What is LLRF doing?
Control and maintain the specified phase and amplitude stability of accelerating field in RF cavity during beam traveling.
Also maintain the filling stage of the RF pulse.
The stability requirement varies in different accelerators, determined by specific application. e.g. XFEL:0.01% and 0.01° ; SARAF: 0.1% and 0.1° ; SNS: 0.5% and 0.5° ; JPARC: 1% and 1°.
Why need LLRF?
Because of the perturbations in real world.
• Synchronous phase • Beam chopping • Pulse beam transient • Charge fluctuations • Non-relativistic beam • Pass band modes • HOMs, wake-field
Phase reference distribution:
• Reference thermal drift • Master oscillator phase noise
• Lorentz force detuning • Microphonics • Thermal effects (Quench…)
• Modulator drop and ripple • Klystron nonlinearity
• Crates power supply noise • Cross talk, thermal drift • Clock jitter, nonlinearity
How does LLRF control?
The LLRF control is actually a loop control: open-loop or closed loop control as shown in the following figure.
In an open loop system, the controller (usually feedforward controller) directly drives the plant without any feedback. In a closed-loop system, the current output is taken into consideration and corrections are made based on feedback. A closed loop system is also called a feedback (FB) control system.
Qiu Feng: RF sources and LLRF system
Rihua Zeng: An Introduction to LLRF Uppsala RF workshop