The initial injection system of the heavy-ion-beam diagnostic (HIBD) of the tokamak ISTTOK provides a 2 mm diameter and 3.6 mrad divergence beam of Cs + or Xe + ions, after extraction from a plasma ion source and acceleration up to 20 keV by a gap lens and diameter definition by a set of apertures. However, the beam current is significantly reduced to 1.5 µA, therefore, complicating the HIBD measurements of the plasma parameters. Creation of the beam with a higher current and the same, or close, geometrical parameters requires a significant improvement of the ion optics of the injection system. This article describes an injection system built in the acceleration tube, and presents results of test experiments with a solid-state thermionic source (SSTS) and a monocusp plasma ion source (MPIS). The improved injection system is characterized by a good match to both ion sources. The use of the long focus ion optics of the acceleration tube allows one to obtain an output beam current up to 11 µA of Cs + ions with SSTS and 39 µA of Xe + ions with MPIS, and dimension preservation at the focus point. Operating on the tokamak ISTTOK, the injection system significantly increased the signal-to-noise ratio and improved the measurements of the plasma parameters.