  We propose a cluster simulation algorithm for statistical ensembles with
fixed order parameter. We use the tethered ensemble, which features Helmholtz's
effective potential rather than Gibbs's free energy, and in which canonical
averages are recovered with arbitrary accuracy. For the D = 2,3 Ising model our
method's critical slowing down is comparable to that of canonical cluster
algorithms. Yet, we can do more than merely reproduce canonical values. As an
example, we obtain a competitive value for the 3D Ising anomalous dimension
from the maxima of the effective potential.
