Abstract
The effect of a higher-order mode upon longitudinal beam stability in an electron storage ring is modeled analytically and with simulations. Narrow band parasitic modes and broadband impedance are considered for the Aladdin and MAX-II electron storage rings. The simulations confirm that a passive harmonic cavity strongly suppresses parasitic coupled-bunch instabilities, in agreement with the analytic model. In the long-bunch regime where the bunch length exceeds the vacuum pipe radius, analytic modeling and simulations indicate that a harmonic cavity also suppresses the microwave instability. In the short-bunch regime where the bunch length is smaller than the vacuum pipe radius, analytic modeling and simulations indicate that tuning in a harmonic cavity may worsen the microwave instability.
Highlights
A passive harmonic radio-frequency cavity may be utilized to lengthen the bunch and suppress longitudinal parasitic coupled-bunch instabilities [1,2,3,4,5]
In this long-bunch regime of the microwave instability, analytic modeling and simulations indicate that tuning in the harmonic cavity suppresses microwave instability
The effect of a passive harmonic cavity upon longitudinal stability has been studied for the Aladdin electron storage ring operated in base-lattice or low-emittance tunings, and for the MAX-II electron storage ring
Summary
A passive harmonic radio-frequency (rf) cavity may be utilized to lengthen the bunch and suppress longitudinal parasitic coupled-bunch instabilities [1,2,3,4,5]. For harmonic-cavity operation at Aladdin (in high- and low- emittance lattices) and the UVSOR electron storage ring, the analytic predictions are in approximate agreement with experiment [8–. For Aladdin operation in either the base-lattice or lowemittance lattice, the natural bunch length exceeds the vacuum-chamber radius Simulations of the bunch length for singlebunch operation of the low-emittance lattice are in approximate agreement with experiment, suggesting that the value of 5:7 is reasonable In this long-bunch regime of the microwave instability, analytic modeling and simulations indicate that tuning in the harmonic cavity suppresses microwave instability. The natural bunch length is smaller than the vacuum chamber radius In this short-bunch regime of the microwave instability, analytic modeling and simulations predict that the microwave instability is worsened as the harmonic cavity is initially tuned in. The microwave instability is not observed in simulations of MAX-II operation with optimally lengthened bunches
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