Experimental demonstration of transient-beam-loading compensation using new digital LLRF system at the Photon Factory storage ring

In ultra-low-emittance synchrotron light sources, the bunch-lengthening technique is useful to mitigate harmful effects due to the intrabeam scattering. The perfomacne of the bunch lengthening can be degraded by the transient beam loading (TBL) effect induced in the cavities. To mitigate the TBL effect, we proposed a TBL compensation technique using a wide-band longitudinal kicker cavity. In this presentation, we report the result of the experimental demonstration of the TBL compensation performed at the KEK PF 2.5 GeV ring. In this experiment, the fill pattern of the electron bunches were customized to enlarge the phase variation of electron bunches induced by the TBL effect. The fundamental cavities and newly developed digital low-level RF (DLLRF) system were used for the experiment. The DLLRF enables the TBL compensation by an arbitrary feedforward pattern of the cavity voltage modulation that is synchronized with the revolution frequency. Although the bandwidth of the fundamental cavity is limited, the variation of the cavity voltage and bunch phase induced by the TBL effect was reasonably mitigated by applying sinusoidal wave modulation of the cavity voltage.