The invention relates to electric propulsion (EP) systems.
EP systems provide small amounts of thrust by high-speed ejection of accelerated ions from an ion engine, and find application in areas such as satellite and space-probe propulsion and satellite station-keeping. The ejected ions act as a propellant in the same way as the combustion products of a chemical rocket. Although the absolute amount of thrust produced by an EP system is very small compared to that of a chemical rocket, the very high velocity with which ions are ejected from the ion engine of an EP system means that the amount of thrust per unit mass flow rate is very large compared to that of a chemical rocket. For example, the Boeing® 702 EP system produces a thrust of 165 mN and has a mass flow rate of approximately 4.4 mg s−1, corresponding to an approximate propellant ejection velocity of 37.5 km s−1. In contrast, a main hydrogen/oxygen engine on a NASA space shuttle produces a thrust of the order of 2 MN and has a mass flow rate of approximately 700 kg s−1, combustion products being expelled at velocity of 2.8 km s−1.
Thrust range and resolution are important characteristics of EP systems. For example a field-effect EP (FEEP) system typically produces several μN of thrust and is capable of μN resolution. The maximum thrust level is however very limited unless multiple systems are employed in parallel. A gridded ion engine system (GIE), can produce a thrust of several tens of mN but thrust resolution is often limited to 10 μN. Furthermore it is usually not possible to reduce the thrust of a GIE below a certain minimum level. This is due to the fact that thrust control is achieved by control of the ion generation process—a relatively high power and inherently difficult process—and it is not possible to control and sustain ion generation to the extent that the thrust is zero.
In some applications it is advantageous for EP systems to produce thrusts on the order of mN with sub-μN resolution and which also have the ability to throttle down from mN thrust levels to zero. Applicant's co-pending application published as WO 2008/009938 proposes an electric propulsion system in which an acceleration and a screen grid are located at an ion output aperture, and whereby the potential between the two grids is varied to control the expulsion of ions, and hence thrust from a plasma chamber. In one embodiment two such ion apertures are arranged about a single plasma chamber to produce substantially anti-parallel thrusts, which can be varied substantially independently.
According to a first aspect of the present invention there is provided an electric propulsion system comprising a plasma chamber having first and second apertures for producing ion beams; a first coil arranged about the chamber and adapted to produce an electromagnetic field in a first region of the chamber adjacent to said first aperture; a second coil arranged about the chamber and adapted to produce an electromagnetic field in a second region of the chamber adjacent to said second aperture; and an RF drive module adapted to drive said first and second coils differentially.
By driving the coils differentially, the electric field in the region of the two apertures can be differentially controlled, and a variation of output thrusts at the two apertures is possible. In this way a net thrust can be produced, which net thrust is varied by controlling the drive to the two coils.
The first and second apertures in one embodiment are arranged to produce ion beams in directions which are substantially anti-parallel. In this way the net thrust remains along a fixed axis, and in certain arrangements its magnitude can be controlled by the differential driving of the two coils as described above.
More complex embodiments may include one or more additional apertures, and one or more corresponding coils arranged around the chamber and adapted to produce an electromagnetic field in a region of the chamber adjacent each such additional aperture. In such embodiments the RF drive module is adapted additionally to provide differential control to each additional coil. More commonly apertures and coils will exist in pairs, and differential control is provided between pairs of coils.
In certain embodiments the drive module is adapted to control the forward power and additionally or alternatively the loss to said first and second coils. Although the signal feed to each coil can be controlled independently, in embodiments of the invention it is not strictly true to consider that the coils are independently controlled due to coupling effects between them. For example a capacitance in a matching circuit for the drive path for a first coil could be adjusted to vary the loss to that coil, but coupling between the coils could result in some change also to the signal observed in the second coil. Nevertheless differential drive is achieved and the ion beams from the corresponding first and second apertures respond differently to the adjustment.
The difference in response of the ion beams to the control of the coils results from non-uniformity, or asymmetry of the plasma density in the plasma chamber.
According to a second aspect of the invention therefore, there is provided a method of operating an electric propulsion system comprising creating a discharge plasma in a plasma chamber; extracting at least two ion beams from said plasma chamber, each ion beam generating a thrust; and controlling an electromagnetic field in the chamber to produce an asymmetry in the plasma density, which asymmetry differentially varies the thrusts of said ion beams.
The electromagnetic field is advantageously controlled to produce a difference in plasma density in the regions from which said at least two ion beams are extracted, and in one embodiment such control can be provided by generating the electromagnetic field in the chamber using at least two differentially controllable coils arranged around said chamber.
The invention extends to methods, apparatus and/or use substantially as herein described with reference to the accompanying drawings.
Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, method aspects may be applied to apparatus aspects, and vice versa.
Preferred features of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which:
Turning to
At the centre of the chamber an annular distributor 112 allows gas flow into the chamber as part of a plasma generation process. Conducting coils 114 and 116 are provided about the chamber and driven by an rf signal to provide an electric field in the chamber which sustains the plasma generation. Coils 114 to the left of the distributor as shown are provided separately from coils 116 occupying a corresponding position on the right, and separate sets of connections are provided for each separate coil.
It is desirable in certain situations to operate using only one side of the chamber, and in such circumstances a gate can be inserted into one of the positions 118, 120. In position 118 for example, the left side of the chamber is isolated from the distributor, and coils 114 are typically not driven or left open circuit, while the device operates using the right side of the chamber and the right aperture only. Using a gate at position 120 allows the left chamber and aperture to be used in an equivalent fashion.
The coils used in generating and sustaining the plasma in the chamber are driven as illustrated in
With reference to
There is a clear correlation between Coil 2 power variation and Beam 2 current (measured at the screen grid of end 2 of the device) which produces a measurable change in probe current (actual Beam 1 thrust). For the reverse case variation of Coil 1 power produces a variation in Beam 1 current but no change in the actual Beam 1 thrust, the expected thrust variation arising at Beam 2. The implication is that there is strong coupling in the system such that the coil on one side of the chamber affects the plasma on the other side.
The interaction between opposite coils and the resultant actual thrust suggests that there is a strong reflection of power by one coil power into the other which produces a level of ionisation in the opposite chamber. The associated beam current induced on the screen grid remains coupled with the input coil power and not with the reflected power and region of increased ionisation. Hence rise in beam current associated with an increasing coil power does not produce an increase in actual output beam current from this side of the chamber.
Neglecting effects of beam divergence and ion species in the extracted beam, the screen grid current, IB, can be related to thrust, F, by the following relationship:
Where Ar is the relative atomic mass of Xenon (0.13129 kg), NA is Avogadro's constant (6.022×1023 atoms/mol), e is electron charge and V is the beam voltage.
It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention. Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.
Number | Date | Country | Kind |
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0823391.8 | Dec 2008 | GB | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/GB2009/002902 | 12/17/2009 | WO | 00 | 7/19/2011 |