  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. 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
coincident positron- and electron- emission characteristics and for the phase
space correlation of lepton vector momenta in free-free pair production.
