Surface loss probabilities of boron-hydride radicals in W7-X and ASDEX Upgrade

• The surface loss probabilities of boron hydride radicals created in boronisation plasmas in W7-X and ASDEX Upgrade were determined experimentally using the cavity technique. • The incident flux responsible for the growth of boron layers consists predominantly of radical ions, with an ion flux contribution of 65–95 %. • The surface loss probability of the ionic species was about 0.2. • The surface loss probability of the neutral species was close to unity. The surface loss probabilities of boron-hydride radicals on silicon surfaces have been measured during boronisations for wall conditioning in the stellarator W7-X and the tokamak ASDEX Upgrade using the cavity technique. The boronisations were performed using a glow-discharge plasma with a mixture of 10 % diborane (B 2 H 6 in W7-X, B 2 D 6 in ASDEX Upgrade) and 90 % Helium at voltages of 300–500 V. Molecular ion species and neutral radicals were distinguished by their incident distributions inside the cavities. In W7-X the incident particles were dominantly ions with 80–95 % contribution to the incident flux, while in ASDEX Upgrade the ion flux contribution was about 65 %. The surface loss probability of the ionic species was about 0.2. One neutral species with surface loss probability close to unity was deduced.