Reivindicaciones

1. A plasma accelerator comprising: - an electrically conductive discharge chamber (11) with an open end, - means for introducing ionizable propellant inside the discharge chamber (11), - an active cathode (19) configured to emit electrons for ionizing the propellant and neutralizing outflowing ions, the active cathode (19) placed outside the discharge chamber (11), - a cathode grid (14) being a passive cathode placed after the open end of the discharge chamber (11), - an electrically conductive control grid (18) placed after the cathode grid (14), - power supply means configured to apply:a potential (V<CT>) between the control grid (18) and the active cathode (19) for controlling thrust of outflowing plasma stream through the open end of the discharge chamber (11);characterised in that said power supply means is further configured to apply: a potential (V<AC>) between the active cathode (19) and the discharge chamber (11) for accelerating electrons into the open end of the chamber and ions towards the open end of the discharge chamber (11) and, a potential (V<DS>) between the discharge chamber (11) and the cathode grid (14) for imparting an electric field between the discharge chamber (11) and the cathode grid (14); and in that the active cathode (19), the cathode grid (14) and the control grid (18) are arranged so as to introduce electrons emitted from the active cathode (19) into the discharge chamber (11) through the control grid (18) and the cathode grid (14). 2. The plasma accelerator according to claim 1, further comprising an inner cathode (13) being a passive cathode electrically connected to the cathode grid (14), the inner cathode (13) placed inside the discharge chamber (11). 3. The plasma accelerator according to claim 1 or 2, wherein the discharge chamber (11) extends lengthwise along an axis of symmetry. 4. The plasma accelerator according to any of claims 1 to 3, wherein the cathode grid (14) and the control grid (18) have their open spaces aligned. 5. The plasma accelerator according to any of claims 1 to 4, further comprising a plurality of magnets (16) configured to confine electrons in the discharge chamber (11). 6. The plasma accelerator according to claim 5, wherein the plurality of magnets (16) is arranged concentrically around the discharge chamber (11) with alternate magnetic poles. 7. The plasma accelerator according to claim 5 or 6, further comprising a casing (20) for magnetically shielding the plurality of magnets (16). 8. The plasma accelerator according to any of claims 1 to 7, wherein the active cathode (19) is a single one. 9. The plasma accelerator according to any of claims 1 to 8, wherein the ionizable propellant is a monatomic or molecular gas. 10. Space borne vehicle comprising at least one plasma accelerator according to any of claims 1 to 9.