The pellet acceleration is realized through a two step acceleration. Pellets are accelerated by the inner driving blade on the disk to the static stop cylinder with a maximum impact velocity of 35 m/s. They slide along the cylinder, and then are stabilized and fixed by little whips to the exit erture of about 60°. This aperture determines the start of the radial main acceleration in the outer acceleration arm.
Driver pump and vacuum system:
The driver pump selected for the centrifuge is a Pfeiffer TPH 5000 turbomolecular pump, which has several favourable features. The rotating part of this pump has very large axial inertial moment compared with that of the acceleration arm (factor 20) and can be balanced very precisely in an upper and lower plane without acceleration arm. The latter was balanced separately together with its hub. Finally, the two were balanced together with a residual imbalance of 100 g mm², which ensures a long life of the bearings. Nevertheless, during operation and stand-by, the oscillation behaviour is continuosly controlled.
The driver pump is also stable in case of accidental thermal breakdown of the cryostat system, which may cause a sudden gas release of about 1 bar litre for a few seconds. The pressure in the vessel containing the whole centrifuge system can rise up to about 3 mbar and subject the rotor to lifting forces of about 100 N. The resulting axial motion is limited to 0,1 mm by the heavy weight of the total rotor and. The duration of such a pressure pulse is limited to about 3 s by the combined pumping system. The same is valid if the vessel exit valve is closed and all accelerated pellets hit the vessel wall. Such short pressure pulses do not heat up the rotor and the cryostates to unstable conditions. The vacuum pump contains the Pfeiffer TPH 5000 rotor drive serving for high-vacuum pumping with a speed of 5700 l/s for H2 at full frequency. A Roots blower with 2000 m³/h and a rotary vane pump with 120 m³/h in series provide the backing vacuum.