Online identification and compensation of nonlinearity in solid-state amplifiers at CAFe2

Purpose The high modularity and redundancy of the solid-state amplifier (SSA) facilitates its flexible output and convenient maintenance, which has made it the main radiofrequency (RF) source at the China Initiative Accelerator Driven System (CiADS) and China Accelerator Facility for superheavy Elements (CAFe2). However, the nonlinear characteristics of high-power RF systems, such as SSA, reduce their potential operational efficiency and increase their operational complexity. In this study, our objective was to accurately measure the SSA nonlinearity and to develop a compensation method.
Methods We utilized a field-programmable gate array (FPGA) in a low-level radiofrequency (LLRF) system to generate a sawtooth driving signal with a tunable amplitude and pulse width. We used this driving signal to measure the nonlinear characteristic curve of the SSA online, calculate the corresponding nonlinear compensation curve, and construct a digital predistortion (DPD) algorithm within the FPGA to compensate for SSA nonlinearity.
Results Our results demonstrated that the DPD algorithm greatly improved the overall linearity of the RF system and thus enhanced its potential operational efficiency. We also observed that the thermal effect of the SSA had a notable impact on the measurement results and effectiveness of the compensation.