A Hilbert-space formulation of and variational principle for spontaneous   wiggler radiation
  Within the framework of Hilbert space theory, we derive a maximum-power variational principle applicable to classical spontaneous radiation from prescribed harmonic current sources. Results are first derived in the paraxial limit, then appropriately generalized to non-paraxial situations. The techniques were developed within the context of undulator radiation from relativistic electron beams, but are more broadly applicable. 
A.E. Charman, J.S. Wurtele