A New Prototype Modulator for the European XFEL Project in Pulse Step Modulator Technology

The European XFEL project at DESY in Germany requires 27 RF stations capable of 10 MW RF power each. Each RF station needs one high voltage modulator that generates pulses up to 12 kV and 2 kA with a duration of 1.7 ms and a nominal repetition rate of 10 Hz. DESY decided to investigate new modulator prototypes and Thomson was awarded to design and build one of these prototype modulators. The Thomson modulator is based on the pulse step modulator (PSM) principle. This technology allows the regulation of the pulse voltage during the pulses, thus achieving a good flatness. In addition to the common PSM technology this modulator design includes additional features. The first one is a constant power regulation system to prevent a 10 Hz loading of the mains. The second one is the extension for 2-quadrant operation in order to demagnetise the core of the pulse transformer between the pulses. The modulator was delivered to the modulator test facility at the DESY Zeuthen site in July 2008 and is now under test. The paper will give a detailed overview on the system and shows the results of the factory and site tests. SYSTEM OVERVIEW The modulator provides high voltage pulses to supply the klystron during the machine pulse. At XFEL the klystrons will be placed in the accelerator tunnel whereas all modulators will be placed in a central modulator hall at the beginning of the accelerator section. As the maximum length will be 1700 m, an impedance matched system is needed for the interconnection. This is achieved by using 4 coaxial cables, each with a nominal impedance of 28 ohms in parallel, resulting in an effective cable impedance of 7 ohms. Figure 1: System overview. Figure 1 shows a simplified block diagram of the overall system. More details are available in [1]. SYSTEM DETAILS PSM technology permits regulation of the pulse voltage during the pulse. This allows full compensation of the voltage droop on the storage capacitors. No other compensation means is needed. The modulator is comprised of 24 active modules. The system is able to operate with 22 modules at full performance, resulting in a redundancy of 2 modules. Each module is equipped with a 20 mF storage capacitor charged up to 700 V prior to the pulse. A full energy pulse discharges it down to 550 V. For the demagnetisation of the pulse transformer between the pulses and for dissipating the inductive stored energy under klystron arcing conditions 4 modules are paired with additional switching modules in a two phase configuration, allowing inverse voltage operation. This makes it possible to set a negative voltage down to 4 modules voltages, which is sufficient for the required operation. Mains Input 690 V Transformer 4 Mod. Pairs 2-Quadr. Conf. 20 Switching Mod. Standard Conf. Low Pass Filter Figure 2: Modulator block diagram. Proceedings of PAC09, Vancouver, BC, Canada TU5PFP101 Radio Frequency Systems T08 RF Power Sources 1075 Figure 3: Complete system view. The Switching Modules The switching modules are designed as pluggable units. As main switching element a 1200 V / 2400 A IGBT transistor module form Semikron is used. The constant power consumption is achieved with a boost converter realised with a 1200 V / 400 A IGBT. All semiconductors are placed on a single water cooled heat sink. Figure 4: Power module.