Laser-generated Plasma Research Articles

Kinetics of Ions Emitted From Nanosecond-Pulsed Laser-Generated Plasma in Broad Range of the Laser Fluence

Carbon plasma was produced by irradiating a graphite target with nanosecond-pulsed Nd:YAG laser beam in the broad range of the laser fluence (25– $2196\,\,\text {J}\cdot \text {cm}^{-2}$ ). A time resolving flat Langmuir probe was used to record the carbon ion pulses. For the investigated range of laser fluence, approximately three orders of magnitude increase in total ion charge (0.03– $2.74~\mu \text{C}$ ) and peak ion energy (4–492 eV) was observed. The ion pulses were deconvoluted using Coulomb–Boltzmann-shifted time function to estimate the number of ion charge states and ion energy of various charge states. The number of available ions charge states in the plasma was found to gradually increase with the laser fluence and C6+ was observed only at the laser fluence of $2196\,\,\text {J}\cdot \text {cm}^{-2}$ . At a given laser fluence, peak energies of the ions are proportional to their charge state. The observed correlation between ions charge state and ions energy points toward the definite electrostatic acceleration mechanism. The equivalent ion accelerating potential was estimated for various values of the laser fluence and compared with the available literature data. These findings are quite useful for the development of laser plasma-based ion sources.

Laser-generated plasmas in length scales relevant for thin film growth and processing: simulation and experiment

In pulsed laser deposition, thin film growth is mediated by a laser-generated plasma, whose properties are critical for controlling the film microstructure. The advent of 2D materials has renewed the interest in how this ablation plasma can be used to manipulate the growth and processing of atomically thin systems. For such purpose, a quantitative understanding of the density, charge state, and kinetic energy of plasma constituents is needed at the location where they contribute to materials processes. Here, we study laser-induced plasmas over expansion distances of several centimeters from the ablation target, which is the relevant length scale for materials growth and modification. The study is enabled by a fast implementation of a laser ablation/plasma expansion model using an adaptive Cartesian mesh solver. Simulation outcomes for KrF excimer laser ablation of Cu are compared with Langmuir probe and optical emission spectroscopy measurements. Simulation predictions for the plasma-shielding threshold, the ionization state of species in the plasma, and the kinetic energy of ions, are in good correspondence with experimental data. For laser fluences of 1–4 J cm−2, the plume is dominated by Cu0, with small concentrations of Cu+ and electrons at the expansion front. Higher laser fluences (e.g. 7 J cm−2) lead to a Cu+ -rich plasma, with a fully ionized leading edge where Cu2+ is the dominant species. In both regimes, simulations indicate the presence of a low-density, high-temperature plasma expansion front with a high degree of ionization that may play a significant role in doping, annealing, and kinetically-driven phase transformations in 2D materials.

Polarized proton beams from laser-induced plasmas

We report on the concept of an innovative source to produce polarized proton/deuteron beams of a kinetic energy up to several GeV from a laser-driven plasma accelerator. Spin effects have been implemented into the particle-in-cell (PIC) simulation code VLPL (Virtual Laser Plasma Lab) to make theoretical predictions about the behavior of proton spins in laser-induced plasmas. Simulations of spin-polarized targets show that the polarization is conserved during the acceleration process. For the experimental realization, a polarized HCl gas-jet target is under construction using the fundamental wavelength of a Nd:YAG laser system to align the HCl bonds and simultaneously circularly polarized light of the fifth harmonic to photo-dissociate, yielding nuclear polarized H atoms. Subsequently, their degree of polarization is measured with a Lamb-shift polarimeter. The final experiments, aiming at the first observation of a polarized particle beam from laser-generated plasmas, will be carried out at the 10 PW laser system SULF at SIOM, Shanghai.

Investigation of Magnetic Fields in Z-Pinches via Multi-MeV Proton Deflectometry

Proton deflectometry is a promising way for mapping electric and magnetic fields in high-density and high-temperature plasmas, where an application of the classical methods (B-dot probes, Faraday rotation, and Zeeman splitting) is limited. It is based on the detection of a multi-MeV proton beam deflected in examined B-fields. In the past years, it has been successfully utilized in laser-generated plasmas for E-field and B-field measurements. Using our numerical code, we investigate the capabilities of proton deflectometry as a diagnostic method of MA Z-pinches. We simulate proton trajectories propagating through typical Z-pinch B-fields in two fundamental experimental setups (radial and axial) in order to study synthetic images (deflectograms). We demonstrate where proton deflectometry might be beneficial for the Z-pinch research. We explain a formation of the key features of deflectograms, which give us information about a profile and strength of the Z-pinch B-fields. We introduce a $B_{L}$ parameter, denoting an effective B-field averaged along the deflected proton orbit and show its importance for the proton deflectometry.

Effects of corrosion-inhibiting surface treatments on irradiated microstructure development in Ni-base alloy 718

The objective of this study is to understand the influence of surface treatments, namely laser shock peening (LSP) and ultrasonic nanocrystal surface modification (UNSM), on the irradiation tolerance of a Ni-base alloy. Surface treatments such as LSP and UNSM have been shown to mitigate the potential for primary water stress corrosion cracking (PWSCC). As such, there is emerging interest in the development and implementation of LSP and UNSM for nuclear reactor components. The LSP process utilizes a laser-generated plasma, while UNSM utilizes mechanical contact at ultrasonic vibration speeds, to create near-surface compressive residual stresses, high dislocation densities, twins and subgrains, and nanostructuring of the workpiece. These resultant microstructural changes can substantially affect the creation and evolution of irradiation damage. Herein, we study precipitation hardened Ni-base Alloy 718+, which is utilized in reactor components exposed to PWSCC-inciting environments. Specimens are treated with LSP or UNSM, then irradiated with 2.0 MeV protons to 7 displacements per atom (dpa) at 500°C. The dislocation line density is roughly an order of magnitude larger in the unirradiated LSP and UNSM specimens than in the baseline (untreated) specimen. Irradiation-induced dislocation loop nucleation results in an increase in the areal density of dislocation-type defects. Irradiation also induces disordering of the γ′ precipitates; this disordering appears more extensive in the baseline than in the LSP and UNSM specimens. Results are considered in the context of overall sink strength. Finally, irradiation-induced softening is observed in all specimens through nanoindentation, and is ascribed to the overall change in sink strength, resulting from the competition between γ′ disordering and dislocation loop nucleation.

Magnetic field enhanced detection of heavy metals in soil using laser induced breakdown spectroscopy

A combination of magnetic field and Laser Induced Breakdown Spectroscopy (LIBS) has been used to improve the detection limit of heavy elements in different Soil samples. The plasma plume was generated by a focused Nd: YAG laser (532 nm) beam on the surface of soil samples in air and the emission spectra were registered in the presence and absence of an external magnetic field (0.3 T) using a set of four spectrometers. The magnetic field is applied transverse to the plasma plume. The emission intensity enhancement factor up to about 8 has been observed and the limit of detection (LOD) of Cr has been improved from 18.2 mg/kg to 7.7 mg/kg in the presence of magnetic field. The effects of magnetic field on the plasma parameters have been studied. The emission intensity enhancement is attributed to magnetic confinement of the laser-generated plasma. The spatial and temporal behavior reveals that the electron number density and electron temperature decreases nearly exponentially in the presence of magnetic field due to the deceleration of the plasma plume.

Maximizing energy deposition by shaping few-cycle laser pulses

We experimentally investigate the impact of pulse shape on the dynamics of laser-generated plasma in rare gases. Fast-rising triangular pulses with a slower decay lead to early ionization of the gas and depose energy more efficiently than their temporally reversed counterparts. As a result, in both argon and krypton, the induced shockwave as well as the plasma luminescence are stronger. This is due to an earlier availability of free electrons to undergo inverse Bremsstrahlung on the pulse trailing edge. Our results illustrate the ability of adequately tailored pulse shapes to optimize the energy deposition in gas plasmas.

Simple magnetic spectrometer for ions emitted from laser-generated plasma at 1010 W/cm2 intensity

AbstractPlasmas were generated by 3 ns pulsed lasers at 1064 nm wavelength using intensities of about 1010 W/cm2 irradiating solid targets with a different composition. The ion emission was investigated with time-of-flight measurements giving information of the ion velocity, charge state generation, and ion energy distribution. Measurements use a coil to generate a magnetic field suitable to deflect ions toward a Faraday cup and/or a secondary electron multiplier.Ion acceleration of the order of hundred eV per charge state, plasma temperature of the order of tens eV, charge states up to about 4+, and Boltzmann energy distributions were obtained in carbon, aluminum, and copper targets.The presented results represent useful plasma characterization methods for many applications such as the new generation of laser ion sources, pulsed laser deposition techniques, and post ion acceleration systems.

Ion energy distribution from laser-generated plasma at intensities of 5 × 109 W/cm2

A ns Nd:YAG laser operating at 5 × 109 W/cm2 is employed to irradiate Au/Al bi-layered targets in high vacuum. The produced laser-generated plasma is analyzed by detecting the emitted ions using an Ion Collector and an Ion Energy Analyzer connected in the time-of flight configuration. The ion spectra were acquired with a fast storage oscilloscope, analyzed in time, velocity and energy and compared with the Coulomb-Boltzmann-Shifted function. The maximum Al ion acceleration was about 1.4 keV while the maximum Au ion acceleration was about 5.0 keV. The plasma temperature was evaluated from the Boltzmann distribution giving about 60 eV for pure Al and up to 150 eV for Al covered by 250 nm Au films.

Lab experiments mimic the origin and growth of astrophysical magnetic fields

A turbulent, laser-generated plasma can amplify magnetic fields to cosmic scales.

Radical Chemistry in a Femtosecond Laser Plasma: Photochemical Reduction of Ag⁺ in Liquid Ammonia Solution.

Plasmas with dense concentrations of reactive species such as hydrated electrons and hydroxyl radicals are generated from focusing intense femtosecond laser pulses into aqueous media. These radical species can reduce metal ions such as Au3+ to form metal nanoparticles (NPs). However, the formation of H2O2 by the recombination of hydroxyl radicals inhibits the reduction of Ag+ through back-oxidation. This work has explored the control of hydroxyl radical chemistry in a femtosecond laser-generated plasma through the addition of liquid ammonia. The irradiation of liquid ammonia solutions resulted in a reaction between NH3 and OH·, forming peroxynitrite and ONOO−, and significantly reducing the amount of H2O2 generated. Varying the liquid ammonia concentration controlled the Ag+ reduction rate, forming 12.7 ± 4.9 nm silver nanoparticles at the optimal ammonia concentration. The photochemical mechanisms underlying peroxynitrite formation and Ag+ reduction are discussed.

Tailoring Laser-Generated Plasmas for Efficient Nuclear Excitation by Electron Capture.

The optimal parameters for nuclear excitation by electron capture in plasma environments generated by the interaction of ultrastrong optical lasers with solid matter are investigated theoretically. As a case study we consider a 4.85keV nuclear transition starting from the long-lived ^{93m}Mo isomer that can lead to the release of the stored 2.4MeV excitation energy. We find that due to the complex plasma dynamics, the nuclear excitation rate and the actual number of excited nuclei do not reach their maximum at the same laser parameters. The nuclear excitation achievable with a high-power optical laser is up to twelve and up to six orders of magnitude larger than the values predicted for direct resonant and secondary plasma-mediated excitation at the x-ray free electron laser, respectively. Our results show that the experimental observation of the nuclear excitation of ^{93m}Mo and the subsequent release of stored energy should be possible at laser facilities available today.

Ion energy distributions from laser-generated plasmas at two different intensities

Laser-generated non-equilibrium plasmas were analyzed at Brookhaven National Laboratory (NY, USA) and MIFT Messina University (Italy). Two laser intensities of 1012 W/cm2 and 109 W/cm2, have been employed to irradiate Al and Al with Au coating targets in high vacuum conditions. Ion energy distributions were obtained using electrostatic analyzers coupled with ion collectors. Time of flight measurements were performed by changing the laser irradiation conditions. The study was carried out to provide optimum keV ions injection into post acceleration systems. Possible applications will be presented.

Magnetic and electric deflector spectrometers for ion emission analysis from laser generated plasma

The pulsed laser-generated plasma in vacuum and at low and high intensities can be characterized using different physical diagnostics. The charge particles emission can be characterized using magnetic, electric and magnet-electrical spectrometers. Such on-line techniques are often based on time-of-flight (TOF) measurements. A 90° electric deflection system is employed as ion energy analyzer (IEA) acting as a filter of the mass-to-charge ratio of emitted ions towards a secondary electron multiplier. It determines the ion energy and charge state distributions. The measure of the ion and electron currents as a function of the mass-to-charge ratio can be also determined by a magnetic deflector spectrometer, using a magnetic field of the order of 0.35 T, orthogonal to the ion incident direction, and an array of little ion collectors (IC) at different angles. A Thomson parabola spectrometer, employing gaf-chromix as detector, permits to be employed for ion mass, energy and charge state recognition. Mass quadrupole spectrometry, based on radiofrequency electric field oscillations, can be employed to characterize the plasma ion emission. Measurements performed on plasma produced by different lasers, irradiation conditions and targets are presented and discussed. Complementary measurements, based on mass and optical spectroscopy, semiconductor detectors, fast CCD camera and Langmuir probes are also employed for the full plasma characterization. Simulation programs, such as SRIM, SREM, and COMSOL are employed for the charge particle recognition.

Diagnostics of Particles emitted from a Laser generated Plasma: Experimental Data and Simulations

The charge particle emission form laser-generated plasma was studied experimentally and theoretically using the COMSOL simulation code. The particle acceleration was investigated using two lasers at two different regimes. A Nd:YAG laser, with 3 ns pulse duration and 1010W/cm2intensity, when focused on solid target produces a non-equilibrium plasma with average temperature of about 30-50 eV. An Iodine laser with 300 ps pulse duration and 1016W/cm2intensity produces plasmas with average temperatures of the order of tens keV. In both cases charge separation occurs and ions and electrons are accelerated at energies of the order of 200 eV and 1 MeV per charge state in the two cases, respectively. The simulation program permits to plot the charge particle trajectories from plasma source in vacuum indicating how they can be deflected by magnetic and electrical fields. The simulation code can be employed to realize suitable permanent magnets and solenoids to deflect ions toward a secondary target or detectors, to focalize ions and electrons, to realize electron traps able to provide significant ion acceleration and to realize efficient spectrometers. In particular it was applied to the study two Thomson parabola spectrometers able to detect ions at low and at high laser intensities. The comparisons between measurements and simulation is presented and discussed.

Graphite oxide based targets applied in laser matter interaction

In the present work, we propose the production of a hybrid graphene based material suitable to be laser irradiated with the aim to produce quasi-monoenergetic proton beams using a femtosecond laser system. The unique lattice structure of the irradiated solid thin target can affect the inside electron propagation, their outgoing from the rear side of a thin foil, and subsequently the plasma ion acceleration. The produced targets, have been characterized in composition, roughness and structure and for completeness irradiated. The yield and energy of the ions emitted from the laser-generated plasma have been monitored and the emission of proton stream profile exhibited an acceleration of the order of several MeVs/charge state.

Effects induced by high and low intensity laser plasma on SiC Schottky detectors

Silicon-Carbide detectors are extensively employed as diagnostic devices in laser-generated plasma, allowing the simultaneous detection of photons, electrons and ions, when used in time-of-flight configuration. The plasma generated by high intensity laser (1016 W/cm2) producing high energy ions was characterized by SiC detector with a continuous front-electrode, and a very thick active depth, while SiC detector with an Interdigit front-electrode was used to measure the low energy ions of plasma generated by low intensity laser (1010 W/cm2). Information about ion energy, number of charge states, plasma temperature can be accurately obtained. However, laser exposure induces the formation of surface and bulk defects whose concentration increases with increasing the time to plasma exposure. The surface defects consist of clusters with a main size of the order of some microns and they modify the diode barrier height and the efficiency of the detector as checked by alpha spectrometry. The bulk defects, due to the energy loss of detected ions, strongly affect the electrical properties of the device, inducing a relevant increase of the leakage (reverse) current and decrease the forward current related to a deactivation of the dopant in the active detector region.

Laser-plasma generation of tunable ultrashort pulses in terahertz and mid-infrared ranges

Laser-plasma generation of tunable ultrashort pulses in terahertz and mid-infrared ranges

Characterization of laser-generated aluminum plasma using ion time-of-flight and optical emission spectroscopy

Laser plasma generated by ablation of an Al target in vacuum is characterized by ion time-of-flight combined with optical emission spectroscopy. A Q-switched Nd:YAG laser (wavelength λ = 1064 nm, pulse width τ ∼ 7 ns, and fluence F ≤ 38 J/cm2) is used to ablate the Al target. Ion yield and energy distribution of each charge state are measured. Ions are accelerated according to their charge state by the double-layer potential developed at the plasma-vacuum interface. The ion energy distribution follows a shifted Coulomb-Boltzmann distribution. Optical emission spectroscopy of the Al plasma gives significantly lower plasma temperature than the ion temperature obtained from the ion time-of-flight, due to the difference in the temporal and spatial regions of the plasma plume probed by the two methods. Applying an external electric field in the plasma expansion region in a direction parallel to the plume expansion increases the line emission intensity. However, the plasma temperature and density, as measured by optical emission spectroscopy, remain unchanged.

Influence of target structure on laser plasma generation efficiency

Spatial restrictions on heat transfer imposed in foil and sintered powder titanium targets have resulted in sufficient increase of efficiency of laser plasma generation as compared to bulk Ti targets. Especially at low laser fluences, where target material input in plasma is higher than ambient air’s. Also, the pattern of momentum coupling coefficient Cm dependency on laser fluence F has changed from convex to concave. Minimum difference in Cm values for bulk and foil targets was 1.68 times, maximum – 1.5 orders of magnitude (always higher for foil). At the impact on sintered porous targets momentum coupling coefficient was lower than for foil, but normalised by mass density, the results were about equal. To our mind, obtained results show that suppression of heat dissipation in porous targets can be same efficient as in foils, but with more benefits for feeding systems and energy efficiency of laser plasma generators.