  We consider the problem of condensation of open string tachyon fields which
have an O(D) symmetric profile. This problem is described by a boundary
conformal field theory with D scalar fields on a disc perturbed by relevant
boundary operators with O(D) symmetry. The model is exactly solvable in the
large D limit and we analyze its 1/D expansion. We find that this expansion is
only consistent for tachyon fields which are polynomials. In that case, we show
that the theory is renormalized by normal ordering the interaction. The
beta-function for the tachyon field is the linear wave operator. We derive an
expression for the tachyon potential and compare with other known expressions.
In particular, our technique gives the exact potential for the quadratic
tachyon profile. It can be used to correct the action which has been derived in
that case iteratively in derivatives of the tachyon field.
