  Abstract Simulation domain in field-aligned coordinates of the electrostatic
gyrokinetic nonlinear turbulence global code, NLT, is extended to include the
magnetic axis. The artificial boundary near the magnetic axis is replaced by
the natural boundary. The singularity at the magnetic axis in Vlasov solver is
treated by considering the spatial relation of fixed grid points in
field-aligned coordinates. A new Poisson's equation solver is developed, the
coefficient matrix of algebraic equations is derived by using Gauss's theorem.
Nonlinear relaxation test of the ITG turbulence with adiabatic electrons is
performed. The gyrocenter conservation is much improved by including the
magnetic axis in the simulation domain. The zonal field and the radial
distribution of the perturbed electrostatic potential are different from
previous results without the magnetic axis.
