Page 202 - Fundamentals of Magnetic Thermonuclear Reactor Design

184 Fundamentals of Magnetic Thermonuclear Reactor Design

6.3 PLASMA IMPACT ON THE FIRST WALL
There are numerous plasma-related physical and chemical processes responsi-
ble for impurity transport into the plasma. They include wall physical sputtering
by α-particles, hydrogen and impurity ions; chemical sputtering; evaporation
and sublimation; material erosion due to plasma current disruption, unipolar
microarcs and ‘runaway’ electrons; surface mechanical destruction as a result of
blistering; as well as a photo-, electron- or ion-driven gas emission [9].
Surface sputtering by incident atoms and ions is described by several param-
eters. The first one is the sputtering coefficient, ρ, the number of sputtered atoms
per ion incident on the target, depending on ion energy, angle of ion incidence
and surface temperature. The sputtering coefficient characterises the ‘particle-
surface’ pair and is also dependent on the surface condition [10].
In the T ≤ 0.7 T range of temperatures (where T is the melting tempera-
m
m
ture), the sputtering coefficient of a metal is T-independent, but grows expo-
nentially with increasing temperature at higher temperatures. In the case of
2
ρ≈ln1/E 2 self-sputtered metals, ρ ≈ Eln[(1/ E)] . If the incidence of medium or heavy ions
is normal to the surface, the angular distribution of sputtered atoms is close to
cosinal. The scattering indicatrix of light ions is clearly deformed and elongated
normally to the surface. The majority of particles sputter at a velocity of 1 to
5 km/s. The sputtering velocity distribution curve has a peak near 2 km/s [11].
Graphite and graphite-containing composites exposed to hydrogen ion bom-
bardment show a peculiar sputtering pattern. Sputtering intensity grows sharply
in temperature range 1000–1300 K, and is accompanied by an increase in vola-
tile hydrocarbon content in the residual gas spectrum. This increase is said to
be due to acceleration of the carbon–hydrogen reactions producing the C H
n
m
compounds (the chemical sputtering). At higher temperatures, a thermal disso-
ciation of such compounds (a phenomenon referred to as a ‘carbon catastrophe’
in the fusion community) prevails.
Hypothetically, a layer sputtered as a result of a graphite wall interaction
7
with plasma during ∼10 s of reactor operation may be a few tens of millimetres
(!) thick. As one can see, not only the impurities affecting plasma performance,
but also the FW integrity is a problem that needs to be tackled. These issues are
discussed in more detail in Sections 7.2.3 and 7.3.3.
Blistering may contribute a lot to the mechanical destruction of the wall and
the formation of impurities. This phenomenon is associated with the behav-
iour of He implanted in the material. He solubility and diffusivity in metals
are very low, and the He atoms’ most favourable lattice positions, in terms of
energy minimisation, are those between the nodes. Atomic migration may only
be caused by the movement of neighbouring vacancies, but at a high accumula-
tion rate, the flow of new vacancies may be insufficient for He transfer through
the vacancy mechanism. As a result, He bubbles originate in the metal, at a
depth where α-particles travel. They tend to migrate and merge into each other
under a mechanical stress field surrounding them. The in-bubble pressure may
be up to 1 GPa. When the bubbles reach the surface, they either cause swelling