Nonlinear dynamics of a magnetic island is simulated using a set of reduced two-fluid equations, and the excitation of the electric field in the vicinity of the magnetic island is observed. An impact of the error field (external symmetry-breaking magnetic perturbation) on the electric field is examined, and its global structure changes with a small error field. A numerical measurement simulating the heavy ion beam probe is carried out on the field given by the nonlinear simulation. An effect of the finite width of the injected beam is taken into account, and the spatial resolution is evaluated. It is found that the measuring error due to the finite beam width is significant when the radial gradient scale length of the electrostatic potential is comparable to the beam width.
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