Diagnostics of First Wall Materials in a Magnetically Confined Fusion Device by Polarization-Resolved Laser-Induced Breakdown Spectroscopy

Abstract

Laser-induced breakdown spectroscopy (LIBS) is a powerful analytical tool for realtime diagnostics and detection of multiple elements deposited at the first wall of magnetically confined plasma fusion devices. Recently, we have tested LIBS in our laboratory for application to in situ real-time diagnostics in the fusion device EAST. In this study, we applied polarizationresolved LIBS (PR-LIBS) to reduce the background continuum and enhance the resolution and sensitivity of LIBS. We used aluminium (Al) (as a substitute for Be) and the first wall materials tungsten (W) and molybdenum (Mo) to investigate polarized continuum emission and signal-tobackground ratio (SBR). A Nd:YAG laser with first, second and third harmonics was used to produce plasma. The effects of the laser polarization plane, environmental pressure and polarizer detection angle were investigated. The spectra obtained without using a polarizer (i.e. LIBS) were compared with those obtained with a polarizer (PR-LIBS). Distribution of emission spectral intensity was observed to follow Malus' law with respect to variation in the angle of detection of the polarizer. The spectra obtained by PR-LIBS had a higher SBR and greater stability than those obtained by LIBS, thereby enhancing the reliability of LIBS for quantitative analyses. A comparison of Al, Mo and W showed that W exhibited a higher continuum with stronger polarization than the low-Z elements.

Numerical simulation of laser ablation of molybdenum target for laser-induced breakdown spectroscopic application

“…Since the 1960s [9], laser-induced breakdown spectroscopy (LIBS) has become a significant tool for component identification on various kinds of samples like metals, alloys [10][11][12][13][14][15]. It has been developed as an analysis technique for the first wall and the region near the diverter in fusion devices such as EAST due to the lower cost, little damage, and no need for pre-treatment [15][16][17][18][19]. Recently, most corresponding studies were carried out by experimental techniques [12,20,21].…”

Relative spectral response calibration using Ti plasma lines

“…Laser induced breakdown spectroscopy (LIBS) is a promising analytical method in many fields [1][2][3][4][5][6][7]. This technique is a type of optical emission spectroscopy that uses a laser pulse as the excitation source.…”

Time resolved anisotropic emission from an aluminium laser produced plasma

“…The reason for this is the observation that the continuum emission from a laser plasma tends to be more polarised than the line emission [1][2][3] which is useful for analytical techniques such as Laser Induced Breakdown Spectroscopy (LIBS). In previous studies, the degree of polarisation (P) of the light emitted from a laser produced plasma was measured for various focal positions, 1,4 laser energies, 4,5 background pressures, 5,6 incident laser polarisations, 3,4 detection directions, 4,6 and pulse durations. 1,6 In many cases, the use of a polariser in front of the spectrometer during a LIBS measurement suppressed the continuum and improved the limit of detection (LOD) by increasing the signal to background ratio.…”

“…In previous studies, the degree of polarisation (P) of the light emitted from a laser produced plasma was measured for various focal positions, 1,4 laser energies, 4,5 background pressures, 5,6 incident laser polarisations, 3,4 detection directions, 4,6 and pulse durations. 1,6 In many cases, the use of a polariser in front of the spectrometer during a LIBS measurement suppressed the continuum and improved the limit of detection (LOD) by increasing the signal to background ratio. In one study, 7 it was found that no significant polarisation of either the continuum or line emissions could be observed.…”

“…This apparent null result compared with previous studies was resolved by taking the laser fluence into account. 2,6 In fact, this technique has birthed a subset of LIBS termed Polarisation-Resolved LIBS (PRLIBS).…”