Elsevier

Journal of Nuclear Materials

Volume 463, August 2015, Pages 684-687
Journal of Nuclear Materials

Mass spectrometry analysis of the impurity content in N2 seeded discharges in JET-ILW

https://doi.org/10.1016/j.jnucmat.2014.12.084Get rights and content

Abstract

Presented are the results of the first mass spectrometry study of impurities at JET with the ILW. Measurements are performed with the newly-installed RGA system that allows for data acquisition in all stages of machine operation. Impurities are predominantly found in the 16–20 AMU range, populated by water, methane and ammonia, and at 28 AMU (CO and N2). The main contaminants in non-seeded discharges are deuterated methane and water, and nitrogen, which are present only in low amounts, and are likely produced by plasma-surface interaction. During N2 seeded discharges, signals increase significantly at 28 AMU, but also in the 16–20 AMU range, indicating conversion of nitrogen to ammonia. In subsequent non-seeded discharges, impurity content is reduced by an order of magnitude, however it exhibits a 5–7 discharge-long legacy.

Introduction

Since the installation of the ITER-like wall (ILW), JET provides a unique test bed to study plasma operation in the ITER material mix: Be plasma-facing components (PFCs) in the main chamber and W components in the divertor [1]. W sputtering which determines the lifetime of the divertor components in the Be/W mix is dominated by intrinsic impurities such as Be, O and C, and extrinsic impurities like N2 and Ar [2], though the C and Be content are orders of magnitude lower in quantity in comparison with the carbon wall [3]. As the JET-ILW does not include carbon-based PFCs, nitrogen is being seeded to replace carbon as the radiating species in the plasma edge and mitigate heat loads at the W target plates [4]. Argon is routinely used as gas for the disruption mitigation system in JET which injects massive amounts of Ar + D2 mixture to reduce or prevent damage to PFCs caused by disruptions [5]. Therefore, a complex set of impurities can interact with the first wall and impact the plasma operation, and identification and quantification of these species and their temporal evolution as legacy gas is of interest.

In this paper, we present a residual gas analysis (RGA) study of the impurity content during N2 seeded discharges at JET. Residual gas analysis relies on the use of mass spectrometers (MS), placed typically downstream of the plasma volume in pump ducts. As the transmission path from the plasma to the MS is long, charged particles, metastable species and neutral non-noble atoms do not reach the MS, and only the content of inert species (stable molecules and noble gases) can be analysed. However, unlike other diagnostic techniques that do not suffer from such limitations (such as VUV spectroscopy), the detection by MS provides information on the impurity content throughout all stages of the tokamak operation (discharge phase, outgassing after a disruption, time between discharges …). This contribution is the first report based on fast RGA measurements in JET-ILW with sufficient temporal resolution to monitor trends in the discharge phase.

Section snippets

Experimental

The impurity content of the tokamak vacuum vessel was analysed with a newly installed sub-divertor residual gas analysis system [6]. It is based on a Hiden Analytical HAL 201 RC mass spectrometer (MS), located in the sub-divertor. The spectrometer is placed inside a soft iron chamber for magnetic shielding, which enables accurate measurements in all stages of the discharge. The pressure in the sub-divertor reaches up to 10−3 mbar during normal discharges, which exceeds the operational range of

Results and discussions

The RGA recordings are studied in the following cases: dry runs (gas injections without plasma operation) which provide information about impurities that are introduced during fueling, non-seeded discharges which provide information about the intrinsic impurity content and N2 seeded discharges, which provide information about the RGA response during different seeding rates, the effect of N2 seeding on the impurity content and the legacy of N2 in subsequent non-seeded discharges.

Conclusions

The RGA measurements indicate low impurity content in JET with the ILW, with the main contaminants being deuterated water and methane, and nitrogen. The dry run recordings reveal that possible sources of impurities are desorption from the gas lines through which D2 is passed and desorption from PFC surfaces, whereas in the non-seeded discharges, the more likely source of impurities is plasma-wall interaction.

In N2 seeded discharges, intensities at 28 AMU clearly dominate the impurity content and

Acknowledgements

This work was supported by EURATOM and carried out within the framework of the European Fusion Development Agreement. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

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1

See the Appendix of F. Romanelli et al., Proceedings of the 24th IAEA Fusion Energy Conference 2012, San Diego, USA.

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