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.

Vacuum tube amplifiers are the most widespread type of Radio Frequency (RF) sources used to produce high power signals needed for beam acceleration in superconducting cavities. At Deutsches Elektronen-Synchrotron (DESY), megawatt rated klystrons are used to produce millisecond-long RF shots for pulsed operation in particle accelerators. In contrast, inductive output tubes are used to provide a continuous RF signal for Continuous Wave (CW) operation. In both cases, the amplifiers suffer from amplitude dependent nonlinearity between the driving and generated signals. This nonlinearity complicates the setup operations of the Low Level RF (LLRF) system and makes it harder to regulate the accelerating field. Therefore a way to linearize the amplifier is highly valuable. This paper covers the design, implementation, and test of an FPGA-based predistortion linearization unit. The first results on the performance of this component in linearizing the amplifiers of running CW and pulsed superconducting accelerators are presented. Such a component uses programmable interpolating Look Up Tables (LUT) that are addressed using the squared value of the requested signal amplitude. This component only adds 64 ns latency to the RF control system without relying on any vendor-dependent FPGA component. Other benefits of using programmable interpolating LUT are low FPGA usage of FPGA resources and flexibility in terms of the type of amplifier to be corrected. The benefits of using this linearizer for klystrons and inductive output tubes are presented and quantified.