Properties of BeD molecules in edge plasma relevant conditions

Abstract

The A 2Π–X 2Σ+ band (the Δv = 0 sequence) emission of beryllium deuteride (BeD) molecules has been observed both in Be-seeded deuterium plasma and in front of Be targets exposed to deuterium plasma. The particle interchange reaction, Be+ + D2 → BeD + D+, is thought to be a dominant process for the BeD formation in the plasma. On the other hand, BeD observed in front of Be targets is a product of chemical sputtering of Be bombarded by deuterium plasma. The photon emission coefficient of the A–X band at λ ∼ 497.3–499.2 nm around the prominent Q branch is estimated to be ∼5 × 10−14 m3 s−1 for electron temperatures ≳8 eV as deduced from particle balance between Be, Be+, and BeD in D2 neutral pressure scans. The surface temperature dependence of chemical sputtering of Be released as BeD is investigated, and the sputtering yield of BeD is found to peak at ∼440 K. This peak temperature is consistent with the onset temperature of the decomposition of BeD2, obtained from thermal desorption spectrometry of BeD2 powder. Also, the chemical sputtering yield is observed to be decreased with increasing incident ion flux, similar to carbon. Vibrational (Tvib) and rotational (Trot) temperatures of BeD molecules are evaluated by fitting a measured A–X band spectrum with a simulated spectrum. Spectra taken from BeD formed in the plasma are well fitted with a single pair of Tvib and Trot, which increase linearly with electron density. However, the vibrational and rotational energy of sputtered BeD is found not to be consistent with a Boltzmann distribution.