  Magnetic reconnection is a plasma phenomenon where a topological
rearrangement of magnetic field lines with opposite polarity results in
dissipation of magnetic energy into heat, kinetic energy and particle
acceleration. Such a phenomenon is considered as an efficient mechanism for
energy release in laboratory and astrophysical plasmas. An important question
is how to make the process fast enough to account for observed explosive energy
releases. The classical model for steady state magnetic reconnection predicts
reconnection times scaling as $S^{1/2}$ (where $S$ is the Lundquist number) and
yields times scales several order of magnitude larger than the observed ones.
Earlier two-dimensional MHD simulations showed that for large Lundquist number
the reconnection time becomes independent of $S$ ("fast reconnection" regime)
due to the presence of the secondary tearing instability that takes place for
$S \gtrsim 1 \times 10^4$. We report on our 3D MHD simulations of magnetic
reconnection in a magnetically confined cylindrical plasma column under either
a pressure balanced or a force-free equilibrium and compare the results with 2D
simulations of a circular current sheet. We find that the 3D instabilities
acting on these configurations result in a fragmentation of the initial current
sheet in small filaments, leading to enhanced dissipation rate that becomes
independent of the Lundquist number already at $S \simeq 1\times 10^3$.
