High-resolution x-ray monochromatic imaging for laser plasma diagnostics based on toroidal crystal

Monochromatic x-ray imaging is an essential method for plasma diagnostics related to density information.Large-field high-resolution monochromatic imaging of a He-like iron(Fe XXV)Kαcharacteristic line(6.701 keV)for laser plasma diagnostics was achieved using a developed toroidal crystal x-ray imager.A high-index crystal orientation Ge(531)wafer with a Bragg angle of 75.37°and the toroidal substrate were selected to obtain sufficient diffraction efficiency and compensate for astigmatism under oblique incidence.A precise offline assembly method of the toroidal crystal imager based on energy substitution was proposed,and a spatial resolution of 3-7μm was obtained by toroidal crystal imaging of a 600 line-pairs/inch Au grid within an object field of view larger than 1.0 mm.The toroidal crystal x-ray imager has been successfully tested via side-on backlight imaging experiments of the sinusoidal modulation target and a 1000 line-pairs/inch Au grid with a linewidth of 5μm using an online alignment method based on dual positioning balls to indicate the target and backlighter.This paper describes the optical design,adjustment method,and experimental results of a toroidal crystal system in a laboratory and laser facility.

Generation and characterization of millimeter-scale plasmas for the research of laser plasma interactions on Shenguang-Ⅲ prototype

In order to produce millimeter-scale plasmas for the research of laser-plasma interactions （LPIs）, gasbag target is designed and tested on Shenguang-III prototype laser facility. The x-ray pinhole images show that millimeter-scale plasmas are produced with the gasbag. The electron temperature inferred from the stimulated Raman scattering （SRS） spectrum is about 1.6 keV. The SRS spectrum also indicates that the electron density has a fiat region within the duration of 200 ps. The obvious differences between the results of the gasbag and that of the void half hohlraum show the feasibility of the gasbag target in creating millimeter-scale plasmas. The LPIs in these millimeter-scale plasmas may partially mimic those in the ignition condition because the duration of the existence of a flat plasma density is much larger than the growth time of the two main instabilities, i.e., SRS and stimulated Brillouin scattering （SBS）. So we make the conclusion that the gasbag target can be used to research the large-scale LPIs.

A 7 T Pulsed Magnetic Field Generator for Magnetized Laser Plasma Experiments

A pulsed magnetic field generator was developed to study the effect of a magnetic field on the evolution of a laser-generated plasma. A 40 kV pulsed power system delivered a fast （-230 ns）, 55 kA current pulse into a single-turn coil surrounding the laser target, using a capacitor bank of 200 nF, a laser-triggered switch and a low-impedance strip transmission line. A one-dimensional uniform 7 T pulsed magnetic field was created using a Helmholtz coil pair with a 6 mm diameter. The pulsed magnetic field was controlled to take effect synchronously with a nanosecond heating laser beam, a femtosecond probing laser beam and an optical Intensified Charge Coupled Device （ICCD） detector. The preliminary experiments demonstrate bifurcation and focusing of plasma expansion in a transverse magnetic field.

Beam Line Design of Compact Laser Plasma Accelerator

A compact laser plasma accelerator that is a novel accelerator based on the interaction of ultra-intense laser and plasmas is being built now at Peking University. According to the results of experiments and numerical simulations, a beam line combining the advantages of quadrupole and analyzing magnets is designed to deliver proton beams with energy ranging from 1 to 44MeV, energy spread within ±5% and 10^6-8 protons per pulse. It turns out that the existence of space charge force of protons can be ignored for the increase of transverse and longitudinal envelopes even in the case of 10^9 protons in one pulse. To cope with the challenge to obtain a uniform distribution of protons at the final experiment target in laser acceleration, we manipulate the envelope beam waist in the Y direction to a proper position and obtain a relatively good distribution uniformity of protons with an energy spread of 0-±5%0.

Experimental Investigation of Glass-Layer Confined Ablation in Laser Plasma Propulsion

Laser plasma propulsion in glass-layer confined ablation was experimentally investigated. The results showed that compared to that of direct ablation, the coupling coefficient was enhanced over ten times. By observing the plasma expansion and calculating the ablation pressure, it was found that a higher ablation pressure and larger glass mass resulted in a higher coupling coefficient in the confined laser ablation.

The Application of Laser Plasma in Ophthalmology

In this paper the production and development of laser plasma is introduced, and the contrlbutlon of laser biomedicine and laser plasma technology to ophthalmology is analyzed. In the end, the latest three progresses (laser photocoagulation, photorefractive keratotomy and laser lridectomy of laser plasma applications in ophthalmology are preserited.

Time Evolution of the Multicomponent Laser Plasma Parameters

The emission of CulnSe2-based laser ablation plasma has been investigated at a distance of I mm from the target surface. The plasma was formed by radiation of the neodymium laser （5 ×10^8 W.cm^-2; 1,064 nm; 20 ns; 12 Hz）. The temporal dynamics have been obtained for the plasma parameters, such as the population of excited states of atoms, electron temperature and density. Two peaks were observed in the temporal profile of the population of excited states. The temporal dependence of electron temperature was singly peaked. During the time interval of 0-300 ns, the electron temperature varied in the range of 1.6-1.2 eV and the density of electrons varied in the range of 1.3 × 10^16 cm^-3 to 9 × 10^14 cm^-3. It has been established that at the selected conditions of plasma formation saved ratio of copper and indium, which correspond to stoichoimetric target composition.

Acceleration and radiation of externally injected electrons in laser plasma wakefield driven by a Laguerre-Gaussian pulse

By using three-dimensional particle-in-cell simulations, externally injected electron beam acceleration and radiation in donut-like wake fields driven by a Laguerre-Gaussian pulse are investigated. Studies show that in the acceleration process the total charge and azimuthal momenta of electrons can be stably maintained at a distance of a few hundreds of micrometers. Electrons experience low-frequency spiral rotation and high-frequency betatron oscillation, which leads to a synchrotron-like radiation. The radiation spectrum is mainly determined by the betatron motion of electrons. The far field distribution of radiation intensity shows axial symmetry due to the uniform transverse injection and spiral rotation of electrons. Our studies suggest a new way to simultaneously generate hollow electron beam and radiation source from a compact laser plasma accelerator.

Filamentation instability in two counter-streaming laser plasmas

The filamentation instability was observed in the interaction of two counter-streaming laser ablated plasma flows, which were supersonic, collisionless, and also closely relevant to astrophysical conditions. The plasma flows were created by irradiating a pair of oppositely standing plastic （CH） foils with Ins-pulsed laser beams of total energy of 1.7 kJ in two laser spots. With characteristics diagnosed in experiments, the calculated features of Weibel-type filaments are in good agreement with measurements.

Development of Debris-free Laser Plasma Sources for EUV Lithography in CIOMP

We have been developing debris-free laser plasma sources for EUV lithography since 1996. Two types of debris-free sources, such as cryogenic target and gas-puff target laser plasma sources, were designed and built up in CIOMP. EUV radiation spectra of the sources with a variety of targets have been obtained by different ways.

Damage characteristics of laser plasma shock wave on rear surface of fused silica glass

The damage to the rear surface of fused silica under the action of high power laser is more severe than that incurred by the front surface,which hinders the improvement in the energy of the high power laser device.For optical components,the ionization breakdown by laser is a main factor causing damage,particularly with laser plasma shock waves,which can cause large-scale fracture damage in fused silica.In this study,the damage morphology is experimentally investigated,and the characteristics of the damage point are obtained.In the theoretical study,the coupling and transmission of the shock wave in glass are investigated based on the finite element method.Thus,both the magnitude and the orientation of stress are obtained.The damage mechanism of the glass can be explained based on the fracture characteristics of glass under different stresses and also on the variation of the damage zone’s Raman spectrum.In addition,the influence of the glass thickness on the damage morphology is investigated.The results obtained in this study can be used as a reference in understanding the characteristics and mechanism of damage characteristics induced by laser plasma shock waves.

Radiation effects of electrons on multilayer FePS3studied with laser plasma accelerator

Space radiation with inherently broadband spectral flux poses a huge danger to astronauts and electronics on aircraft,but it is hard to simulate such feature with conventional radiation sources. Using a tabletop laser-plasma accelerator, we can reproduce exponential energy particle beams as similar as possible to these in space radiation. We used such an electron beam to study the electron radiation effects on the surface structure and performance of two-dimensional material(Fe PS3).Energetic electron beam led to bulk sample cleavage and damage between areas of uneven thickness. For the Fe PS3sheet sample, electron radiation transformed it from crystalline state to amorphous state, causing the sample surface to rough.The full widths at the half maximum of characteristic Raman peaks became larger, and the intensities of characteristic Raman peaks became weak or even disappeared dramatically under electron radiation. This trend became more obvious for thinner samples, and this phenomenon was attributed to the cleavage of P–P and P–S bonds, destabilizing the bipyramid structure of [P2S6]4-unit. The results are of great significance for testing the maximum allowable radiation dose for the two-dimensional material, implying that Fe PS3cannot withstand such energetic electron radiation without an essential shield.

X-Ray and VUV Spectra from the Laser Plasma Produced with “Kanal-2” Facility

The paper presents experimental results obtained on “Kanal-2” facility. Laser radiation focusing on the surface of plane magnesium targets created the high temperature plasma, which emitted X-ray and vacuum ultraviolet (VUV) radiation. This radiation spectrum was investigated with two spectrographs: the mica crystal spectrograph (working range 8.2 ? - 9.6 ?) and the grazing incidence VUV spectrograph (working rage 30 ? - 130 ?). A set of beryllium stepwise attenuators appended the diagnostic complex and allowed us to get an approximated picture of a continuous spectrum within the range of 2.2 ? - 6.2 ?. The estimation of the plasma electron temperature Te from the ratio between the intensity of the dielectronic satellites and the resonance line gives Te ~ 180 eV. The ratio between the intensity of the resonance and intercombination lines gives the electron density of the emitting zone ne ~ 2 × 1019 cm-3. Some lines observed within the spectral range of 8.5 ? - 9.1 ? belong to none of the transitions of Mg ions. Perhaps the observed spectrum is determined by the transitions in so-called hollow ions of Mg, i.e. in the ions with unfilled inner shells. The spectra obtained with the grazing incidence spectrograph and with the minimum-directioned discrepancy iteration method of spectrum reconstruction from the attenuation curve in the beryllium stepwise attenuators are also presented.

Investigation on the hydrodynamics of slab x-ray laser plasma by nonuiform line focused laser irradiation

Based on the two-dimensional model, this paper compares the hydrodynamics of slab x-ray laser plasma produced by different nonuniform line focused irradiations. It finds that the average intensity and the duration of laser pulse and the overall shape of the intensity distribution in the focal line have different influences on the plasma. Calculations show that the evolution of temperature variation is more sensitive to the pulse duration and the electron density variation is more sensitive to the pulse intensity. Pulses with duration of 200 ps to 500 ps and with intensity of 0.2 TW/cm2 to 1.0 TW/cm2 are proved acceptable in slab x-ray lasers.

Prospect of ultrahigh-resolution fast neutron absorption spectroscopy based on a laser plasma electron accelerator

Fast neutron absorption spectroscopy is widely used in the study of nuclear structure and element analysis. However,due to the traditional neutron source pulse duration being of the order of nanoseconds, it is difficult to obtain a high-resolution absorption spectrum. Thus, we present a method of ultrahigh energy-resolution absorption spectroscopy via a high repetition rate, picosecond duration pulsed neutron source driven by a terawatt laser. The technology of single neutron count is used, which results in easily distinguishing the width of approximately 20 keV at 2 MeV and an asymmetric shape of the neutron absorption peak. The absorption spectroscopy based on a laser neutron source has one order of magnitude higher energy-resolution power than the state-of-the-art traditional neutron sources, which could be of benefit for precisely measuring nuclear structure data.

High performance micromachining of sapphire by laser induced plasma assisted ablation(LIPAA)using GHz burst mode femtosecond pulses

GHz burst-mode femtosecond(fs)laser,which emits a series of pulse trains with extremely short intervals of several hundred picoseconds,provides distinct characteristics in materials processing as compared with the conventional irradiation scheme of fs laser(single-pulse mode).In this paper,we take advantage of the moderate pulse interval of 205 ps(4.88 GHz)in the burst pulse for high-quality and high-efficiency micromachining of single crystalline sapphire by laser induced plasma assisted ablation(LIPAA).Specifically,the preceding pulses in the burst generate plasma by ablation of copper placed behind the sapphire substrate,which interacts with the subsequent pulses to induce ablation at the rear surface of sapphire substrates.As a result,not only the ablation quality but also the ablation efficiency and the fabrication resolution are greatly improved compared to the other schemes including single-pulse mode fs laser direct ablation,single-pulse mode fs-LIPAA,and nanosecond-LIPAA.

Carrier-Envelope Phase-Dependent Phase Matching of Terahertz Emission in Laser Plasma

We investigate off-axis phase-matched terahertz(THz)radiation in laser plasma pumped by few-cycle laser pulses.We find that the THz amplitude and angular distributions in the far field are sensitively dependent on the pump pulse’s focal carrier-envelope phase(CEP).Ring-like profiles of THz radiation are obtained at CEP values of 0.5πand 1.5π,due to the inversely symmetric local THz waveforms emitted before and after laser focus.Off-axis phase-matched THz radiation offers a tool to accurately measure the CEP of few-cycle pulses at the center of a medium.

Generation and regulation of electromagnetic pulses generated by femtosecond lasers interacting with multitargets

Ultrashort and powerful laser interactions with a target generate intense wideband electromagnetic pulses(EMPs).In this study,we report EMPs generated by the interactions between petawatt(30 fs,1.4×10^(20) W/cm^(2))femtosecond(fs)lasers with metal flat,plastic flat,and plastic nanowire-array(NWA)targets.Detailed analyses are conducted on the EMPs in terms of their spatial distribution,time and frequency domains,radiation energy,and protection.The results indicate that EMPs from metal targets exhibit larger amplitudes at varying angles than those generated by other types of targets and are enhanced significantly for NWA targets.Using a plastic target holder and increasing the laser focal spot can significantly decrease the radiation energy of the EMPs.Moreover,the composite shielding materials indicate an effective shielding effect against EMPs.The simulation results show that the NWA targets exert a collimating effect on thermal electrons,which directly affects the distribution of EMPs.This study provides guidance for regulating EMPs by controlling the laser focal spot,target parameters,and target rod material and is beneficial for electromagnetic-shielding design.