Results on FPGA-based High Power Tube Amplifier Linearization at DESY
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.
Klystron, IOT, Particle accelerators, LLRF