Inverse Bremsstrahlung Absorption Research Articles

Critical assessment of the issues in the modeling of ablation and plasma expansion processes in the pulsed laser deposition of metals

This paper presents a review on the modeling of ablation and plasma expansion processes in the pulsed laser deposition of metals. The ablation of a target is the key process that determines the amount of material to be deposited; while, the plasma expansion governs the characteristics of the deposited material. The modeling of ablation process involves a study of two complex phenomena: (i) laser-target interaction and (ii) plasma formation and subsequent shielding of the incoming radiation. The laser-target interaction is a function of pulse duration, which is captured by various models that are described in this paper. The plasma produced as a result of laser–target interaction, further interacts with the incoming radiation, causing the shielding of the target. The shielding process has been modeled by considering the various photon absorption mechanisms operative inside the plasma, namely: inverse Bremsstrahlung, photoionization, and Mie absorption. Concurrently, the plasma expands freely until the ablated material gets deposited on the substrate. Various models describing the plasma expansion process have been presented. The ability of the theoretical models in predicting various ablation and plasma characteristics has also been compared with the relevant experimental data from the literature. The paper concludes with identification of critical issues and recommendations for future modeling endeavors.

Energy conversion in a glass-laser-induced blast wave in air

Near-infrared solid laser-induced blast waves were investigated using a Q-switched Nd:glass laser oscillating at 1053 nm wavelength, up to 2.0 J/pulse laser energy Ei, and 33 ns pulse duration (full width at half maximum). Half-shadowgraph half-self-emission visualization elucidated laser detonation waves and blast waves. Laser output and transmission through the waves were measured using pairs of a photodetectors and energy meters. Results show volumetric absorption in this near-infrared region, which is mainly attributable to low inverse-Bremsstrahlung absorption coefficients at short wavelengths. Some input laser energy was converted into blast wave energy: 59% at Ei=1.0 J/pulse, which approximates that of CO2 laser-induced blast waves.

Plasma currents and inverse bremsstrahlung absorption under strong dc/ac electric fields

Plasma current induced by a strong dc electric field is studied by Fokker-Planck (FP) simulation. Based upon this, a hydrodynamic-like equation, similar to Spitzer's but without the weak-field limit, is given for calculating the current. This equation is suitable for application in hybrid-PIC simulations relevant to the fast ignition (FI) scheme. Furthermore inverse bremsstrahlung (IB) absorption is studied in a wide laser intensity range. In particular, we introduce an IB operator which is similar to Langdon's, but without the low laser intensity limit. This operator enables one to treat IB absorption properly in the inertial confinement fusion (ICF) schemes with the NIF/LMJ-scale laser intensity beyond 1015 W/cm2.

Inverse bremsstrahlung absorption with nonlinear effects of high laser intensity and non-Maxwellian distribution

Inverse bremsstrahlung (IB) absorption and evolution of the electron distribution function (EDF) in a wide laser intensity range (10;{12}-10;{17} W/cm;{2}) have been studied systematically by a two velocity-dimension Fokker-Planck code. It is found that Langdon's IB operator overestimates the absorption rate at high laser intensity, consequently with an overdistorted non-Maxwellian EDF. According to the small anisotropy of EDF in the oscillation frame, we introduce an IB operator which is similar to Langdon's but without the low laser intensity limit. This operator is appropriate for self-consistently tackling the nonlinear effects of high laser intensity as well as non-Maxwellian EDF. Particularly, our operator is capable of treating IB absorption properly in the indirect and direct-drive inertial confinement fusion schemes with the National Ignition Facility and Laser MegaJoule laser parameters at focused laser intensity beyond 10;{15} W/cm;{2} .

Rapid self-magnetization of laser speckles in plasmas by nonlinear anisotropic instability

Presented here are the first kinetic two-dimensional Vlasov– Fokker–Planck calculations of inertial confinement fusion-related laser–plasma interactions, to include self-consistent magnetic fields, hydrodynamic plasma expansion and anisotropic electron pressure. An underdense plasma, reminiscent of the gas fill of a hohlraum, is heated by a laser speckle with Iλ2=1.0×1015 W cm− 2μm2 and radius w0=5 μm. Inverse bremsstrahlung absorption of the laser and non-local electron transport lead to the development of a collisional analogue of the Weibel electromagnetic instability. The instability is seeded by magnetic fields, generated in an initial period of linear growth due to the anisotropic electron distribution arising in a laser speckle. Using the circular polarization does not generate significant fields. For linear polarization, the field generally saturates when the magnetization is ωτei> 1, and the effective growth rate is similar to the coherence time of typical laser speckles. The presence of these magnetic fluctuations significantly affects the heat fluxes and hydrodynamics in the plasma.

Analysis of weld pool dynamic during stationary laser–MIG hybrid welding

A mathematical model was established to simulate the weld pool development and dynamic process in stationary laser–metal inert gas (MIG) hybrid welding. Surface tension and buoyancy were considered to calculate liquid metal flow pattern; moreover, typical phenomena of MIG welding such as filler droplets impinging weld pool, electromagnetic force in the weld pool, and typical phenomena of laser beam welding such as recoil pressure, inverse Bremsstrahlung absorption, and Fresnel absorption were all considered in the model. The laser beam and arc couple effect was introduced into this model by the plasma width during hybrid welding. Transient weld pool shape and complicated liquid metal velocity distribution from two kinds of weld pool to a unified weld pool were calculated. Furthermore, the simulated weld bead geometries were in good agreement with experimental measurement.

Theoretical model for double pulse laser-induced breakdown spectroscopy

We present a simple theoretical model for the emission from double pulse laser-induced plasmas that was developed to better understand the processes and factors involved in enhancement of plasma emission. In this model, the plasma emission is directly proportional to the square of plasma density, its volume, and the fraction of second laser pulse absorbed through inverse bremsstrahlung absorption by the plasma plume of the first laser pulse. The electron-ion collision frequency determines the profile and location of the peak of emission enhancement with respect to the delay between the two lasers, whereas the amplitude of the enhancement is mainly dependent on the increase in the mass ablation rate after the second laser pulse. The effects of increase in temperature and in plasma volume after the second laser pulse are also discussed in light of this model.

Distribution of the intensity absorbed by evaporative front in laser cutting nonmetallic material

The distribution of the intensity absorbed by the evaporative cutting front is investigated using ray-tracing method after the laser beam undergoes multiple reflections in an actual kerf photographed experimentally. The laser beam is characterized by focused Gauss beam. Fresnel absorption on the kerf is taken into account, while inverse bremsstrahlung absorption is negligible. The influences of the times of multiple reflections and laser beam's position on the absorbed intensity are also discussed. Results indicate front intensity absorption is mainly determined by the first three incident beams. The laser axis moves toward the front, which is affected by the cutting speed.

Inverse bremsstrahlung absorption with full electron-electron collisions operator

A two-dimensional Fokker-Planck program is developed, in which full e-e collisions are taken into account self-consistently with arbitrary anisotropy of electron distribution function (EDF), to investigate inverse bremsstrahlung and the evolution of EDF. The numerical results show that e-e collisions will enhance inverse bremsstrahlung absorption. The evolution of EDF can be divided into two stages distinguished by different absorption rates. During the first stage, an initially Maxwellian EDF transforms rapidly into an anisotropic one with two temperatures. With the increase of anisotropy, the absorption rate decreases dramatically while the contribution ratio of e-e collisions increases rapidly. In particular, we find that with the increase of ion charge state Zi the contribution ratio of e-e collisions increases in the high laser frequency regime, while it decreases in the low frequency regime.

Mitigation of stimulated Raman scattering in hohlraum plasmas

One aspect of recent research to control Stimulated Raman Scattering (SRS) in hohlraum plasmas is the investigation of risk mitigation strategies for indirect drive inertial confinement fusion. Experimental tests of these strategies, based on prior theoretical and experimental knowledge of SRS, are performed in hohlraum experiments. In the last year, two strategies have been investigated. The first is the use of high Z dopants to reduce SRS backscatter. Forward stimulated Brillouin scattering (FSBS) could lead to beam spray reducing SRS. Since FSBS depends on the electron temperature and thermal effects depend strongly on Z2, a small amount of a high Z dopant, 1-2%, can have a large effect. Experiments have been conducted at the Omega laser to test this theory by varying the amount of Xe dopant in neo-pentane gas filled hohlraums. The experimental measurements do show a decrease in SRS backscatter as Xe dopant is added. However, there are still uncertainties regarding the responsible mechanism since increases inverse-Bremsstrahlung absorption of the SRS light may play a role. The second strategy investigated is using high kλD plasmas to reduce SRS backscatter. Experiments conducted at the Omega laser facility in hohlraum plasmas determined the critical onset intensity for a range of kλD. A scaling of the critical onset intensity as a function of kλD has been determined.

A novel “sandwich” method for observation of the keyhole in deep penetration laser welding

A sandwich method was used to observe the keyhole in deep penetration laser welding, which provided an effective way to analyze both the Fresnel and inverse Bremsstrahlung absorption. In the transparent metal-analog system, different densities of metal vapor, ionized atoms, and free electrons in the keyhole can be simulated by changing the thickness of aluminum films. The research results show that inverse Bremsstrahlung absorption exerts a tremendous influence on the energy absorption of the laser beam for CO 2 laser welding. Low density of keyhole plasma benefits the incident laser energy coupling to the materials. However, excess density of keyhole plasma baffles the transmission of the incident laser beam to the interior material. By comparing inflow energy and outflow energy, there exits an energy balance on the keyhole wall by balancing the absorbed laser intensity and heat flux on the wall.

Spectroscopic studies of laser induced aluminum plasma using fundamental, second and third harmonics of a Nd:YAG laser

In the present work, we have studied the spatial evolution of the aluminum plasma produced by the fundamental (1064 nm), second (532 nm) and third (355 nm) harmonics of a Q-switched pulsed Nd:YAG laser. The experimentally observed line profiles of neutral aluminum have been used to extract the excitation temperature using Boltzmann plot method whereas the electron number density has been determined from the Stark broadened profiles. Besides we have studied the variation of excitation temperature and electron number density as a function of laser irradiance at atmospheric pressure. In addition, we have performed quantitative analysis of photon absorption and vapor ionization mechanism at three laser wavelengths and estimated the inverse bremsstrahlung (IB) absorption and photoionization (PI) coefficients. The validity of the assumption of local thermodynamic equilibrium is discussed in the light of the experimental results.

Anisotropic instability in a laser heated plasma

The theory of the Weibel instability induced by the inverse Bremsstrahlung absorption of a laser light in an underdense plasma is revisited. It is shown that previous analyses have strongly overestimated the effect by neglecting the stabilizing term related to the interaction of the generated quasistatic magnetic field with the laser-heated electrons. The revised model leads to a reduction of the growth rate by more than a factor of 10, to strong reduction of the domain of unstable modes and to inversion of the direction of the unstable wave vectors in the long wavelength limit. The consequences of this instability on the laser plasma interaction are also discussed.

Si-Kα radiation generated by the interaction of femtosecond laser radiation with silicon

Controlling the absorption mechanism, e.g. inverse Bremsstrahlung and resonant absorption, the emission of Si-Kα-radiation using focused femtosecond laser radiation (tp = 100fs, λ = 850 nm, I = 20 PW/cm2) has been investigated. The emission Si-Kα-radiation has been improved with double pulses, varying the delay, the energy ratio between pre- and main-pulse and the focal position. The efficiency for double-pulse-generation of Si-Kα-radiation has been increased 4 times compared to single pulse generation.

Inverse bremsstrahlung absorption and the evolution of electron distributions accounting for electron-electron collisions

Two-dimensional Fokker-Planck simulations have been conducted to investigate the inverse bremsstrahlung absorption and the evolution of the electron distribution function (EDF), where the electron-electron (e-e) collisions are taken into account, allowing for highly anisotropic electron distributions. The numerical results show that the anisotropic part of the EDF is comparable to the isotropic part even for a moderate laser field. The resulting EDF is no longer a simple super-Gaussian, but a hybrid of the super-Gaussian and Maxwellian distributions. Furthermore, the e-e collisions tend to enhance the inverse bremsstrahlung absorption; the contribution ratio of e-e collisions to the inverse bremsstrahlung absorption rate increases with increasing ion charge state Zi in the high laser frequency regime, while it decreases in the low laser frequency regime. It indicates that one cannot simply neglect the e-e collisions in high Zi cases.

Laser light backscatter from intermediate and high Z plasmas

In experiments at the Omega Laser Facility [J. M. Soures et al., Fusion Technol. 30, 492 (1996)], stimulated Brillouin backscatter (SBS) from gasbags filled with krypton and xenon gases was ten times lower than from CO2-filled gasbags with similar electron densities. The SBS backscatter was a 1%–5% for both 527 and 351nm interaction beams at an intensity of ∼1015W∕cm2. The SRS backscatter was less than 1%. The 351nm interaction beam is below the threshold for filamentation and the SBS occurs in the density plateau between the blast waves. Inverse bremsstrahlung absorption of the incident and SBS light account for the lower reflectivity from krypton than from CO2. The 527nm interaction beam filaments in the blowoff plasma before the beam propagates through the blast wave, where it is strongly absorbed. Thus, most of the 527nm SBS occurs in the flowing plasma outside the blast waves.

Nonlinear propagation of ultraintense and ultrashort laser pulses in a plasma channel limited by metallic walls

The nonlinear propagation of an ultraintense and ultrashort (UIUS) laser pulse in a metallic capillary is investigated using a classical model which takes into account the inverse bremsstrahlung absorption (IBA) in the formed plasma. The attenuation of the laser pulse due to the IBA in the plasma and to the laser energy dissipation in the metallic walls is shown. The guiding length and the twist of the laser pulse temporal envelope are presented for several values of the parameters of the plasma, the laser pulse and the metal. The numerical treatment shows that the guiding length increases when the pulse duration becomes shorter. This calculus shows also that in the case of moderate electronic densities, ne<1017m−3, the formed plasma has a negligible effect compared to that of the metallic walls.

Transport Phenomena and Keyhole Dynamics during Pulsed Laser Welding

Numerical and experimental studies were conducted to investigate the heat transfer, fluid flow, and keyhole dynamics during a pulsed keyhole laser welding. A comprehensive mathematical model has been developed. In the model, the continuum formulation was used to handle solid phase, liquid phase, and mushy zone during melting and solidification processes. The volume-of-fluid method was employed to handle free surfaces. The enthalpy method was used for latent heat. Laser absorptions (Inverse Bremsstrahlung and Fresnel absorption) and thermal radiation by the plasma in the keyhole were all considered in the model. The results show that the recoil pressure is the main driving force for keyhole formation. Combining with the Marangoni shear force, hydrodynamic force, and hydrostatic force, it causes very complicated melt flow in the weld pool. Laser-induced plasma plays twofold roles in the process: (1) to facilitate the keyhole formation at the initial stage and (2) to block the laser energy and prevent the keyhole from deepening when the keyhole reaches a certain depth. The calculated temperature distributions, penetration depth, weld bead size, and geometry agreed well with the corresponding experimental data. The good agreement demonstrates that the model lays a solid foundation for the future study of porosity prevention in keyhole laser welding.

Cluster-assisted multiple ionization of xenon and krypton by a nanosecond laser

Multicharged xenon and krypton ions with charge states up to ${\mathrm{Xe}}^{11+}$ and ${\mathrm{Kr}}^{11+}$ have been observed in laser ionization of a pulsed xenon or krypton beam by a $25\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ Nd-YAG laser with laser intensity of ${10}^{10}--{10}^{11}\phantom{\rule{0.3em}{0ex}}\mathrm{W}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ at $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. There is strong evidence to support that those multicharged ions come from cluster-assisted electron recolliding ionizations inside the cluster after multiphoton ionization of atoms in the cluster, the electron can gain its kinetic energy by inverse bremsstrahlung absorption from a laser field quickly.

Laser ablation of Cu and plume expansion into 1atm ambient gas

A one-dimensional gas-dynamic model is presented for the laser ablation of Cu and the expansion of the Cu vapor in a background gas (He) at 1atm. The ionization of Cu and He, the inverse bremsstrahlung absorption processes and photoionization process, and the back flux onto the target are considered simultaneously. The binary diffusion, the viscosity, and the thermal conduction including the electron thermal conduction are considered as well. Numerical results show that the consideration of ionization and laser absorption in the plume greatly influences the gas dynamics. The ionization of Cu enables the recondensation at the target surface to happen even during the laser pulse. The ionization degree of Cu and He may change greatly with the location in the plume. For laser irradiances ranging from 2to9×1012W∕m2, the simulations show that the second-order ionization of Cu competes with the first-order ionization. In the region close to the target surface, the first-order ionization of Cu dominates. In the core of the plasma, the second-order ionization of Cu may dominate over the first-order ionization at laser irradiances higher than 7×1012W∕m2. In the mixing layer, the first-order ionization of Cu is always more important than the second-order ionization although the latter increases monotonously with laser irradiance. The ionization of He is only important in the mixing layer. The plume expansion velocity is much larger than that without ionization and laser absorption by the plume. The relative importance of different laser absorption mechanisms may change with time. Close to the surface photoionization and electron-neutral inverse bremsstrahlung are always important. Once the ionization in the plume starts, at later time, electron-ion inverse bremsstrahlung can become more important than photoionization in the plume core until the shock wave front. Unlike in the vacuum case, electron-neutral inverse bremsstrahlung is very strong due to the relatively high number density of neutral atoms in the plume in the presence of a dense ambient gas. A similar laser irradiance threshold is found for the ablation rate and the plasma formation in the plume, which agrees well with the case of nanosecond laser ablation of metals in vacuum.