Neutral beam current drive (NBCD) on the EAST tokamak is studied by using
Monte-Carlo simulation. 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 net currents driven by the
beam are off-axis although the fast ion currents are on-axis. This is found to
be due to the electron shielding effect being strong near the magnetic axis. We
also investigate the dependence of NBCD efficiency on the plasma density. The
results indicate that the NBCD efficiency decreases with the increase in plasma
density. A simple semi-analytic estimation of the dependence of NBCD efficiency
on the density is proposed and is in reasonable agreement with the results
directly obtained in the simulations