Change in the relief of a target surface treated by compression plasma flows

The change in the surface relief of a steel-3 target (GOST 380) treated by compression plasma flows is experimentally studied. The energy density absorbed by the target varies in the range of 10–35 J/cm² and the pulse duration is 100 μs. It is shown experimentally and numerically that the development of KelvinHelmholtz instability strongly affects the formation of the target surface treated with compression plasma flows: a large-scale wave-like relief with characteristic sizes of 200 × 1000 μm is formed on the target surface and, as a result, the roughness of the surface increases. However, the microrelief at the scale of individual elements is smoothed to a maximum roughness of about 0.5 μm.     

Modifications of AISI 1045 steel by picosecond Nd:YAG laser at 266 nm; comparison with 532 and 1064 nm pulses

Interaction of Nd:YAG laser, operating at 266 nm wavelength and a pulse duration of 40 ps, with AISI 1045 steel was studied. Surface damage threshold was estimated to be 0.14 J/cm². The steel surface modification was studied at the laser fluence of ∼1.0 J/cm². The energy absorbed from Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following AISI 1045 steel surface morphological changes and processes were observed: (i) intensive damage of the target in the central zone of irradiated area; (ii) appearance of periodic surface structures at nano-level, with periodicity in agreement with the used wavelength; (iii) reduction of oxygen concentration in irradiated area; and (iv) development of plasma in front of the target. Generally, interaction of laser beam with AISI 1045 steel (at 266 nm) results in a near-instantaneous creation of damage, meaning that large steel surfaces can be modified in short times.     

Surface modifications of AISI 1045 steel created by high intensity 1064 and 532 nm picosecond Nd:YAG laser pulses

Interaction of Nd:YAG laser, operating at 1064 or 532 nm wavelength and a pulse duration of 40 ps, with AISI 1045 steel was studied. Surface damage thresholds were estimated to be 0.30 and 0.16 J/cm² at the wavelengths of 1064 and 532 nm, respectively. The steel surface modification was studied at the laser energy density of 10.3 J/cm² (at 1064 nm) and 5.4 J/cm² (at 532 nm). The energy absorbed from Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following AISI 1045 steel surface morphological changes and processes were observed: (i) both laser wavelengths cause damage of the steel in the central zone of irradiated area; (ii) appearance of a hydrodynamic feature in the form of resolidified droplets of the material in the surrounding outer zone with 1064 nm laser wavelength; (iii) appearance of periodic surface structures, at micro- and nano-level, with the 532 nm wavelength and, (iv) development of plasma in front of the target. Generally, interaction of laser beam with the AISI 1045 steel (at 1064 and 532 nm) results in a near-instantaneous creation of damage, meaning that large steel surfaces can be processed in short time.     

Phonon–particle coupling effects in odd–even double mass differences of magic nuclei

It has been proposed to use the formation of a magnetized plasma of laser-accelerated ions and electrons at the irradiation of the curved surface of the inner cavity of the target by a petawatt laser pulse to initiate a neutronless nuclear reaction of protons with boron nuclei. The possibility of an additional increase in the intensity of the reaction owing to the compression of the plasma at the irradiation of the outer surface of the target by a second terawatt laser pulse synchronized in time with the plasma-forming pulse has been discussed. The parameters of laser pulses and a target have been determined at which the ignition of a pB plasma occurs; i.e., the energy released in reactions is equal to the energy of the plasma.     

Double-pulse laser ablation applied to reactive PLD method for synthesis of carbon nitride film: A second laser shot delay

Carbon nitride films were deposited using ablation of graphite target by second harmonic radiation of Nd:YAG laser in nitrogen atmosphere. To produce high hardness films, the deposited particles should have sufficient kinetic energy to provide their efficient diffusion on a substrate surface for formation of crystal structure. However, a shock wave is arisen in ambient gas as a consequence of laser plasma explosive formation. This shock wave reflected from the substrate interacts with plume particles produced by the first laser pulse and decreases their kinetic energy. This results in decrease of film crystallinity. To improve film quality, two successive laser pulses was proposed to be used. At adjusting time delay, the particles induced by the second pulse wilt serve as a piston, which will push forward both stopped particles ablated by the first pulse and arisen from chemical reactions in ambient gas. An X-ray photoelectron spectroscopy (XPS) analysis of deposited films has shown an increase of content of sp ³ carbon atoms corresponding to crystalline phase, if double-pulse configuration is employed. The luminescence of excited C₂ and CN molecules in laser plume at different distances from the target was studied to optimize the delay between laser pulses.     

Picosecond laser ablation of nano-sized WTi thin film

Interaction of an Nd:YAG laser, operating at 532 nm wavelength and pulse duration of 40 ps, with tungsten-titanium (WTi) thin film (thickness, 190 nm) deposited on single silicon (100) substrate was studied. Laser fluences of 10.5 and 13.4 J/cm² were found to be sufficient for modification of the WTi/silicon target system. The energy absorbed from the Nd:YAG laser beam is partially converted to thermal energy, which generates a series of effects, such as melting, vaporization of the molten material, shock waves, etc. The following WTi/silicon surface morphological changes were observed: (i) ablation of the thin film during the first laser pulse. The boundary of damage area was relatively sharp after action of one pulse whereas it was quite diffuse after irradiation with more than 10 pulses; (ii) appearance of some nano-structures (e.g., nano-ripples) in the irradiated region; (iii) appearance of the micro-cracking. The process of the laser interaction with WTi/silicon target was accompanied by formation of plasma.     

Surface modifications of TiN coating by pulsed TEA CO2 and XeCl lasers

Interactions of a transversely excited atmospheric (TEA) CO₂ laser and an excimer XeCl laser, pulse durations ∼2 μs (initial spike FWHM ∼100 ns) and ∼20 ns (FWHM), respectively, with polycrystalline titanium nitride (TiN) coating deposited on high quality steel AISI 316, were studied. Titanium nitride was surface modified by the laser beams, with an energy density of 20.0 J/cm² (TEA CO₂ laser) and 2.4 J/cm² (XeCl laser), respectively. The energy absorbed from the CO₂ laser beam is partially converted to thermal energy, which generates a series of effects such as melting, vaporization of the molten material, shock waves, etc. The energy from the excimer XeCl laser primarily leads to fast and intense target evaporation. The calculated maximum temperatures on the target surface were 3770 and 6300 K for the TEA CO₂ and XeCl lasers, respectively. It is assumed that the TEA CO₂ laser affects the target deeper, for a longer time than the XeCl laser. The effects of the XeCl laser are confined to a localized area, near target surface, within a short time period.Morphological modifications of the titanium nitride surface can be summarized as follows: (i) both lasers produced ablation of the TiN coating in the central zone of the irradiated area and creation of grainy structure with near homogeneous distribution; (ii) a hydrodynamic feature, like resolidified droplets of the material, appeared in the surrounding peripheral zone; (iii) the process of irradiation, in both cases, was accompanied by appearance of plasma in front of the target.Target color modifications upon laser irradiation indicate possible chemical changes, possibly oxidation.     

Erosion of the GaAs target under irradiation by a high-power pulsed ion beam

The results of examination of the GaAs-target erosion under irradiation by a high-power pulsed ion beam are reported. In the experiments, use was made of a high-power pulsed ion source with the following parameters: ion energy — 250 keV, target current density — 350 A/cm², pulse duration — 80 ns, target energy density — up to 7 J/cm². The target erosion coefficient and its dependence on the number of successive pulses are measured. It is found that the surface roughness parameter is increased with the number of successive beam pulses. A regular structure of surface relief is observed to form in the case where the number of pulses > 20–40. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 66–70, January, 2007.     

Pulse regime in formation of fractal fibers

The pulse regime of vaporization of a bulk metal located in a buffer gas is analyzed as a method of generation of metal atoms under the action of a plasma torch or a laser beam. Subsequently these atoms are transformed into solid nanoclusters, fractal aggregates and then into fractal fibers if the growth process proceeds in an external electric field. We are guided by metals in which transitions between s and d-electrons of their atoms are possible, since these metals are used as catalysts and filters in interaction with gas flows. The resistance of metal fractal structures to a gas flow is evaluated that allows one to find optimal parameters of a fractal structure for gas flow propagation through it. The thermal regime of interaction between a plasma pulse or a laser beam and a metal surface is analyzed. It is shown that the basic energy from an external source is consumed on a bulk metal heating, and the efficiency of atom evaporation from the metal surface, that is the ratio of energy fluxes for vaporization and heating, is 10^–3–10^–4 for transient metals under consideration. A typical energy flux (~10⁶ W/cm²), a typical surface temperature (~3000 K), and a typical pulse duration (~1 μs) provide a sufficient amount of evaporated atoms to generate fractal fibers such that each molecule of a gas flow collides with the skeleton of fractal fibers many times.     

Mathematical modeling of the heating of a fast ignition target by an ion beam

We develop a BIN computer code for simulating the interaction of a monochromatic ion beam with a plasma, which takes into account changes in the spatial distribution of the heated-plasma temperature. This enables us to calculate the heating of both homogeneous and inhomogeneous plasmas with parameters corresponding to their real spatial distributions at the time of maximum compression of the inertial confinement fusion (ICF) target. We present the results of a numerical simulation using the BIN code for the heating of a homogeneous deuterium–tritium plasma by a short pulse of monochromatic ions at various ion velocity and plasma–electron thermal velocity ratios. We also present the results of calculations for the heating of an inhomogeneous plasma of a non-cryogenic target formed as a beryllium deuteride–tritide shell by beams of light, medium, and heavy ions. As the initial distributions, we use the results of numerical simulations for such a target, precompressed by a laser pulse (carried out at the M. V. Keldysh Institute of Applied Mathematics using the DIANA code). We demonstrate the possibility of forming the central ignitor with the parameters sufficient for igniting the targets by beams of ions with energies E ~ 100 ? 400 MeV/u and specific energy densities of the beam Q ～ 5?20 GJ/cm². The required specific energy density drops with increase in the ion energy; however, due to the increased path length, larger-charge ions have to be used.     

Bubble creation and collapse during excimer laser ablation of weak absorbing polymers

We present evidence suggesting that XeCl laser ablation of a weakly absorbing poly-methyl-methacrylate (PMMA) polymer, done by chemical, thermal bond breaking of the polymer chain or optical breakdown of the material, which involves plasma generation, creates a cloud of small asymmetric near the surface bubbles, which subsequently expand and aggregate during the same laser pulse duration or in subsequent pulses depending on the laser pulse energy. When a critical volume is reached each bubble collapses in a high pressure and temperature central point and rebounds ejecting a hot jet of material on the non-irradiated area of the polymer and creating craters on the surface. A characteristic bipolar pressure wave corresponding to the bubble collapse, explosion and rebound is observed. The number density of the craters on the surface is a function of the laser pulse sequence number and the laser pulse energy density.     

Peculiarities of pulsed ion implantation from a laser plasma containing multiply charged ions

A mathematical model describing the dynamics of a pulsed laser plasma with multiply charged ions, as well as the formation of the accelerated ion flow in an external magnetic field, is developed. Experimental studies and mathematical simulation by the particle-in-cell method are used to determine the role of multiply charged ions in the process of ion implantation into a silicon substrate from the pulsed plasma containing singly and doubly charged titanium ions. The plasma spreads between parallel-plate electrodes (Ti target and Si substrate) along the normal to the surface of the target. Ions are accelerated by high-voltage negative pulses applied to the substrate. It is found that doubly charged ions effectively participate in the implantation process when an external electric field is applied very soon after the laser action on the target. The application of a high-voltage pulse with an amplitude of 50 kV 0.5 μs after a laser pulse leads to ion implantation with an energy close to 100 keV. With increasing delay in the application of the high-voltage pulse, the upper boundary of the energy spectrum of implanted ions is displaced towards lower energies. Comparison of the depth profiles of titanium distribution in silicon calculated from the results of simulation are compared with the experimental profiles shows that the model developed here correctly describes the formation of the high-energy component of the ion flow, which is responsible for defect formation and doping of deep layers of the substrate.     

Structural and phase changes in single-crystalline silicon treated with compression plasma flows

The structural-phase changes in p-type single-crystalline silicon treated with compression plasma flows (CPFs) with an energy density of 5–12 J/cm² are investigated by the X-ray diffraction method depending on the crystallographic orientation of the silicon and the plasma energy density. In addition, the conductivity type on the treated silicon surface is determined by means of measuring the sign of the thermopower.; the surface morphology, by scanning electron microscopy; and the open-circuit voltage, upon illumination of the treated silicon surface (AM1.5 spectrum). It is found that treatment with CPFs results in the occurrence of the photovoltaic effect conditioned by the formation of an n-type modified surface layer. Depending on the crystallographic orientation, the modified layer either remains single crystalline (for the initial orientation (111)) or is subjected to amorphization (for the initial orientation (100)). At an energy density of ～8–9 J/cm² the action of CPFs leads to texture formation on the silicon surface.     

Dynamics of laser-induced cavitation bubble during expansion over sharp-edge geometry submerged in liquid – an inside view by diffuse illumination

Laser ablation in liquids is growing in popularity for various applications including nanoparticle production, breakdown spectroscopy, and surface functionalization. When laser pulse ablates the solid target submerged in liquid, a cavitation bubble develops. In case of “finite” geometries of ablated solids, liquid dynamical phenomena can occur inside the bubble when the bubble overflows the surface edge. To observe this dynamics, we use diffuse illumination of a flashlamp in combination with a high-speed videography by exposure times down to 250 ns. The developed theoretical modelling and its comparison with the experimental observations clearly prove that this approach widens the observable area inside the bubble. We thereby use it to study the dynamics of laser-induced cavitation bubble during its expansion over a sharp-edge (“cliff-like” 90°) geometry submerged in water, ethanol, and polyethylene glycol 300. The samples are 17 mm wide stainless steel plates with thickness in the range of 0.025–2 mm. Bubbles are induced on the samples by 1064-nm laser pulses with pulse durations of 7–60 ns and pulse energies of 10–55 mJ. We observe formation of a fixed-type secondary cavity behind the edge where low-pressure area develops due to bubble-driven flow of the liquid. This occurs when the velocity of liquid overflow exceeds ~20 m s⁻¹. A re-entrant liquid injection with up to ~40 m s⁻¹ velocity may occur inside the bubble when the bubble overflows the edge of the sample. Formation and characteristics of the jet evidently depend on the relation between the breakdown-edge offset and the bubble energy, as well as the properties of the surrounding liquid. Higher viscosity of the liquid prevents the generation of the jet.     

fs Laser surface nano-structuring of high refractory ceramics to enhance solar radiation absorbance

The surface and structural modification of titanium (Ti) has been explored after the interaction of ultrashort laser pulses with the surface target. The targets were exposed by femtosecond Ti: Sapphire laser pulses in liquid (ethanol) and dry (air) environment. In order to explore the effect of pulse energy, the targets were exposed to 1,000 succeeding pulses for various pulse energies ranging from 200 to 500 μJ for pulse duration of 25 fs. SEM analyses were performed for central as well as the peripheral ablated areas of the target. It was found that in the case of ethanol (both for central and peripheral ablated areas) there is a grain growth along with nanoscale pores and dots when the target was irradiated for 200 μJ. For intermediate energies (300–400 μJ), grains of 1–2 μm with distinct boundaries are formed in the central ablated area. Whereas in the peripheral ablated area, laser-induced periodic surface structures (LIPSS) and globules are grown. For the highest pulse energy (500 μJ), distinct grains are observed for both regions. However, in the peripheral area the grains are of bigger size with cracks along the boundaries. In case of ablation in air, in the center of ablated areas, island-like structures with multiple ablative layer or LIPSS and nanoscale spheres are observed both for lower and intermediate pulse energies. For the highest pulse energy only nanoscale LIPSS could be observed. For ablation in air at the peripheral areas, well-defined, laser-induced periodic surface structures are observed for all pulse energies. Raman spectroscopy reveals that the liquid (ethanol) environment forms the carbonyl compounds with the metal and induces C–C stretching vibration, whereas in case of air, hydroxo complexes are formed. It has been found that surface treatment of Ti with ultrashort (25 fs) laser radiation in ethanol environment allows the growth of particular surface structures in the form of grains and simultaneously induces changes in its chemical composition.     

Enhanced production of fast multi-charged ions from plasmas formed at cleaned surface by femtosecond laser pulse

We present atomic, energy, and charge spectra of ions accelerated at the front surface of a silicon target irradiated by a high-contrast femtosecond laser pulse with an intensity of 3×10¹⁶ W/cm², which is delayed with respect to a cleaning nanosecond laser pulse of 3-J/cm² energy density. A tremendous increase in the number of fast silicon ions and a significant growth of their maximum charge in the case of the cleaned target from 5+ to 12+ have been observed. The main specific features of the atomic, energy, and charge spectra have been analyzed by means of one-dimensional hydrodynamic transient-ionization modeling. It is shown that fast highly charged silicon ions emerge from the hot plasma layer with a density a few times less than the solid one, and their charge distribution is not deteriorated during plasma expansion.This revised version was published online in August 2005 with a corrected cover date.     

Neutron production in picosecond laser-generated plasma on a be target

Experimental data on neutron production in a plasma generated on a Be target by a picosecond laser of intensity 2 × 10¹⁸ W/cm² are presented. In contrast to previous measurements, a Ta converter is not used in this study to generate γ rays. The neutron yield is equal to 2 × 10³ over a solid angle of 4π steradians per laser pulse. A simultaneous measurement of the maximum energy of hard x rays gave E _γmax ～ 6 MeV, the number of these photons being 5 × 10⁸ over an angle of 4π steradians per laser pulse. The energy distributions of fast electrons and photons are estimated theoretically.     

Investigation of different surface morphologies formed on AISI 304 stainless steel via millisecond Nd:YAG pulsed laser oxidation

Different surface morphologies on AISI 304 stainless steel have been obtained after millisecond Nd:YAG pulsed laser oxidation. The effects of laser processing parameters, especially pulse width and laser energy density on the surface morphologies of the stainless steel were emphatically investigated. The results showed that surface morphologies were significantly changed with increasing laser pulse widths and laser energy densities. When the pulse width was 0.2–1.0 ms and laser energy density was 4.30×10⁶–7.00×10⁶ J/m², the surface was obviously damaged and the morphologies varied gradually from craters to ripple structures. However, when the pulse width was longer than 1 ms and the laser energy density was increased from 1.90×10⁷ to 3.16×10⁷ J/m², the sizes of craters got smaller until disappeared and the surface became flatter and smoother. Nevertheless, the smooth surface was not obtained under overhigh laser energy densities. In addition, the schematic relationship was used to describe the formation process and mechanism of different surface morphologies.     

Quantitative studies of copper plasma using laser induced breakdown spectroscopy

In the present work, we present the spatial evolution of the copper plasma produced by the fundamental harmonic (1064 nm) and second harmonic (532 nm) of a Q-switched Nd:YAG laser. The experimentally observed line profiles of neutral copper have been used to extract the electron temperature using the Boltzmann plot method, whereas, the electron number density has been determined from the Stark broadening. Besides we have studied the variation of electron temperature and electron number density as a function of laser energy at atmospheric pressure. The Cu I lines at 333.78, 406.26, 465.11 and 515.32 nm are used for the determination of electron temperature. The relative uncertainty in the determination of electron temperature is ≈10%. The electron temperature calculated for the fundamental harmonic (1064 nm) of Nd:YAG laser is 10500–15600 K, and that for the second harmonic (532 nm) of Nd:YAG laser is 11500–14700 K at a Q Switch delay of 40 μs. The electron temperature has also been calculated as a function of laser energy from the target surface for both modes of the laser. We have also studied the spatial behavior of the electron number density in the plume. The electron number densities close to the target surface (0.05 mm), in the case of fundamental harmonic (1064 nm) of Nd:YAG laser having pulse energy 135 mJ and second harmonic (532 nm) of Nd:YAG laser with pulse energy 80 mJ are 2.50×10¹⁶ and 2.60×10¹⁶ cm⁻³, respectively.