During the operation of a fusion machine, it is important to know the composition of the surface of the Plasma Facing Components (PFCs) in interaction with the plasma. Indeed, it is likely to evolve (erosion, oxidation...) modifying the interaction conditions. On ITER, we will also try to determine:

• the PFCs tritium concentration in order to monitor the tritium inventory within the ITER vessel, which is limited,
• the presence of helium bubbles in the PFCs, helium coming from the interaction of fusion neutrons with the materials or from the decay of tritium trapped in the material. The helium bubbles degrade the thermomechanical properties of the PFCs designed to sustain high heat fluxes (~10 MW/m²). 
  
   The LIBS (Laser-Induced Breakdown Spectroscopy) technique is particularly well suited to perform these measurements. LIBS consists in focusing a nano, pico or even femtosecond pulsed laser beam on a material to be analyzed in order to obtain a high laser intensity on its surface. The laser/matter interaction leads to the ablation of the material and the creation of a plasma plume. The spectral analysis of the plasma emission gives access to the elemental composition of the ablated material and so to the composition of the material under study. This technique is widely used in industrial applications that require regular control of the composition of materials. It is currently in its heyday as it allows the remote characterization of the composition of rocks on Mars [see the CEA/CNES ChemCam project for the Curiosity mission https://mars.nasa.gov/msl/spacecraft/instruments/chemcam/ or the Supercam project for the Perseverance mission https://mars.nasa.gov/mars2020/spacecraft/instruments/supercam/]. 

The first in-situ tests took place in December 2021 in WEST and were performed at atmospheric pressure. The nanosecond laser source and the spectrometer for the optical analysis of the LIBS plasma were placed in the machine hall, outside the WEST vacuum chamber. The AIA equipped with the fiber LIBS tool has been deployed in the tokamak vacuum vessel to perform LIBS analysis on a tungsten PFCs of the divertor. These first measurements are currently being analyzed.

The CEA has an inspection robot with articulated arms, called the AIA (Articulated Inspection Arm), which allows inspecting during in between plasma the status of the CFPs thanks to an embedded camera. A fibered LIBS tool has been installed on the AIA in order to be able to characterize the CFPs surface during operation. It was designed and tested by the CEA in close collaboration with the CORIA.

In the following figure, the evolution of the intensity of copper, nickel and boron lines are presented as a function of thickness, each point corresponding to a laser pulse. These elements are impurities present in a deposited layer on the surface of the analyzed PFCs. This layer results from the erosion of the material due to the plasma-wall interaction and from its deposition after transport by the plasma.

From this figure, we can deduce that the layer deposited during the plasmas has a thickness of about 600 nm.

After this first experiment of fibered LIBS in a tokamak, the CEA and CORIA are now aiming at the following steps:

• to finalize the developments in order to be able to deploy LIBS measurements on a regular basis on WEST,
• to consider the integration of picosecond lasers allowing precise measurements of hydrogen concentrations (H, D, T), an important measurement in view of ITER operation.