  This paper theoretically proves that an electron storage ring can generate
coherent radiation in THz region using a quick kicker magnet and an ac
sextupole magnet. When the vertical chromaticity is modulated by the ac
sextupole magnet, the vertical beam collective motion excited by the kicker
produces a wavy spatial structure after a number of longitudinal oscillation
periods. We calculate the radiation spectral distribution from the wavy bunch
in Hefei Light Source(HLS). If we reduce electron energy to 400MeV, it can
produce extremely strong coherent synchrotron radiation(CSR) at 0.115THz.
