Improving the spectral intensity of aluminum plasma by applied-magnetic field in laser-induced breakdown spectroscopy

Abstract

In this report, an impact of applied-magnetic field on laser induced breakdown spectroscopy (LIBS) emission with different air pressures and several time delays was explored by analyzing the solid aluminum (Al) sample. A Neodymium-doped yttrium aluminum garnet (Nd:YAG) laser at its basic wavelength (1064 nm) was utilized to generate the plasma and its optical emission was recorded with the help of an optical emission spectroscopy (OES) technique. A notable improvement was observed primarily at low pressure and shorter delay time for Al atomic lines due to the effect of magnetic field (0.5 T). For the air pressure of 100 Pa and delay time of 100 ns, the signal enhancement of approximately 2-fold was obtained for Al (I) 396.1 nm spectral line. The signal enhancement due to the magnetic confinement was attributed to an increase in electron-impact excitation rate and recombination processes. The plasma parameters like, electron temperature (T_e) and electron number density (n_e) were also affected under the applications of magnetic field. A considerable enhancement was noticed in comparison to the field-free case. The plasma temperature attained their highest values at 500 Pa only, however, the electron density was found to be completely enhanced across the magnetic field. The optical enhancement is accredited to the plasma-confinement by applied field and has scope in improving the LIBS sensitivity.

Introduction

Laser-induced breakdown spectroscopy (LIBS) is one of the most effective technique which uses a powerful laser beam onto the sample material and generates the plasma that consisting of atoms, electrons, ion and excited species. The optical emission spectrum during the excitation of plasma provides necessary information about the composition of material, which can be utilized for plasma diagnostic and analytical purposes [1]. LIBS is a spectroscopic technology that has precedence over other traditional techniques since it is applicable for all types of materials [2], [3], [4]. This tool carries the ability of multi-elemental analysis, real-time response, experimental flexibility and simplicity [5]. It has ascertained its applications in the fields of biotechnology [6], environmental monitoring [7], remote sensing industry [8] and substantial analysis [9]. The performance of traditional LIBS can be improved to enhance the intensity of spectral lines from plasma. Different procedures that can be utilized to improve the sensitivity of LIBS, include in-transparent laser occurrence, double pulse, nanoparticle-enhanced LIBS, usage of purge gas, spatial and magnetic confinements [10], [11], [12], [13], [14], [15] etc.

The use of magnetic field in LIBS to enhance the plasma’s optical emission is a simple and inexpensive method. This subject has useful applications in nanoparticle production, debris dispersal, thin film removal, provision of intense ultra-violet (EUV) lithographic sources along with surface amendments [16], [17], [18], [19], [20]. The expansion of laser-induced plasma (LIP) across a magnetic field involves various physical phenomena, like, plume imprisonment, change of thermal energy of plasma to kinetic energy, ion acceleration, emission development and plasma instabilities [21]. The plasma-magnetic field interactions help to comprehend the inertial imprisonment fusion plasmas [22], [23], [24], aids in improvement of solar wind and astrophysical jets [25], benefits in promulgation of bipolar flows and charged particle beams linked to young stellar objects [26], [27]. High-intensity pulsed magnetic fields were widely used to enhance the emission in LIBS [28], [29], [30], [31]. Previously, different researchers intended to understand the enhancement effects in LIP across an-applied magnetic field. For instance, Arshad et al. [32] studied the influence of transverse magnetic field (0.5 T) on the emission spectra of graphite plasma as a function of laser fluence in background ambient gas. The laser fluence was ranged from 0.4 to 2.9 J/cm² under two different environments of air and Ar at a pressure of 150 and 760 torr. It was found that the plasma intensity, electron temperature and electron number density were enhanced at all fluences and environmental conditions due to the application of magnetic field. This effectiveness of plasma parameters was ascribed to the joule heating effect and magnetic confinement. Rai et al. [33] explored the role of steady magnetic field (∼5 kilogauss) on LIBS emission from certain elements (Mn, Mg, Cr and Ti) in aqueous solution. Nearly 1.5 times higher enhancement in the emission intensity was seen in the presence of magnetic field. Moreover, the temporal profile for Mg line emission exhibited a significant enhancement between 2 and 7 µs time delays due to the presence of magnetic field. This enhancement in plasma emission is associated to an increase in the rate of recombination due to the existence of magnetic field. Recently, Abbasi et al. [34] reported an optical emission improvement from palladium (Pd) sample by applying magnetic field-assisted laser-induced breakdown spectroscopy (MF-LIBS). A significant enhancement (3–4 fold) in plasma emission was observed at low values of laser fluence (12.6 J/cm²) for both Pd-I and Pd-II lines. The plasma parameters; the temperature and electron density were also considerably increased in the presence of magnetic field as compared to the without case. In another study of plasma enhancement effects due to the applications of magnetic field, Liu et al. [35] employed a Nd:YAG laser to measure the optical emission from Al-alloy target at different ambient air pressures. They observed that the spectral emission from Al and Li lines was significantly improved under the variation of magnetic field strength. Further, the temporal evolution of different plasma parameters like, electron temperature, electron number density, expansion velocity and plasma lifetime were all significantly affected by the field.

In this report, we explore the role of an applied-magnetic field on the emission aspects of laser-induced Al plasma with different air pressures and several time delays, in order to optimize the spectroscopic performance, and investigate the influence of magnetic field on the plasma parameters such as T_e & n_e. The main objective of the study is to improve the spectroscopic performance of LIBS by utilizing a combination of magnetic field and ambient air pressures at different time delays. A Nd:YAG laser in its fundamental wavelength (1064 nm) was employed to generate the plasma plume inside the vacuum chamber and its optical emission recorded with a OES technique. The field strength of 0.5 T was generated with the help of two permanent magnets and measured with a gauss tesla meter.