Ambient gas; Magnetic field; LIBS; EAST; ICRF wall cleaning;TRANSVERSE MAGNETIC-FIELD; INDUCED PLASMA SPECTROSCOPY; OPTICAL-EMISSION; PLUME; PERFORMANCE

In this paper, laser-induced breakdown spectroscopy (LIBS) under magnetic field condition has been studied in laboratory and EAST tokamak. The experimental results reveal that in helium ambient gas, the magnetic field significantly enhances the LIBS signal intensity (similar to 3 times). The effect of time delay and laser fluence on the intensity of LIBS has been investigated for optimizing the signal to background ratio (S/B). The developed LIBS approach has been applied to monitor the cleaning performance of the first wall in the fusion device of EAST using the ion cyclotron range of frequency (ICRF). The experimental results demonstrate that the cleaning performance for Li/D co-deposition layer is effective under helium ambient gas. The removing rate of Li on the surface of W tile is faster than that on Mo tile in He-ICRF cleaning and the D/(D + H) ratio on Mo tile is higher by similar to 1.2 times than that on W tile. This work would indicate the feasibility of using LIBS to monitor the wall cleaning processes in EAST tokamak. (C) 2017 Elsevier B.V. All rights reserved.

Magnetic field; Lithium plasma; Excited state; Recombination cross-section; Libs;INDUCED BREAKDOWN SPECTROSCOPY; OPTICAL-EMISSION SPECTROSCOPY

In this paper, the effect of magnetic field (1.1 T) on the atomic and ionic spectral emission of a laser produced lithium plasma at low pressure has been investigated. The experimental results indicate that magnetic field enhances the intensities of Li I spectral lines but reduces the Li II spectral lines intensities. In this study, two narrowband filters were placed before the ICCD camera to observe the evolution feature of Li II spectral line (548.39 nm, 2p(3)P(2,1,0)-> 2s(3)S(1)) and Li I spectral line (610.30 nm, 3d(2)P(3/2,5/2) -> 2p(2)P(1/2,3/2)), respectively. The plasma dynamic images show that with the magnetic field, the number density of luminous Li atoms is higher, while the number density of luminous Li ions is lower in comparison to the field-free case. The reduced Li II spectral intensities indicate that the quenching rate of Li ions in the excited state is greater than that without the magnetic field. The enhanced impact frequency of recombination indicates that magnetic field increases the recombination process of electron and Li ions. All of these observations strongly suggest that magnetic confinement increases the recombination process of the electrons with Li ions in the plasma, which results in the decrease in the intensity of Li II line. The results are useful for applying laser-induced breakdown spectroscopy (LIBS) to in-situ diagnose the processes of lithium wall conditioning in EAST tokamak. (C) 2017 Elsevier B.V. All rights reserved.

Time-of-flight mass spectroscopy; Laser induced breakdown spectroscopy; Tungsten plasma; Multi-charged ions; Dynamic plasma sheath;INDUCED-BREAKDOWN-SPECTROSCOPY; ABLATION; SPECTROMETRY; LIBS

In this work, the plasma was produced by irradiating a tungsten target with an 8 ns pulsed Nd:YAG (lambda = 1064 nm) laser in a vacuum chamber under the pressure of 4 x 10(-4) Pa. The optical and particle emissions were systematically investigated using laser induced breakdown spectroscopy and time-of-flight mass spectroscopy respectively. The results showed that not only there were neutral and single ionized atoms in the laser induced plasma, but also quite a number of multi-charged ions were observed. The ion charge state was even up to 6 at the laser power density of similar to 11 GW/cm(2). Time and space resolved optical spectroscopy was investigated by using a bundle of lined fibers. Meanwhile, the time-resolved mass spectrometric study of laser produced tungsten plasma was carried out. The variation in intensities of the different species with time showed that higher charged ions reached their peak intensities earlier. This demonstrated that the higher charged ions had higher velocities and the different charged ions were separated during the expansion process. The kinetic energy corresponding to the velocity of the ions was found to increase exponentially with ionic charge state which was related to the acceleration of the dynamic plasma sheath. (C) 2017 Elsevier B.V. All rights reserved.

tokamak; laser-induced breakdown spectroscopy; impurity deposition; fuel retention; plasma wall interaction;ADVANCED SUPERCONDUCTING TOKAMAK; PLASMA-FACING COMPONENTS; CO-DEPOSITED LAYERS; FUSION-REACTORS; RETENTION; REMOVAL

Post-mortem methods cannot fulfill the requirement of monitoring the lifetime of the plasma facing components (PFC) and measuring the tritium inventory for the safety evaluation. Laser-induced breakdown spectroscopy (LIBS) is proposed as a promising method for the in situ study of fuel retention and impurity deposition in a tokamak. In this study, an in situ LIBS system was successfully established on EAST to investigate fuel retention and impurity deposition on the first wall without the need of removal tiles between plasma discharges. Spectral lines of D, H and impurities (Mo, Li, Si, ...) in laser-induced plasma were observed and identified within the wavelength range of 500-700 nm. Qualitative measurements such as thickness of the deposition layers, element depth profile and fuel retention on the wall are obtained by means of in situ LIBS. The results demonstrated the potential applications of LIBS for in situ characterization of fuel retention and co-deposition on the first wall of EAST.