FPGA-Based RF and Piezocontrollers for SRF Cavities in CW Mode

Modern digital low level radio frequency (RF) control systems used to stabilize the accelerating field in facilities, such as free electron laser in Hamburg or the European X-ray free electron laser, are based on the field programmable gate array (FPGA) technology. Presently, these accelerator facilities are operated with pulsed RF. In the future, these facilities will operate with the continuous wave (CW), which requires significant modifications on the real-time feedbacks realized within the FPGA. For example, higher loaded quality factor of the superconducting RF cavities operated in the CW mode requires sophisticated resonance control methods. However, iterative learning techniques widely used for machines operated in pulsed mode are not applicable for the CW. In addition, the mechanical characteristic of the cavities now have a much more important impact on the choice of the feedback scheme. To overcome the limitations of classical proportional-integral controllers, a novel real-time adaptive feed-forward algorithm is implemented in the FPGA. Also, the high power RF amplifier, which is an inductive output tube (IOT) for CW operation instead of a klystron for the pulsed mode, has a major impact on the design and implementation of the firmware for regulation. In this paper, we report on our successful approach to control the multicavity vector sum with an ultrahigh precision (amplitude error <;0.01% rms and phase stability <;0.02° rms), using a single IOT source and the individual resonance control through piezoactuators. Performance measurements of the proposed solution were conducted at the cryomodule test bench facility.