  Exploring the nucleon's sea quark and gluon structure is a prime objective of
a future electron-ion collider (EIC). Many of the key questions require
accurate differential semi-inclusive (spin/flavor decomposition, orbital
motion) and exclusive (spatial distributions of quarks/gluons) DIS measurements
in the region 0.01 < x < 0.3 and Q^2 ~ few 10 GeV^2. Such measurements could
ideally be performed with a high-luminosity collider of moderate CM energy, s ~
10^3 GeV^2, and relatively symmetric configuration, e.g. E_e/E_p = 5/30-60 GeV.
Specific examples are presented, showing the advantages of this setup
(angular/energy distribution of final-state particles, large-x coverage)
compared to typical high-energy colliders.
