Simulations of the pressure profile in the Petra III frontends

Petra III will be a high brilliance, small emittance, third generation synchrotron radiation source. The undulators will deliver photon beams with small cross sections and therefore the components in the frontend will be as compact as feasible. The resulting narrow cross sections of the vacuum system will yield a small conductivity in the whole beamline. The design of the frontends is now at an advanced state so that the initial design of the vacuum system has to be revised. The vacuum specification demands for a hydrocarbon and dust free vacuum system. To provide this, the beamline will be initially pumped down with a dry pumping station to a pressure of 10-6mbar. After reaching this pressure, the pumping station will be switched off and a set of ion getter pumps will pump the beamline continuously. To protect the ion getter pumps, it is necessary that during operation the pressure in the pumps will be below 10-6 mbar. The simulation shows that especially at the high power slit systems and during the start of operation, a high amount of gas will be photodesorbed. To cope with this, additional pumps will be installed in the frontends as compared to the first design step. To control the pressure inside some beamline components an optional pressure control system is also introduced. The simulations of the pressure profiles were done with a Monte Carlo simulation program