Divergence of the beam. Potential of the beam. The main sources of errors are the electrons of the plasma beam and secondary electron emission. When you are measuring the ion current density of plasma beam the probe reacts to both ions and electrons of plasma. The electron and ion currents flow in opposite directions and you register the total current that is less than ion current you need to register.
Second problem is that of secondary electron emission from the probe surface due to ion bombardment.
These electrons increase current density reading. You can ignore this error when you measure the current density of ions with the energy of several hundred eV or less, but if you measure the ions with the energy of several keV, the error may be significant. However the grid can decrease the ions flow on probe. This problem may be solved with Faraday cup shown in Fig.3. It works like black body in optic absorbs all particles that pass into collector and significantly decrease the output of secondary particles created due to interaction of primary particles with the surface of collector.