Date of this Version
Optics Express 19:15 (2011), pp.14067-14075.
Within the past decades, laser-induced breakdown spectroscopy (LIBS) has become a well-established and powerful optical emission spectroscopy (OES) analytical technique [1–5]. As a useful analytical tool, LIBS is used extensively in various areas such as remote detection, hostile environment monitoring, and cultural heritage conservation [6–10]. Researchers are paying more attention to LIBS as a diagnostic method for elemental analysis as it is characterized by fast, real-time, in situ, low invasiveness, and multi-elemental diagnosis, normally without the need for sample preparation [11,12]. However, one of its major drawbacks is its low sensitivity, which seriously hinders further improving the limit of detection (LOD) and restricts the further development and application of LIBS. To enhance the LIBS sensitivity, ultrashort-pulse and dual-pulse LIBS have been developed in recent years [13–18]. The signal and sensitivity of LIBS can be significantly increased by using ultrashort-pulse and dual-pulse schemes. However, there are two drawbacks: one is the increased complexity of the LIBS setup, and the other is the increased cost of using more than one laser. Besides adopting the newest laser or adding lasers, another handy, flexible, and cost effective method is combining the spatial and magnetic confinement of plasmas, which can effectively improve the sensitivity of LIBS in a facile way. As is well known, a shock wave is produced with a plasma when a sample is ablated by a laser pulse . On one hand, the plasma is confined by the magnetic field; on the other hand, the shock wave spreads out at a very high speed, and will be reflected back when encountering walls and will compress the plasma . Combining these two effects, the plasma is compressed into the center, resulting in highly increased collision rates among particles within the plasma which leads to an increase in the number of atoms in high-energy states and, hence, enhanced emission spectra intensity [21, 22]. In the past, researchers only studied the plasma confinement effects using either spatial or magnetic confinement. For instance, our previous research  studied the enhancement of plasmas confined with an aluminum hemispherical cavity, resulting in an enhancement factor of 12 for Mn lines of low-concentration Mn element. Rai et al.  have studied the magnetic field confinement effect in air with a homemade magnetic device, achieving a maximum enhancement factor of 2 for metal alloy samples. The aim of this work was to investigate the enhancement effects by applying combined spatial and magnetic confinements in LIBS. Laser-induced plasmas were produced in a hemispherical cavity located in an externally applied static magnetic field between a pair of permanent magnets. The OES and fast imaging of the plasma plumes were investigated to study the evolution of the plasmas.