Mitigation of collective instability in the transient process after swap-out injection by RF modulation

Abstract

On-axis swap-out injection schemes have become the baseline for several ultralow-emittance storage ring-based synchrotron light sources, such as APS-U, ALS-U, and HEPS. This scheme naturally requires that the injector (usually consisting of an electron gun, a LINAC and a booster synchrotron) has the capability of providing full-charge bunches. However, it is nontrivial to inject a high-charge bunch into the ring, even when using on-axis injection. One of the difficulties is the transverse single-bunch instability that arises in the transient process immediately after injection due to injection imperfections. To mitigate the aforementioned injection-induced transient instability under the condition of reasonable injection imperfections, we propose to implement the RF modulation technique in the booster before bunch extraction. The blowup of longitudinal emittance of the bunch before extraction from the booster is expected under carefully adjusted modulation settings. Simulations demonstrate the effectiveness of the proposed technique.