Laser-produced Plasma Plume Research Articles

Application of laser-produced extended plasma plumes for generation and characterization of the high-order harmonics of 64 fs pulses

Harmonics with the photon energy of up to ~150 eV were studied using extended ablation plumes as the nonlinear media. The application of the ablation pulses of different duration covering the range between a few tens of femtoseconds and a few tens of nanoseconds revealed the advanced features of the extended plasmas produced by the subnanosecond pulses for efficient harmonic generation. The examples of the quasi-phase-matching of a group of harmonics in the plateau range and the advantages of using the two-color pump and cluster-containing plasmas are presented. We analyze the spatial and coherence characteristics of the high-order harmonics produced during propagation of the 64 fs pulse through the extended plasma plume produced by 370 ps pulses. It is shown that the divergence of plasma harmonics in the plateau range is 7 times smaller than the divergence of the driving radiation used for high-order harmonic generation. The measurements of the coherence properties of the lower-order harmonics showed that the visibility of interference fringes in the far field was in the range of 0.54–0.73.

Investigation on recovery of gap insulation strength and EUV radiation in the post-discharge stage of a laser-triggered discharge produced tin plasma EUV source

This paper presents an investigation on the recovery of gap insulation strength and extreme ultraviolet (EUV) radiation in the post-discharge stage of a laser-triggered discharge produced tin plasma (LTDPP) EUV source. Gap insulation strength recovers more quickly under a shorter gap length. The gap breakdown voltage recovered to 19.4 kV with a breakdown probability of 25% at 80 µs after an 8 J discharge under a 2.5 mm gap. The same discharge energy under a 5 mm gap required 140 µs for equal recovery. The EUV radiation recovery process was measured utilizing a double laser triggered discharge system. EUV radiation recovery lagged behind the gap insulation strength recovery for both the 2.5 and 5 mm gap. In the case of the 8 J input pulse energy, EUV radiation recovered to the same level as the first EUV pulse at about 600 and 160 µs for the 5 and 2.5 mm gaps, corresponding to a 1.67 kHz and 6.25 kHz repetition rate, respectively. In the case of the 2.5 mm gap and 4 J input pulse energy, our EUV source could work with a repetition rate of upto 23 kHz, enabling the EUV output to reach 345 W/2πsr which was 4.5 times as the value of the 5 mm gap and 8 J input energy. To discuss the slower EUV radiation process, the initial laser trigger phase and plasma pinch stage during the first and second laser-triggered discharge were visualized by means of high speed visible light imaging and laser Schlieren imaging. Comparative results suggest that tin droplets influenced the initial laser-produced plasma plume, consequently affecting the Z-pinch plasma dynamics and its EUV radiation which resulted in the delayed EUV recovery process.

Magnetic reconnection between colliding magnetized laser-produced plasma plumes.

Observations of magnetic reconnection between colliding plumes of magnetized laser-produced plasma are presented. Two counterpropagating plasma flows are created by irradiating oppositely placed plastic (CH) targets with 1.8-kJ, 2-ns laser beams on the Omega EP Laser System. The interaction region between the plumes is prefilled with a low-density background plasma and magnetized by an externally applied magnetic field, imposed perpendicular to the plasma flow, and initialized with an X-type null point geometry with B=0 at the midplane and B=8 T at the targets. The counterflowing plumes sweep up and compress the background plasma and the magnetic field into a pair of magnetized ribbons, which collide, stagnate, and reconnect at the midplane, allowing the first detailed observations of a stretched current sheet in laser-driven reconnection experiments. The dynamics of current sheet formation are in good agreement with first-principles particle-in-cell simulations that model the experiments.

Fast ion generation in femtosecond laser ablation of a metallic target at moderate laser intensity

The generation of ions during laser ablation of a metallic target (copper) with ≈50 fs Ti:Sa laser pulses of moderate intensity (≈1014 W cm−2) is studied by simultaneous fast-imaging and ion-probe techniques. The spatiotemporal distribution of excited ions and neutrals in the laser-produced plasma plume is analyzed by exploiting appropriate band-pass filters in the imaging set-up, while the ion flux angular distribution is characterized by angle-resolved ion probe measurements. An interesting feature of our results is the generation of a fast ion population separated from the neutral component of the atomic plasma plume and characterized by sub-keV kinetic energies, which is interpreted in the frame of a simple model of ambipolar diffusion.

Dynamics of femtosecond laser-produced plasma ions

We investigated the ion laser-produced plasma plume generated during ultrafast laser ablation of copper and silicon targets in high vacuum. The ablation plasma was induced by ≈50 fs, 800 nm Ti:Sa laser pulses irradiating the target surface at an angle of 45°. An ion probe was used to investigate the time-of-flight profiles of the emitted ions in a laser fluence range from the ablation threshold up to ≈10 J/cm2. The angular distribution of the ion flux and average velocity of the produced ions were studied by moving the ion probe on a circle around the ablation spot. The angular distribution of the ion flux is well described by an adiabatic and isentropic model of expansion of a plume produced by laser ablation of solid targets. The angular distribution of the ion flux narrows as the laser pulse fluence increases. Moreover, the ion average velocity reaches values of several tens of km/s, evidencing the presence of ions with kinetic energy of several hundred eV. Finally, the ion flux energy is confined in a narrow angular region around the target normal.

High-quality electron beam from laser wake-field acceleration in laser produced plasma plumes

Generation of highly collimated (θdiv ∼10 mrad), quasi-monoenergetic electron beam with peak energy 12 MeV and charge ∼50 pC has been experimentally demonstrated from self-guided laser wake-field acceleration (LWFA) in a plasma plume produced by laser ablation of solid nylon (C12H22N2O2)n target. A 7 TW, 45 fs Ti:sapphire laser system was used for LWFA, and the plasma plume forming pulse was derived from the Nd:YAG pump laser of the same system. The results show that a reproducible, high quality electron beam could be produced from this scheme which is simple, low cost and has the capability for high repetition rate operation.

New Approaches in Frequency Conversion of Laser Radiation in Plasma Plumes

Recent studies of plasma harmonic generation are reviewed. We show the advances of this technique for generation of coherent ultrashort pulses in the extreme ultraviolet range. Among the achievements highlighted in this review are the comparative experimental and theoretical studies of high-order harmonic generation in silver plasma, isolated subfemtosecond pulse generation in Mn plasma ablation, stable generation of high-order harmonics of femtosecond laser radiation from laser produced plasma plumes at 1 kHz pulse repetition rate, and high-order harmonic generation in fullerenes using few- and multi-cycle pulses of different wavelengths. We show that new developments of plasma harmonic studies can lead to the creation of an attractive field of laser ablation induced high-order harmonic generation spectroscopy.

Stable generation of high-order harmonics of femtosecond laser radiation from laser produced plasma plumes at 1 kHz pulse repetition rate

We present a method for the creation of stable weakly ionized plasmas from laser ablation of solid targets using a 1 kHz pulse repetition rate laser, which can be used for stable high-order harmonic generation from plasma plumes. The plasma plumes were generated from cylindrical rotating targets. Without target rotation the intensity of harmonics in the 40-80 nm range drops by more than one order of magnitude during less than 10(3) shots, while, with rotation of the target at typically 30 revolutions per minute, stable emission of high-order harmonics from aluminum plasma plumes with variation of less than 10% was maintained for >10(6) laser shots.

Expansion and backscattering of laser produced Fe plasma plume

Forward and backward moving atoms within the laser produced plasma plume were studied by means of a cavity ringdown spectroscopy. The plume was produced using a nanosecond Nd-YAG laser pulse illuminating stainless steel target in a vacuum or helium background gas. Measurements were done at pressures ranging from 10−5 to 1mbar. Atomic absorption line shapes of iron around 388nm were measured above and below the target at different times after the ablation initiation. Changes in absorption line shapes were used to estimate kinetic parameters of the plasma plume. The observations were interpreted through modeling which takes into account the angular and velocity distributions of atoms in the expanding plume. The amount of backward scattered atoms was about 10% of the total number of particles.

Beyond Zeeman spectroscopy: Magnetic-field diagnostics with Stark-dominated line shapes

A recently suggested spectroscopic approach for magnetic-field determination in plasma is employed to measure magnetic fields in an expanding laser-produced plasma plume in an externally applied magnetic field. The approach enables the field determination in a diagnostically difficult regime for which the Zeeman-split patterns are not resolvable, as is often encountered under the conditions characteristic of high-energy-density plasmas. Here, such conditions occur in the high-density plasma near the laser target, due to the dominance of Stark broadening. A pulsed-power system is used to generate magnetic fields with a peak magnitude of 25 T at the inner-electrode surface in a coaxial configuration. An aluminum target attached to the inner electrode surface is then irradiated by a laser beam to produce the expanding plasma that interacts with the applied azimuthal magnetic field. A line-shape analysis of the Al III 4s–4p doublet (5696 and 5722 Å) enables the simultaneous determination of the magnetic field and the electron density. The measured magnetic fields are generally found to agree with those expected in a vacuum based on the pulsed-power system current. Examples of other transitions that can be used to diagnose a wide range of plasma and magnetic field parameters are presented.

Image analysis of expanding laser-produced lithium plasma plume in variable transverse magnetic field

AbstractUsing fast imaging technique, the effect of transverse magnetic field on the shape and dynamics of the lithium plasma plume has been studied. Enhancement in the overall emission intensity as well as appearance of distinct structures (lobes) in the plasma plume in the presence of magnetic field has been observed. By introducing a variable magnetic field, the influence of J × B force in expanding plasma plume across the transverse magnetic field has been explored. It appears that J × B force does not has a substantial role in the observed structures in the present case rather different atomic processes, which get affected due to change in plasma parameters, are responsible for appearance of these structures.

Systematic studies of two-color pump-induced high-order harmonic generation in plasma plumes

High-order harmonic generation (HHG) has been studied in various laser-produced plasma plumes using a two-color orthogonally polarized beam with a 12:1 energy ratio between the fundamental and second-harmonic (SH) components. The influence of the relative phase between the fundamental and SH waves on the HHG efficiency has been investigated. Odd and even harmonic generation in plasma plumes containing nanoparticles, fullerenes, carbon nanotubes, and other samples was optimized. The effect of the variation in the SH intensity on the HHG conversion efficiency in carbon aerogel and silver plasma plumes has also been studied. It is shown that by increasing the SH intensity, one can generate only even harmonics by suppressing the odd harmonics.

Ablation yield and angular distribution of ablated particles from laser-irradiated metals: The most fundamental determining factor

Five metals (Zn, Cu, Ni, Ti, and Mo) were irradiated with 150 shots of a Q-switched Nd:YAG pulsed laser in a vacuum of 10−3torr. The ions projected out of the laser-produced plasma (LPP) plume were detected by CR-39 detectors positioned at −15°, 0°, 30°, 60°, and 90° with respect to the target-surface normal at a distance of 5cm from the target in each case. The angular distribution of LPP ions, which is characterized by the exponent n of cosnθ distribution, is given by n=2.5–11 for the five target metals. The value of the exponent n has no systematic correlation with the square-root of atomic mass of the target metals but exhibits systematic dependence on the room temperature Debye–Waller's thermal parameter B or the mean-square amplitude of atomic vibrations 〈u2〉. Likewise, the ablation yield (atoms/shot) of the twelve target metals investigated by Thestrup et al. (2002) [8] under identical irradiation conditions is a function of the room temperature B-factor or 〈u2〉.

Fast ion generation in the cathode plasma jet of a multipicosecond laser-triggered vacuum discharge

Ion generation in the cathode plasma jet of a moderate-current (approximately 2.3 kA), low-energy (< or = 20 J) vacuum spark discharge triggered by approximately 27 ps, 10 mJ laser pulses is studied using time of flight technique. Fastest ion velocity and velocity corresponding to the peak of the time of flight signals for Al cathode were measured to be approximately 5.25x10(8) cm/s (energy of approximately 143 keV/u) and approximately 8.1x10(7) cm/s (energy of approximately 3.4 keV/u), respectively. Corresponding velocities in the case of ions generated from laser-produced Al plasma (energy of approximately 550 mJ, intensity of approximately 10(14) W/cm(2)) were found to be much smaller, viz., approximately 1.05x10(8) cm/s (energy of approximately 5.75 keV/u) and approximately 2.63x10(7) cm/s (energy of approximately 0.36 keV/u), respectively. Study shows efficient acceleration of ions in a current-carrying cathode plasma jet of a small-energy multipicosecond laser-triggered spark discharge as compared with that in a high-energy multipicosecond laser-produced plasma plume.

Simultaneous Multitemperature Measurements of Thermal Diffusivity and Composition

It has been well established that thermal forcing of disordered multielement metallic alloys results in permanent modification of their thermal transport properties. The mechanism depends on the detail of heating and its duration, and entails rearrangement of the spatial distribution of constituent atoms within the material proper. It presents a serious complication in developing a body of properties data as a function of temperature because establishment of a thermal state for a specimen is often cast into question. A new general technique is presented for simultaneous multiple-temperature measurements of thermal diffusivity and local composition for a single specimen. The specimen is heated steeply into a state of temperature nonuniformity. The measurement is carried out by time-resolved spectroscopy of single-shot laser-produced plasma (LPP) plume emissions; analysis is made of the emissions as a function of position within the laser focal area. The procedure for analysis is presented together with the results for 80 mass% Ni-20 mass% Cr Nichrome specimens.

Laser induced plasma plume imaging and surface morphology of silicon

Shot-to-shot variation in the characteristics of laser produced plasma plume and surface profile of N-type silicon (1 1 1) are investigated. In order to produce plasma, a Q-switched Nd: YAG laser (1064 nm, 10 mJ, 9–14 ns) is tightly focused on silicon target in air at room temperature. Target was exposed in such a way that number of laser shots was increased from point to point in ascending order starting from single shot at first point. Target was moved 2 mm after each exposure. In order to investigate shot-to-shot variation in the time integrated emission intensity regions within the plasma plume, a computer controlled CCD based image capture system was employed. Various intensity regimes were found depending strongly on the number of incident laser pulses. Plasma plume length was also found to vary with the number of pulses. The topographic analysis of the irradiated Si was performed by Scanning Electron Microscope (SEM) which shows the primary mechanisms like thermal or non-thermal ablation depend on the number of shots. Surface morphological changes were also studied in terms of ripple formation, ejection, debris and re-deposition of material caused by laser beam at sample surface. The micrographs show ripples spacing versus wavelength dependence rule [ Λ ≈ λ/(1 − sin θ)]. Intensity variations with number of shots are correlated with the surface morphology of the irradiated sample.

Time resolved Nomarski interferometery of laser produced plasma plumes

We report results from optical interferometric probing of a laser generated Zn plasma plume. The experiment was performed in a vacuum and O 2 rich environments where the background pressure of O 2 was maintained at 1000 Pa and the results from both regimes are compared. The focus of our work is very much on the early stages in the life of the plasma plume which remains, to date, a largely unexplored area of study, at least in the pulsed laser deposition research domain. It was found that the electron density profile normal to the target is different in the background gas at early times (∼30 ns) compared to that of the vacuum case. At later times (∼80 ns) both profiles have a very similar shape. We also observe the formation of a shock wave at the plasma–gas interface shortly after plasma breakdown (<15 ns).

Dependence of high order harmonics intensity on laser focal spot position in preformed plasma plumes

The dependence of the high-order harmonic intensity on the laser focal spot position in laser produced plasma plumes is experimentally studied. High order harmonics up to the 59th order (λ∼13.5 nm) were generated by focusing 48 fs laser pulses from a Ti:sapphire laser system in silver plasma plume produced using 300 ps uncompressed laser radiation as the prepulse. The intensity of harmonics nearly vanished when the best focus was located in the plume center, whereas it peaked on either side with unequal intensity. The focal spot position corresponding to the peak harmonic intensity moved away from the plume center for higher order harmonics. The results are explained in terms of the variation of phase mismatch between the driving laser beam and harmonics radiation produced, relativistic drift of electrons, and defocusing effect due to radial ionization gradient in the plasma for different focal spot positions.

Substrate heating effects on the propagation dynamics of laser produced plume during pulsed laser deposition of oxides

We report on the expansion dynamics of laser-produced plasma plumes of complex oxides in an oxygen atmosphere. In particular, we have studied the combined effects of background gas pressure and substrate heating on the plume propagation in typical pressure and temperature regimes of oxides thin film deposition by pulsed laser deposition. Our results evidence a reduced resistance of the background gas to the plume propagation as the substrate temperature increases. The experimental data are analyzed in the frame of a model describing the plume propagation into the background gas. Our experimental findings clearly indicate that the deposition temperature might influence film growth, not only through its direct thermal effect on the surface kinetics of adatoms, but also by affecting the energetic properties of the precursors in the gas phase.

Benard–Marangoni Instability as Possible Modifier of Surface Alloy Composition

Thermophysical properties of amorphous alloys represent the features of a given material specimen, and, as such, they are dependent, in general, on their elemental composition. Some properties are measured at surfaces, and others are measured for the bulk as a whole. Complications arise when the elemental composition varies as a function of position within the material specimen, as demonstrated by simultaneous measurements of thermal diffusivity and elemental composition by time-resolved spectroscopy of laser-produced plasma (LPP) plume emissions. To further understand the source of a rather common near-surface elemental composition anomaly, the evolution of the surface composition of Wood’s alloy under the influence of thermal cycling with, and without, a temperature gradient over the specimen has been investigated. Surface composition modifications have been found to take place by accumulation of irregularly spaced gray patches of an inhomogeneous composition on the surface in the presence of a temperature gradient. Determination of elemental composition by LPP spectroscopy shows the three-dimensional structure of the patches.