Isolated regions of nano-tendril bundles (NTBs) have been grown on tungsten surfaces exposed to low-pressure direct-current helium plasma discharges operating with the addition of impurity gases, including neon, argon, nitrogen and residual air. Through the variation of impurity gas partial pressures and bombarding ion energies, threshold conditions and operational ranges for the growth of NTB's have been investigated. It was found that increasing the vacuum base-pressure from ~10−5 to ~10−4 Pa during the experiment could give rise to NTB formation without the need for other additional gases. In general, the required impurity gas ratio and the incident ion energy necessary to form these structures was found to vary across the range of different additional gas species, with a key parameter being the net erosion yield (Y) due to ion bombardment, Y ~ 10−2–10−3, with the morphology altering with changing net erosion yield. It is predicted that erosion-deposition processes, acting upon existing He-induced morphology changes, are the precursors for NTB growth.
Observation of the bulk ion density peaking in a discharge with an impurity hole in the LHD