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.
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Longitudinal shunt impedance
To produce longitudinal Coulomb forces which add up to the (longitudinal) acceleration voltage
with the longitudinal effective acceleration voltage
The time-independent shunt impedance,
One can use the quality factor
where W is the maximum energy stored. Since the quality factor is the only quantity in the right equation term that depends on wall properties, the quantity
Transverse shunt impedance
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.
This does not necessarily imply a change in particle energy since a particle can also be deflected by magnetic fields (see Panofsky-Wenzel theorem).
Polarization angle
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.