HLS Digital Transverse Bunch By Bunch Feedback System

Hefei Light Source (HLS) is with injection at 200MeV, operation at 800MeV, and the transverse coupled-bunch instabilities (TCBI) of the storage ring influence the injection process and steady operation. HLS now is using the octopole magnet to suppress the TCBI during injection and increases the beam current from 100mA to more than 250mA. However, the nonlinearity of the octopole magnet changes the dynamic aperture, and lowers the injection efficiency. The octopole magnet limits the further increase of the beam current during injection. According to the HLS upgrade plan, the emittance of the beam will be 30nm.rad, much smaller than 160nm.rad now, and lower emittance makes the octopole magnet impossible to be used as an operational tool to suppress TCBI. Therefore, we need to develope a transverse bunch-by-bunch feedback (TBF) system to cure TCBI during injection and operation. And this work is not only the problem which is needed to be solved in nowaday operation of HLS, but also will be an important technology for the upgrade of HLS.TBF systems could suppress the TCBI and are developed in the accelerator labs around the world. Based on the development of the analog TBF system, we developed a digital TBF, which is simpler and more reliable than the analog one. And the digital TBF system is a better platform for feedback with ability of higher taps of digital filter, which has a better performance in feedback. The digital feedback processor has been developed with flexibility in tuning and versatile tools for diagnostics, which is new development for feedback technology. The HLS digital TBF system is based on the TED digital feedback processor. Considering the working points and harmonic number of bunches, we modify the FPGA program of the feedback processor to assign the 6 ADCs to two independent feedback loops for both horizontal and vertical feedback. The RF direct sampling scheme is used in HLS digital TBF system, and simplifies the electronics circuits and saves the cost for tuning and maintaining. The filter design is optimized for injection and time domain least square fitting meathod is used to generate the coefficients of the digital FIR filter to cure the tune shift during injection.The HLS digital TBF system could suppress TCBI during injection and operation, and could improve the beam current during injection. Without octopole magnet, the feedback could store more than 250mA current. And with octopole magnet and feedback on, the beam current was over 350mA during injection. The beam lost during ramping due to the tune change.Longitudinal feedback with transverse stripline kicker is also discussed in this paper. A further study of the digital TBF system is needed to make it as a necessary tool for the operation of the storage ring in HLS.