Technical Design Report Super Fragment Separator

DRAFT
The slow extraction mode would result in a smaller beam spot but a simple estimate of the heat
diffusion time, following refs. [49,50], shows that the thermal diffusion time (tD) can be fast
enough to distribute the heat from a smaller volume, tD = σ 2 Cp ρ/(4λ). Here σ is the spot size, the
specific heat Cp = 1.83 J.kg -1 .K -1 at 1100 K, density ρ = 1.84 g/cm 3 and the thermal conductivity
λ = 70 W.m -1 .K -1 . For σ = 0.5 cm, tD becomes 0.28 s. In case of 1 cm it would be tD = 1.1 s. This
means that the temperature will not rise to values higher than in the fast extraction mode with a
repetition rate of 1 s.
The simulations also show that it is important for a tolerable maximum temperature in the centre to
have the cooling water close, but not too close to avoid boiling of the water on the walls of the hot
tubes which would drastically reduce the heat transfer.
The thermal coupling from the graphite to the aluminium block needs further investigations. A
liquid metal layer may be used. The heat transfer to the cooling tubes is increased by turbulent
flow in special swirl tubes providing a steady turbulent flow [51]. A prototype must be built and
heated with external heat sources.
The thermal expansion of the graphite will lead to mechanical stress. The analysis with the ANSYS
code shows a maximum deformation in the center of up to 0.1mm. This corresponds to an
equivalent von-Mises stress of 10-20 MPa. In the fast extraction mode on top of this value the
pressure wave from the instantaneous heating must be added. Together these pressures stay still
below a critical limit for cyclic stress of ~ 60 MPa. In the DC mode the mechanical stress stays
below the critical limits for a full graphite/aluminium construction. Nevertheless the mounting
should be done on a cold side which will not increase the stress too much. A large uncertainty is the
additional swelling of the material due to radiation damage. To allow more deformation the
graphite block can be divided in an upper and a lower half, cut into many slices. This detailed
design work is still in progress. Special care has to be taken about tensile forces such as from
rarefaction waves following the initial positive pressure wave. A simulation of the distribution and
damping of such waves is ongoing.
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