Experimental investigation on the effect of surface electric field in the growth of tungsten nano-tendril morphology due to low energy helium irradiation

Abstract

The mechanisms responsible for and controlling the growth of tungsten nano-tendrils (or "fuzz") under low-energy helium plasma exposure remain unclear. For the first time in nanotendril experiments, the plasma sheath-produced electric field and the helium (He) ion energy have been decoupled, showing that the sheath electric field has little impact on nano-tendril growth, eliminating a possible cause for tendril growth. The well-established necessary growth conditions for W fuzz were maintained with He ion flux density Γ He > 10 21 He m -2 s -1 , surface temperature T s = 1273 K, He ion energy E He = 64 eV, and He ion fluence Φ He > 10 24 He m -2 . A grid is situated between the tungsten sample and plasma, with the grid and sample potentials independently controlled in order to control the electric field at the surface of the sample while maintaining the same incident He ion energy to the surface. A calculation of the potential profile in the drift space between the grid and sample was used to account for space charge and calculate the electric field at the surface of the sample. Tungsten fuzz formed at all electric fields tested, even near zero electric field. Also, the depth of the resulting W fuzz layer was unaltered by the electric field when compared to the calculated depth determined from an empirical growth model. The conclusion is that the sheath electric field is not necessary to cause the changes in surface morphology.

He-ion induced surface morphology change and nanofuzz growth on hot tungsten surfaces

“…Finally, the possibility that electrostatic effects (Karpov 2014) may contribute to fuzz growth was been recently explored in two separate investigations (Fiflis et al 2016, Woller et al 2016. No evidence for such a mechanism was found in either work at the electric field magnitudes typically generated in plasma sheaths.…”

Low energy helium ion irradiation induced nanostructure formation on tungsten surface

“…In fact, many transient events due to plasma instability such as ELMs and disruptions can create enough stresses and thermal loads to break or easily remove the W fuzz [17]. Therefore, contamination of plasma due to the presence of fuzzy W surfaces is always a serious concern [8,13,18]. Additionally, the presence of W fuzz on a W surface may alter and significantly degrade the W thermal properties [17].…”

“…To the date, researchers have concluded that the major factors significantly influence the W fuzz growth are surface temperature, incident He ion-energy, -flux, and -fluence [18]. The lower energy threshold for fuzz growth is 12 to 30 eV [2,19,20] depending up on sample grade, plasma conditions, and surface temperature used during the experiments.…”

A review of late-stage tungsten fuzz growth

“…The interaction with the electric field from the plasma present could accelerate this. However, two studies have shown that electric field variation has no effect on fuzz growth [30,31], so this mechanism has been rejected.…”