Topographical, structural and hardness changes in surface layer of stainless steel-AISI 304 irradiated by fusion-relevant high energy deuterium ions and neutrons in a low energy plasma focus device

Abstract

This paper aims to investigate the effects of fusion relevant intense pulse of energetic and high fluence deuterium ions and neutrons generated in a low energy (3.0kJ) plasma focus device (UNU/ICTP PFF) on stainless steel-AISI 304 samples. The irradiation was performed using different number of deuterium plasma focus shots (1, 3, 5 and 10) at a fixed axial distance of 8cm from the anode tip. X-ray diffraction spectra of irradiated samples indicated the structural phase transition from mixed ferittic-austenitic form in virgin AISI 304 SS sample to only austenitic form indicated by the presence of only γ-Fe diffraction peaks and increase in corresponding peak intensities. A small shift in all austenitic diffraction peaks towards the higher diffraction angles was observed indicating the decrease in lattice spacing by vacancy defects most probably due to energetic fusion neutrons. Field emission scanning electron microscopy and atomic force microscopy images showed the formation of nanometer sized particles on irradiated samples' surfaces. The size of nanoparticles initially decreased as the number of focus shots was increased from 1 to 3 and to 5, and later as the number of focus shots was increased to 10, the aggregations of the particles occurred to form bigger clusters. The sample surface hardness was found to decrease with increasing number of irradiation shots which can be attributed to the induction of vacancy defects by 2.45MeV fusion neutron irradiations and the increasing deposition of the copper ablated from the anode-top on the irradiated samples. The study clearly established that the plasma focus device provides intense fusion relevant pulses suitable for causing the topographical, structural and hardness changes on the surface of the irradiated SS AISI 304 which is one of the candidate materials for plasma facing components in fusion reactors.