Digital Self-Excited Loop

The high-Q superconducting cavities being developed at JLab have complicated RF control, with large Lorentz detuning at start up. Typically, Lorentz detuning can be much larger than the loaded cavity bandwidth. Several near-term (e.g. JLab 12 GeV project) and longer-term (e.g. ERLs) projects will involve operation of a large number of high-Q superconducting cavities. Of particular importance in these machines is the stability with respect to ponderomotive instabilities, rapid turn-on time and recovery from a trip.

We have developed and demonstrated a control system based on a digital self-excited loop (SEL) for the 12 GeV Upgrade. The digital SEL solves the problem of high-Q superconducting cavity turn on by eliminating the need for external tuning devices or other procedures to get the cavity back to nominal gradient. In addition, the digital SEL could also be used in other superconducting accelerator projects, such as the Facility for Rare Isotope Beams.

The SEL has been used in analog form for many years for accelerating cavity field control. In essence, the SEL uses the cavity as a resonant circuit -- much like a resonant (tank) circuit is used to build an oscillator. An oscillating resonant circuit can be forced to oscillate at different, but close, frequencies to resonance by applying a phase shift in the feedback path. This allows the circuit to be phase-locked to a master reference, which is crucial for multiple cavity accelerators. For phase and amplitude control, the SEL must be forced to the master reference frequency and feedback provided for in both dimensions.

To build the digital SEL, engineers at JLab embedded the analog SEL function into a Field Programmable Gate Array (FPGA). The program can be quickly reconfigured to different cavities and environments.

Reference: 
C. Hovater, T. L. Allison, J.R. Delayen, J. Musson, T.E. Plawski, “A Digital Self Excited Loop for Accelerating Cavity Field Control”, Proceedings of the 2007 Particle Accelerator Conference, Albuquerque, NM