Intense Laser-solid Interaction Research Articles

Energy coupling in intense laser solid interactions: Material properties of gold

In the double-cone ignition inertial confinement fusion scheme, a high density deuterium–tritium (DT) fuel is rapidly heated with high-flux fast electrons, which are generated by short and intense laser pulses. A gold cone target is usually used to shorten the distance between the critical surface and the compressed high density DT core. The material properties of the solid gold may affect the generation and transport of fast electrons significantly, among which the effects of ionization and collision are the main concerns. In this work, the effects of ionization, collision, and blow-off plasma on the laser energy absorption rate are investigated using the LAPINS code: A three-stage model is adopted to explain the mechanism of fast electron generation and the change in the laser energy absorption rate. With the increase in the charge state of Au ions, the laser–plasma interaction transfers to the later stage, resulting in a decrease in the laser energy absorption rate. Collision has both beneficial and harmful effects. On the one hand, collision provides a thermal pressure, which makes it easier for electrons to escape into the potential well in front of the target and be accelerated in the second stage. On the other hand, collision increases stopping power and suppress electron recirculation within the target in the third stage. The vacuum sheath field behind the target enhances the electron circulation inside the target and, thus, improves the laser energy absorption; however, this effect will be suppressed when the blow-off plasma density behind the target increases or collision is considered.

Maximizing MeV x-ray dose in relativistic laser-solid interactions

Simulations of intense laser-solid interactions reveal the optimal configuration of laser intensity and duration at constant energy to maximize 1--5 MeV x-ray yield. A phenomenological model predicts the hot-electron temperature as a function of the laser parameters and density scale length.

On the control of electron heating for optimal laser radiation pressure ion acceleration

We study the onset of electron heating in intense laser–solid interactions and its impact on the spectral quality of radiation pressure accelerated ions in both hole boring and light sail regimes. Two- and three-dimensional particle-in-cell (PIC) simulations are performed over a wide range of laser and target parameters and reveal how the pulse duration, profile, polarization and target surface stability control the electron heating, the dominant ion acceleration mechanisms and the ion spectra. We find that the onset of strong electron heating is associated with the growth of the Rayleigh–Taylor-like instability at the front surface and must be controlled to produce high-quality ion beams, even when circularly polarized lasers are employed. We define a threshold condition for the maximum duration of the laser pulse that allows mitigation of electron heating and radiation pressure acceleration of narrow energy spread ion beams. The model is validated by three-dimensional PIC simulations, and the few experimental studies that reported low energy spread radiation pressure accelerated ion beams appear to meet the derived criteria. The understanding provided by our work will be important in guiding future experimental developments, for example for the ultrashort laser pulses becoming available at state-of-the-art laser facilities, for which we predict that proton beams with $\sim$ 150–250 MeV, $\sim$ 30% energy spread, and a total laser-to-proton conversion efficiency of $\sim$ 20% can be produced.

Off-harmonic optical probing of high intensity laser plasma expansion dynamics in solid density hydrogen jets

Due to the non-linear nature of relativistic laser induced plasma processes, the development of laser-plasma accelerators requires precise numerical modeling. Especially high intensity laser-solid interactions are sensitive to the temporal laser rising edge and the predictive capability of simulations suffers from incomplete information on the plasma state at the onset of the relativistic interaction. Experimental diagnostics utilizing ultra-fast optical backlighters can help to ease this challenge by providing temporally resolved inside into the plasma density evolution. We present the successful implementation of an off-harmonic optical probe laser setup to investigate the interaction of a high-intensity laser at 5.4times 10^{21},hbox {W/cm}^{2} peak intensity with a solid-density cylindrical cryogenic hydrogen jet target of {5},{upmu }mathrm{m} diameter as a target test bed. The temporal synchronization of pump and probe laser, spectral filtering and spectrally resolved data of the parasitic plasma self-emission are discussed. The probing technique mitigates detector saturation by self-emission and allowed to record a temporal scan of shadowgraphy data revealing details of the target ionization and expansion dynamics that were so far not accessible for the given laser intensity. Plasma expansion speeds of up to (2.3 pm 0.4)times 10^{7},hbox {m/s} followed by full target transparency at {1.4},{mathrm{ps}} after the high intensity laser peak are observed. A three dimensional particle-in-cell simulation initiated with the diagnosed target pre-expansion at {-0.2},{mathrm{ps}} and post processed by ray tracing simulations supports the experimental observations and demonstrates the capability of time resolved optical diagnostics to provide quantitative input and feedback to the numerical treatment within the time frame of the relativistic laser-plasma interaction.

Extended particle absorber for efficient modeling of intense laser–solid interactions

An extended thermal particle boundary condition is devised to more efficiently and accurately model laser–plasma interactions in overdense plasmas. Particle-in-cell simulations of such interactions require many particles per cell, and a large region of background plasma is often necessary to correctly mimic a semi-infinite plasma and avoid electron refluxing from a truncated plasma. For long-pulse lasers of many picoseconds, such constraints can become prohibitively expensive. Here, an extended particle boundary condition (absorber) is designed that instantaneously stops and re-emits energetic particles streaming toward the simulation boundary over a defined region, allowing sufficient time and space for a suitably cool return current to develop in the background plasma. Tunable parameters of the absorber are explained, and simulations using the absorber with a 3-ps laser are shown to accurately reproduce those of a causally separated boundary while requiring only 20% the number of particles.

Modulation of Oxygen Vacancy on Midinfrared Light‐Induced Ferroelectricity of KTaO3

Nonlinear phononics plays an important role in intense laser–solid interactions, especially in light‐induced superconductivity, polarized and ferroelectric phases. Herein, by nonlinear coupling of phonon modes, the modulation of oxygen vacancy defects on the dynamic properties of midinfrared light‐induced ferroelectricity of cubic KTaO3 is studied. It is found that oxygen vacancy causes the coupling constant l between the high‐frequency phonon mode and the low‐frequency phonon mode to be negative, which means that the lowest frequency infrared‐active phonon mode (Qlz) is softened when highest‐frequency infrared‐active phonon mode (Qhx) is exteriorly pumped; it is proved that the Qlz mode can be softened when the KTaO3 contains 1/8, 1/4, and 2/3 of oxygen vacancies. When the KTaO3 contains a quarter of oxygen vacancies, it can effectively enhance the coupling constant, reduce the damping effect, and induce the ferroelectricity of KTaO3. But the Qlz mode cannot be softened when there are 3/8 of oxygen vacancies in KTaO3. Overall, the findings strongly support that turning the photoinduced ferroelectricity of KTaO3 by oxygen vacancy defects is desirable, which provides an effective strategy for controlling the light‐induced ferroelectricity in the all‐optical devices.

Dispersion calibration for the National Ignition Facility electron-positron-proton spectrometers for intense laser matter interactions.

Electron-positron pairs, produced in intense laser-solid interactions, are diagnosed using magnetic spectrometers with image plates, such as the National Ignition Facility Electron-Positron-Proton Spectrometers (EPPSs). Although modeling can help infer the quantitative value, the accuracy of the models needs to be verified to ensure measurement quality. The dispersion of low-energy electrons and positrons may be affected by fringe magnetic fields near the entrance of the EPPS. We have calibrated the EPPS with six electron beams from a Siemens Oncor linear accelerator (linac) ranging in energy from 2.7 MeV to 15.2 MeV as they enter the spectrometer. A Geant4 traveling-wave optical parametric amplifier of superfluorescence Monte Carlo simulation was set up to match depth dose curves and lateral profiles measured in water at 100cm source-surface distance. An accurate relationship was established between the bending magnet current setting and the energy of the electron beam at the exit window. The simulations and measurements were used to determine the energy distributions of the six electron beams at the EPPS slit. Analysis of the scanned image plates together with the determined energy distribution arriving in the spectrometer provides improved dispersion curves for the EPPS.

Electron deflection driven by terahertz pulses from laser-plasma interactions

Traditional electron accelerators usually use the radio-frequency field as the driving field. In contrast, the terahertz radiation, with a shorter wavelength and a higher accelerating gradient, has the capability to work as the driver of compact electron accelerators in the future. In addition, the terahertz pulse can also provide an ultrafast modulated field to compress and measure the electron pulse width. Recently, the interactions between terahertz filed and electrons have attracted increasing attention. Intense laser-solid interactions can produce both a high-energy terahertz pulse and a high-charge electron beam simultaneously. This advantage enables a unique platform for the terahertz control of electrons and the terahertz pump-electron probe experiments. Here, by taking a feasible experimental layout as an example, we first investigate the interactions of a terahertz pulse with a co-propagating electron beam. Systematic discussions on the influences of several parameters on the terahertz field-induced electron deflection are presented via numerical simulations. We find that a strong terahertz electric field or the long pulse duration will enhance the deflection. Furthermore, the deflection depends strongly on the terahertz waveform. A preliminary experiment is performed to qualitatively demonstrate the relevant analysis.

Direct electron attachment to fast hydrogen in 10−9 contrast 1018 W cm−2 intense laser solid target interaction

The interaction of an ultra-short (<30 fs), high-contrast (<10−9), high-intensity (>1018 W cm−2) laser pulse with a solid target is not generally known to produce and accelerate negative ions. The transient accelerating electrostatic-fields are so strong that they ionize any atom or negative ion at the target surface. In spite of what may appear to be unfavourable conditions, here it is reported that H− ions extending up to 80 keV are measured from such an interaction. The H− ion flux is about 0.1% that of the H+ ions at 20 keV. These measurements employ a recently developed temporally-gated Thomson parabola ion spectrometry diagnostic which significantly improves signal-to-noise ratios. Electrons that co-propagate with the fast protons cause a two-step charge-reduction reaction. The gas phase three-body attachment of electrons to fast neutral hydrogen atoms accounts for the measured H− yield. It is intriguing that such a fundamental gas-phase reaction, involving the attachment of an electron to a hydrogen atom, has not been observed in laboratory experiments previously. Laser-produced plasma offers an alternative environment to the conventional charged particle beam experiments, in which such atomic physics processes can be investigated.

Proton beams from intense laser-solid interaction: Effects of the target materials

We report systematic studies of laser-driven proton beams produced with micrometer-thick solid targets made of aluminum and plastic, respectively. Distinct effects of the target materials are found on the total charge, cutoff energy, and beam spot of protons in the experiments, and these are described well by two-dimensional particle-in-cell simulations incorporating intrinsic material properties. It is found that with a laser intensity of 8 × 1019 W/cm2, target normal sheath acceleration is the dominant mechanism for both types of target. For a plastic target, the higher charge and cutoff energy of the protons are due to the greater energy coupling efficiencies from the intense laser beams, and the larger divergence angle of the protons is due to the deflection of hot electrons during transport in the targets. We also find that the energy loss of hot electrons in targets of different thickness has a significant effect on the proton cutoff energy. The consistent results obtained here further narrow the gap between simulations and experiments.

Toward attosecond control of electron dynamics in two-dimensional materials

Attosecond motion of strongly driven electrons encodes information on intense laser–solid interactions, leading to material- and laser-dependent photoemission and high-harmonic generation (HHG). Here, we investigate sub-cycle control over the electron dynamics in two-dimensional (2D) materials via adjusting the relative phase of two-color pulses. Electrons in 2D solids are sensitive to the exact shape of the optical field, showing properties similar to those of isolated atoms. Accompanied by spectra modulations, the resultant harmonic yields are enhanced and the cutoff energy increases to a ratio as large as 24% when an optimized phase is applied. Different from the atomic-like HHG, however, the cutoff energy is linearly dependent on the maximum strength of the light electric field, in line with that shown in conventional bulk solids. Thus, two-dimensional materials provide a unique platform where both bulk and atomic electron dynamics can be investigated. Our work suggests a simple way to generate optimized harmonic emission with existing experimental laser technology and offers a powerful tool for analyzing attosecond quantum dynamics during laser–solid interactions.

Ion-acoustic gardner solitons and double layers in magnetized electron-positron-ion quantum plasma

The nonlinear propagation of ion-acoustic Gardner solitons (GSs) and double layers (DLs) in electron-positron-ion (EPI) plasma in the presence of an external static magnetic field has been examined. The Korteweg–de Vries (KdV), modified KdV (mKdV), and Gardner equations have been derived by the reductive perturbation technique. It is seen that the amplitude, polarity, speed, width of such solitons are significantly modified by the species densities and by the obliquity angle. Compressive or rarefactive KdV solitons are obtained, but only compressive solitons are obtained in the mKdV case, wherever Gardner positive and negative DLs exist. The results of our present investigation may be useful for understanding the nonlinear wave propagation in dense quantum plasmas such as those existing in white dwarfs, neutron stars and intense laser-solid matter interaction experiments as an example of laboratory plasmas.

Modeling terahertz emission from the target rear side during intense laser-solid interactions.

Relativistic laser-solid target interaction is a powerful source of terahertz radiation where broadband terahertz radiation is emitted from the front and rear surfaces of the target. Even though several experimental works have reported the generation of subpicosecond duration gigawatt peak power terahertz pulses from the target rear surface, the underlying physical process behind their origin is still an open question. Here we discuss a numerical model that can accurately reproduce several aspects of the experimental results. The model is based on the charged particle dynamics at the target rear surface and the evolution of the charge separation field. We identify the major contributors that are responsible for broadband terahertz emission from the rear surface of the target.

Proton beam emittance growth in multipicosecond laser-solid interactions

High intensity laser-solid interactions can accelerate high energy, low emittance proton beams via the target normal sheath acceleration (TNSA) mechanism. Such beams are useful for a number of applications, including time-resolved proton radiography for basic plasma and high energy density physics studies. In experiments using the OMEGA EP laser system, we perform the first measurements of TNSA proton beams generated by up to 100 ps, kilojoule-class laser pulses with relativistic intensities. By systematically varying the laser pulse duration, we measure degradation of the accelerated proton beam quality as the pulse length increases. Two dimensional particle-in-cell simulations and simple scaling arguments suggest that ion motion during the rise time of the longer pulses leads to extended preformed plasma expansion from the rear target surface and strong filamentary field structures which can deflect ions away from uniform trajectories and therefore lead to large emittance growth.

Effects of resistive magnetic instability and electron refluxing on fast electron dynamics and bremsstrahlung radiation characteristics in relativistic laser-solid interaction

Fast electron transport as well as bremsstrahlung emissions induced by relativistic intense laser solid interactions are investigated with a newly developed particle-in-cell (PIC) simulation code. This PIC code is capable of simulating intense laser-solid interactions by taking into account many coupled physical processes, like laser-plasma interaction, ionization dynamics, collisional dynamics, electron transport and photon emission. The target effects can be distinguished according to their intrinsic atomic properties. In this work, aluminium, copper, and gold targets with different thickness are considered. It is shown that with the fixed laser intensity of 1020 W cm−2, the angular distribution of bremsstrahlung emission, significantly depends on target material and target thickness. For low Z material, the electrons and the emitted photons tend to be collimated along the laser propagation direction. For high Z material, the angular distribution is broader, which is attributed to the self-generated resistive magnetic field-induced electron momentum divergence. Moreover, the simulation result shows that for Al target, the total emission energy increases with decreasing target thickness, because electron refluxing induces considerable backward emission. Comparatively, for Au target, the electron refluxing effect on the total emission energy can be ignored, because the backward emission is somewhat buried within the broad distribution.

Parametric study of a high amplitude electromagnetic pulse driven by an intense laser

An investigation of the electromagnetic (EM) pulse produced by high intensity laser-solid interactions has been carried out by employing the proton probing technique. Laser parameters including energy, pulse duration, and intensity were varied to investigate the influence on the EM pulse amplitude. The data reveal that the amplitude of the EM pulse depends on the incident laser energy and the pulse duration. The optimum pulse length for a given laser energy is found to be close to 100 fs. The net charge associated with the traveling EM pulse has been found to be dependent on the laser intensity, in a good agreement with a semiempirical model. The understanding of the EM pulse is important for the postacceleration of laser driven proton beams.

Experimental study of fast electron generation in intense short duration laser solid interaction at grazing incidence

We report a study on the generation of relativistic fast electrons during the interaction of an ultrashort (45 fs) laser pulse with a variety of targets (Al, Cu, Mo, Si, polyvinyl chloride, and Perspex), at a nearly grazing angle of incidence (∼70°) at an intensity of ∼1.2 × 1018 W/cm2. The fast electron emission direction was found to be dependent on the laser pulse amplified spontaneous emission (ASE) contrast, its extent, and laser pulse duration. For a 45 fs laser pulse and an ASE extent of 1 ns, the electron emission direction changes from a nearly target surface (∼7° ± 2°, divergence: ∼30°) to a specular direction (∼20° ± 2°, divergence: ∼20°) when the contrast was decreased from ∼10−7 to ∼10−6. It further shifts toward target normal with an increase in the ASE duration from 1 ns to 5 ns. For an ASE contrast of ∼10−7 and ∼10−6 with a 1 ns duration, preplasma density scale lengths of ∼30 μm and ∼45 μm were estimated, respectively, from the HELIOS hydrodynamic code. For a longer ASE duration in the range of 1–5 ns, the density scale length was in the range of ∼45–140 μm. Further, for a 45 fs pulse and a 1 ns ASE level, the electron beam charge was found to be higher in the case of Mo (∼52 pC) and Si (∼122 pC) targets compared to the other targets (∼23–34 pC). A further increase in the charge of the electron beam of ∼1.45× (∼75 pC) and ∼2.7× (∼141 pC) was also observed with Mo for a longer pulse duration of ∼170 fs (ASE 1 ns) and a larger ASE duration of 5 ns at (45 fs pulse), respectively. The electron beam properties were found to be independent of laser polarization (p, s, and circular) except a slightly higher charge (∼1.5×) was observed in the case of p-polarized laser light. The fast electron spectrum showed a maximum energy of 1 MeV with electron temperatures of ∼390 keV and ∼320 keV for the target surface and specular direction, respectively. Electron acceleration and their emission are explained in terms of applicable mechanisms in preformed plasma on the target surface considering the role of surface fields which is also established by the 2D particle in cell simulation.

Bremsstrahlung emission profile from intense laser-solid interactions as a function of laser focal spot size

The bremsstrahlung x-ray emission profile from high intensity laser-solid interactions provides valuable insight to the internal fast electron transport. Using penumbral imaging, we characterise the spatial profile of this bremsstrahlung source as a function of laser intensity by incrementally increasing the laser focal spot size on target. The experimental data shows a dual-source structure; one from the central channel of electrons, the second a larger substrate source from the recirculating electron current. The results demonstrate than an order of magnitude improvement in the intensity contrast between the two x-ray sources is achieved with a large focal spot, indicating preferable conditions for applications in radiography. An analytical model is derived to describe the transport of suprathermal electron populations that contribute to substrate and central channel sources through a target. The model is in good agreement with the experimental results presented here and furthermore is applied to predict laser intensities for achieving optimum spatial contrast for a variety of target materials and thicknesses.

Characterization of fast electron divergence and energy spectrum from modeling of angularly resolved bremsstrahlung measurements

Characteristics of fast electrons generated in an intense laser-solid target interaction are studied by modeling angularly resolved bremsstrahlung measurements with a hybrid Particle-In-Cell code, Large Scale Plasmas. The experiment was performed using the 50 TW Leopard laser at the Nevada Terawatt Facility. A 100 μm thick Cu foil was irradiated by the 15 J, 0.35 ps laser at a peak laser intensity of 2 × 1019 W/cm2. Bremsstrahlung produced by transport and recirculation of the fast electrons in the foil was measured with two differential filter-stack x-ray spectrometers at 22° and 40° from the laser axis. The two-spectrometer signals simultaneously fit by varying single slope temperatures (Thot) and divergence angles (θ) enable for determining Thot and θ to be 1.1 ± 0.3 MeV and 15° ± 8°, respectively. The Thot inferred from the bremsstrahlung signals agrees with that from an escaped electron measurement, suggesting that that the bremsstrahlung is predominantly produced by the transport of the high energy fast electrons in the first pass.

Novel scintillator-based x-ray spectrometer for use on high repetition laser plasma interaction experiments.

The characterisation of x-rays from laser-plasma interactions is of utmost importance as they can be useful for both monitoring electron dynamics and also applications in an industrial capacity. A novel versatile scintillator x-ray spectrometer diagnostic that is capable of single shot measurements of x-rays produced from laser-plasma interactions is presented here. Examples of the design and extraction of the temperature of the spectrum of x-rays produced in an intense laser-solid interaction (479 ± 39 keV) and the critical energy from a betatron source (30 ± 10 keV) are discussed. Finally, a simple optimisation process involving adjusting the scintillator thickness for a particular range of input spectra is demonstrated.