Research Of Barrier Bucket Digital Low Level RF System In HIRFL-CSRe

The Institute of Modern Physics,Chinese Academy of Sciences will use the Moving Barrier Bucket(BB)stacking scheme in High Intensity heavy-ion Accelerator Facility(HIAF).BB stacking scheme is introduced to overcome the limitation of the traditional beam stacking method and accumulate the heavy ion beam to higher intensity.BB stacking method is very sensitive to RF voltage,the distortion of RF voltage will lead to the degradation of the beam quality.The amplitude,phase and periodic adjustable BB voltage produced by the Radio Frequency(RF)control system are the core of this accumulation mode.The research of Barrier Bucket is carried out on Cooling Storage Experimental Ring of Heavy Ion Research Facility in Lanzhou(HIRFL-CSRe).We review the character of BB RF system and measure the frequency response of RF system.The distortion expression of BB voltage is obtained by using the harmonic analysis method,the results of simulation and experiment show that the main reason of BB voltage distortion is the frequency response of RF system.The formula of static predistortion method is given to overcome the nonlinear of RF system.Then the nonlinear characteristic of amplifier is considered,to improve control accuracy,an Nth-order power series was chosen to describe the nonlinear of amplifier and make it into look-up table to adjust the static predistortion voltage dynamically.Finally,a LLRF control system based on predistortion is designed,the LLRF system is programmed by predistortion method to correct BB voltage.The measurement results show that the predistortion LLRF control system can effectively correct the BB voltage distortion.The results will be applied to the BB stacking mode of HIRFL-CSRe and HIAF.