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Ions produced by collisions between high-energy electron beams and residual gas molecules can be trapped along the beam trajectory by the strong electric potential well of the electron beam, leading to detrimental effects on beam dynamics. Ion trapping is therefore a relevant issue for high-duty continuous-wave (CW) machines, such as energy recovery linacs intended for use as high-average-current FEL drivers.
To mitigate trapped ions, we investigate the introduction of bunch gaps between bunch trains as an alternative to clearing electrodes, motivated by concerns over wake fields excited by electrode structures. The use of bunch gaps is well established in storage rings, where periodic focusing from successive bunches is interrupted, rendering ion trapping unstable. In linac-based FELs, however, the introduction of bunch gaps gives rise to variations in beam loading in accelerating cavities, which is likely to affect beam and RF stability.
In this study, assuming an average beam current of 10 mA and a bunch repetition frequency of 162.5 MHz, the stability of trapped ions is numerically evaluated for various bunch-gap patterns using a linear optics model. In addition, beam-loading variations in accelerating cavities are analytically estimated under conditions where ion trapping is expected to be suppressed. The feasibility of introducing bunch gaps for ion clearing in the designed ERL-FEL machine at KEK is discussed, considering the associated beam-loading effects.
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