  In the merging-compression method of plasma start-up, two flux-ropes with
parallel toroidal current are formed around in-vessel poloidal field coils,
before merging to form a spherical tokamak plasma. This start-up method, used
in the Mega-Ampere Spherical Tokamak (MAST), is studied as a high Lundquist
number and low plasma-beta magnetic reconnection experiment.
  In this paper, 2D fluid simulations are presented of this merging process in
order to understand the underlying physics, and better interpret the
experimental data. These simulations examine the individual and combined
effects of tight-aspect ratio geometry and two-fluid physics on the merging.
The ideal self-driven flux-rope dynamics are coupled to the diffusion layer
physics, resulting in a large range of phenomena. For resistive MHD
simulations, the flux-ropes enter the sloshing regime for normalised
resistivity eta < 1E-5. In Hall-MHD three regimes are found for the qualitative
behaviour of the current sheet, depending on the ratio of the current sheet
width to the ion-sound radius. These are a stable collisional regime, an open
X-point regime, and an intermediate regime that is highly unstable to
tearing-type instabilities.
  In toroidal axisymmetric geometry, the final state after merging is a
MAST-like spherical tokamak with nested flux-surfaces. It is also shown that
the evolution of simulated 1D radial density profiles closely resembles the
Thomson scattering electron density measurements in MAST. An intuitive
explanation for the origin of the measured density structures is proposed,
based upon the results of the toroidal Hall-MHD simulations.
