Grounding for EMC at the European XFEL

The European X-Ray Free Electron Laser (XFEL) research facility is currently being built at Deutsches Elektronensynchrotron (DESY) in Hamburg, Germany. In this large underground facility high power RF stations are operated next to high precision diagnostics instruments. This made electromagnetic compatibility (EMC) an important issue from the very beginning of the project. A well-designed grounding system with a hierarchical structure is an essential ingredient for safety and EMC. Important parts of the facility ground were implemented in the course of the civil construction work. The specialties of tunnel boring and building far below the ground water level had to be taken into account. Some aspects of facility grounding were investigated at the free electron laser facility FLASH which operates for users at DESY for some years.