  Tokamak plasma current start-up assisted by Electron Bernstein waves (EBW)
has been demonstrated successfully in a number of experiments. The dynamic
start-up phase involves a change in field topology, as the initially open
magnetic field lines form closed flux surfaces (CFS) under the initiation of a
plasma current. This change in field topology will bring about a change in the
current drive (CD) mechanism, and, although various mechanisms have been
proposed to explain the formation of CFS, no detailed theoretical studies have
previously been undertaken. Here, we report on the development of a start-up
model for EBW-assisted plasma current start-up in MAST. It is shown that
collisions are responsible for only a small part of the CD, while the open
magnetic field line configuration leads to an asymmetric confinement of
electrons, which is responsible for the greater part of the CD.
