The design criteria for a future lepton-hadron collider detector
dedicated for QCD studies have to take into account that the vast domain
of the strong interaction phenomena remain experimentally unexplored.
The standard model and its extensions, which provide bench mark processes
for optimizing the future high energy ee and pp collider detectors (paying
the price of reducing the research scope to verification of a priori predefined
scenarios), can serve here only as qualitative exploration guides. The
challenge is thus to design a facility optimized for a generic research
program. In my talk I shall argue that the optimal detector for such a
program is a 4-pi detector capable to measure and reconstruct complete collision
events of momentum and spin tunable beams of leptons, nucleons and ions.
The design of such a detector imposes important constraints on the collider
design: on its energy range, the optics of the interaction point, insertion
magnet aperture, their combined functions as low angle particle spectrometers,
bunch separations bunch sizes, beam pipe size and shape etc... I shall
discuss these constraints using a generic model of such a detector and try
to define areas in which common effort of accelerator and detector designers
is indispensable.