1. A plasma accelerator comprising:an electrically conductive discharge chamber with an open end,
means for introducing ionizable propellant inside the discharge chamber,
an active cathode configured to emit electrons for ionizing the propellant and neutralizing outflowing ions, the active cathode placed outside the discharge chamber,
a cathode grid being a passive cathode placed after the open end of the discharge chamber,
an electrically conductive control grid placed after the cathode grid,wherein the plasma accelerator further comprises:power supply means configured to apply:
a potential (VCT) between the control grid and the active cathode for controlling thrust of outflowing plasma stream through the open end of the discharge chamber,
a potential (VAC) between the active cathode and the discharge chamber for accelerating electrons into the open end of the chamber and ions towards the open end of the discharge chamber and,
a potential (VDS) between the discharge chamber and the cathode grid for imparting an electric field between the discharge chamber and the cathode grid;wherein the active cathode, the cathode grid and the control grid are arranged so as to introduce electrons emitted from the active cathode into the discharge chamber through the control grid and the cathode grid.