Superconducting cavity driving with FPGA controller

Abstract

The digital control of several superconducting cavities for a linear accelerator is presented. The laboratory setup of the CHECHIA cavity and ACC1 module of the VU-FEL TTF in DESY-Hamburg have both been driven by a Field Programmable Gate Array (FPGA) based system. Additionally, a single 9-cell TESLA Superconducting cavity of the FNPL Photo Injector [1] at FERMILAB has been remotely controlled from WUT-ISE laboratory with the support of the DESY team using the same FPGA control system. These experiments focused attention on the general recognition of the cavity features and projected control methods. An electrical model of the resonator was taken as a starting point. Calibration of the signal path is considered key in preparation for the efficient driving of a cavity. Identification of the resonator parameters has been proven to be a successful approach in achieving required performance; i.e. driving on resonance during filling and field stabilization during flattop time while requiring reasonable levels of power consumption. Feed-forward and feedback modes were successfully applied in operating the cavities. Representative results of the experiments are presented for different levels of the cavity field gradient.Keywords: TESLA cavity control, CHECHIA driving, FPGA, system identification, LLRF control, INTRODUCTIONThe LLRF -Low Level Radio Frequency cavity control system is still under development in order to improve regulation of accelerating fields in the resonators [1] (figure 1). The controlled section, powered by one klystron, may consist of many cavities. The fast amplitude and phase control of the cavity field is accomplished by modulation of the klystrons driving signal using a vector modulator. The cavities are driven with pulses of 1.3 ms duration to an average accelerating gradient of 25 MV/m. The cavity RF pickup signal is down-converted to an intermediate frequency of 250 KHz while preserving the amplitude and phase information. The ADC and DAC converters link the analog and digital parts of the system with a sampling interval of 1 µs. The digital signal processing is executed in the FPGA system to obtain field vector detection (I/Q Detector), calibration and filtering. The control feedback system regulates the vector sum of the pulsed accelerating fields in multiple cavities. The FPGA based controller stabilizes the detected real (in-phase) and imaginary (quadrature) components of the incident wave according to the given set point. Additionally, the adaptive feed-forward is applied to improve the compensation of repetitive perturbations induced by the beam loading and by the dynamic Lorentz force detuning. The control block applies the value of the cavity parameters estimated in the identification system and generates the required data for the FPGA based controller.A comprehensive system model was developed for investigating the optimal control method of the cavity [3]. The design of a fast and efficient digital controller is a challenging task and it is an important contribution to the op...

Compact Muon Solenoid Decade Perspective and Local Implications

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Institute of electronic systems in CARE and EuCARD projects accelerator and FEL research, development and applications in Europe

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