The injected current is flat. For single-run convergence, simply going by the Poisson solver error tolerance is adequate.
If the Poisson solver is not settled, the user should try the following:
Add more cycles if the Poisson solver iterations are trending down. See Solver Setup > Basic > Cycles > Maximum Number of Relaxation Cycles.
Reduce the Poisson solver error tolerance (set to a larger number).
Increase the number of particles injected.
Improve the mesh in the injected region or anywhere where the beam region may not be adequately resolved.
Consider a quadratic field solution, with perhaps an even lower Poisson solver error tolerance like 1.0E-17 to 1.0E-20.