The superconducting cavities operated at high Q level need to be precisely tuned to the RF frequency. Well tuned cavities assure the good field stability and require a minimum level of RF power to reach the operating gradient level. The TESLA cavities at XFEL accelerator are tuned using slow (step motors) and fast (piezo) tuners driven by the control system. The goal of this control system is to keep the detuning of the cavity as close to zero as possible even in the presence of disturbing effects (LFD - Lorentz Force Detuning and microphonics). The step motor tuners are used to coarse cavity tuning while piezo actuators are used to fine-tuning and disturbance compensation. The crucial part of the piezo control system is the piezo driver. To compensate LFD the piezo driving with relatively high voltage (up to 100V) and high current (up to 1A) is needed. Since the piezo components are susceptible to destruction with overvoltage, overcurrent, and also overtemperature, one has to pay special attention to keep the piezos healthy. What makes things worse and more critical, is that the piezo exchange is not possible after the module is assembled. Therefore the special hardware must be assisting the power amplifier, detecting the dangerous conditions and disabling piezo operation when needed. It must be fail-safe, so even in a case of failure the piezos shall survive. It must be also robust and it must not disturb or disable normal operation. Due to many channels (16 for master/slave RF), the hardware solution must be well scalable. The paper discuss the design of XFELs piezo driver together with PEM (Power and Energy Monitor) supervising the driver operation and preventing piezos from destruction. The achieved results and operation of the complete system are demonstrated.
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