Measurement of bunch length and energy spread for single bunch in HLS

Based on the principle that the synchrotron light has the same time structure with the beam signal, bunch length can be calculated by measuring the full width half height of the synchrotron light pulse. The variation of bunch length with beam current and RF loltage in single bunch mode was measured. And the result shows that the bunch length is in reverse proportion to the RF voltage to the power of 0.3 and it is smaller than the theoretical result. The growth rate of bunch length with the beam current is 2.0 ps/mA. The energy spread was measured by measuring the longitudinal quantum lifetime, which indicates that the bunch lengthening is the result of energy spread and potential well distortion．