An experiment HERA-
utilising an internal polarized nucleon target in the 820 GeV
HERA proton beam would constitute a natural extension of the studies
nucleon spin structure in progress at DESY with the HERMES
Conceivably, this would be the only place where to study high energy
nucleon-nucleon spin physics besides the dedicated RHIC spin program at BNL
supposed to start early in the next decade.
An internal polarized nucleon target offering unique features such as
polarization above 80% and no or small dilution, can be safely
in a proton ring at high densities up to atoms/cm.
The integrated luminosity which could be accumulated in
the experiment is estimated to be
and corresponds to about 3 real years running under
present HERA conditions.
As long as the polarized target would be used in conjunction with
HERA proton beam, the `phase-I' physics programme of HERA-
could be started and
focused to measurements of single spin asymmetries.
Once having available a polarized proton beam at HERA,
all combinations of beam and target polarization
() could be possible and correspondingly double spin asymmetries
, and would be accessible at HERA-.
measurements would constitute an alternative
-fixed target- approach to similar physics which will be accessible to
the collider experiments STAR and PHENIX at the low end of
the RHIC energy scale ( GeV).