Page 257 - Fundamentals of Magnetic Thermonuclear Reactor Design

238 Fundamentals of Magnetic Thermonuclear Reactor Design

TABLE 7.6 Methods for Increasing the Erosion Lifetime of Divertor Targets
Concept Method Technical solutions
Erosion rate Material selection Tungsten armour
minimisation Optimisation of edge Edge plasma cooling down and reduction
plasma parameters of oxygen content
Increase Erosion area Target movement
of initial expansion Separatrix displacement Rotating target
mass of the Material re-usage Re-melted target
eroding Building up the Use of first wall material
material sacrificial layer Tile design optimisation
Multi-channel target
Addition Restoring deposition Plasma feeding
of fresh Facilitate Chemical vapour deposition Replacement
material replacement of of tiles/target Liquid metal films
armour/target Liquid metal jets
Continuously Liquid or solid droplet curtain
renewed armour Evaporating target with capillary substrate
Plasma spray
Thermal evaporation/deposition

establish physical conditions at the plasma periphery (low-edge plasma’s tem-
perature and reduced oxygen impurity concentration) such that tungsten could
present itself to its best advantage.
The sacrificial material’s initial mass can be increased by spreading the
plasma particle flux over a larger area, providing a quasiclosed atom circulation
cycle (sputtering–redeposition), and building up the sacrificial layer.
Eroded surface extension is achieved by displacing the target and the peak of
a separatrix parallel to the plasma particle flow against each other (Fig. 7.3). To
this end, either rotating targets (Fig. 7.9) or moving the separatrix by poloidal
magnetic field variation are employed. The rate of such relative displacement
must be optimised (see comment to Table 7.7).
Sputtered atoms deposit on the FW surface parts, where erosion is slow
or absent. They can be driven back to intense-erosion regions by aligning the
axes of rotating targets along the toroidal magnetic field direction. In this case,
the concentration of the ‘returnee’ atoms over the target surfaces will be more
uniform. The initial topography of the sputtered surface can also be restored by
thermo-gravitational methods that get a layer of deposited atoms melted and
then redeposited under gravity (Fig. 7.10).
The possibilities of increasing the sacrificial layer by optimising the combi-
nations of properties related to heat transfer (thermal conductivity—the largest
permissible surface temperature) have largely been exhausted. One opportunity
that remains unexploited is the multichannel target that employs consecutive
switching of the coolant path (Fig. 7.11).