The cancellation effect has been an important and long-standing controversial issue in the study of coherent synchrotron radiation (CSR) induced electron bunch dynamics in bends. In this talk, in order to illustrate the physical picture of the cancellation effect, equations of motion are derived from the canonical formulation of the dynamics for an ultrarelativistic electron bunch under collective interaction on a curved trajectory. It is shown that two seemingly unrelated physical quantities for a bunch in a bend, i.e., the kinetic energy change induced dispersion effect due to the potential energy change, and the radial component of the collective radiative force, are actually strongly correlated. Using retarded potentials, we show that the logarithmic singularities in these two dynamical functions (resulted from the local interaction contributions) are actually cancelled, thus the local interactions influence the particle transverse optics only via the second or higher order terms. The application of the cancellation effect in the CSR studies and measurements is also highlighted.


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