  The validity of a closure called the minimal tau approximation (MTA), is
tested in the context of dynamo theory, wherein triple correlations are assumed
to provide relaxation of the turbulent electromotive force. Under MTA, the
alpha effect in mean field dynamo theory becomes proportional to a relaxation
time scale multiplied by the difference between kinetic and current helicities.
It is shown that the value of the relaxation time is positive and, in units of
the turnover time at the forcing wavenumber, it is of the order of unity. It is
quenched by the magnetic field -- roughly independently of the magnetic
Reynolds number. However, this independence becomes uncertain at large magnetic
Reynolds number. Kinetic and current helicities are shown to be dominated by
large scale properties of the flow.
