  Longitudinal bunch lengthening via higher harmonic cavities is essential for
the new state-of-the-art 4th generation of synchrotron light storage rings, as
it can effectively improve the Touschek lifetime and mitigate the transverse
emittance growth due to intrabeam scattering. In general, the optimum or
near-optimum bunch lengthening condition is widely adopted for the double
radio-frequency system. This paper reveals, under this optimum lengthening
condition, that the short-range effect of resonant modes of the main and
harmonic cavities has the potential to enhance or suppress the bunch
lengthening significantly. Using the planned Hefei Advanced Light Facility
storage ring as an example, it is particularly demonstrated that the
short-range effects of the main and harmonic fundamental modes can dramatically
degrade the bunch lengthening for the assumed case of high-charge bunches. This
degradation of bunch lengthening is again presented with a realistic example of
PETRA-IV that operated in timing mode with high bunch charge. It is found that
there exists a setting of harmonic voltage and phase quite different from the
conventional optimum lengthening setting, to get optimum bunch lengthening.
