Compensation of emittance variation in a diffraction-limited storage ring

In diffraction-limited storage rings (DLSRs), due to the comparable energy loss from bending magnets to that from insertion devices (IDs), the independent adjustments of ID gaps can introduce random emittance variations, which need to be compensated. Hefei Advanced Light Facility (HALF) is a soft x-ray DLSR with both 20 long dispersion-free and 20 short dispersive straight sections. Its original emittance compensation scheme is based on the radiation damping effect, achieved by adjusting the gaps of two damping wigglers (DWs) located in long straight sections. This paper proposes a novel emittance compensation scheme for HALF based on the quantum excitation effect. In this new scheme, a short variable-gap wiggler is installed in a dispersive straight section, with the two DWs operating at their minimum gaps. The proposed scheme achieves emittance compensation comparable to the original scheme in both cases with and without intrabeam scattering, even though the wiggler is significantly shorter. It also yields significantly smaller energy spread variations and consequently, very small effective emittance variations at short dispersive straight sections, with the effective emittance considered basically compensated. Moreover, fixing the minimum gaps of the two DWs can provide additional beamlines for users as well as enhanced radiation damping, which not only compensates for the emittance growth caused by the short wiggler due to quantum excitation but also reduces the damping times to suppress intrabeam scattering. For DLSRs without dispersive straight sections, the proposed quantum-excitation-based emittance compensation scheme can be applied by converting a dispersion-free straight section into a dispersive one simply using a dipole chicane.