In accelerator physics, shunt impedance is a measure of the strength with which an eigenmode of a resonant radio frequency structure (e.g., in a microwave cavity) interacts with charged particles on a given straight line, typically along the axis of rotational symmetry.
If not specified further, the term is likely to refer to longitudinal effective shunt impedance.
, an eigenmode of the resonator has to be excited, leading to power dissipation
When a particle is deflected in transverse direction, the definition of the shunt impedance can be used with substitution of the (longitudinal) acceleration voltage by the transverse effective acceleration voltage, taking into account transversal Coulomb and Lorentz forces.
Because the transverse deflection can be described with polar coordinates, one may define a deflection or polarization angle using the transverse acceleration voltage components.
Polar coordinates are used because it is possible to add up voltage components like vectors, but not shunt impedances.