The effect of D2 partial pressure, deposition rate, and sample temperature on D retention in W codeposited layers

Abstract

Hydrogen-isotope (HI) fuel retention in the walls of magnetic fusion devices must be minimized to achieve tritium self-sufficiency and for safety considerations. Retention in codeposited layers (i.e., layers formed from eroded material while simultaneously exposed to HIs) is projected to be a dominant source. This work investigates retention in tungsten (W)–deuterium (D) codeposited films as a function of D2 partial pressure. Tungsten films were deposited onto quartz substrates via magnetron sputtering in an Ar-D2 mixed background with the D2 partial pressure, pD2, varied from 0.13 to 1.6 Pa; the substrate temperature, Ts, varied from 310 K to 623 K; and the W deposition rate, rd, varied from ∼1.0 to 4.2×1019 m−2s−1. After deposition, D release was measured using thermal desorption spectroscopy (TDS). For films deposited at Ts∼ 310 K, D/W increased from 0.13 to 0.44 Pa, was constant to 0.89 Pa, and decreased at 1.6 Pa. However, the TDS spectra for films at all pD2 showed a similar distribution of D in traps, with a single release peak at 400–700 K. For films with pD2 = 0.89 Pa, D/W decreased exponentially with increasing Ts, in agreement with the Ts scaling in the literature, while the release peak in the TDS spectra shifted to higher desorption temperatures. For films deposited near room temperature, the measured D/W ratio decreased by 3× as rd increased, in agreement with the rd−0.8 scaling in the literature, while the tail of the D release increased from 700 to 1200 K for the higher rd. Hence, retention mitigation strategies for ITER (e.g., baking of the vacuum vessel at 493 K) may not be sufficient, especially for films formed with high rd.

Keywords

• Tungsten
• Deuterium
• Retention
• Codeposition
• Plasma-material interactions