Morphologies of co-depositing W layer formed during He plasma irradiation (vol 58, 106002, 2018)

Tungsten (W) samples were exposed to helium (He) plasmas with precipitation of W particles. It was shown that visible large-scale fiberform nanostructures (LFNs) were formed on the surface. The height of the LFN increased exponentially with the irradiation time in the initial phase of the growth. The growth rate was totally different from that of the conventional W nanostructure (fuzz) growth by He plasma irradiation, where the thickness increased in proportion to the square root of time. The LFNs growth had a directionality in a millimeter global scale; the direction was always similar to 45-60 degree rotated in clockwise direction from the magnetic field. The plasma flow measured spectroscopically near the sample was found to significantly increase when inserting a negatively biased sample, and the growth direction of the LFN was consistent with the direction of the plasma flow. Moreover, highly porous fiberform nanostructures or cauliflower-like structures comprised the deposition layer even when the He incident ion energy was lower than the threshold energy of fuzz growth (20-30 eV).