Necessary and sufficient conditions for the solvability of the Gauss variational problem for infinite dimensional vector measures

We continue our investigation of the Gauss variational problem for infinite dimensional vector measures associated with a condenser $(A_i)_{i\in I}$. It has been shown in Potential Anal., DOI:10.1007/s11118-012-9279-8 that, if some of the plates (say $A_\ell$ for $\ell\in L$) are noncompact then, in general, there exists a vector $\mathbf a=(a_i)_{i\in I}$, prescribing the total charges on $A_i$, $i\in I$, such that the problem admits no solution. Then, what is a description of all the vectors $\mathbf a$ for which the Gauss variational problem is nevertheless solvable? Such a characterization is obtained for a positive definite kernel satisfying Fuglede's condition of perfectness; it is given in terms of a solution to an auxiliary extremal problem intimately related to the operator of orthogonal projection onto the cone of all positive scalar measures supported by $\bigcup_{\ell\in L}A_\ell$. The results are illustrated by examples pertaining to the Riesz kernels.