Possible Measurements of Single and Double Spin Asymmetries with   HERA--$\vec{N}$
  The physics scope of a possible future experiment utilizing an internal polarized nucleon target in the HERA proton beam is discussed. By measuring single spin asymmetries in inclusive particle production at 820~GeV beam energy the higher-twist sector of perturbative QCD can be probed with good statistical sensitivity. To support the physics case for proton polarization at HERA, we consider the measurement of double spin asymmetries in photon plus jet production. It appears possible to determine the polarized gluon distribution in the range 0.1~$\leq x_{gluon} \leq$~0.4 with good accuracy. 
Wolf-Dieter Nowak (DESY-IfH Zeuthen)