Electron cyclotron resonance magnetic nozzle thruster

Abstract

An Electron Cyclotron Resonance, ECR, magnetic nozzle thruster comprising: a thruster chamber; an antenna disposed at least partially within the thruster chamber and configured to emit radio frequency radiation into the thruster chamber; a magnetic field source configured to generate a magnetic field within the thruster chamber, the magnetic field comprising a resonance region; and a ferromagnetic structure, wherein the ferromagnetic structure is positioned relative to the magnetic field source so as to increase a volume of the resonance region within the thruster chamber.

Description

[0001] Electron Cyclotron Resonance Magnetic Nozzle Thruster

[0002] Field of the Invention

[0003] The present invention relates to an Electron Cyclotron Resonance (ECR) magnetic nozzle thruster and in particular to an ECR thruster having a ferromagnetic structure which increases the thickness and volume of the resonance region within the thruster.

[0004] Background

[0005] The lifetime of satellites is fundamentally limited by their thrusters. Thruster lifetime is limited by how fast it depletes its propellant or by how long it can survive before it is critically eroded. Many satellites use electric propulsion thrusters. Hall Effect thrusters and Gridded Ion thrusters are in widespread use, but ECR magnetic nozzle thrusters have yet to be used commercially. This is primarily due to historically low performance, e.g. low power efficiency.

[0006] An Electron Cyclotron Resonance (ECR) magnetic nozzle thruster uses a combination of microwaves and magnets to heat and accelerate its fuel. The microwaves generate a low potential plasma (ions and electrons) with a decreased erosion rate compared with Hall Effect and Gridded Ion thrusters. These existing thruster types have the additional disadvantages of having a relatively complex design, high costs and requiring an external cathode (neutralizer). There is therefore a need to increase the performance of ECR magnetic nozzle thrusters so that they may become a competitive option for spacecraft thrusters for satellite manufacturers.

[0007] Summary of the Invention

[0008] A first aspect of this disclosure provides an Electron Cyclotron Resonance, ECR, magnetic nozzle thruster comprising : a thruster chamber; an antenna disposed at least partially within the thruster chamber and configured to emit radio frequency radiation into the thruster chamber; a magnetic field source configured to generate a magnetic field within the thruster chamber, the magnetic field comprising a resonance region; and a ferromagnetic structure, wherein the ferromagnetic structure is positioned relative to the magnetic field source so as to increase a volume of the resonance region within the thruster chamber.

[0009] The magnetic field source may be disposed forward of the thruster chamber. The ferromagnetic structure may be disposed rearward of the magnetic field source. The magnetic field source may comprise at least one permanent ring magnet. The at least one permanent ring magnet may be a Samarium Cobalt magnet. The at least one permanent ring magnet may be a Neodymium magnet.

[0010] The ferromagnetic structure may be secured directly to a rearward facing surface of permanent ring magnet. The ferromagnetic structure may comprise at least one iron ring.

[0011] The ECR magnetic nozzle thruster may comprise a central axis, the magnetic field source may comprises a permanent ring magnet and the permanent ring magnet, thruster chamber and at least one iron ring may each be centred on the central axis.

[0012] An inner diameter of the iron ring may be larger than an outer diameter of the thruster chamber. The iron ring may overlap the base of the thruster chamber such that the thruster chamber is partially recessed inside the iron ring.

[0013] The ferromagnetic structure may comprise a first ring made of a ferromagnetic material, the first ring comprising an annular protrusion extending radially inwards. The annular protrusion may extend underneath the base of the thruster chamber.

[0014] The ferromagnetic structure may comprise a second ring made of a ferromagnetic material, the second ring arranged concentrically with the first ring. The second ring may be disposed underneath the base of the thruster chamber.

[0015] The radio frequency radiation may be between 2.40 GHz and 2.50 GHz. The radio frequency radiation may have a -3 dB bandwidth of less than 0.1 MHz. The radio frequency radiation may comprise multiple different input frequencies in the range 2.40 GHz to 2.50 GHz.

[0016] A gradient of the magnetic field strength within the resonance region may be less than 1 T / m, less than 0.8 T / m or less than 0.75 T / m.

[0017] The ECR magnetic nozzle thruster may comprise a body supporting the thruster chamber and the thruster chamber may comprise a backplate made of Boron Nitride and a frontplate made of Isostatic Graphite. The body and frontplate may define a fuel line for introducing a propellant into the thruster chamber. The ECR magnetic nozzle thruster may comprise a propellant tank in fluid communication with the fuel line. The propellant may be Xenon. Brief Description of the Figures

[0018] So that the general concepts set out in the foregoing sections can be more fully understood, embodiments thereof will be described with reference to the accompanying drawings, in which:

[0019] Figure 1 is a cross-sectional view of a new design of ECR thruster according to the present invention;

[0020] Figure 2 is a cut-through perspective view of the ECR thruster of Figure 1, according to the present invention;

[0021] Figure 3 is a graph showing a comparison of the magnetic field strength for an unmodified thruster and the ECR thruster of the present invention;

[0022] Figure 4 is an axisymmetric magnetic field model of an unmodified thruster;

[0023] Figure 5a is an axisymmetric magnetic field model of the ECR thruster of Figures 1 and 2;

[0024] Figure 5b shows the axisymmetric magnetic field model of Figure 5a with exemplary dimensions marked;

[0025] Figure 6 shows a comparison of the ECR thruster design of the present invention using a Samarium Cobalt magnet and a Neodymium magnet;

[0026] Figure 7 shows an alternative ECR magnetic nozzle thruster according to the present invention;

[0027] Figure 8 shows the axisymmetric magnetic field model for the unmodified thruster of Figure 4, with areas of different magnetic field strength illustrated;

[0028] Figure 9 shows the axisymmetric magnetic field model for the ECR thruster of Figures 1, 2, 5a and 5b, with areas of different magnetic field strength illustrated;

[0029] Figure 10 shows the axisymmetric magnetic field model for the alternative ECR thruster of Figure 7, with areas of different magnetic field strength illustrated;

[0030] Figure 11 is a graph showing how the thrust varies for different propellant mass flow rates for both the ECR thruster of Figures 1 and 2 and the unmodified thruster;

[0031] Figure 12 is a graph showing how the specific impulse varies for different propellant mass flow rates for both the ECR thruster of Figures 1 and 2 and the unmodified thruster;

[0032] Figure 13 is a graph showing how the thrust efficiency varies for different propellant mass flow rates for both the ECR thruster of Figures 1 and 2 and the unmodified thruster;

[0033] Figure 14 is a graph showing the thruster floating potential for different propellant mass flow rates for both the ECR thruster of Figures 1 and 2 and the unmodified thruster; Figure 15 is a graph showing the ion current density measured at a range of rotation angles for different propellant mass flow rates;

[0034] Figure 16 is a graph showing how the thrust varies for different thruster powers for both the ECR thruster of Figures 1 and 2 and the unmodified thruster;

[0035] Figure 17 is a graph showing how the specific impulse varies for different thruster powers for both the ECR thruster of Figures 1 and 2 and the unmodified thruster; Figure 18 is a graph showing how the thrust efficiency varies for different thruster powers for both the ECR thruster of Figures 1 and 2 and the unmodified thruster; Figure 19 is a graph showing the thruster floating potential for different thruster powers for both the ECR thruster of Figures 1 and 2 and the unmodified thruster; Figure 20 is a graph 2000 showing the ion current density measured at a range of rotation angles for different thruster powers; and Figure 21 is a graph 2100 showing how the measured thrust varies with background pressure for both the ECR thruster of Figures 1 and 2 and the unmodified thruster.

[0036] Detailed description

[0037] When in a magnetic field, electrons rotate around the magnetic field lines at their cyclotron frequency. The electron cyclotron frequency is directly proportional to the local magnetic field strength. Resonant heating of the electrons occurs when the frequency of the microwaves matches the cyclotron frequency of the electrons, this is called electron cyclotron resonance (ECR). When using a microwave frequency of 2.45 GHz, ECR occurs at a magnetic field strength of 875 Gauss. By correctly sizing a magnet, the resonance region can be positioned within a chamber of a thruster, intersecting the microwave antenna, as shown in Figure 3. When gaseous propellant is injected into the chamber, free electrons enter the resonance region and are heated. These high temperature electrons collide with neutral particles and ionise them. These electrons are confined to the magnetic field lines, rotating tightly around them, this prevents the electrons from colliding with the chamber walls. The magnetic field gradient in the magnetic nozzle acts to accelerate these electrons out of the chamber. The potential difference created by this mass exodus of electrons acts to accelerate the ions out of the chamber after the electrons. This is known as ambipolar acceleration.

[0038] The energy transfer from the microwaves to the propellant is highly dependent on the resonance region. Until now, the design of ECR magnetic nozzle plasma thrusters has not accounted for the thickness of the resonance region, often assuming it to be near zero. However, as electron cyclotron resonance is the primary mechanism for heating the propellant, the effect of the volume of the resonance region on thruster performance is significant. An ECR magnetic nozzle thruster typically uses microwaves at a single frequency, usually 2.45 GHz. From equation 1 it can be seen that the driving microwave frequency, f , defines the magnetic field strength at which ECR occurs.

[0039] .ft

[0040] Where B is the magnetic field strength in Tesla at which the electrons are resonantly heated by the microwaves, meis the mass of an electron in kilograms, f is the driving microwave frequency in Hertz and e is the elementary charge in Coulombs.

[0041] As meand e are constants, it follows that the location of the ECR region in the magnetic field, B, is solely dependant on the driving microwave frequency, f . A zero bandwidth driving microwave frequency therefore produces a single, finite magnetic field strength at which ECR occurs, producing an ECR surface of zero thickness and volume. However no signal has zero bandwidth, instead there is a range in driving microwave frequency that produces a range in magnetic field strength at which ECR occurs.

[0042] The range of frequencies outputted by a microwave generator is defined by its -3 dB bandwidth. While magnetron microwave generators have -3 dB bandwidths on the scale of hundreds of MHz, modern solid-state microwave generators have -3 dB bandwidths of less than 0.1 MHz. This reduces the theoretical thickness of the resonance region by a factor of one thousand, from approximately 1 millimetre to 1 micrometer for the thruster described in the embodiments below. By decreasing a microwave signal's bandwidth, its amplitude is increased so that total power remains constant. Therefore, decreasing the thickness of a thruster's resonance region by decreasing the bandwidth of the microwave signal will increase the power density available within the resonance region.

[0043] The thruster's magnetic nozzle acts to accelerate the electrons downstream, away from the thruster. However, this charge separation creates an ambipolar electric field that acts to pull the electrons back upstream towards the thruster. The higher magnetic field strength upstream then repels the electrons via the magnetic mirror force. This bouncing of electrons between the magnetic mirror and the electrostatic field creates high velocity electrons in the resonance region. These high velocity electrons are subject to a doppler shifted driving microwave frequency. The large range in electron velocities has the effect of artificially broadening the microwave bandwidth, thickening the resonance region. The thickness of the resonance region due to Doppler broadening can be calculated using equation 2.

[0044] Where AXD is the resonance region thickness due to Doppler broadening in meters, v|| is the electrons mean axial velocity in meters per second, fc is the electron cyclotron frequency in Hertz, Bo is the magnetic field strength at resonance in Tesla and 3B / 3x is the gradient of the magnetic field in Tesla per meter. Due to the Doppler broadening, electrons can gain energy in regions surrounding the thin resonance region, effectively increasing the thickness of the resonance region.

[0045] Figures 1 and 2 show a cross-sectional and cut-through perspective view respectively of a new design of ECR thruster 100 according to the present invention. The ECR thruster 100 comprises a coaxial connector 102, for example a 7 / 16 coaxial connector. This is connected to a coaxial line comprising an inner conductor 108, which may be made of copper, a dielectric layer 106, which may be made of Boron Nitride and an outer conductor 104. The ECR thruster 100 comprises a magnetic field source, which is this embodiment is a permanent ring magnet 110. The ring magnet 110 may be a Samarium Cobalt magnet of grade Sm2Col7 28 / 20. Other kinds of magnet or electromagnets may be used.

[0046] The ECR thruster 100 comprises a body 114, which may be made of aluminium. The body 114 may comprise a tube or pipe defining a propellant inlet 116. Although only one propellant inlet 116 is shown in Figure 1, multiple propellant inlets may be provided. The ECR thruster 100 comprises a thruster chamber 125 into which propellant is introduced via one or more propellant injectors 122. In some embodiments, eight propellant injectors 122 are disposed around the base of the thruster chamber at equal angular intervals. The thruster chamber 125 has a base or backplate 126, which may be made of Boron Nitride. The ECR thruster 100 also comprises a frontplate 128, which may be made of Isostatic graphite. The frontplate 128 may also form the side walls of the thruster chamber 125. The frontplate 128 may be secured to the body 114 by a number of bolts, visible in Figure 2. The ECR thruster 100 also comprises a circular o-ring 118, which may be made of rubber. This forms an airtight seal between the body 114 and the frontplate 128, to prevent any gaseous propellant from escaping. A channel 120 may be defined between the body 114 and the frontplate 128. The channel 120 forms a propellant manifold to allow propellant to flow from the propellant inlet(s) 116 to the propellant injectors 122.

[0047] Arrow 'A' in Figure 1 defines a rearward direction, such that the thruster chamber 125 is located at the rear of the ECR thruster 100, thrust is produced in a rearward direction and the coaxial connector 102 is located at the front of the ECR thruster 100.

[0048] In the centre of the thruster chamber 125 is an antenna 124, which may be made of Isostatic graphite. The antenna 124 is connected to the inner conductor 108 and is configured to emit radio frequency radiation into the thruster chamber 125. The ECR thruster 100 may be circularly symmetric about a central axis (axisymmetic). The antenna 124 is located along the central axis and the thruster chamber 125 is centred on the central axis. The coaxial connector 102 may be connected to a solid-state microwave generator configured to generate Radio Frequency radiation. The frequency of the radiation may be in the microwave range. The frequency of the microwaves may be tuneable in order to optimise the thruster performance. The frequency may be tuned dynamically to account for changes in temperature during operation. In some embodiment a frequency of between 2.40 GHz and 2.50 GHz is used. In some embodiments, the input frequency may be centred at 2.45 GHz. The -3 dB bandwidth of the microwaves may be less than 0.1 MHz.

[0049] The ring magnet 110 is located forwards of the thruster chamber 125. The magnetic field generated by the ring magnet 110 is present inside the thruster chamber 125. The thruster chamber 125 is located relative to the ring magnet 110 such that there is a region inside the thruster chamber 125 where the magnetic field strength causes ECR to occur, for a given input frequency.

[0050] The ECR thruster 100 also comprises a ferromagnetic structure, which in the embodiments of Figures 1 and 2 is an iron ring 112. The iron ring 112 may be made of pure DT4 grade iron. Other ferromagnetic materials or alloys could be used in place of pure iron. The iron ring 112 is positioned relative to the ring magnet 110 so as to increase a volume of the resonance region within the thruster chamber 125. The iron ring 112 may be disposed forwards of the thruster chamber 125, but may also have a height sufficient to overlap with the base of the thruster chamber 125. The iron ring 112 may have an inner diameter which is larger than an outer diameter of the thruster chamber 125 such that the thruster chamber 125 is partially recessed inside the iron ring 112.

[0051] The iron ring 112 may be secured directly to the rearwards facing surface of the ring magnet 110. In some other embodiments, an insulator may be provided between the ring magnet 110 and the iron ring 112. The iron ring 112 is also centred on the central axis of the ECR thruster 100.

[0052] The improvements in performance which have been achieved with the ECR thruster 100 shown in Figures 1 and 2 will now be discussed. Figure 3 is a graph showing a comparison of the magnetic field strength, measured along the central axis of the thruster chamber 125, for an unmodified thruster having a thin resonance region and the ECR thruster 100 of the present invention, having a thick resonance region. First trace 302 shows how the magnetic field strength for the ECR thruster 100 of the present invention varies with displacement from the back of the thruster chamber 125. Second trace 304 shows how the magnetic field strength for an unmodified thruster varies with displacement from the back of the thruster chamber 125. The axial position of the thruster chamber that provided the greatest performance was determined experimentally. For the unmodified thruster, the base of thruster chamber was located 30 mm from the ring magnet and no ferromagnetic structure is present. For the ECR thruster 100 with the iron ring 112, the base of the thrust chamber was located 22 mm from the ring magnet 110.

[0053] The dashed line in Figure 3 shows the magnetic field strength at which resonance occurs, 875 Gauss (0.0875 T), given the input frequency of 2.45 GHz. As can be seen, the gradient of the magnetic field strength, 3B / 3x, is less for the ECR thruster trace 302 than for the unmodified thruster trace 304.

[0054] For both the unmodified ("thin") thruster and the ECR thruster 100 ("thick"), the axial magnetic field strength gradient at resonance, 3B / 3x, can be calculated. From this, the doppler broadened resonance region thickness, AXD, can be calculated, using equation (2) and assuming an axial electron velocity of 3.0 x 106m / s. These values are shown in table 1.

[0055] As the rate of change of magnetic field strength gradient is relatively small, we can make the assumption that magnetic field strength gradient remains constant within the resonance region. This allows for the calculation of the lower and upper bounds of the doppler broadened resonance regions, see equations (3) and (4).

[0056] (3) (4)

[0057] Where BDI and BDUare the lower and upper bounds of the doppler broadened resonance region in Tesla, Bo is the magnetic field strength at resonance in Tesla, dB / dx is the gradient of the magnetic field in Tesla per meter and AXD is the resonance region thickness due to Doppler broadening in meters.

[0058] Table 1

[0059] The size of the doppler broadened resonance region can be visualised as the region between the lower and upper bounds of the doppler broadened resonance region BDI and BDU. This is shown as the red regions 406, 502 in Figures 4 to 10. This assumes that there is a negligible change in magnetic field strength gradient with radial displacement. Although the range of magnetic field strengths over which resonance occurs is greater in for the unmodified ("thin") thruster, because the magnetic field strength gradient is lower in the ECR thruster 100 ("thick"), and this region of lower magnetic field strength gradient is positioned over the resonant magnetic field strength, the total resonance volume inside the thruster chamber is increased, leading to an increase in the thrust, specific impulse and thrust efficiency. As can be seen in Table 1, the gradient of the magnetic field strength in the resonance region for the ECR thruster 100 is 0.73 T / m compared with a gradient of 1.68 T / m for the unmodified thruster. Depending on the exact design used in the ECR thruster 100 and the operating conditions (e.g. ambient magnetic field, temperature etc.), the gradient of the magnetic field strength may be less than 0.75 T / m, less than 0.8 T / m or less than 1 T / m. This represents a significant flattening of the magnetic field gradient at and around the resonant frequency when compared to an unmodified thruster, leading to a thickening of the resonance region and an increase in the volume of the resonance region.

[0060] Figure 4 is an axisymmetric magnetic field model of an unmodified thruster 400. Only the key elements of the thruster 400 are shown. The thruster 400 has a ring magnet 404 and a thruster chamber 402. The base of the thruster chamber 402 is located 30 mm from the top of the ring magnet 404, such that the resonance region 406 is located within the thruster chamber 402. The thickness of the resonance region 406 along the central axis is approximately 8mm (see Table 1). Regions 408 are the resonance regions which lie outside of the thruster chamber 402.

[0061] Figure 5a is an axisymmetric magnetic field model of the ECR thruster 100 of Figures 1 and 2. Again only the key elements of the thruster 100 are shown. The thruster 100 has a ring magnet 110, a thruster chamber 125 and an iron ring 112 located between the ring magnet 110 and the thruster chamber 125. The base of the thruster chamber 125 is located 22 mm from the top of the ring magnet 110, such that the resonance region 502 is located within the thruster chamber 125. The resonance region 502 is thickened in several places, such that the total volume of the resonance region 502 is increased. The thickness of the resonance region 502 along the central axis is approximately 12mm (see Table 1). Regions 504 are the resonance regions which lie outside of the thruster chamber 125.

[0062] Figure 5b shows the axisymmetric magnetic field model of Figure 5a with exemplary dimensions marked. The inner diameter of the iron ring 112 is 104mm and the outer diameter of the thruster chamber 125 is 74mm. The base of the thruster chamber 125 is located 22 mm from the surface of the ring magnet 110 and the thruster chamber 125 is 20mm high. These dimensions are exemplary and dependent on the topology of the magnetic field produced by the ring magnet 110 and the input frequency of electromagnetic radiation.

[0063] A Samarium Cobalt Magnet is just one example of a suitable permanent magnet to act as magnetic field source. Samarium Cobalt magnets have a high maximum working temperature of approx. 300 °C. Since the magnet is not mechanically connected to the thrust chamber, low temperatures of well under 100 °C were observed during testing. Therefore a Neodymium magnet could be used instead of the Samarium Cobalt magnet. The higher magnetic coercivity of the Neodymium magnet allows it to be smaller while creating a slightly larger resonance region 602. Figure 6 shows a comparison of the ECR thruster design using the Samarium Cobalt magnet 110 on the left and a NdFeB (N50H) grade Neodymium magnet 600 on the right. The smaller size and lower mass density of the Neodymium magnet results in a lower magnet mass, see table 2. The thruster can also be made slightly more compact. A small increase in the height of the iron ring is required, increasing its mass from 0.23 kg to 0.26 kg. coercivity, JB8X working tempSC 120

[0064] Table 2

[0065] Figure 7 shows an alternative ECR magnetic nozzle thruster 700 according to the present invention. The alternative ECR thruster 700 has the same ring magnet 110 and thruster chamber 125 as the previous embodiment. The ferromagnetic structure comprises a first ring 702 made of a ferromagnetic material. Again, this may be pure iron or another ferromagnetic metal or alloy. The first ring 702 has an annular protrusion 703 extending radially inwards. The annular protrusion 703 may be disposed approximately half way between the base and top of the first ring, or approximately one third of the way between the base and the top of the first ring. The exact position of the annular protrusion 703 which produces the greatest thickening of the resonance region 706 will depend on the other dimensions of the ECT thruster 700, the strength of the ring magnet 110 and the input frequency of electromagnetic radiation. The annular protrusion 703 extends underneath the base of the thruster chamber 125.

[0066] The ECR thruster 700 also comprises a second ring 704 made of a ferromagnetic material. Again, this may be pure iron or another ferromagnetic metal or alloy. The second ring 704 is arranged concentrically with the first ring 702. The height of the second ring is less than the distance between the ring magnet 110 and the base of the thruster chamber 125, such that the second ring 704 is disposed entirely underneath the base of the thruster chamber 125. A can be seen, the alternative arrangement of the ferromagnetic structure shown in Figure 7 produces a different resonance region 706 inside the thruster chamber 125, which may further increase the volume of the resonance region 706.

[0067] Other arrangements of ferromagnetic structure are contemplated. For example, a thruster may comprise first and second rings arranged concentrically. Neither ring may have any annular protrusion. Alternatively, the outer ring may have an annular protrusion, as shown in Figure 7. Alternatively or in addition, an annular protrusion extending radially outwards may be provided on the outer ring. The inner ring may also have one or more annular protrusions directed inwardly or outwardly. The annular protrusions may be located at any axial position on the inner and / or outer ring. For example, the annular protrusions may be located at the top and / or bottom of the ring, such that the ring has an "L" or "T" shaped cross-section. In some other embodiments, three or more rings may be arranged concentrically. Each ring may have one or more annular or radial protrusions as described above.

[0068] Figures 8, 9 and 10 illustrate how the resonance region is thickened by being stretched between two regions or relatively lower magnetic field strength. Figure 8 shows the axisymmetric magnetic field model for the unmodified thruster 400 of Figure 4, with areas of magnetic field strength below, within and above the resonance strength illustrated. The ring magnet produces an area 802 of lower magnetic field strength which is centred on the central axis, above the surface of the ring magnet 404 facing the base of the thruster chamber 402. The resonance region 406 along the central axis of the thruster is stretched towards this area 802, as indicated by the double headed arrow. However, there is only one point of stretching.

[0069] Figure 9 shows the axisymmetric magnetic field model for the ECR thruster 100 of Figures 1, 2, 5a and 5b, with areas of magnetic field strength below, within and above the resonance strength illustrated. The presence of the iron ring 112 cases an annular area 902 of relatively low magnetic field strength adjacent the inner surface of the iron ring 112. This decreases the magnetic field gradient towards the edges of the thruster chamber 125, resulting in a stretching of the resonance region, such that it occupies a greater proportion of the thruster chamber 125. Thus, an additional point of stretching is introduced.

[0070] Figure 10 shows the axisymmetric magnetic field model for the ECR thruster 700 of Figure 7, with areas of magnetic field strength below, within and above the resonance strength illustrated. The presence of the annular protrusion 703 introduces a third area 1002 of lower magnetic field strength and therefore a third point of stretching. The second ring 704 may have the effect of decreasing the already present magnetic filed low 1004 around the central axis of the ring magnet 110, to increase the stretching effect here and / or to balance the increased stretching effect produced by the first ring.

[0071] As discussed above, by modifying the geometry of the ferromagnetic structure even further, more regions of low magnetic field strength can be introduced, further stretching the resonance region.

[0072] The ECR magnetic nozzle thruster 100 has also been tested with Krypton, Argon and Water Vapour as propellants and found to work well with these. As the thruster 100 is electrodeless, almost any gaseous propellant can be used.

[0073] The strong magnetic field generated by the ring magnet can introduce significant torque on a satellite as it orbits the Earth's magnetic field. This torque can be negated if an identical magnet is placed on the satellite in a mirrored position. Modelling has shown that placing such a magnet 400 mm away from the thruster's magnetic circuit results in a negligible change in magnetic field topology at the thruster. Alternatively, another identical thruster could be provided in a mirrored position on the opposite side of the satellite to the first.

[0074] An ECR magnetic nozzle thruster 100 according to the designs shown in Figures 1 and 2 and having the dimensions shown in Figure 5b has been manufactured and tested. The ECR thruster 100 tested comprised a boron nitride backplate 126 and a 2.5mm diameter isostatic graphite antenna 124 that was 20mm long. The thruster chamber 125 comprised an isostatic graphite chamber, 74mm in diameter and 20mm long. Xenon was injected into the bottom of the thruster chamber 125 through eight rectangular holes of 1mm x 2mm dimensions. The magnetic nozzle was generated by a Samarium Cobalt permanent magnet (Sm2Col7 28 / 20) with an outer diameter of 132mm, inner diameter of 18mm and height of 27mm.

[0075] Figures 11-21 are graphs illustrating the results obtained from these tests. Figures 11 to 13 and 16 to 18 show the measured thrust performance. The thruster being tested was mounted to a torsional thrust balance, the displacement of which was proportional to the thrust generated by the thruster.

[0076] Figure 11 is a graph 1100 showing how the thrust (in mN) varies for different propellant mass flow rates for both the ECR thruster 100 (solid circles) and the unmodified thruster (empty circles). Figure 12 is a graph 1200 showing how the specific impulse varies for the same range of mass flow rates for both the ECR thruster 100 (solid circles) and the unmodified thruster (empty circles). Figure 13 is a graph 1300 showing how the thrust efficiency varies for the same range of mass flow rates for both the ECR thruster 100 (solid circles) and the unmodified thruster (empty circles). Three readings were taken for each mass flow rate tested. The ECR thruster 100 was not tested at 0.59 mg / s flow rate. As can be seen, the thrust, specific impulse and thrust efficiency are all increased for the ECR thruster tested compared to the unmodified thruster at all mass flow rates.

[0077] The thruster becomes positively charged as electrons are accelerated out of the thruster. This positive potential accelerates positive ions out of the thruster which generates the thrust. The larger the floating potential of the thruster, the more ions are accelerated and the higher the velocity of these ions, increasing thruster performance. The floating potential of the thruster was measure by connecting a multimeter to the thruster. Figure 14 is a graph 1400 showing the thruster floating potential for different propellant mass flow rates for both the ECR thruster 100 (solid circles) and the unmodified thruster (empty circles).

[0078] Figure 15 is a graph 1500 showing the ion current density measured at a range of rotation angles for different propellant mass flow rates. Ion measurements were taken with a faraday probe located 427 mm from the thruster exit. The rotation angle corresponds to the angular displacement of the probe from the central axis of the thruster. The dashed lines show the results from the unmodified thruster while the solid lines show the results from the ECR thruster 100. The mass flow rates tested were 0.34 mg / s, 0.39 mg / s, 0.44 mg / s, 0.49 mg / s and 0.54 mg / s. The traces on the graph 1500 are labelled accordingly.

[0079] It can be seen in Figure 15 that the ion current density near the central axis is increased for the ECR thruster tested compared to the unmodified thruster at all mass flow rates. The unmodified thrust traces can be seen to produce a higher ion current density at high rotation angles. This is a disadvantage, as it shows that the ion plume has high divergence, which reduces the thruster's total efficiency. The ion plume produced by the ECR thruster 100 can be seen to be more concentrated around the central axis.

[0080] The greatest increase in ion current density demonstrated by the ECR thruster 100 over the unmodified thruster can be seen at the lowest mass flow rate of 0.34 mg / s. This is advantageous, as the lower mass flow rates provide the highest performance. Figure 16 is a graph 1600 showing how the thrust (in mN) varies for different thruster powers for both the ECR thruster 100 (solid circles) and the unmodified thruster (empty circles). The thruster power is a measure of the input power of the microwave generator. Figure 17 is a graph 1700 showing how the specific impulse varies for the same range of thruster powers for both the ECR thruster 100 (solid circles) and the unmodified thruster (empty circles). Figure 18 is a graph 1800 showing how the thrust efficiency varies for the same range of thruster powers for both the ECR thruster 100 (solid circles) and the unmodified thruster (empty circles). Three readings were taken for each thruster power tested. As can be seen, the thrust, specific impulse and thrust efficiency are all increased for the ECR thruster tested compared to the unmodified thruster at all thruster powers.

[0081] Figure 19 is a graph 1900 showing the thruster floating potential for different propellant thruster powers for both the ECR thruster 100 (solid circles) and the unmodified thruster (empty circles).

[0082] Figure 20 is a graph 2000 showing the ion current density measured at a range of rotation angles for different thruster powers. Ion measurements were taken with a faraday probe located 427 mm from the thruster exit. The rotation angle corresponds to the angular displacement of the probe from the central axis of the thruster. The dashed lines show the results from the unmodified thruster while the solid lines show the results from the ECR thruster 100. The input thrust powers tested were 30 W, 61 W, 91 W, 119 W and 143 W. The traces on the graph 2000 are labelled accordingly.

[0083] It can be seen in Figure 20 that the ion current density near the central axis is increased for the ECR thruster tested compared to the unmodified thruster at all thrust powers tested. The unmodified thrust traces can be seen to produce a higher ion current density at high rotation angles. This is a disadvantage, as it shows that the ion plume has high divergence, which reduces the thruster's total efficiency. The ion plume produced by the ECR thruster 100 can be seen to be more concentrated around the central axis.

[0084] The greatest increase in ion current density demonstrated by the ECR thruster 100 over the unmodified thruster can be seen at the highest thruster power of 143 W. This is advantageous, as the highest thruster powers provide the highest performance. ECR magnetic nozzle thrusters are sensitive to background pressure. At higher background pressures, more electrons collide with neutral particles, reducing the strength of the ambipolar electric field and therefore the ion acceleration. This effect should be taken into account when attempting to predict the in-space performance of these thrusters. Figure 21 is a graph 2100 showing how the measured thrust varies with background pressure for both the ECR thruster 100 (solid circles) and the unmodified thruster (empty circles). The background pressure was artificially increased by introducing an auxiliary flow of Xenon into the vacuum chamber housing the thruster. This test was conducted using a mass flow rate of 0.39 mg / s and a thruster power of 145 W. The decrease in thrust for higher background pressures can be seen. It can also be seen that the thrust is increased for the ECR thruster 100 over the unmodified thruster for all background pressures.

Claims

Claims1. An Electron Cyclotron Resonance, ECR, magnetic nozzle thruster comprising: a thruster chamber; an antenna disposed at least partially within the thruster chamber and configured to emit radio frequency radiation into the thruster chamber; a magnetic field source configured to generate a magnetic field within the thruster chamber, the magnetic field comprising a resonance region; and a ferromagnetic structure, wherein the ferromagnetic structure is positioned relative to the magnetic field source so as to increase a volume of the resonance region within the thruster chamber.

2. The ECR magnetic nozzle thruster of claim 1, wherein the magnetic field source is disposed forward of the thruster chamber.

3. The ECR magnetic nozzle thruster of claim 1 or claim 2, wherein the ferromagnetic structure is disposed rearward of the magnetic field source.

4. The ECR magnetic nozzle thruster of any preceding claim, wherein the magnetic field source comprises at least one permanent ring magnet.

5. The ECR magnetic nozzle thruster of claim 4, wherein the at least one permanent ring magnet is a Samarium Cobalt magnet.

6. The ECR magnetic nozzle thruster of claim 4 or claim 5, wherein the ferromagnetic structure is secured directly to a rearward facing surface of permanent ring magnet.

7. The ECR magnetic nozzle thruster of any preceding claim, wherein the ferromagnetic structure comprises at least one iron ring.

8. The ECR magnetic nozzle thruster of claim 7, comprising a central axis and wherein: the magnetic field source comprises a permanent ring magnet; and the permanent ring magnet, thruster chamber and at least one iron ring are each centred on the central axis.

9. The ECR magnetic nozzle thruster of claim 7 or claim 8, wherein an inner diameter of the iron ring is larger than an outer diameter of the thruster chamber.

10. The ECR magnetic nozzle thruster of any of claims 7 to 9, wherein the iron ring overlaps the base of the thruster chamber such that the thruster chamber is partially recessed inside the iron ring.

11. The ECR magnetic nozzle thruster of any preceding claim, wherein the ferromagnetic structure comprises a first ring made of a ferromagnetic material, the first ring comprising an annular protrusion extending radially inwards.

12. The ECR magnetic nozzle thruster of claim 11, wherein the annular protrusion extends underneath the base of the thruster chamber.

13. The ECR magnetic nozzle thruster of claim 11 or claim 12, wherein the ferromagnetic structure comprises a second ring made of a ferromagnetic material, the second ring arranged concentrically with the first ring.

14. The ECR magnetic nozzle thruster of claim 11, wherein the second ring is disposed underneath the base of the thruster chamber.

15. The ECR magnetic nozzle thruster of any preceding claim, wherein the radio frequency radiation is between 2.40 GHz and 2.50 GHz.

16. The ECR magnetic nozzle thruster of claim 15, wherein the radio frequency radiation has a -3 dB bandwidth of less than 0.1 MHz.

17. The ECR magnetic nozzle thruster of claim 15 or claim 16, wherein the radio frequency radiation comprises multiple different input frequencies in the range 2.40 GHz to 2.50 GHz.

18. The ECR magnetic nozzle thruster of any preceding claim, wherein a gradient of the magnetic field strength within the resonance region is less than 1 T / m, less than 0.8 T / m or less than 0.75 T / m.

19. The ECR magnetic nozzle thruster of any preceding claim, comprising a body supporting the thruster chamber, wherein the thruster chamber comprises a backplate made of Boron Nitride and a frontplate made of Isostatic Graphite.

20. The ECR magnetic nozzle thruster of claim 19, wherein the body and frontplate define a fuel line for introducing a propellant into the thruster chamber.

21. The ECR magnetic nozzle thruster of claim 20, comprising a propellant tank in fluid communication with the fuel line, wherein the propellant is Xenon.

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