Page 2 - Outer Divertor Damage Characterization from Deuterium Plasma Bombardment in Graphene-Coated Tungsten

OUTER DIVERTOR DAMAGE CHARACTERIZATION FROM DEUTERIUM PLASMA BOMBARDMENT · NAVARRO et al. 543
Fig. 1. CAD drawing of C-2W experiment at TAE Technologies.
PFCs in future fusion reactors. The sputter etching of tungsten with helium has been extensively
8–10
as tungsten is the main PFC candidate in most large nuclear fusion experiments across the world. We transferred a sheet of graphene onto the surface of polycrystalline tungsten (PCW) before irradiation. Samples of graphene-coated W were placed in the outer southern divertor (Fig. 2). Section II provides the details of the experimental setup and sample preparation. Results are discussed in Sec. III. This is the first large-scale plasma experiment where a graphene coating was tested.
II. EXPERIMENTAL II.A. Materials
Materials used in this study are >99.95% PCW
plates, 1 × 1 cm, 1 mm thick (provided by Alfa Aesar).
They were mechanically polished using silicon carbide
(from 120 to 1200 grit) and finished with a 1-μm particle-
sized diamond suspension to a mirror polished surface.
Graphene was grown via chemical vapor deposition on
30-μm-thick copper (99.99% purity) using conventional
growth methods [500 standard cubic centimeters
per minute (sccm) of 5% H2: 95% Ar, 500 sccm of 5%
CH4: 95% Ar] (Ref. 23). The graphene is coated with
polymethylmethacrylate (PMMA) and placed in an FeCl3
bath, which etches the copper, leaving the graphene
suspended in the solution. The combination is then
transferred to the tungsten substrates, the PMMA is
removed with acetone, the surface is cleaned with
isopropanol, and the sample is rinsed in deionized
water. The quality of the transfer was checked via
24,25
investigated.
creates vacancies and interstitials as it undergoes a damage cascade in the tungsten lattice, which can lead to mass losses. Surface morphology changes on the surface can occur at any helium energy, creating tungsten “fuzz” structures at low helium energies [tens to hundreds electron-volts (Ref. 11)], and “grass” structures at high helium energies [>10 keV (Refs. 12 and 13)]. Fuel loading in the walls is also a concern, particularly for fusion devices that intend to burn tritium because it increases the T2 inventory.
Irradiation with high-energy helium
We have investigated the use of graphene to
suppress the development of surface structures caused
by helium and fuel loading. Graphene is a two-
dimensional carbon allotrope that has a very tightly
packed sp2 honeycombed structure, resulting in the
smallest bond distance between carbon atoms.
14,15
This
structure has demonstrated extraordinary properties; in
particular, graphene’s radiation resistance under these
extreme conditions is of particular interest for its use
16–20
in fusion devices.
and Michely investigates the effect of a graphene coating on an iridium substrate, bombarded by xenon and argon over a range of energies (200 to 500 eV) and relatively low fluences (1013 ions/cm2), claiming a reduction in sputtering between coated and uncoated materials of almost two orders of magnitude. While current computational studies demonstrate a thorough theoretical understanding on graphene’s radiation tolerance,21,22 more experimental evidence is needed to make graphene a viable coating for materials exposed to a nuclear environment. In particular, we are interested in how tungsten performs with a graphene coating in a low-energy, moderate-fluence plasma environment,
One of such studies by Herbig
FUSION SCIENCE AND TECHNOLOGY · VOLUME 75 · AUGUST 2019
Raman spectroscopy.
uniform over 95% of the surface and poor at the edges of the tungsten sample. Two tungsten samples were
The graphene coverage was