Due to the hight voltage electrostatic acceleration like in a TV, is not practical at BESSY. As an alternative BESSY employs micro-waves or, more accurately, radio-frequent (rf) waves. The rf-waves are stored in a rf-cavity. Electrons entering this cavity at the correct moment feel an accelerating force similar the the force a surfer feels when he is surfing down a wave. Electrons can only be accelerated, however, when they enter at the maximum of the wave. They can thus only be accelerated at specific periods in time. The rf-accelerated electron-beam is therefore not continuous but consists of a train of short pulses.