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The effects of shape and thickness of a tin surface layer and of the energy of a 170 ps neodymium:yttrium-aluminum-garnet laser pulse on the conversion efficiency (CE) into extreme ultraviolet emission in the 13.5 nm region is investigated. Whereas a CE of up to 1.16% into the 2% reflection band of multilayer Mo/Si optics was measured for a bulk Sn target at a laser energy of 25 mJ, significant CE enhancement up to 1.49% is demonstrated for a 200-nm-thick Sn layer on a microstructured porous alumina substrate.
A study of ASP 30 steel surface modification with high intensity Ti:sapphire laser, operating at 804 nm wavelength and pulse duration of 60 fs, in vacuum ambient, is presented. ASP 30 steel surface variations were studied at laser intensities of 1014 and 1013 W/cm2. The steel target specific surface changes and phenomena observed are: (i) Creation of craters at 1014 W/cm2 intensity; (ii) formation of periodic surface structures only at the reduced intensity of 1013 W/cm2; (iii) chemical surface changes registered only at higher laser intensity, and (iv) occurrence of plasma in front of the surface, including its emission in X-ray region. It can be concluded from this study that the reported laser intensities can effectively be applied for ASP 30 steel surface modification. Careful choosing of laser intensity and pulse count can lead to precise superficial material removal, for example laser intensity ~1013 W/cm2 and low pulse count can lead to ultra-precise surface processing. Generally, femtosecond laser surface modification of ASP 30 steel is non-contact and very rapid compared with traditional modification methods.
A double layer a-CN/TiAlN coating deposited on ASP30 steel substrate was irradiated by femtosecond laser and surface modification effects were observed. Moderate laser intensities used were in the range of 1014–1013 W/cm2, while the total thickness of double layer coating was 4.8 µm (a-CN = 0.6 and TiAlN = 4.2 µm). Laser-induced changes of the surface showed dependence on laser intensity and number of accumulated pulses. Irradiations at the highest intensity resulted in preservation of one or both layers up to 10 pulses, while at lower intensity (1013 W/cm2) a-CN layer is removed after several pulses and TiAlN is preserved up to 50 pulses. Evaluated damage threshold of the target was 0.49 J/cm2. Lower laser intensity irradiation produced periodic surface structures (LIPSS) over the entire irradiated spot with periodicity of ~700 nm, almost in agreement with the laser wavelength used. Irradiations carried out at the highest laser intensity (1014 W/cm2) and laser pulse count of ≥50 resulted in the creation of crater like damages with depth up to 20 µm. Craters were conically shaped, implying intensive processes which took place at the surface. Generation of LIPSS as well as craters can be of great interest for contemporary technologies.
Laser interactions with spray targets (clouds of submicron droplets) are studied here via numerical simulations using two-dimensional particle-in-cell codes. Our simulations demonstrate an efficient absorption of laser pulse energy inside the spray. The energy absorption efficiency depends on the inter-droplet distance, size of the cloud of droplets, and laser pulse intensity, as well as on the pre-evaporation of droplets due to laser pulse pedestal. We investigate in detail proton acceleration from the spray. Energy spectra of protons in various acceleration directions vary significantly depending on the density profile of the plasma created from the droplets and on laser intensity. The spray target can be alternative of foil targets for high intensity high repetition ultrahigh contrast femtosecond lasers. However, at intensities >1021 W/cm2, the efficiency of laser absorption and ion acceleration from the droplets drops significantly in contrast to foils.
Surface periodic structures are generated upon irradiation of a silicon carbide (SiC) thin film by the plasma produced by 40 fs pulses from a Ti:Sapphire laser focused onto a thick low density polyethylene (LDPE) foil facing the SiC film. Independently of the number of laser pulses applied, these structures, with average regular periodicity of 710 nm, are evident throughout all irradiated areas. We attribute their formation to the efficient coupling of the unfocused femtosecond laser pulse with the incoherent extreme ultraviolet component of the laser-generated LDPE plasma.
The response of titanium surface irradiated with high intensity (1013 – 1015 W/cm2) Ti:sapphire laser was studied in vacuum. Most of the reported investigations were conducted with nano- to femtosecond lasers in gas atmospheres while the studies of titanium surface interacting with femtosecond laser in vacuum are scarce. The laser employed in our experiment was operating at 800 nm wavelength and pulse duration of 60 fs in single pulse regime. The observed surface changes and phenomena are (1) creation of craters, (2) formation of periodic surface structures at the reduced intensity, and (3) occurrence of plasma in front the target. Since microstructuring of titanium is very interesting in many areas (industry, medicine), it can be concluded from this study that the reported laser intensities can effectively be applied for micromachining of the titanium surface (increasing the roughness, formation of parallel periodic surface structures etc.).
Production of sharply collimated high velocity outflows – plasma jets from massive planar targets by a single laser beam at PALS facility is clarified via numerical simulations. Since only a few experimental data on the intensity distribution in the interaction beam near the focus are available for the PALS facility, the laser beam profile was calculated by a numerical model of the laser system and the interaction optics. The obtained intensity profiles are used as the input for plasma dynamic simulations by our cylindrical two-dimensional fluid code PALE. Jet formation due to laser intensity profile with a minimum on the axis is demonstrated. The outflow collimation improves significantly for heavier elements, even when radiative cooling is omitted. Using an optimized interaction beam profile, a homogeneous jet with a length exceeding its diameter by several times may be reliably generated for applications in laboratory astrophysics and impact ignition studies.
Laser interactions with mass-limited targets are studied here via numerical simulations using our relativistic electromagnetic two-dimensional particle-in cell code including all three-velocity components. Analytical estimates are derived to clarify the simulation results. Mass-limited targets preclude the undesirable spread of the absorbed laser energy out of the interaction zone. Mass-limited targets, such as droplets, are shown here to enhance the achievable fast ion energy significantly due to an increase in the hot electron concentration. For given target dimensions, the existence is demonstrated for an optimum laser beam diameter when ion acceleration is efficient and geometrical energy losses are still acceptable. Ion energy also depends on the target geometrical form and rounded targets are found to enhance the energy of accelerated ions. The acceleration process is accompanied by generation of the dipole radiation in addition to the ordinary scattering of the electromagnetic wave.
We present a series of experimental results, and their interpretation,
connected to various aspects of the hydrodynamics of laser produced
plasmas. Experiments were performed using the Prague PALS iodine laser
working at 0.44 μm wavelength and irradiances up to a few
1014 W/cm2. By adopting large focal spots and
smoothed laser beams, the lateral energy transport and lateral expansion
have been avoided. Therefore we could reach a quasi one-dimensional regime
for which experimental results can be more easily and properly compared to
available analytical models.
The experience of target fabrication with low-density and cluster
heterogeneity is presented. Cluster plasma research is strongly dependent
on target fabrication development and target structure characterization.
Ten more target parameters should be measured for experiment interpreting
in case of micro-heterogeneous plasma. Foam and foil targets, high-Z doped
also, are produced and irradiated on the existing laser facilities. The
density of 4.5 mg/cc cellulose triacetate in the form of regular
three-dimensional polymer networks are achieved which is as low as plasma
critical density for the third harmonic of iodine laser light. The
possibilities of varying important target parameters, methods of their
monitoring are discussed. Experiments with underdense foam targets with or
without clusters irradiated on Prague Asterix Laser System (PALS) laser
facility are analyzed preliminary for target optimization. Under-critical
foams of varying structure (closed-cell foam or three-dimensional
networks) and densities are reported for plasma experiments. Thermal and
radiation transport in such targets are considered.
Acceleration of quasineutral plasma blocks by ponderomotive force
induced by normally incident short laser pulse is studied here via 1D3V
Particle-In-Cell (PIC) simulations. Very high current densities
109–1011 Acm−2 of accelerated
ions are observed for maximum laser intensities in the range
1016–1018 Wcm−2 on solid
hydrogen target. Ion acceleration process is traced here via evolution of
ion density and of ion velocity distribution. Basic parameters of the
accelerated plasma blocks are determined from temporally integrated ion
distributions. Our results provide more detailed information than the
previous analytical estimates (Hora, 2003) and
the two-fluid 1D hydrodynamic simulations (Glowacz
et al., 2004).
Interactions of sub-nanosecond pulses of kJ-class iodine laser
“PALS” with low-density foams and acceleration of Al foils by
the pressure of the heated foam matter are investigated here, both
experimentally and theoretically. X-ray streak camera is used for
evaluation of the speed of energy transfer through the porous foam
material. The shock-wave arrival on the rear side of the target is
monitored by optical streak camera. Accelerated foil velocities, measured
by three-frame optical interferometers, and shadowgraphs, reach up to
107 cm/s. The accelerated foil shape is smooth without any
signature of small-scale structures present in the incident laser beam.
Conversion efficiencies as high as 14% of the laser energy into the
kinetic energy of Al foil are derived. Experimental results compare well
with our two-dimensional hydrodynamics simulations and with an approximate
Hot electrons may significantly influence interaction of ultrashort
laser pulses with solids. Accurate consideration of resonant absorption
of laser energy and hot electron generation at a critical surface was
achieved through the developed physical and mathematical models. A
two-dimensional (2D) ray-tracing algorithm has been developed to
simulate laser beam refraction and Bremsstrahlung absorption with
allowance for nonlinear influence of a strong electromagnetic field.
Hot electron transport was considered as a straight-line flow weakening
by a friction force calculated in the approximation of the average
state of ionization. Developed models were coupled with the 2D
Lagrangian gas dynamic code “ATLANT” that takes into
account nonlinear heat transport. The developed program has been
applied to simulate irradiation of Al foils by picosecond laser double
pulses. Hot electron transport and heating resulted in thin foil
explosions. The transition from the exploding foil regime to the
ablative one with foil thickening has been simulated and analyzed at
various values of laser light intensity. In second series of
calculations we have modeled the interaction of a nanosecond iodine
laser with a two-layered target.
K-α emission is an intense short-pulse line source well
suited for X-ray diagnostic techniques with subpicosecond and
micrometer resolution. Numerical simulations are performed here in a
search for laser–target interaction regimes where both high
efficiency of laser energy transformation to X-ray emission and
ultrashort X-ray pulses are achieved. We use the one-dimensional PIC
code for the description of the laser interaction with the plasma layer
at the target surface. Fast electron transport into the target is
treated by our newly developed Monte Carlo code with temporal
resolution that is described here in detail. Our simulations reveal
extremely short ∼200 fs FWHM bright K-α X-ray pulses
emitted from targets heated by 120-fs pulses of a table-top laser.
Laser energy conversion efficiency to K-α line emission as
high as 6 × 10−5 is noticed. Integration of the
emitted energy over the focal spot is carried out to improve the
simulation accord with published experimental data. Negligible impact
of self-induced electric fields on K-α emission is found
for conducting target materials at moderate laser intensities
[lsim ]1017 W/cm2.
The absolutely calibrated K-shell spectra emitted from
short-living aluminum plasmas at laser intensities of 5 ×
1015–4 × 1018 W/cm2
are reported. The experiments performed with the constant energy,
variable-length laser pulses (1.5 ps–1 ns) are modeled by the
one-dimensional (1D) hydrodynamics code, including nonlinear resonance
absorption of the laser radiation, fast electron acceleration, and
energy transfer into the target. The characteristic features of the
measured and the postprocessed spectra are outlined. The spatial
and temporal profiles of the emitted spectra are presented;
the scaling rules for the conversion efficiency of the laser
radiation into the line X-ray emission are discussed.
Formation of transverse inhomogeneities in the corona of a
solid target irradiated by an inhomogeneous main laser pulse
and a uniform background pulse was observed experimentally via
side-on shadowgraphy. The experimental results were successfully
interpreted using a two-dimensional hydrodynamics code. Our
simulations identified the onset of sharp contact boundaries
between plasma streams of different expansion velocities. The
formation and the decay of the contact boundaries is investigated
in detail. When the background pulse is used as a laser prepulse,
a layer of coronal plasma is formed that enhances main pulse
collisional absorption in underdense plasma and creates conditions
for an efficient thermal smoothing of the transverse inhomogeneities.
Laser prepulse effect on thermal smoothing of non-uniformities
of target illumination is studied by means of 2D Lagrangian
hydrodynamics simulation, based on parameters of real experiment.
A substantial smoothing effect is demonstrated in case
of an optimum delay between the prepulse and main heating
laser pulse. The cause of enhancement of thermal smoothing
effect by laser prepulse is the formation of long hot layer
between the region of laser absorption and ablation surface.
Comparison with experimental results is presented.
Emission of energetic ions from solid targets irradiated by intense ultrashort
laser pulses is studied in the framework of a one-dimensional self-consistent
hydrodynamical model. The computed ion spectra reproduce well the basic parameters
of published experimental results. Optimum conditions are found for the
generation of MeV ions by picosecond laser pulses.
The interaction of femtosecond laser pulses with solid-state density plasmas in regime of normal skin effect is investigated by means of numerical simulation. For short-wavelength lasers and laser pulses with length ≲ 120 fs full width at half maximum, the regime of normal skin effect is shown to hold for peak intensities up to 1017 W/cm2. The basic characteristics of the interaction are revealed and certain departures from simplistic models in electron distribution function, in plasma dielectric constant, and in laser absorption are pointed out. Comparison with the published experimental results is made.
The interaction of ultrashort laser pulses with a fully ionized plasma is investigated in the plane geometry by means of numerical simulation. The impact of the space oscillations in the amplitude of the laser electric field on the shape of the electron distribution function, on laser beam absorption, and on electron heat transport is demonstrated. Oscillations in the absorption rate of laser radiation with the minima coincident to the maxima of the laser electric field lead to a further decrease in the absorption of laser radiation. Heat flux in the direction of increasing temperature in the underdense region is caused by the modification of the electron distribution function and by the density gradient. A limitation of heat flux to the overdense plasma isobserved with the flux limiter in range 0.03–0.08, growing moderately with the intensity 1014–1016 W/cm2 of the incident 1.2-ps laser pulse.