Investigating helium-deuterium synergies in plasma-exposed tungsten using laser ablation techniques

This article describes the application of laser ablation based techniques to quantify the depth-dependent concentrations of gaseous species below a tungsten surface. In this work, we use a Quantel QSMART Nd:YAG 60 mJ, 5 ns frequency doubled laser that operates at a 532 nm wavelength, which we couple to an MVAT energy modulator to decrease the laser energy to 1.8 mJ. We use this laser to ablate tungsten and then characterized the resulting laser-induced plasma via laser-induced breakdown spectroscopy (LIBS). The ablated gases are pumped into a quadropole mass spectrometer to perform laser ablation mass spectroscopy (LAMS). The coupled LIBS-LAMS technique is demonstrated using a polycrystalline tungsten specimen that was helium ion implanted with variable helium ion energies to create an approximately 200 atomic parts per million flat-top concentration profile. LIBS-LAMS measurements are then performed on polycrystalline tungsten exposed to helium only, deuterium only, or 90% deuterium-10% helium plasmas on the linear plasma device PISCES/A using either a 75 or 250 eV bias voltage. The LIBS-LAMS measurements clearly indicate that exposure to helium during mixed deuterium-helium plasma exposure leads to an increased near surface deuterium concentration but reduces deuterium permeation below the tungsten surface.