Page 254 - Fundamentals of Magnetic Thermonuclear Reactor Design

First Wall Components Chapter | 7 235

FIGURE 7.8 Types of thermal mechanical defects occurring in armour tiles.

an incorrect machining of tile sides, may lead to a considerable contamination
and cooling of the plasma. An extreme case of side cracking is a complete or
partial chipping of a tile (Fig. 7.8D and E). If this defect is accidental and the
affected tile does not face the plasma directly, the flaked-off piece is heated and
evaporated slowly, without affecting the plasma behaviour. But if the tile is from
the upper reactor part, a chipped off piece of tile may get into the plasma column
and cause a current disruption.
If chipping is intense or affects tiles that face the plasma directly, the reactor
may go out of service. The detachment of an entire tile is dangerous not only
because the tile can get inside the plasma (which in some cases is tolerable). The
greatest risk is the substrate’s lengthy exposure to the plasma particle fluxes and
potential leakage of copper into the plasma column.
Let us make some quantitative estimates. We start with assuming that brems-
strahlung radiation accounts for the largest part of plasma energy losses. Brems-
2
strahlung radiation power is proportional to Z . Next, we assume that beryllium,
graphite and copper have commensurate sputtering coefficients. With these as-
sumptions in hand, it is clear that one can ‘open’ just 4% of the heat sink panels’
2
surface area (∼25 m ) to double the theoretical radiative losses.
If we use tighter criteria and assume that the losses are mostly caused by
4
recombination radiation with power proportional to Z , then the ‘allowable’ loss