Oct 4, 2017 - (Al) to A¼184 (W), melting points from Tm ¼ 430 K (In) to .... dence of the oscillation period on the atomic mass of the tar- .... plotted in linear and semi-logarithmic coordinates in Figs. ... 6. Total collected charge vs. probe potential in linear and semi-logarithmic coordinates .... For the first element this leads.
Investigation of femtosecond laser-produced plasma from various metallic targets using the Langmuir probe characteristic P. Nica, S. Gurlui, M. Osiac, M. Agop, M. Ziskind, and C. Focsa
Citation: Physics of Plasmas 24, 103119 (2017); doi: 10.1063/1.5006076 View online: http://dx.doi.org/10.1063/1.5006076 View Table of Contents: http://aip.scitation.org/toc/php/24/10 Published by the American Institute of Physics
PHYSICS OF PLASMAS 24, 103119 (2017)
Investigation of femtosecond laser-produced plasma from various metallic targets using the Langmuir probe characteristic P. Nica,1 S. Gurlui,2 M. Osiac,3 M. Agop,1 M. Ziskind,4 and C. Focsa4 1
Department of Physics, Technical “Gh. Asachi” University, Iasi 700050, Romania Faculty of Physics, “Al. I. Cuza” University, 11 Blvd. Carol I, Iasi 700506, Romania 3 Department of Physics, Faculty of Science, University of Craiova, 13 Al. I. Cuza str, Craiova 200585, Romania 4 University Lille, CNRS, UMR 8523, PhLAM—Physique des Lasers, Atomes et Mol ecules, CERLA—Centre d’Etudes et de Recherches Lasers et Applications, F-59000 Lille, France 2
(Received 2 March 2017; accepted 21 September 2017; published online 4 October 2017) The Langmuir probe is used to characterize the plasma produced by fs-laser ablation from pure metallic targets. Time dependence of the probe current and the total collected charge is discussed in terms of a shifted Maxwell–Boltzmann distribution function, and from probe characteristics the plasma temperature and average charge state are calculated. Target materials of various physical properties (atomic mass, thermal constants) are used to find possible correlations with resulting plasma parameters. By positively biasing the probe, the collected charge –probe voltage characteristic is in general vertically shifted, and for low negative probe potentials an effect consisting in an abnormal decrease of the ion current is observed. Periodic falls of the total collected charge vs. probe voltage are experimentally recorded, the effect being more significant at high background pressure. They are tentatively attributed to secondary ionization. Published by AIP Publishing. https://doi.org/10.1063/1.5006076 I. INTRODUCTION
Understanding the mechanisms of interaction between ultra-short laser pulses and a target surface is a challenge for scientists when relating to fundamental studies, and it has received a growing interest due to the great number of potential applications. For example, the possibility of reaching high irradiances led to inertial confinement fusion experiments, where the formation, propagation, and stability of plasma jets are key issues.1,2 There is also a high interest in experimental and theoretical investigations of low-intensity pulsed laser ablation and deposition.3–5 The physical processes at the surface of irradiated materials are strongly dependent on the ultrafast time-scale, non-equilibrium energy distributions with large excess of populations in the excited states being produced. This consequently influences the expanding plasma parameters like average charge state, plasma potential, temperature, density, and generally the entire space-time plume dynamics.3–6 The difference of fs-ablation compared to longer pulse length is that it offers plasma of high density, because the laser energy is delivered before significant thermal conduction occurs, and there is no interaction between the laser beam and the ablated plume.7,8 Therefore, ultrafast lasers became essential through their widespread use in various applications and scientific researches, although the physical mechanisms in fs-laser ablation plasma are not yet fully understood and still require further studies of fundamental aspects.9 In order to investigate the transient fs-laser ablation plasmas, the Langmuir probe (LP) maybe used as one of the simplest techniques, since it consists of placing a metallic electrode into the plasma and measuring the current to it at various applied voltages.10–15 However, it is an intrusive technique and the electrode must be carefully designed to 1070-664X/2017/24(10)/103119/10/$30.00
avoid significant interaction with the plasma, or even its damage.11 Moreover, the interpretation of the currentvoltage (I-V) curves is difficult, especially for transient plasmas (as those considered in this paper). Langmuir probe diagnostic systems are typically applied to stationary and homogenous plasma, which can be maintained for considerable time.14 Application of a Langmuir probe to the laser ablation process must take into account that plasma parameters are time- and space-dependent during the expansion, plasma is non-homogeneous, it has nonzero drift velocities, and lifetimes typically in the microsecond range. Such apparently non-ideal conditions do not in fact preclude the useful application of Langmuir probes in the study of laser-produced plasmas. For example, in Ref. 16 charged particle measurements employing the Langmuir probe and Faraday cup are compared with ion dynamics resulting from optical emission spectroscopy (OES) and spectrally resolved 2D imaging, good correlation being showed for various laser fluencies. At moderate laser intensities (1014 W cm2), similar with those of our experiments, the LP method revealed the presence of a sizeable population of fast ions flying ahead of the main neutral component, a general feature of ultrashort laser ablation of metallic targets with few mJ pulses.17 Moreover, it has been shown that the ejected charges decrease with increasing target at. wt. For example, the charge yield resulting from laser-carbon interaction being one order of magnitude higher than the charge yield obtained for the gadolinium target.9 Also, in Ref. 18, using various targets, a nonMaxwellian (fast peak) component was evidenced in the ion velocity distribution and a Maxwellian (slow peak) component was evidenced, the temporal separation of the two peaks growing with atomic mass.
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In the present paper, the LP is used to characterize the plasma produced by fs-laser ablation from various metallic targets, the probe current being discussed in terms of a shifted Maxwell–Boltzmann distribution function. The recording time-scales are analyzed by coupling the distribution functions of electrons and ions through an effective mass, and special attention is given to the convolution process induced by the recording system. The metallic targets were grounded or positively biased to observe the influence on the probe current temporal trace, on the time-integrated collected charge dependence vs. probe biasing, and generally on the expansion process. The studied target materials are chosen based on various physical properties, having atomic mass numbers from A ¼ 27 (Al) to A ¼ 184 (W), melting points from Tm ¼ 430 K (In) to Tm ¼ 3683 K (W), or boiling points from Tb ¼ 1261 K (Te) to Tb ¼ 5828 K (W). Correlations of expanding plasma parameters (temperature and average charge state) with those of the target material are investigated. II. EXPERIMENTAL SET-UP
A schematic view of the experimental set-up developed mainly for analytical purposes, described in detail elsewhere,19–21 is shown in Fig. 1. The experiments have been performed in a stainless steel vacuum chamber at 105 Torr residual pressure. The Ti:Sa femtosecond laser (800 nm, 60 fs, 100 Hz repetition rate, 1.7 mJ/pulse) has been focused by a f ¼ 25 cm lens at quasi-normal incidence onto various metallic targets (Al, Mn, Ni, Cu, In, Te, and W) placed in the vacuum chamber. The estimated spot diameter at the impact point has been 160 lm. Thus, the laser intensity resulted to be IL ¼ 0:15 PW=cm2 . The targets were placed in vacuum, on a rotating, electrically isolated X-Y-Z micrometric stage. The current extracted from the plasma plume was measured by a cylindrical Langmuir probe made of stainless steel with 0.8 mm diameter and 5 mm length. It has been placed perpendicular to the plume expansion direction, on the plume axis at 3.5 mm from the target surface. The Langmuir probe
FIG. 1. Experimental set-up.
Phys. Plasmas 24, 103119 (2017)
and the target were biased with stabilized dc power sources, at voltages VP and VT , respectively. The transitory signals of the probe have been recorded from the voltage drop across a 4.6 kX resistor (Rp) by a digital 600 MHz, 2.5 GS/s oscilloscope (LeCroy, Wave Surfer 62XS). Input impedances of 50X or 1 MX were used, and a fast photodiode was used to trigger the oscilloscope on the laser pulse. We note that the target-probe distance is usually chosen of few centimeters,11,12 considering that the probe does not influencing the plume expansion and local plasma parameters. For example, in Ref. 22 it was reported that a perturbation of the plume expansion happened due to interference by the probe when placed close to the target (
