  It is shown that a relevant control of Hamiltonian chaos is possible through
suitable small perturbations whose form can be explicitly computed. In
particular, it is possible to control (reduce) the chaotic diffusion in the
phase space of a Hamiltonian system with 1.5 degrees of freedom which models
the diffusion of charged test particles in a turbulent electric field across
the confining magnetic field in controlled thermonuclear fusion devices. Though
still far from practical applications, this result suggests that some strategy
to control turbulent transport in magnetized plasmas, in particular tokamaks,
is conceivable. The robustness of the control is investigated in terms of a
departure from the optimum magnitude, of a varying cut-off at large wave
vectors, and of random errors on the phases of the modes. In all three cases,
there is a significant region of maximum efficiency in the vicinity of the
optimum control term.
