Spatial analysis of impurities on the surface of flange and optical window of the tokamak using laser induced breakdown spectroscopy

Highlights

• Identification of constituents of impurity layer on the surface of optical window and flange of Aditya tokamak.

• Spatial distribution of impurities on the optical window of the tokamak.

• Variation of spectral intensities of constituents with radial distance and color.

• Relative variation of spectral intensity of impurity constituents.

Abstract

Laser induced breakdown spectroscopy (LIBS) spectra of optical window and flange of Aditya tokamak have been recorded in the spectral range of 200–500 nm in open atmosphere. For qualitative analysis of these samples, the intensity variation of elemental impurities (Fe, Cr, Ni, Mn, Mo, Cu and C) on the surface of optical window and flange with radial distance has been studied. Depending on thickness of thin films of impurity, four different concentric color rings are observed on the surface of the flange. On moving from reddish brown to indigo color ring, the trends in spectral intensity of these impurities are found in decreasing order. Comparative spectral intensity variation of impurities in both the samples reveals that the impurity deposition on the flange surface is more in comparison to the optical window. Our study demonstrates the capability of LIBS to analyze the spatial distribution of deposited impurities on window and flange samples.

Introduction

Laser-induced breakdown spectroscopy (LIBS) is a very popular technique from the last decades due to its numerous advantages described in several books [1], [2] and research articles [3], [4], [5]. In nanosecond laser, the laser pulse vanishes within 10–20 ns and within the first 100 ns the LIBS spectra of ablated material are dominated by continuum radiation [6]. But after 1 μs delay, their characteristic spectral emission lines start appearing and therefore the light detection should be delayed to improve signal to background ratio. This timing known as gate delay changes with different target materials and laser properties (power density and wavelength of the laser) [5]. Due to its unique properties, LIBS is a potential tool for in-situ and real-time analysis of the plants, bio- and nuclear material [7], [8], [9], [10], [11].

Aditya tokamak is a moderate field and the first indigenously built medium size non-superconducting tokamak capable of producing 250 kA of plasma current with 300 ms of flattop duration. Its working fuel gas is hydrogen at pressure 1.06×10^-4–1.33×10^-4 hPa. It was designed, fabricated, erected and commissioned at the Institute for Plasma Research, Bhat, Gandhinagar (IPR) [12]. In the tokamak like International Thermonuclear Experimental Reactor (ITER), LIBS has been started as a potential diagnostic tool for characterization of the wall conditions which is performed under ultrahigh vacuum conditions and the toroidal magnetic field [5]. The LIBS has also been used for identification of impurities in Aditya tokamak [13]. In the present study, we have taken the optical window and the flange samples for studying spatial distribution of impurity materials on their surfaces. In the tokamak due to plasma–wall interaction, a small amount material from the plasma facing components is eroded and re-deposited at relatively cooler area/parts of the vacuum vessel in the form of a thin layer. To increase the lifetime and maintain the optical components ready for further study, it is desirable to identify these deposited impurities. Because of long term contamination, the lifetime of optical components unprotected from reactor grade plasmas will be very short [14]. In addition to this, the visibility of the optical window and other optical parts decreases due to deposition of thin film [15], [16]. Mirrors and optical windows are essential components in spectroscopy and imaging systems both in industrial instruments for plasma processing and in controlled thermonuclear fusion devices (e.g., tokamak) [14], [17], [18], [19]. In the last couple of years, work has been initiated in our laboratory. In the present manuscript, an attempt has been made to identify the impurities spatially distributed on the surface of optical window and flange using the LIBS technique.