  A modified quasi periodic undulator (QPU) is developed and to be installed at
Hefei Light Source (HLS).Magnet dimensions optimization is applied. High
harmonics contamination is eliminated from the fundamental emission effectively
according to the field tests. The depression ratio of 3rd harmonic is increased
by an order of magnitude than the current device with other harmonics well
reduced simultaneously. The significance of the modification measure is
verified practically. The design, measurement and commissioning of the device
is described.
