Cavity Control Algorithms
A LLRF system typically incorporates either a heterodyne or direct sampling technique, followed by fast ADCs, then an FPGA and finally a transmitting DAC. An ultra low jitter clock system (sub-picoseconds) is required for fast sampling ADCs/DACs to preserve necessary signal to noise ratio. As a result we have universal hardware able to accommodate a variety of firmware for the specific application. The cavity control algorithm, greatly released from hardware constraints, can be developed much faster for very specific and demanding requirements. Based on the type of the accelerator (Pulsed, CW) and nature of the cavity field disturbances ( Lorentz Forces, microphonics, helium drift, temperature drift, beam load etc.) one can select a suitable controller. A generic PID feedback mechanism may be the first and easiest choice for most cases, while Kalman filtering or Adaptive control require more involved system modeling, and extra computational effort. A pulse system usually requires an additional feed forward mechanism while for a CW machine an SEL (Self Excited Loop) can be an excellent approach. Selecting the correct gains for effective control can be challenging and it depends on the beam energy spread requirements, system stability and control system scheme ( I&Q, Amplitude & Phase, SEL). Finally, the computational capability of the FPGA allows it to incorporate digital signal processing (for instance CIC(Cascaded Integrator-Comb filter), tunable IIR) as well as CORDIC ( COordinate Rotation DIgital Computer) to calculate trigonometric quantities. In this talk, a summary of popular algorithms is presented along with an example of the Jefferson Lab 12 GeV cavity field control system.
- Research Organization:
- Thomas Jefferson National Accelerator Facility (TJNAF), Newport News, VA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Nuclear Physics (NP)
- DOE Contract Number:
- AC05-06OR23177
- OSTI ID:
- 1996821
- Report Number(s):
- JLAB-PHY-09-1317; DOE/OR/23177-7061
- Resource Relation:
- Conference: LLRF09 Workshop, Tsukuba/Japan, October 19, 2009
- Country of Publication:
- United States
- Language:
- English
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