Study of bunch lengthening with harmonic cavities for HLS-III

NSRL recently proposed a future plan to further upgrade the HLS to an EUV diffraction-limited storage ring, named HLS-III. To improve the Touschek lifetime and suppress beam instabilities, HLS-III will employ a bunch-lengthening harmonic cavity system. Based on theoretical analysis, this work evaluates the bunch lengthening performance under five distinct double RF configurations involving different frequencies and harmonic orders, identifies the required harmonic cavity voltage to achieve the target bunch lengthening, and quantifies the corresponding improvement factor in Touschek lifetime. These findings provide valuable guidance for the selection of RF frequencies and the design of harmonic cavity parameters for HLS-III.