Coupled-bunch instability study of multi-cell deflecting mode cavities for the Advanced Photon Source

Abstract

The short-pulse X-ray project at the Advanced Photon Source (APS) uses four room-temperature, three-cell, 2.815-GHz, deflecting-mode cavities in two consecutive straight sections. Undamped, these cavities' higher-order and lower-order resonator modes will cause multi-bunch instabilities in longitudinal and transverse planes for any bunch pattern of a 100-mA store. Damping of these modes must be part of the design of the cavities. We report calculations of instability growth rates that were essential in specifying and checking the rf design of the damping structures. We used various operating bunch patterns and scanned levels of damping of the cavities. Because one of the operating bunch patterns is not symmetric, we used a normal mode analysis implemented in the APS code clinchor. Our calculation included random sampling of resonator frequencies in a reasonable range.

RF design of normal conducting deflecting structures for the advanced photon source

“…Beam dynamics in APS sets strict requirements on damping of of low-order-modes (LOM) and high-ordermodes (HOM) in the cavity [10]. We found that the best way to address these requirements was to use a pair of cavities instead of one long cavity.…”

“…The operating frequency of the deflector is the 8th harmonic of the APS ring RF frequency [10]. For this design we chose power and repetition rate similar to that of a commercially available klystron [12].…”

The wakefield effects of pulsed crab cavities at the Advanced Photon Source for short-x-ray pulse generation

“…With the h=8 as the chosen frequency, an in-depth study based on Monte-Carlo simulation has been performed, and the results are reported in Ref. [4]. …”

“…Progress made in the various aspects of the project are reported in these proceedings, which include the overall configuration of a pulsed crab-cavity system and its implementation in the APS storage ring [2], rf design of crab-cavity system [3], and multi-bunch instability analysis for the APS storage ring with crabcavity installed [4]. Here we describe the single-bunch instability caused by a crab-cavity system.…”

“…[4], where the analysis is based on the analytic formula. Here we investigate the instability by tracking multiple bunches, each consisting of multiple macroparticles.…”