Laser ablation molecular isotopic spectroscopy: A novel tool to characterise the distribution of 13C and 12C on graphite after 13CH4 tracer injection in Wendelstein 7-X

Abstract

Wendelstein 7-X (W7-X) operation with inertially cooled graphite Test Divertor Unit (TDU) was finalised in a series of identical hydrogen plasmas with injection of 13C isotopically marked methane. The break-up of 13CH4, injected through gas inlets at one toroidal position of W7-X, and the subsequent transport and deposition of 13C in the device was studied ex-situ. The TDU was therefore extracted from the vessel and some of the removed TDU elements were examined regarding their 13C distribution in deposited layers on the surface using Laser Ablation Molecular Isotopic Spectroscopy (LAMIS). This study shows the 13C/12C distinction capabilities of LAMIS used to analyse the carbon deposition pattern on Plasma-Facing Components (PFCs) on two of the removed TDU elements. The resulted pattern was compared with complementary results from Nuclear Reaction Analysis (NRA) and simulations of 13C deposition by ERO2.0, a material transport and plasma-surface interaction code. Ultra- short laser pulses of a Nd:YAG laser were used in collinear double-pulse configuration for LAMIS. The first pulse (l1 = 355 nm, t1 = 35 ps, F = 2.3 J/cm2) produced a laser- induced plasma on the surface and 50 ns later, the second laser pulse (l2 = 1064 nm, t2 = 35 ps) was directed into this plasma to enhance the signal. Each pulse pair ablates in total 200 nm of the surface material. The analysis is conducted in 1.5 mbar N2 environment for improved signal-to-noise ratio relative to vacuum conditions. A high throughput custom-made spectrometer in Littrow-arrangement (f = 750 mm, A = 6000 at l = 473 nm, lspan = 14 nm) was used to analyse the C2 Swan bands (d3⊓g - a3⊓u, v = 1 to v’ = 0) of laser-induced plasma emission.

Keywords

• LAMIS
• LIBS
• W7-X