Beam halo measurements for special bunches in a storage ring by using a coronagraph.

We demonstrate an experimental methodology for measuring the halo distribution of special bunches in a storage ring using a synchrotron radiation coronagraph composed of the objective lens and a re-diffraction system. The optimum parameters for the coronagraph were investigated within several boundary conditions by applying a paraxial Fourier transformation sequentially from one plane to the next plane. In addition, the effect of Mie-scattering was estimated for different polishing-quality lenses and it shows that a high-quality lens is capable of achieving a dynamic range of the monitor of about 10. The capability of the halo monitor has been demonstrated by measuring the horizontal particle distribution of special bunches in beam experiments at the BESSY II storage ring. This monitor offers a new opportunity for continuous monitoring of special bunches in the storage ring such as transverse resonance island buckets and pulse-picking by resonant excitation, which open new horizons for storage rings that are capable of sophisticated experiments using a single bunch signal as well as serving high-flux users simultaneously.