Single-crystal and polycrystalline diamond erosion studies in Pilot-PSI

Abstract

Diamond is a promising candidate for enhancing the negative-ion surface production in the ion sources for neutral injection in fusion reactors; hence evaluation of its reactivity towards hydrogen plasma is of high importance. High quality PECVD single crystal and polycrystalline diamond samples were exposed in Pilot-PSI with the D + flux of (4-7)·10 24 m -2 s -1 and the impact energy of 7-9 eV per deuteron at different surface temperatures; under such conditions physical sputtering is negligible, however chemical sputtering is important. Net chemical sputtering yield Y = 9.7·10 -3 at/ion at 800°C was precisely measured ex-situ using a protective platinum mask (5x10x2 µm) deposited beforehand on a single crystal followed by the post-mortem analysis using Transmission Electron Microscopy (TEM). The structural properties of the exposed diamond surface were analyzed by Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Gross chemical sputtering yields were determined in-situ by means of optical emission spectroscopy of the molecular CH A-X band for several surface temperatures. We observed a bell shape dependence of the erosion yield versus temperature between 400°C and 1200°C, with a maximum yield of ~1.5·10 -2 at/ion attained at 900°C. The yields obtained for diamond are relatively high (0.51.5)·10 -2 at/ion, comparable with those of graphite. XPS analyses show amorphization of diamond surface within 1 nm depth, in good agreement with molecular dynamics (MD) simulation. MD was also applied to study the hydrogen impact energy threshold for erosion of [100] diamond surface at different temperatures.

Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas

“…Therefore, carbon materials, which show yields one or two orders of magnitude higher than metals, would sit between clean metals and cesiated surfaces in terms of NI yield, at least under high energy positive ion bombardment (few tens of eV). However, cesiated surfaces have proven to be efficient also at low energy particle bombardment (few eV), have demonstrated very low number of co-extracted electrons [60], and have the advantage to be continuously renewed by Cs evaporation while it might be difficult to keep a defect free diamond layer under plasma exposure due to its inherent reactivity with hydrogen, even at low energy [48].…”

“…Kogut et al [48] demonstrated with MD simulation that the chemical sputtering threshold for diamond at room temperature is around 4 eV. Still with MD, de Rooij found chemical sputtering threshold at 5 eV and penetration threshold for H in diamond around 7 eV [49].…”

Particle flux characteristics of a compact high-field cascaded arc plasma device

“…where S L is the nominal speed of roots pump. In this compact vacuum system, the effective pumping speed is calculated by equations (3)- (5). Calculation results show that 798 L s −1 of the effective pump speed is obtained in the compact vacuum chamber.…”

“…Due to the capacity to produce high density plasma, cascaded arc plasma sources are selected to simulate the divertor plasma stream. Plasma-wall interactions (PWI) experiments with high particle flux have been performed in cascaded arc source type linear plasma devices such as Magnum-PSI [4], Pilot-PSI [5], CIMPLE-PSI [6], SCU-PSI [7], and DUT-PSI [8]. It has been demonstrated that the achievement of large ion flux requires both high magnetic field and huge gas fueling rate (typically several to tens of standard liters per minute (SLM)).…”