  For the HLS-II bunch current measurement system, in order to obtain the
absolute value of bunch current, the calibration factor should be determined by
using DCCT. At the HLS storage ring, the stretch effect of bunch length is
observed and the change rate is about 19% when the bunch current decays over
time and this will affect the performance of bunch current detection. To
overcome the bunch stretch influence in the HLS-II bunch current measurement,
an evaluation about pickup type and signal processing is carried out.
Strip-line pickup and button pickup are selectable, and the theoretical
analysis and demonstration experiment are performed to find out an acceptable
solution for the bunch current measurement system at HLS-II. The experimental
data analysis shows that the normalized calibration factor will change by about
27% when the bunch length change by about 19% if using the button pickup and
processing by peak value of bunch signal, the influence will be reduced to less
2% if adopting the strip-line pickup and integral.
