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Within a class of superstring vacua which have an additional non-anomalous$U(1)'$ gauge factor, we address the scale of the $U(1)'$ symmetry breaking andconstraints on the exotic particle content and their masses. We also show thatan extra gauge $U(1)'$ provides a new mechanism for generating a naturallysmall effective $\mu$ term. In general, existing models are not consistent withall phenomenological constraints; however, they do provide a testing ground toaddress the above issues, yielding a set of concrete scenarios. Under theassumptions that the spontaneous $U(1)'$ breaking takes place in the observablesector and that the supersymmetry breaking scalar mass square terms arepositive at the string scale, the breaking of $U(1)'$ symmetry is radiative. Itcan take place when the appropriate Yukawa couplings of exotic particles are oforder one, which occurs for $Z_2\times Z_2$ fermionic orbifold constructions atsymmetric points of moduli space. The $Z'$ mass is either of ${\cal O}(M_Z)$,when the symmetry breaking is due to a single standard model singlet, or is ofa scale intermediate between the string and electro-weak scales, determined bythe radiative corrections (or by competing non-renormalizable operators), whenthe breaking is due to two or more mirror-like singlets. In the former case,the $M_{Z'}/M_Z$ hierarchy achievable without excessive fine tuning is withinfuture experimental reach.Comment: 20 Pages, uses RevTeX (additional references and comments added

Implications of Abelian extended gauge structures from string models