A tuning method based on the coupled mode frequencies in biperiodic coupled-cavity linear accelerator

A coupled-cavity linac (CCL) is widely used around the world in many fields. The radio frequency design is always a fundamental and crucial problem in CCL. In biperiodic structures, especially side-coupled structures, entirely different structures between accelerating cavities (ACs) and coupled cavities (CCs) will cause different frequency drops (delta f) with the introduction of coupling slots. In order to achieve precise and consistent frequencies in each cell, the rule of individual cavity frequency drops should be figured out. A model of lumped resonant circuits is chosen to provide accurate prediction for normal mode frequencies. Through solving equivalent circuit equations, a parameter called frequency gap difference (Delta F-g) is proposed, whose value is directly proportional to the frequency difference (df) between AC and CC and the ratio is one-to-one when AC frequency (f(a)) is close to CC frequency (f(c)). Based on the property of Delta F-g, a simplified method on how to design and tune a triplet CCL is proposed; instead of tuning each individual cell, the new method only requires measuring the frequencies of the coupled normal modes, which considerably simplifies the tuning process and reduces the turn-around time. The particular design flow and tuning method have been successfully applied in the Nanjing University CCL Project. In addition, through comparing cavities with and without coupling slots, the value of frequency drops with different coupling coefficients is revealed and analyzed. Published by AIP Publishing.