Breakdown of classical bunch length and energy spread formula in a quasi-isochronous electron storage ring

Originating from the stochastic nature of the photon emission process, the longitudinal coordinates of electrons diffuse even if the global phase slippage of a storage ring is zero, as we cannot zero all the local phase slippages simultaneously. This quantum diffusion is viewed as the most fundamental effect limiting the lowest bunch length realizable in an electron storage ring from single-particle dynamics perspective. Energy spread diverges when bunch length is pushed to the limit given by this diffusion. Here we report the first experimental evidence supporting the existence of this effect. The measurements confirm the breakdown of the classical formula of bunch length and energy spread in an electron storage ring with globally small and locally large phase slippage.