-state emissive target fluorescent molecule exhibited significative aggregation-induced emission enhancement (AIEE) feature. Furthermore, the luminogen showed reversible mechanochromic luminescence behavior involving color change from orange to red. In addition, the powder X-ray diffraction (XRD) test results verified that the mechanofluorochromic phenomenon of luminogen 1 was attributed to a morphological transformation between the crystalline and amorphous states.

Laser Irradiated Foam Targets: Absorption and Radiative Properties

An experimental campaign to characterize the laser radiation absorption of foam targets and the subsequent emission of radiation from the produced plasma was carried out in the ABC facility of the ENEA Research Center in Frascati (Rome). Different targets have been used: plastic in solid or foam state and aluminum targets. The activated different diagnostics allowed to evaluate the plasma temperature, the density distribution, the fast particle spectrum and the yield of the X-Ray radiation emitted by the plasma for the different targets. These results confirm the foam homogenization action on laser-plasma interaction, mainly attributable to the volume absorption of the laser radiation propagating in such structured materials. These results were compared with simulation absorption models of the laser propagating into a foam target.