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High precision temperature control of the RF GUN is necessary to optimally accelerate thousands of electrons within the injection part of the European X-ray free-electron laser XFEL and the Free Electron Laser FLASH. A difference of the RF GUN temperature from the reference value of only 0.01 K leads to detuning of the cavity and thus limits the performance of the whole facility. Especially in steady-state operation there are some undesired temperature oscillations when using classical standard control techniques like PID control. That is why a model based approach is applied here to design the RF GUN temperature controller for the free-electron lasers. A thermal model of the RF GUN and the cooling facility is derived based on heat balances, considering the heat dissipation of the Low-Level RF power. This results in a nonlinear model of the plant. The parameters are identified by fitting the model to data of temperature, pressure and control signal measurements of the FLASH facility, a pilot test facility for the European XFEL. The derived model is used for controller design. A linear model predictive controller was implemented in MATLAB/Simulink and tuned to stabilize the temperature of the RF GUN in steady-state operation. A test of the controller in simulation shows promising results.

High Precision Temperature Control of Normal-conducting RF GUN for a High Duty Cycle Free-Electron Laser