Analysis of impurities on contaminated surface of the tokamak limiter using laser induced breakdown spectroscopy

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Highlights

  • The manuscript deals with analysis of tokamak limiter, as wall protection is one of the prime concerns in tokamak operation.

  • Qualitative and quantitative analysis of contaminated surface of the limiter have been performed.

  • Depth profile analysis of limiter surface is performed which gives the pattern of deposition of plasma dust on its surface.

Abstract

Laser induced breakdown spectroscopy (LIBS) analysis of plasma dust/impurities deposited on the surface of graphite limiter of Aditya tokamak is presented in the manuscript. LIBS spectra of contaminated curved surface of the limiter show the presence of Fe, Cr, Ni, Mo, Mn, Cu and C. The depth profile analysis of impurities has been performed by recording LIBS spectra with successive number of laser shots. Variation of spectral line intensity of impurities on its surface with distance from one end of the curved surface to the other end has been studied which shows spatial analysis i.e. deposition pattern of plasma dust/impurities on its surface. The concentration of constituents has been calculated using calibration free LIBS (CF-LIBS) method. The study demonstrates the capability of LIBS for depth profile and spatial analysis of deposited impurities on tokamak limiter.

Introduction

In modern tokamak operation, the wall protection is one of the prime concerns. In recent years, graphite/tungsten coated graphite are commonly used as limiter and first wall material for complete coverage of the internal vacuum vessel surfaces of the tokamak [1], [2], [3], [4]. The hot plasma interacts with the limiter and vacuum vessel wall introducing impurities in the plasma [5]. These impurities get deposited on limiter, vacuum vessel wall and other internal components like optical windows etc. Limiter made of graphite is used in Aditya tokamak to reduce plasma-wall interaction and protect the vacuum vessel wall as well as inside components [5]. To understand the plasma-wall interaction, it is necessary to identify the deposited impurities and their pattern of deposition, if possible in real time during plasma discharge. The shape of the plasma-facing surface of the limiter of Aditya tokamak, IPR is curved. Hence in present work, contaminated curved surface of the limiter obtained from IPR, Gandhinagar has been analyzed for detection and quantification of impurities deposited on its surface.

LIBS is a very popular technique due to its versatility described in books [6], [7] and research articles [8], [9], [10], [11], [12], [13], [14], [15]. The major advantages of LIBS are its speed, ability to obtain multi-elemental analysis in a single laser shot, a very small amount of material (~ 1 μg) or volume is required, and can be used for in-situ analysis. It is readily applied to the real site with the help of suitable instrument and optical fiber [10]. Due to its unique properties, LIBS has become a potential tool for in-situ and real-time analysis of the nuclear material [13], [16], [17], [18], [19], [20], [21]. Using LIBS technique, Karhunen et al. [22] have performed the depth profiles analysis of different ITER-relevant material mixtures, Gasior et al. [23] have used LIBS for diagnostics of fuel retention and removal and wall composition in ITER, Mercadier et al. [24] have performed time and space-resolved studies of PFCs and Grisolia et al. [7] have studied the capability of the heating diagnostic coupled with LIBS analysis in different PFCs. In the present work, the limiter of the Aditya tokamak, IPR, Gandhinagar, Gujarat, India has been studied.

Section snippets

Material and method

The experimental setup for recording LIBS spectra of limiter is shown in Fig. 1. It consists of a frequency-doubled (532 nm) Nd:YAG laser (Continuum Surelite III-10), capable of delivering maximum energy of 425 mJ with pulse width 4 ns (FWHM) and variable repetition rate of 1–10 Hz. The laser is focused on the surface of the limiter using a convex lens of focal length 15 cm, which produces the plasma on its surface. Characteristic photons emitted from the plasma is collected at an angle of 45° to

Qualitative analysis

We have recorded LIBS spectra of impurities deposited on the surface of the limiter by keeping the limiter at suitable sample stage so that every time the laser hits at fresh surface of the limiter. LIBS spectra (Fig. 2) clearly show the presence of spectral lines of Fe, Cr, Ni, Mo, Mn, Cu, C, Ca and Mg.

These elements are also present in LIBS spectra of impurities deposited on the other plasma-facing components like optical window and flange obtained from Aditya tokamak as discussed in our

Conclusion

In the present paper, identification and quantification of impurities deposited on limiter surface have been performed using LIBS. In LIBS spectra of the impurity layer, different elements like Fe, Cr, Ni, Mo, Mn, Cu, C, Ca and Mg are identified. We have performed the depth profile analysis of the limiter by recording LIBS spectra with successive number of laser shots at the same point on its surface. We found that the spectral line intensity of impurity elements decreases with successive laser

Acknowledgement

Board of Research in Fusion Science and Technology (BRFST), Institute for Plasma Research, Gandhinagar, Gujarat, India is gratefully acknowledged for financial assistance. We are thankful to Dr. Ashok Kumar Pathak for his help and valuable suggestions. Authors Dr. Gulab Singh Maurya and Dr. Rohit Kumar acknowledge BRFST (NFP-DIAG-F11-03) for providing financial assistance as SRF.

References (29)

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