Ion thruster for interplanetary space mission

Abstract: An ion thruster is operable in an interplanetary space with plasma generated by microwae in a propellant atmosphere. A vessel (2) defines first (40) and second (41) hollow spaces and ends at an opening (43). A microwave generating unit (44) generates the microwave in the first hollow space as a standing wave penetrating into the second hollow space. A propellant supplying unit (3) supplies the propellant into the second hollow space to make the propellant absorb the standing wave and produce main plasma comprising main ions and main electrons. An accelerating unit (17) accelerates only the main ions into an ion beam to inject the ion beam through the opening into the interplanetary space. Preferably, a neutralizing unit (19) defines a third space (52) which is in communication to the first space and into which the standing wave penetrates. The propellant comes into the third space to produce neuralizing ions and electrons. The ions are pulled by the ion beam to leave the neutralizing electrons, which neutralize the vessel.

Description: This invention relates to an ion thruster which is operable for interplanetary space mission. A conventional ion thruster comprises a vessel, a cathode unit adjacent to the vessel, and a propellant supplying unit connected to the vessel. The cathode unit comprises a hollow cylindrical cathode. The vessel defines a hollow space which ends at an opening. The hollow space includes an anode. Electrical potential is applied between the anode and the vessel. The cathode unit emits thermoelectrons into the hollow space. The propellant supplying unit supplies a propellant into the hollow space to form a propellant atmosphere in the hollow space. The thermoelectrons in the propellant atmosphere are accelerated by the electrical potential between the anode and the cathode and come into collision with the propellant to produce plasma which comprises plasma ions and plasma electrons. An accelerating unit is placed at the opening. The accelerating unit accelerates only the plasma ions to form and eject an ion beam through the opening towards a surrounding space. On the other hand, it is known to generate plasma in an inert gas atmosphere by using microwave. The inert gas atmosphere may be got by introducing an inert gas into a hollow space. It is, however, to be noted in connection with the conventional ion thruster that the plasma is generated using the electrodes. The cathode must be preheated before operation so that its quick start is possible. It is difficult to prolong the life of the ion thruster because degradation of the electrodes is inevitable. US-A-3866414 discloses an ion engine for inducing microwave power in a second hollow space via a first hollow space and a coupling element. The third hollow space is coupled to the second hollow space via channels for the propellant ; there is no connection between the first and the third hollow spaces. It is therefore an object of the present invention to provide an ion thruster for producing plasma without the electrodes. This object is solved with the features of the claims. The ion thruster according to the invention has a simple structure, a prolonged life and a high propulsion capability. In accordance with this invention, there is provided an ion thruster which is operable for interplanetary space mission and comprises a vessel defining a first and a second hollow space and a window between the first and the second hollow spaces. The second hollow space has an opening. Microwave is generated by a microwave generating unit and is transmitted into the first hollow space. The first hollow space is operable as a cavity resonator for the microwave so that a standing wave is produced in the first hollow space to penetrate and induce electric field power into the second hollow space through the window. A propellant supplying unit supplies a main propellant into the second hollow space. The main propellant absorbs the microwave power to produce main plasma in the second hollow space. The main plasma comprises main plasma ions and main plasma electrons. An accelerating unit is related to the opening and accelerates only the main plasma ions to form and eject an ion beam through the opening into the surrounding space. Fig. 1 shows a schematic section of a conventional ion thruster; Fig. 2 schematically shows a section of an ion thruster according to an embodiment of this invention; and Fig. 3 is a schematic perspective and sectional view of the ion thruster depicted in Fig. 2. Referring to Fig. 1, a conventional ion thruster (see JP-U-140 437/85) will be described at first in order to facilitate an understanding of this invention. The conventional ion thruster comprises a vessel 2, a cathode unit 1 adjacent to the vessel 2, and a propellant supplying unit 3 connected to the vessel 2 directly and through the cathode unit 1. The vessel 2 defines a main discharge space 11. The cathode unit 1 comprises a hollow cylindrical cathode 4 and a cathode keeper 5 having an opening collinear with the cylindrical cathode 4. The cathode unit 1 defines a cathode hollow space 6 connected to the main discharge space 11. The hollow cylindrical cathode 4 is connected to a cathode power supply 7. The cathode keeper 5 is connected to a cathode keeper power supply 8. The propellant supplying unit 3 comprises a propellant supplying tank 9 to supply a main propellant into the main discharge space 11. The propellant supplying tank 9 is connected to the vessel 2 directly and through the cathode hollow space 6. The hollow cylindrical cathode 4 is heated by the cathode power supply 7 to emit thermoelectrons. The cathode keeper power supply 8 produces electric discharge between the hollow cylindrical cathode 4 and the cathode keeper 5. The electric discharge generates a cathode plasma 10 in the cathode hollow space 6 by the use of the thermoelectrons and the propellant. The cathode plasma 10 comprises cathode plasma electrons. The main discharge space 11 includes an anode 12 and ends at an opening 13. The anode 12 is connected to an anode power supply 14 for accelerating the cathode plasma electrons as accelerated electrons from the cathode hollow space 6 towards the anode 12 in the main discharge space 11. The accelerated electrons come into collision with the main propellant in the main discharge space 11 to produce main plasma 15. The main plasma comprises main ions and main electrons. The vessel 2 is surrounded by a magnetic field supplying unit 16. The magnetic field supplying unit 16 produces a magnetic field in the main discharge space 11 to give a spiral movement to the accelerated electrons. The spiral movement is useful to prolong a travel length towards the anode 12 so as to increase collision probability of the main propellant with the accelerated electrons. An accelerating unit 17 is placed at the opening 13. The accelerating unit 17 accelerates only the main plasma ions to form and eject an ion beam 18 through the opening 13 into the surrounding space. The conventional ion thruster further comprises a neutralizing unit 19. The neutralizing unit 19 is supplied with the main propellant as a neutralizing propellant by the propellant supplying unit 3 and comprises a neutralizing cathode 20 and a neutralizing keeper 21 having an opening collinear with the neutralizing cathode 20. The neutralizing cathode 20 and the neutralizing keeper 21 are connected to a neutralizing cathode power supply 22 and a neutralizing keeper power supply 23. The neutralizing cathode 20 is heated by the neutralizing cathode power supply 22 to emit neutralizing thermoelectrons. The neutralizing keeper power supply 23 produces neutralizing electric discharge between the neutralizing cathode 20 and the neutralizing keeper 21. The neutralizing electric discharge generates a neutralizing plasma 24 by the use of the neutralizing thermoelectrons and the neutralizing propellant. The neutralizing plasma 24 comprises neutralizing ions, neutralizing electrons, and thermoelectrons. The thermoelectrons are pulled by the ion beam from the opening of the neutralizing keeper 21 for neutralization. Referring to Fig. 2, an ion thruster according to a preferred embodiment of this invention comprises similar parts designated by like reference numerals. The vessel 2 defines first and second hollow spaces 40 and 41 and a window between the first and the second hollow spaces 40 and 41. In the example being illustrated, a quartz plate is placed at the window and will be designated by the reference numeral 42. The second hollow space 41 has an opening 43 opposite to the quartz plate 42. The ion thruster further comprises a microwave generating unit 44 connected to the vessel 2. The microwave generating unit 44 comprises a microwave oscillator 45, an oscillator power supply 46, and a waveguide 47. The microwave oscillator 45 is put into operation by the oscillator power supply 46 and produces a microwave which propagates into the first hollow space 40 through the waveguide 47. The first hollow space 40 is operable as a cavity resonator for the microwave so that a standing wave is produced in the first hollow space 40 to penetrate and induce electric field power into the second hollow space 41 through the quartz plate 42. Turning to Fig. 3, the first hollow space 40 has a length adjusted by a plunger 55 to effectively become the cavity resonator. Turning back to Fig. 2, the propellant supplying tank 9 is connected to the second hollow space 41. A main flow controller 48 controls a flow of the main propellant. Therefore, the main propellant is supplied into the second hollow space 41 and absorbs the electric field power to produce the main plasma in the second hollow space 41. The accelerating unit 17 accelerates only the main ions to form and eject an ion beam through the opening 43 into the surrounding space. More specifically, the accelerating unit 17 comprises first and second grid electrodes 49 and 50 at the opening 43. The first grid electrode 49 is contiguous to the second hollow space 41. The second grid electrode 50 is away from the second hollow space 41. The accelerating unit 17 further comprises an electric potential supplying unit 51. The electric potential supplying unit 51 supplies an electrical potential difference between the first and the second grid electrodes 49 and 50 so that the first grid electrode 49 has a higher potential having a range between 1kV and 2kV and the second grid electrode 50 has a lower potential of about -500V. The ion thruster further comprises a neutralizing unit which is somewhat different from the neutralizing unit 19 described with reference to Fig. 1 but will be designated by the reference numeral 19. More specifically, the neutralizing unit 19 comprises the neutralizing cathode 20 as illustrated in Fig.1. Heated by the heating power supply 22, the neutralizing cathode 20 produces thermoelectrons for use in neutralizing the vessel 2. It is unnecessary to use the neutralizing keeper 21 and the neutralizing keeper power supply 23. The neutralizing unit 19 defines a third hollow space 52 connected to the first hollow space 40 through the quartz plate 42 for the microwave and ends at an orifice 53. The standing wave penetrates and induces electric field power into the third hollow space 52 through the quartz plate 42. The propellant supplying tank 9 supplies the propellant into the third hollow space 52. The neutralizing propellant absorbs the electric field power to produce the neutralizing plasma in the third hollow space 52. The neutralizing electrons are pulled by the ion beam through the orifice 53. In other words, the microwave generating unit 44 can generate simultaneously the main plasma and the neutralizing plasma in the vessel 2 and in the neutralizing unit 19, respectively. Therefore, the ion thruster comprises drastically reduced numbers of the power supplies and the electrodes so as to reduce the total weight and improve the reliability of the ion thruster. The quartz plate 42 is operable as a protection wall for diffusion of the main propellant and the main plasma towards the second hollow space 41. If desired, it is possible not to use the quartz plate 42 but to leave the window open.This is because the main propellant and the main plasma do not significantly diffuse into the second hollow space 41 even when the window is left wholly open. An insulator 54 is used for insulating between the vessel 2 and the propellant supplying unit 3 and consists of a plurality of wire nets. This is because the main plasma has a potential of about 1 kV and there exists a large potential difference between the main plasma and the propellant supplying unit 3. An optimum density is about 10¹¹ particles cm⁻³ which is achieved when the microwave is used to generate the plasma for ion thrusters.

Claims: An ion thruster which is operable in an interplanetary surrounding space and comprises a) a first hollow space (40), a second hollow space (41) having an opening (43) and a third hollow space (52) having an orifice (53), said opening (43) and said orifice (53) being open to said surrounding space, b) a window between said first and said second hollow spaces, c) a propellant supplying unit (3) for supplying a supply propellant to said second hollow space (41) as a main propellant, d) a microwave generating unit (44) for producing main plasma ions and electrons in said main propellant, e) an accelerating unit (17) at said opening for accelerating said main plasma ions to form and eject an ion beam through said opening (43) to said surrounding space, and f) the first hollow space (40) is operable as a cavity resonator for said microwave so that a standing wave is produced in said first hollow space (40) for penetration and induction of an electric field into the second hollow space (41) through said window, characterized in that g) said third hollow space (52) is coupled to said first hollow space (40) through said window with said propellant supplying unit (3) made to supply said supply propellant to said third hollow space (52) as a neutralizing propellant and with said microwave generating unit (44) made to produce neutralizing plasma ions and electrons in said neutralizing propellant so that said neutralizing plasma electrons are pulled by said ion beam through said orifice (53) to leave said neutralizing plasma ions in said third hollow space (52) for neutralization of said second hollow space (41). An ion thruster as claimed in claim 1, wherein a thermoelectron producing unit (20, 22) is used to produce thermoelectrons in said third hollow space (52) to be pulled by said ion beam together with said neutralizing plasma electrons. An ion thruster as claimed in claim 1 or 2, wherein said window is left wholly open between said first hollow space (40) and said second and said third hollow spaces (41 and 52, respectively). An ion thruster as claimed in claim 1 or 2, wherein said window is a quartz plate (42) disposed between said first hollow space (40) and said second and said third hollow spaces (41 and 52, respectively) . An ion thruster as claimed in any one of claims 1 to 3, wherein said window is left open between said first hollow space (40) and said second (41) and said third (52) hollow spaces. An ion thruster as claimed in any one of claims 1 to 5, wherein said propellant supplying unit (3) comprises a propellant supplying tank (9) connected to said third hollow space (52) and, through an insulator (54), to said second hollow space (41).

Inventors: IIDA HIROSHI, KUNINAKA HITOSHI, KURIKI KYOICHI, IIDA, HIROSHI, KUNINAKA, HITOSHI, KURIKI, KYOICHI, IIDA, HIROSHI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: F03H1
IPC codes: F03H1