Impedance of the Pulse Power Converter for the Sis100 Bipolar Extraction Kicker System

SIS100 will be operated with high intensity heavy-ion and proton beams. In order to avoid coherent beam instabilities, the reduction of the overall ring impedance is of great importance. The extraction kicker system is one of the main contributors to the overall ring impedance. This paper focuses on the contribution of the external electrical network to the kicker impedance. Previous calculations as well as measurements of the kicker impedance have already been carried out for the SIS18 and ESR kickers and these showed a significant contribution to the impedance influencing the beam [1]. The SIS100 extraction and emergency kicker magnet system will be equipped with a bipolar pulse power generator. Thereby, the beam can be deflected in two directions by the same kicker magnets. Depending on the desired deflection direction, the current fed into the magnet has a positive or negative polarity. A Pulse Forming Network (PFN) will be used as energy storage. To provide the different polarities of the current the PFN is connected to a transformer. This transformer is also needed to isolate the PFN and the kicker magnets galvanically from each other. Since this setup is new in several ways it is important to determine its contribution to the coupling impedance of the kicker system. THEORETICAL CONSIDERATION The ion beam circulating in SIS100 will induce a voltage within the kicker magnet. A well known technique to represent the frequency components of this induced voltage is a schottky spectrum. The induced fields in the kicker may affect the particle motion by itself. The threshold for beam instability and growth rate is dependent on the transverse coupling impedance. In accordance to [2] only magnetic deflections are further considered. In SIS100, the kicker magnet is designed as a window frame magnet, compare figure 1. Figure 1: Sketch of the cross-section of a SIS100 window frame kicker magnet. As described by Nassibian and Sacherer in [2] two effects are observed when particles pass a kicker magnet. Firstly, a longitudinal impedance is generated by a magnetic flux induced in the core. This impedance is mainly inductive but has also a resistive part due to core losses. Since it is not linked with the magnet coil this magnetic flux is independent of the external network load impedance Zg. Secondly, there is a differential flux induced in the core which results in a transverse impedance. This impedance is partially originated by the core losses and partially by the coupling to the magnet winding and thereby to the electrical circuit of the kicker magnet system. If the impedances arising from core losses are not considered, the longitudinal and transverse impedances respectively can be derived as describe in [2] to   Ω 4 2 2 2 0 2 0 2