Development of Control System for Fast Frequency Tuners of Superconducting Resonant Cavities for FLASH and XFEL Experiments

This dissertation covers the recent research and development (R&D) activities of control systems for the fast frequency tuners of TESLA cavities and predicts the implications foreseen for large scale machines such as the FLASH and the planned XFEL. In particular, the framework of the presented activities is the effort toward the: 1. R&D of the driving circuit, 2. R&D of the control algorithm, 3. R&D of the control system. The main result of these activities is the permanent installation of the target piezo control system and its commissioning for 40 cavities divided into 5 accelerating modules at the DESY FLASH facility. The author’s contribution was the study of possible designs of high-voltage, high-current power amplifiers, used for driving the fast frequency tuners, shows that several parameters of such a device needs to be considered. The most important parameter is the input and output power estimation. This arises from the fact that the estimation is the most crucial issue for both power supply and power amplifier unit design. Furthermore, the integrated overvoltage and overcurrent, as well as monitoring circuits, cannot be omitted, since the piezo tuners should be monitored and protected. The results of experiments, jointly performed by the author, proved the possibility for designing of an 8-channel piezo driver unit (with true bipolar output voltage and current (±100 V, 1.5 A) and significantly low crosstalk levels (>60 dB) between integrated driving circuits), that enables simultaneous compensation of single accelerating module composed of 8 superconducting cavities. The current installation of these devices for the FLASH facility covers 4 units packed into single crate of Eurocard standard with an integrated power supply unit. Another topic which is elucidated in this dissertation is digital controller development. This mainly covers: 1. cavity detuning computation and its hardware implementation, 2. automatic control of fast frequency tuners for multi-cavity configuration. The cavity detuning computation is the most important input to the fast frequency tuners controller, because it gives the information about the cavity response to the applied compensation. With the use of LLRF control systems that are commonly based on fast FPGA devices, the possibility of hardware implementation of detuning computation has been considered. The author worked especially on applying the serial pipelined computations. This work made it possible to define the computation core latency as well as its hardware resource usage according to the multi-cavity configuration of the controller. This hardware resource usage is such that the piezo controller integration with RF field controller can be considered. The main difficulty in applying the hardware implementation of cavity detuning has been further exploited in the case of the automatic control of fast frequency tuners. The experimental analysis of DC characteristics of the piezo elements has shown the benefits of applying a small number of hardware resources for implementing of the control algorithm based on the proportional gain control scheme. As a proof of this principle, the experimental results from the first tests of the controller have been successfully performed and demonstrated for high-gradient operations of TESLA resonators. The digital controller development would not be possible without dedicated digital hardware platforms. The 8-channel control system has been developed by the author of this thesis using Simcon DSP boards designed by LLRF control group. The experience gained in the field of the design and development of the digital control systems makes it possible to design a dedicated control board equipped with integrated 32-channel driving and sensing circuits. The designed hardware platform has been integrated with LLRF control systems and installed permanently at the FLASH facility. Finally, the experience gained with control system commissioning during machine operation close to the maximum operating conditions has been reported. Preliminary tests performed during high-gradient, high-current experiment (9 mA) show that the installed system: 1. is capable of compensating of dynamic as well as static cavity detuning over the flattop region for less than 10 Hz, 2. is suitable for compensating simultaneously of up to 32 cavities, 3. can be operated efficiently with RF pulse repetition rates of up to 10 Hz, 4. allows for fast cavity tuning and by-passing in a demand range of ±1 kHz. Measuring effectiveness of piezo compensation system has been one of the important issues studied during the machine operation. The RF control was measured first without piezo compensation and next with piezo compensation for a single accelerating module using forward power, cavity power as well as reflected power measurements. The obtained results show that RF control efforts for large scale facilities (such as the XFEL) can be reduced in the range of 10-20 MW. This is especially true for facilities containing one hundred accelerating modules. For single accelerating module equipped with high accelerating field gradient cavities, the reduced input power can be used efficiently for improving the beam acceleration especially when the module is last part of the FEL machine.