We report on ion emission from plasma produced on thick targets irradiated with nanosecond and femtosecond pulses delivered by mid-ultraviolet and soft x-ray lasers, respectively. To distinguish between different ion acceleration mechanisms, the maximum kinetic energy of ions produced under different interaction conditions is plotted versus laser fluence. The transformation of the time-of-flight detector signal into ion charge density distance-of-flight spectra makes it possible to determine the mean kinetic energy of the fastest ion groups based on the influence of the acoustic velocity of ion expansion. This allows obtaining additional characteristics of the ion production. The final energy of the group of fast ions determined using the ion sound velocity model is an order of magnitude larger in the fs-XFEL interaction than in the ns-UV one. On the contrary, the ablation yield of ions in our experiment is seven orders of magnitude greater when applying ns-UV laser pulses, not only due to higher energies of UV laser pulses, but also due to a significant difference in interaction and ion formation mechanisms.
Laser-target interaction experiments demonstrated that the return target current, jTC(t), which neutralizes the target charge appearing when the fastest electrons escape the plasma, is one of principal characteristics of the laser-matter interaction. jTC(t) flowing between the target and the ground is emerging just when the laser intensity exceeds the threshold intensity of the plasma formation. The experimental determination of the number of escaped electrons is primarily based on precise target current observations. We present the experimental observation of jTC(t) neutralizing metallic and plastic targets irradiated with low intensity ranging from 108 to 1013 W/cm2 delivered by KrF and iodine lasers operated at 248-nm and 1315-nm wavelengths, respectively. Our experiments show that the charge appears on targets continuously not only during the laser-plasma interaction but also during the plasma expansion into the vacuum. The analysis of jTC(t) allows us to determine the level of influence of the target surface pollution by chemisorbed hydrocarbons on the plasma production, which is also needed for elucidation of specific processes leading to the target charge and ion emission. Very specific feature of jTC(t) was found for plastic targets. Our experiments also demonstrate that the accompanying phenomenon of low-intensity laser-target interaction is the generation of electromagnetic pulses that are emitted at frequencies coinciding also with the resonant frequency modes of the interaction vacuum chamber.
Josef Krása, Daniel KlÍr, Karel Řezáč, Jakub Cikhardt, Andriy Velyhan, Miroslav Pfeifer, Jan Dostál, Miroslav Krůs, Roman Dudžák, Simona Buryšková, Vincenso Nassisi, Domenico Delle Side
A resistive target probes were employed to obtain unique characteristics of the dynamic of the laser-matter interaction through the observation of a return current neutralizing the positive target charge caused by electrons definitely leaving the plasma. Experimental observations show that three phases of the laser-produced plasma are well recognized, i.e. the plasma ignition phase, active plasma phase, and afterglow phase already for the lowest laser intensity at the focal point of 108 Wcm-2 . Then, at higher intensities the first two phases strongly dominate and the target current reaches a value of 10 kA. A throughput of plasma barrier for the fastest electrons leaving the plasma is defined, and its value is experimentally estimated. We also present key experiments proving that the duration of the electrical charge on the target is much longer than the duration of the laser-plasma interaction for the entire intensity range.
A UV pulsed lasers was employed to produce C and Ti ions of different charge with current densities of the order of 10 mA/cm2. A post ion acceleration, up to 30 kV, was employed to increase the ion energy and to implant polyethylene biocompatible surfaces up to a depth of about 200 nm. Preliminary results about surface properties indicate that implanted surfaces have higher wetting and micro-hardness with respect to un-implanted ones.
A novel technique for ion implantation of electronics materials by means of a laser ion source emitting multi-energetic ion streams was investigated. A UV pulsed laser beam, at intensities of the order of 108 W/cm2, was employed to produce plasma in a vacuum from a Ge target. The apparatus utilized was very versatile and able to contain an expansion chamber in order to allow the plasma to be diluted before the application of an accelerating voltage. The mean ion energy increased with the laser pulse energy and the ion charge state, and ranged between about 100 eV and 1 keV. To increase the ion energy a post-acceleration up to 50 kV was employed, which resulted in ion energies from about 50 keV to about 150 keV, depending on the charge state. The multi-energetic ion beam, with current density of the order of 10 mA/cm2, was employed to irradiate silicon substrates and to obtain surface implantations up to a depth of about 150 nm. During the implantation process the ion beams were generated with a repetition rate of the laser pulse of 1 Hz. The depth profiles of the ion implants were investigated by Rutherford backscattering spectrometry and laser ablation - inductively coupled plasma - mass spectrometry.
Results about the efficiency of the laser cleaning on the reduction of corrosion products from the surface of ancient coins are reported. In this work an ancient copper coin datable from 1500 to 1600 A.D. and a UV excimer laser were utilized. The goal of this work consists to study the potentiality of UV laser treatment in the reduction of the chlorine concentration on the coin surface which is the main responsible of the corrosion processes of the ancient coins. We used Energy Dispersive X-Ray Fluorescence (EDXRF) and X-Ray Diffraction (XRD) techniques to estimate the chemical composition of the coin surface, before and after UV excimer laser cleaning. In particular, we measured the chlorine, copper and calcium concentrations. We found that a radiation dose of about 19 J/cm2 was able to reduce the chlorine concentration from 2.3 % w/w to 0.6 % w/w without damaging the metal bulk.
In this work the mutagenic effect on Escherichia coli strains induced by UV radiation emitted by a XeCl laser (λ =
308 nm) has been analysed as a function of the exposure dose and compared with the effect induced by 254 nm
radiation emitted by a conventional germicidal lamp. E. coli strains, wild-type (recA+) and mutant (recA1, defective in
DNA damage repair systems), plated on LB agar, supplemented with rifampicin when requested, were irradiated by
means of a germicidal lamp in the dose range 0 - 9 mJ/cm2. Similar strains were exposed to 308 nm pulsed laser
radiation (τ = 20 ns FWHM; max. pulse energy: 100 mJ) in the dose range 0-1.0 x 104 mJ/cm2. The discrepancy
between the results obtained with the lamp and the laser on the mutation frequency, suggested that the biological
response to the two radiation sources involves distinct mechanisms. This hypothesis was supported by the evidence that
exposure to near-UV 308 nm induced mutagenesis in the recA-defective strain at an extent considerably higher than in
the recA-proficient strain.
In this work we report the preliminary experimental results on the selective ablation of sulphur in ancient stones. The sulphur concentration was reduced after laser action. For this goal an excimer laser operating at 308 nm wavelength and time duration of 20 ns was used. In order to estimate the sulphur concentration before and after laser cleaning, a portable apparatus for energy-dispersive X-ray fluorescence (EDXRF) was utilised. The processed sample were characterized by an initial sulphur concentration of 2.8% w/w. After the laser treatment, sulphur concentration decreased after a total deposited energy of about 30 J/cm2 up to 1.2% w/w value. Due to the porosity of the stone, in fact, it is difficult to eliminate completely the S presence in the composition of the stones. It was also observed that after a few laser shots the initial black area of the stone became white showing in this way the great potential of the laser action on the cleaning process of the pietra leccese.
In this work an ion acceleration system based on a laser ion source was studied. It was able to generate ion beams utilizing as a source a laser plasma produced by a XeCl laser from a copper target. The focused laser beam provided a power density on the target surface of about 3.5x108 W/cm2. Laser wavelength and pulse duration were 308 nm and 20 ns, respectively. The experimental apparatus consisted substantially of a plasma generation chamber, a drift tube and an expansion chamber mounted on the target stem inside the generation chamber. The expansion chamber end formed the acceleration gap together with a grounded bored electrode, placed in front of it at a distance of 1.3 cm. A Faraday cup placed at the end of the drift tube was used to reveal the ion intensity.
Many attempts were done in order to accelerate plasma ions without the expansion chamber, but arcs were present. The maximum accelerating voltage applied to the extraction gap was 18 kV, resulting in an ion bunch of about 4.2 nC and a peak current of 220 μA.
We report here the results about the sulphur concentration in ancient stones and its removal after the application of UV laser action. A portable apparatus for energy-dispersive X-ray fluorescence (EDXRF) was utilised to measure the sulphur concentration before and after laser cleansing. After laser application the sulphur concentration decrased to 1.2% w/w whilst the initial value was 2.8% w/w. The laser energy density was about 1 J/cm2 and the efficient shot number was approximately 30 with a rate of 1Hz.
We generated electron beams by a Nb polycrystalline photocathode illuminated by different wavelength excimer lasers; an XeCl and a KrCl. The cathode surface roughness was 0.09. At low accelerating voltage, the electron beams were governed by the space-charge effect and their intensity never resulted clipped as previous by the Child-Langmuir law. Instead, the output current waveform was similar to the laser one under saturation regime. The quantum efficiencies were 3.2 x 10-5 and 6.7 x 10-7 for the KrCl and XeCl irradiation, respectively.
Experimental results concerning the morphologic modification and the presence of new elements in palladium films loaded in hydrogen gas and processed by an excimer laser are presented. The Pd films were deposited on Si(100) wafers and imbedded in hydrogen up to 50 days. Before to be analyzed the films were irradiated with 500 laser shots. The laser fluence was lower than 25 mJ/cm2 in order to avoid the film ablation. The gas pressure was 5 bar. The samples were analyzed by Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX). The films changed their morphology after about eight days whereas grains with new elements were found in samples processed more than thirty days.
We present the experimental results concerning the study and the development of a Laser Ion Source (LIS). By means of an excimer laser we irradiated a metal target at high power density, realizing an efficient source of multiple charged ions. The analysis of the generated plasma plume was performed for three different laser spot sizes determining the threshold conditions of the ablation process. A diagnostic system with a Faraday cup was developed in order to detect the ion current along the propagation tube. Time-of-flight measurements were performed, also inserting in front of the cup an adjustable voltage electrostatic barrier in order to get quantitative information about the ion flux and the kinetic energy of the produced ions. To study the plasma characteristics we evaluated the total etched material per pulse, 0.25 μg, and the fractional ionization, 12%. A modified Maxwell-Boltzmann distribution was applied to provide a consistent description of the velocity distributions in the plume. The ablated material was spatially monitored by optical transmission analysis of a deposited film. Applying the high voltage to the LIS extraction gap, an ion beam containing Cu+1 (0.44mA), Cu+2 (0.34mA), Cu+3(0.99mA), and Cu+4(0.01mA) ions was obtained.
Experimentally we observed stochastic resonance-like (SR) phenomena of diffusion transport of silica microspheres in water solution in the spatially distributed potential created by Bessel light beam. One-dimensional approximation of this optical field is periodic potential of washboard type with static force applied. We observed two distinguished types of SR--enhanced particle mobility in the direction of static force and a cooperative behavior of self-ordering patterns.
We present the experimental results concerning the study and the development of a Laser Ion Source (LIS). By means of an excimer laser we irradiated a metal target at high power density, realizing an efficient source of multiple charged ions. The analysis of the generated plasma plume was performed for three different laser spot sizes, determining the threshold conditions of the ablation process. A diagnostic system with a Faraday cup was developed in order to detect the ion current along a propagation tube. Time-of-flight (TOF) measurements were performed, also inserting in front of the cup an adjustable voltage electrostatic barrier in order to get quantitative information about the ion flux and the kinetic energy of the produced ions. To study the plasma characteristics we evaluated the total etched material per pulse, 0.25 μg, and the fractional ionization, 12%. The ablated material distribution was monitored by optical transmission analysis of a deposited film. Applying a high voltage to the LIS extraction gap, an ion beam containing Cu+1(0.44mA), Cu+2(0.34mA), Cu+3(0.09mA), and Cu+4(0.01mA) was obtained.
In this paper we report on the biological effects of XeCL laser irradiation on Staphylococcus epidermidis and Escherichia coli. UV interaction with cellular systems is responsible for photochemical, photothermal or photodecomposition processes. When short-wavelength UV radiation strikes biological material, the DNA is damaged causing cell killing, mutagenesis or carcinogenesis. We report on different effects of XeCl laser irradiation on two microbial systems; collection strain of Staphylococcus epidermidis (in suspension) and collection strains of Eschericha coli proficient or deficient in DNA recombination/repair pathways (irradiated on solid surfaces). In S epidermidis the 308 nm radiation can significantly enhanced the proliferation rates. In wild type E. coli cells the radiation did not stimulate the growth rates. Surprisingly, the 308 nm radiation elicited a very strong lethal effect on DNA recombination/repair-defective strains (harbouring the recA56 null mutation), even more pronounced than irradiation with a UV 254 nm germicidal lamp. The unknown mechanism responsible for this biological response is currently under investigation.
In this work the electron beam current and beam emittance measured for an Al and a diamond film cathode irradiated with two different UV excimer lasers, XeCl and KrCl, are presented. The output current was measured with a fast Rogowski coil while the emittance was determined with a new diagnostic setup composed by two movable slit arrays and an array of small cups. The phase space area occupied by the electrons at the output was determined by the current measured with the cups and by the position of the slits. With a 4 mm2 beam spot the maximum current from an Al cathode was 740 mA and from a diamond cathode was 560 mA with the KrCl laser. The corresponding emittance values were 130 and 50 for the Al and the diamond cathode, respectively. From these values the normalized beam brightness were estimated to be 0.18 X 109 A((pi) m rad)-2 for Al cathode and 0.92 X 109 A((pi) mrad)-2 for diamond cathode.
Results of experiments investigating the influence of the XeCl and KrCl laser beams on the generation of electron beams up to short threshold are reported. The cathode was a lead- lanthanum-zirconium-titanate PLZT ceramic having the emission side coated with 200 micrometer large strips made up gold film. Under the laser irradiation the emission current increased more than 40%.
In this experimental work, a new and very compact home made XeCl laser has been used for the ion beam generation by metallic targets. Multiply-charge heavy ion pulses have been extracted by the plasma produced by a focused laser beam at relatively low flux (approximately 30 MW/cm2) on Si, Ge, Mg and Zn targets. An output peak current of Si3+ ions of 375 mA has been recorded at an acceleration voltage of 200 V only. The insertion of a variable capacitance between the target holder and the acceleration electrode allowed a self-bunching of the ion beam. Besides, a peak current of 1.4 A of Pb3+ ions was obtained by increasing the laser flux to 86 MW/cm2 and the acceleration voltage to 500 V.
In this experimental work, the SBS and SRS processes were used in order to produce UV laser beams of good optical quality and short duration time. An XeCl oscillator and a double-pass amplifier with a phase-conjugate mirror via stimulated Brillouin scattering, generate the laser beam at 308 nm to pump a Raman cell. The oscillator pulse was 11 nsec long, while the amplified phase-conjugate beam duration could vary from 3.3 to 1.5 nsec, by using the compression effect operated by the Brillouin mirror. When this last laser beam was focused into a Raman cell containing methane at 30 atm, the shortest backward stimulated Raman scattering pulse at 338.4 nm was 170 psec long with a brightness in excess of 1013 W(DOT)cm-2(DOT)sr-1. The 338.4 nm wavelength is interesting for the production of short bunches of cold electrons from Mg and Zn targets.
This paper reports the behavior of an XeCl laser having two independent preionizers which generate UV radiation into the discharge region in the perpendicular and parallel direction to the electrode surfaces. Output energy and other parameters have been measured using a single preionizer and both preionizers, changing the power-supply polarity and varying the delay time between the preionization and the main discharge. When the preionization system generated photons only in the direction perpendicular to the electrode surfaces and a negative power supply was used, a high discharge current and a high output laser energy were obtained. Besides, the diffusion coefficient of the photo-preionized electrons played an important role in the laser behavior, particularly when a consistent delay time between the onset time of the preionization and the main discharge was present. During the breakdown the discharge impedance is time dependent and its value changes as the preionizers change position. By comparing the experimental results obtained by the equation of the discharge electric circuit, it is possible to get exactly the electric element value which simulates the discharge impedance.
This paper reports the behavior of an XeCl laser having two independent preionizers which generate UV radiation into the discharge region in the perpendicular and parallel direction to the electrode surfaces. Output energy and other parameters have been measured using a single preionizer and both preionizers, changing the power supply polarity and varying the delay time between the preionization and the main discharge. When the preionization system generated photons only in the direction perpendicular to the electrode surfaces and a negative power supply was used, a high discharge current and a high-output laser energy were obtained. Besides, diffusion coefficient of the photo-preionized electrons played an important role in the laser behavior, particularly when a consistent delay time between the onset time of the preionization and the main discharge was present.
Mode locking of a laterally UV prelonized discharge XeC1 laser
has been obtained by using a phase conjugate mirror as full reflector
in the laser cavity. The phase conjugate mirror via stimulated Brilbum
scattering presents a time dependent reflectivity. It acts as a
modulator and provides amplitude modulation at the cavity round trip
time. An output beam made of 5 short pulses separated by 10 nsec has
been obtained. The shortest pulse is 2 nsec FWHM long.
A compact XeCl laser system made up of an oscillator and an amplifier is described. By applying a stimulated Brillouin scattering mirror (SBSM) to the amplifier, an output laser beam of optical and spectral characteristics very close to those of the oscillator has been obtained. By applying to the amplifier a phase conjugate cavity, 1.7m long, formed by an SBSM and a quartz flat window, a pulse train made of pulses separated by the cavity round trip time has been obtained. The first pulse was 8 nsec long while the last pulse was shorter than 2 nsec.
The operation of a narrow-bandwidth Self Filtering Unstable Resonator
(SFUR) of magnification I M I = 10 applied to a UV-preionized XeC1 laser has
been demonstrated. An intracavity grating (2400 lines/mm)) at a grazing
incidence angle (80 ) has been employed to get a narrow-bandwidth SFUR. A
4.5 mm diameter output beam of 100 pJ, 19 ns long and with a linewidth of
0. 13 A has been obtained. Measurements of the spectral and optical laser
radiation characteristics are reported. The narrow-bandwidth SFUR operation
has been compared with the operation of a plane-plane cavity employing the
same grazing incidence grating and 1 mm diameter intracavity spatial
apertures.
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