Technical Design Report Super Fragment Separator

DRAFT
The target will be a graphite wheel with a diameter of 45 cm. Since it will be operated with a wide
variety of incident-beam elements, the target thickness must be adjustable. This has been realized.
by subdividing the graphite ring into five concentric circles, each 16 mm wide, with thicknesses of
5.4, 13.5, 21.6, 32.4, and 43.2 mm, corresponding to 1, 2.5, 4, 6, and 8 g/cm 2 , respectively. Following
the PSI experience, the best choice for the graphite material is R6510P [67].The most
important property of this graphite is isotropic expansion during heating thus avoiding excessive
radial amplitudes during target rotation. The target thickness can be changed by shifting the entire
wheel assembly orthogonal to the beam direction. In the same way, the target can be removed
entirely from the beam. The 238 U primary-beam spot is assumed to have a Gaussian shape with a
width of 1 mm in horizontal (bending) direction and of 2 mm in vertical direction, leading to a
specific energy deposition of 72 W/g.
The target will be cooled only by thermal radiation from the surface; no additional cooling will be
applied. The advantage of operating graphite at elevated temperatures lies in the fact that the
thermal conductivity of graphite decreases with increasing radiation doses, but the residual thermal
conductivity after irradiation is higher if the irradiation occurs at high temperature. The maximum
operating temperature is given by a tolerable level of the sublimation rate of graphite (about 10 mg
g -1 year -1 ), and should not exceed 1800 K [65].
While the graphite rings have to be operated at high temperatures to facilitate thermal radiation, the
spokes and bearings of the target wheel have to be kept at relatively low temperatures. This will
lead to different thermal expansions and consequently to stress. To solve these problems, the
graphite target ring is connected to another one by graphite spokes to allow thermal expansion.
Minimum heat transfer to the axle is guaranteed by connecting this ring by thin hollow spokes
made of INCONEL 600. The heat radiated from the graphite target has to be absorbed by water-cooled
copper blocks to protect the bearings from the radiated heat.
These elements were taken into account for the following ANSYS calculations. Assuming that the
target wheel rotates with a speed of 60 rpm, maximum temperatures of about 750 o C for the
graphite wheel have been obtained (see Figure 2.4.120). The most critical part in this arrangement
is the bearing of the axle close to the shaft. It was investigated at PSI that temperatures of about
120 °C should not be exceeded. To assure this more detailed calculations have been done. The
temperature distribution for the axle with bearings was calculated with the temperature distribution
from the model above (see Figure 2.4.120). Temperatures of only 35 o C have been obtained (see
Figure 2.4.121) so that the target wheel can be connected to the PSI drive without overheating the
bearings.
The rotation of the wheel will be induced by an electrical motor running outside the vacuum; the
torque will be transmitted via steel drive shafts and a 90-degree gear. Some important components
are the ball bearings of the target wheel; at PSI these are made by GMN Co. [68] and consist of
silver-coated rings and cages with MoS2 lubrication and silicon nitride balls. They have been
proven to run in vacuum during more than one year without failure [65].
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