Nanostructuring of molybdenum and tungsten surfaces by low-energy helium ions

Abstract

Document VersionPublisher's PDF, also known as Version of Record (includes final page, issue and volume numbers)Please check the document version of this publication:• A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.• The final author version and the galley proof are versions of the publication after peer review.• The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. The formation of metallic nanostructures by exposure of molybdenum and tungsten surfaces to high fluxes of low energy helium ions is studied as a function of the ion energy, plasma exposure time, and surface temperature. Helium plasma exposure leads to the formation of nanoscopic filaments on the surface of both metals. The size of the helium-induced nanostructure increases with increasing surface temperature while the thickness of the modified layer increases with time.In addition, the growth rate of the nanostructured layer also depends on the surface temperature. The size of the nanostructure appears linked with the size of the near-surface voids induced by the low energy ions. The results presented here thus demonstrate that surface processing by low-energy helium ions provides an efficient route for the formation of porous metallic nanostructures.

Comparison of tungsten nano-tendrils grown in Alcator C-Mod and linear plasma devices

“…This layer depth is in-line with the calculations from the Baldwin formula and proof that a significant layer of W fuzz can grow over a short timescale. It must be mentioned that the Baldwin formula is based on data only at surface temperatures of 1120 K and 1320 K. Some work has shown this formula to overestimate growth rates for T surf > 1400 K [3,12]. It has been suggested that a factor dependent on He ion energy might be missing from the formula [12] and that re-crystallization effects might be competing with the surface changes at higher temperatures [3].…”

“…It must be mentioned that the Baldwin formula is based on data only at surface temperatures of 1120 K and 1320 K. Some work has shown this formula to overestimate growth rates for T surf > 1400 K [3,12]. It has been suggested that a factor dependent on He ion energy might be missing from the formula [12] and that re-crystallization effects might be competing with the surface changes at higher temperatures [3]. Despite concerns about the formula raised in the literature, the comparison with the Alcator C-Mod results, which were also performed at T surf > 1400 K, and the Baldwin formula resulted in good agreement.…”

“…[1][2][3]). The main difference is in the comparison of the surface morphologies for the maximum surface temperatures in the transient experiments (Alcator C-Mod and Pilot-PSI) and the equivalent steady-state surface temperatures from other experiments.…”

“…Despite concerns about the formula raised in the literature, the comparison with the Alcator C-Mod results, which were also performed at T surf > 1400 K, and the Baldwin formula resulted in good agreement. It is also worth noting that the data for the Baldwin formula [1] as well as from [3] and [12] were taken at steady-state surface temperatures over one continuous plasma exposure while the CMod exposures had transient surface temperatures and were the integration of 14 short discharges.…”

“…The growth of tungsten (W) nano-tendrils (or "fuzz") has been well documented in a wide range of linear plasma devices [1][2][3]. The growth conditions used for W fuzz in these devices are elevated surface temperatures (T s = 1000-2000 K) of a bare tungsten substrate while implanting with helium ions at sufficient incident energies (E He+ > 20 eV).…”