Investigating graphite surface and structural modifications induced by hydrogen ion bombardment in a plasma focus device

This study investigates the effects of high-energy hydrogen ions generated by a Mather-type plasma focus device on graphite, a critical plasma-facing material (PFM) in fusion reactors. Graphite samples were irradiated with hydrogen ions at incremental fluences (1, 10, 20 shots). Surface modifications (voids, cracks, melting) were analyzed using optical microscopy (OM) and scanning electron microscopy (SEM). The resulting surface changes were clearly visible in the SEM and OM images. Microscopic analysis of the samples subjected to gradual irradiation and point sputtering revealed voids and localized melting on the sample surfaces, which were correlated with increasing ion fluence, particularly after 20 shots. Structural changes in the graphite caused by high-energy proton irradiation were quantified via X-ray diffraction (XRD). The XRD spectrum of the irradiated samples exhibited shifts in peak positions and recrystallization (crystal growth) due to transient thermal annealing, indicating significant structural alterations. To characterize the hydrogen ions produced by the plasma focus device, the Lee code and a Faraday cup detector were utilized. The experimental results showed that the average ion energy was approximately 46 keV. Ion penetration depth, hydrogen retention, and radiation damage were simulated using the SRIM code. The simulation results indicated that the maximum damage occurred at a depth of 200 nm, with a damage rate of 0.024 displacements per atom (dpa) per shot. The highest concentration of hydrogen ions, measured at 0.6 %, was found at a depth of 220 nm. These findings demonstrate graphite's susceptibility to hydrogen-induced damage under fusion-relevant conditions. The study validates plasma focus devices as effective tools for PFM testing.