Abstract
High-speed digital filter systems for bunch-by-bunch feedback systems have been developed at KEK. A two-tap finite impulse response filter with simple hardware realizes the functions of a 90\ifmmode^\circ\else\textdegree\fi{} phase shift, suppression of the static component and digital delay of up to a few hundred turns for the KEKB rings. Difficulties in the circuit board, such as the trimming of the timing skews or the problem of long-term reliability, have been solved using custom GaAs large-scale integrated circuits which demultiplex and multiplex fast parallel-digital data coming from the analog-to-digital converter and going to the digital-to-analog converter. Two major applications of the filter board, the bunch current monitor and the bunch oscillation recorder with 20 MB memories for transient-domain analysis of the instabilities, are also described.
Highlights
In a storage ring which stores many bunches at high beam current, especially in particle-factory machines, such as KEKB, PEP-II, or DAFNE, it is very likely that many strong coupled-bunch instabilities occur in both the transverse and longitudinal planes
It supplies trigger signals to the 1:16 DMUX blocks, the synchronization timing signals (SYNCO and SYNCON) which are used to synchronize between multiple fast data demultiplexer (FDMUX) and fast data multiplexer (FMUX), and the slow timing signals used in the low-voltage transistor-transistor logic (LvTTL) circuit (SLCK and SLCKN)
An example of the filling pattern of the KEKB low energy ring (LER) during a physics run is shown in Fig. 9, where the total current was 265 mA
Summary
In a storage ring which stores many bunches at high beam current, especially in particle-factory machines, such as KEKB, PEP-II, or DAFNE, it is very likely that many strong coupled-bunch instabilities occur in both the transverse and longitudinal planes. In KEKB, we use higher-order-mode-free cavities [1,2,3] in both low energy ring (LER) and high energy ring (HER), and take special care of the structure of the vacuum components Even with these efforts, we have observed strong transverse coupled-bunch instabilities in both rings, the sources of which are presumed to be mainly from the beam-ion effect in the KEKB HER and the photoelectron effect [4] in the KEKB LER. For this purpose, we have developed a digital filter system which realizes the functions of phase shift by any desired value, dc suppression, and an adjustable digital delay without down sampling.
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