LCLS RF gun feedback control

The LCLC RF gun requires a water based thermal system to tune the resonance frequency of the cavity to 2856.03 MHz. The RF system operates in pulsed mode with bursts of 2 - 3 musec. duration at a repetition rate of 30 - 120 Hz. The thermal system operates in combination with the low-level RF system (LLRF) to set the operation point of the cavity. The LLRF system controls the amplitude and phase of the cavity voltage and defines the necessary slow signals to the thermal system. The thermal system operates by pre-heating / pre-cooling the water and mixing both channels to achieve the temperature to control the cavity resonant frequency. The tune control of the RF gun includes two systems with different dynamics. The dynamics of the thermal system is slow while the RF system is fast. Additionally, different actuators in the system present limits that introduce non-linearities to be taking into account during the start up process. Combining these characteristics, a controller is designed for the resulting hybrid system that allows convergence in large for all the operation conditions and achieve the performance in the magnitude and phase of the cavity voltage required around the operation point.