  We report an analysis of electron-optical properties of a toroidal magnetic
sector spectrometer and examine parameters for its implementation in a
relativistic heavy-ion storage ring like HESR at the future FAIR facility. For
studies of free-free pair production in heavy-ion atom collisions, this
spectrometer exhibits very high efficiencies for coincident $e^+e^-$ pair
spectroscopy over a wide range of momenta of emitted lepton pairs. The high
coincidence efficiency of the spectrometer is the key for stringent tests of
theoretical predictions for the phase space correlation of lepton vector
momenta in free-free pair production.
