Plasma thruster with birdcage antenna

Abstract

A plasma thruster comprises a cylindrical discharge channel (1), an injector (4), a RF antenna surrounding the discharge channel (1) and a device (3) for generating an axial static magnetic field in the discharge channel (1). The RF antenna is a cylindrical birdcage antenna (2) formed of several electrically conductive parallel legs (10) connected by two end rings (11) including capacitors (12) between adjacent legs (10) in each case. The two end rings (11) with the capacitors (12) are formed on two printed circuit boards (14) to which the legs (10) are attached, said printed circuit boards (14) having a through opening for the discharge channel (1). The antenna maximizes electrical coupling efficiency and provides resulting electromagnetic fields for quasi-neutral plasma acceleration along with the magnetic field effect provided by the externally applied magnetic field. This plasma thruster allows an easy upscaling or downscaling due to the printed circuit boards and is particularly suitable for low power applications like propulsion for smaller spacecrafts or satellites.

Claims

1. A plasma thruster comprising: a cylindrical discharge channel (1) having an inlet for a propellant and an outlet, an injector (4) for injecting the propellant through the inlet into the cylindrical discharge channel (1), an RF antenna surrounding the cylindrical discharge channel (1), said RF antenna when fed with RF power generating electromagnetic fields that ionize the propellant forming a plasma (8) in the cylindrical discharge channel (1) that is then ejected through the outlet to generate thrust, a magnetic field generator (3) for generating an axial static magnetic field in the cylindrical discharge channel (1), said axial static magnetic field providing boundary conditions for formation of helicon waves within the plasma (8), as well as providing a magnetic nozzle effect at the outlet for quasi-neutral plasma acceleration, wherein the RF-antenna is a cylindrical birdcage antenna (2) formed of a number of electrically conductive parallel legs (10) connected by two end rings (11) on each side, said two end rings (11) comprising one or several capacitors (12) between adjacent legs (10) among the number of electrically conductive parallel legs in each case, said one or several capacitors (12) defining a resonance frequency of the cylindrical birdcage antenna (2), and the injector (4) is made of an electrically conductive material and mounted movably along a cylinder axis of the cylindrical birdcage antenna (2) for fine tuning of the resonance frequency, wherein the two end rings (11) with the one or several capacitors (12) are soldered on two printed circuit boards (14) to which the number of electrically conductive parallel legs (10) are attached, said two printed circuit boards (14) having a through opening for the cylindrical discharge channel (1), wherein the number of electrically conductive parallel legs (10) comprise feet (13) for fastening on both ends of each of the number of electrically conductive parallel legs, and the two printed circuit boards (14) on which the one or several capacitors (12) are soldered, are placed on isolating flanges (15) to which the number of electrically conductive parallel legs (10) are fixed by means of screws and nuts (17) through holes in the feet (13), in the two printed circuit boards (14) and in the isolating flanges (15).

2. The plasma thruster according to claim 1, characterized in that the legs (10) are formed by a combination of 3D-printing and casting technology.

3. The plasma thruster according to claim 1, characterized in that the two printed circuit boards (14) and isolating flanges (15) have the form of rings.

4. The plasma thruster according to claim 3, characterized in that in cross-section perpendicular to the cylinder axis, the number of electrically conductive parallel legs (10) and feet (13) have a cross-sectional shape corresponding to the cross-section of cylinder jacket sections of a hollow cylinder.

5. The plasma thruster according to claim 1, characterized in that the injector (4) is provided with an electric drive a stepper motor or piezoelectric actuator, for movement of the injector (4) along the cylinder axis of the cylindrical birdcage antenna (2).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The proposed plasma thruster is described in the following in detail by means of an exemplary embodiment and corresponding figures. The figures show:

(2) FIG. 1 an example of a plasma thruster system in schematic representation;

(3) FIG. 2 a schematic view of a birdcage antenna in high pass configuration as used in the present invention;

(4) FIG. 3 a schematic view of the electromagnetic fields generated via the birdcage antenna and the externally applied axial static magnetic field;

(5) FIG. 4 an example for the components and design of the birdcage antenna of the proposed plasma thruster;

(6) FIG. 5 a photo of one of the printed circuit boards used for the birdcage antenna of FIG. 4 in plan view;

(7) FIG. 6 a side view and a cross sectional view of one the legs of the birdcage antenna of FIG. 4; and

(8) FIG. 7 a cross sectional view of an example of the proposed plasma thruster.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) FIG. 1 shows an example of a complete plasma thruster system using a plasma thruster according to the present invention. The plasma thruster consists of the cylindrical discharge channel 1 surrounded by the birdcage antenna 2. The birdcage antenna 2 on the other hand is surrounded by a solenoid 3 (alternatively by an arrangement of permanent magnets) applying an axial static magnetic field along the discharge channel 1. The propellant is injected into the discharge channel 1 by an injector 4. An RF generator 5 combined with a matching network 6 provides the RF power to the birdcage antenna 2. The solenoid 3 is supplied by a DC power supply 7. By applying the RF power to the birdcage antenna 2 the propellant in the discharge channel 1 is converted into a plasma 8 by the electromagnetic waves generated with the birdcage antenna 2. The axial magnetic field provides the boundary condition for the generation of helicon waves in the plasma 8. The birdcage antenna-generated electromagnetic fields in conjunction with the applied static magnetic field provide the acceleration of the quasi-neutral plasma 8 which is ejected through the outlet of the discharge channel 1 as a plasma jet 9 generating the corresponding thrust.

(10) FIG. 2 shows a schematical representation of a birdcage antenna 2 in a high pass configuration used in the proposed plasma thruster. In this example the birdcage antenna 2 consists of eight legs 10 connected by two end rings 11 on each side. At each end ring 11, between each two legs 10, capacitors 12 are included in the end ring 11. These capacitors 12 are used to provide the desired resonant frequency of the birdcage antenna 2, which is related to the excitation frequency of the power unit (RF generator 5), for example a frequency of 40.68 MHz. In the present example, the birdcage antenna 2 comprises eight legs 10. The proposed plasma thruster may however also have a birdcage antenna 2 with a higher or lower number of legs 10.

(11) By operating at one of the resonant frequencies of the birdcage antenna 2, the antenna is optimized, also from the point of view of minimizing the reflected power loss and ohmic losses. This means that an antenna impedance Z.sub.antenna=50+j0? can be achieved. Other resonant modes have reactance of X.sub.antenna=0?, but R.sub.antenna can be different than 50?. Standard RF generators, which have an output impedance of Z.sub.RF=50+j0? can provide an electric power coupling of more than 99.99%. At the desired resonance mode of the birdcage antenna, this also results in homogeneous and linearly polarized magnetic and electric fields (E.sub.1, B.sub.1) in the transverse cross section of the plasma cylinder (discharge channel 1). Such configurations provide drift velocity to both ions and electrons at the same time and in the thrust direction and, therefore, increases the thrust generated by the plasma thruster. Furthermore, the static magnetic field B.sub.0 provides a divergence at the outlet of the discharge channel 1 and, thus, the effect of a magnetic nozzle, which further accelerates the quasi-neutral plasma and, thus, also increases the thrust. The corresponding electromagnetic fields (E.sub.1, B.sub.1) as well as the externally applied static magnetic field B.sub.0 are schematically indicated in FIG. 3.

(12) A very important aspect of the present invention is the design of the birdcage antenna 2. FIG. 4 exemplary shows an explosion view of components for mounting the birdcage antenna 2. Essential features of this antenna are the two printed circuit boards 14 (PCB), on which the antenna legs 10 are directly attached and to which the capacitors 12 between the antenna legs are soldered. The end ring of the birdcage antenna 2 is also formed on the corresponding circuit boards 14. In the present example a very advantageous form of the legs 10 with feet 13 on each side is shown. In cross section perpendicular to the cylinder axis, the legs 10 and feet 13 have a cross sectional shape corresponding to the cross section of cylinder jacket sections of a hollow cylinder with the same cylinder axis. The legs are preferably formed by combining a 3D-printing and casting technology to produce the antenna legs out of copper. This system is particularly suitable for low power applications and thus for small satellites.

(13) The 3D-printing and casting technology guarantees the conductivity of the copper and at the same time the desired shape of the legs (and feet). The printed circuit boards 14 are connected to the antenna by means of PEEK screws and corresponding nuts 17 as indicated in FIG. 4. These withstand high temperatures and at the same time insulate electrically. The capacitors 12 are soldered to the PCBs (as shown in FIG. 5). RF ground 19 and RF input 18 are connected to the antenna legs 10. The grounding uses an electrically conductive screw to dissipate any reflected power. The RF input connector is directly soldered or clamped. Two aluminum oxide flanges 15 are used to hold the structure together. These are electrically insulating and resistant to high temperatures. The PTFE flange 16 attached at the top, has the task of holding the overall structure while providing electrical insulation. As can be seen from FIG. 4, the legs 10 are fixed via through holes in the feet 13, the PCB board 14 and the aluminum oxide flanges 15 with the screws and nuts 17.

(14) FIG. 5 shows a photo of one of the PCB boards 14 in plan view. In this plan view the end ring 11 of the birdcage antenna together with the capacitors 12 can be recognized. The figure also shows the through holes 20 for fixing the legs 10.

(15) FIG. 6 shows a side view and a top view on one of the legs 10 with the feet 13 on both ends together with exemplary dimensions in millimeters. The antenna itself is preferably isolated from the outside by a Faraday shield. This is made of a conductive material andin the application on a satelliteis very advantageous as it isolates the satellite electronics from the electromagnetic fields of the plasma thruster. It also isolates the birdcage antenna from parasitic external electromagnetic fields.

(16) FIG. 7 shows a cut view of an example of the proposed plasma thruster in which the Faraday shield 21 is also indicated. The figure also indicates the movement possibility of the injector 4 by the double arrow in the figure. The plasma thruster is designed to resonate at a specific frequency, for example 40.68 MHz. The birdcage antenna 2 can be designed to resonate at this frequency by appropriate selection of the capacitors 12 in the end rings 11. However, as the electronic components used in practice are not ideal and have always some deviations as well as the assembly and integration always have some tolerances, a fine adjustment of the resonance frequency is required. This is achieved by the movable injector 4 made of an electrically conductive material, in the present example brass. This injector can be moved along the thruster's axis of symmetry or cylindrical axis of the birdcage antenna as indicated in FIG. 7. When this injector 4 is moved within/very close to the antenna region, the thruster's resonance frequency shifts upwards. The extent of movement defines the increment of frequency that can be finally controlled. With such a movable injector, for example, a fine tuning of the resonance setting of the antenna with an interval range of 0.01-0.05 MHz is possible. The movement of the injector can be made manually or can also be achieved with an appropriate electric drive, in particular a stepper motor, to automatize the adjustment.

LIST OF REFERENCE SIGNS

(17) 1 discharge channel 2 birdcage antenna 3 solenoid 4 injector 5 RF generator 6 matching network 7 DC power supply 8 plasma 9 plasma jet 10 leg 11 end ring 12 capacitor 13 foot 14 PCB board Al.sub.2O.sub.3 flange 16 PTFE flange 17 PEEK screws and nuts 18 RF input 19 RF ground 20 through hole 21 Faraday shield