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 10⁴. 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.