  Neutral beam current drive (NBCD) on the EAST tokamak is studied by using
Monte-Carlo test particle code TGCO. Phase-space structure of the steady-state
fast ion distribution is examined and visualized. We find that trapped ions
carry co-current current near the edge and counter-current current near the
core. However, the magnitude of the trapped ion current is one order smaller
than that of the passing ions. Therefore their contribution to the fast ion
current is negligible (1% of the fast ion current). We examine the dependence
of the fast ion current on two basic plasma parameters: the plasma current I_p
and plasma density n_e. The results indicate that the dependence of fast ion
current on I_p is not monotonic: with I_p increasing, the fast ion current
first increases and then decreases. This dependence can be explained by the
change of trapped fraction and drift-orbit width with I_p. The fast ion current
decreases with the increase of plasma density n_e. This dependence is related
to the variation of the slowing-down time with n_e, which is already well known
and is confirmed in our specific situation. The electron shielding effect to
the fast ion current is taken into account by using a fitting formula
applicable to general tokamak equilibria and arbitrary collisionality regime.
The dependence of the net current on the plasma current and density follows the
same trend as that of the fast ion current.
