Artificial satellites generally use auxiliary engines or thrusters to carry out trajectory or attitude correction maneuvers. In the same way, space probes intended for the exploration of the solar system have thrusters that allow them to position themselves very precisely around a planet, or even to land on an asteroid in order to take samples of material from it.
As a general rule these thrusters, known as chemical or propellant thrusters, provide thrusts of a few Newtons at most by using liquid propellants such as hydrazine (N2H2) or hydrogen peroxide (oxygenated water). During the decomposition of these propellants, the chemical energy is converted into heat then into thrust during the expansion of the hot gases in a nozzle. The main drawbacks of these chemical thrusters are that their specific impulse is limited, that the propellants needed to operate them represent half of the total mass of the satellite and that their high propellant consumption limits the service life of the satellite.
In order to make it possible for space missions to go further and be of longer duration, recent years have seen the emergence of plasma thrusters which have the advantage over chemical thrusters that they provide greater specific impulses, significantly increase the mass of the payload compared with the mass of the propulsion system as well as the service life of the satellite. Their main drawbacks, as will be seen, are the lack of reliability of the ignition, in particular when the propellant gas pressure is low, their limited service life due to the ion bombardment of certain elements, and their miniaturization limitations for their use for example on miniature satellites. It should be noted that if their energy yield, although better than that of chemical thrusters, were improved, even further or longer missions could be envisaged.
Plasma thrusters can be classified in different ways according to whether their mode of igniting the plasma or the mode of accelerating the plasma towards the outlet of the nozzle is taken into consideration. It should be noted that these two criteria are relatively independent of one another and just as important as one another. In fact, the ignition mode determines the completeness of the ionization of the propellant gas and the reliability of this ignition, thus the reliability of the thruster, and can determine the size of the plasma discharge chamber, the space requirement, weight and energy yield of the thruster. With respect to the mode of accelerating the plasma, this determines the thrust, the specific impulse and the energy yield, and can determine the space requirement, weight and service life of the thruster.
If their mode of igniting the plasma is taken into consideration as the classification criterion, a first category of plasma thruster is the thruster known as an “arcjet” thruster, as described in patent application U.S. Pat. No. 5,640,843, the principle of which is the ignition of the plasma by an electric arc in the jet of propellant gas. The advantage of this category of thruster is that, all things being equal otherwise, it provides higher thrusts than other types of plasma thrusters, but it has the following major drawbacks: these thrusters have a low specific impulse compared with that of other plasma thrusters; consume a lot of electric current; have a limited service life due to the bombardment of the electrodes and of the internal walls of the discharge chamber by the ions and electrons which reach temperatures in the order of a few thousands to a few tens of thousands of degrees; require the excess heat to be evacuated into space, which results in a poor energy yield. Moreover, the ignition of plasma when the partial pressure of propellant gas is low lacks reliability.
According to this same criterion, a second category of plasma thruster is that of plasma thrusters that ignite their plasma solely by the resonance of an electromagnetic (EM) wave, often a microwave, in a discharge chamber containing a propellant gas to be ionized. The major drawback of the thrusters of this category is the relatively low energy yield since only a small fraction of the EM energy is absorbed by the plasma. Moreover, the ionization of the propellant gas is rarely complete, in particular when the propellant gas flow rate is high, and the ignition of the plasma lacks reliability when the partial pressure of propellant gas is low.
According to this same criterion, a third category of plasma thruster is that of plasma thrusters using “gyromagnetic resonance” of the magnetized free electrons of the plasma or ECR (“Electron Cyclotron Resonance”). As the application of a magnetic field to the plasma causes its free electrons to spin in one and the same determined direction and at one and the same determined frequency, the plasma can theoretically be ignited there, then sustained with an energy yield equal to 1 by the total absorption of an electromagnetic wave, the electric field of which rotates at the same speed and in the same direction as these magnetized electrons. In order to maximize this energy yield in practice, the length of the discharge chamber is substantially equal to an integer number of the half-wavelength of the electromagnetic wave in a vacuum, which poses the problem of miniaturization of the discharge chamber and therefore of the thruster. In fact, in order to be able to increase the resonance frequency of the EM wave while still having ECR conditions, it is necessary to increase correlatively the intensity of the magnetic field, which initially presupposes the use of powerful electromagnetic coils but the space requirement and weight of these coils runs counter to the objective of miniaturizing the thruster. This miniaturization problem is complicated moreover by the multiplicity of sources having to emit into the discharge chamber: propellant gas source, EM wave source and magnetic field source. Patent EP 0 505 327 describes such a thruster. Other technical fields also use ECR plasma sources, such as for example that of the production of integrated circuits. Patent application US 2005 0 287 describes an ECR ion source, equipped with electromagnetic coils, for ion implantation in microelectronics. The use of electromagnetic coils results in a significant weight and space requirement for a relatively low energy yield because of the losses by Joule effect, which is ill-suited to a use as a space thruster. Moreover, the ionization of the propellant gas is rarely complete, in particular when the propellant gas flow rate is high, and the ignition of the plasma lacks reliability when the partial pressure of propellant gas is low. Finally, these thrusters often suffer from the existence of parasitic plasma jets directed upstream known under the name ion pump effect.
Whatever the manner in which their plasma is ignited, plasma thrusters can also be classified according to the second criterion, which is the mode of accelerating the plasma in the nozzle.
According to this second criterion, a first family is that of plasma thrusters known as “electrostatic”, which are characterized by the electrostatic nature of the force accelerating the plasma towards the outlet of the nozzle. This family can in turn be divided into three categories: accelerator grid thrusters, Hall effect thrusters and field effect thrusters.
The category of accelerator grid thrusters is characterized by the fact that the ions coming from a discharge chamber are accelerated by a system of electrically polarized grids. It should be noted that the ejected plasma is not electrically neutral. Accelerator grid thrusters have the following drawbacks, which limit their effectiveness and service life: the positive ion beams passing through the accelerator grid erode it, which limits the service life of these thrusters; the ejected ions recombine with the ejected electrons and generate obscuring deposits of material on the solar panels of the satellites on which they are fitted; the discharge chamber must have a large volume; the energy yield is relatively low because of plasma leaks at the walls of the discharge chamber and the accelerator grid; and the thrust is limited by the limitation of the density of the ions inside the grids due to the secondary electrons. Examples of accelerator grid thrusters are given in patent applications JP 01 310 179 and US 2004/161579 A1, in patent U.S. Pat. No. 7,400,096 B1, and in the article by MORRISON N. A. et al. “High rate deposition of ta-C:H using an electron cyclotron wave resonance plasma source”, published in THIN SOLID FILMS, ELSEVIER-SEQUOIA S. A. LAUSANNE, C H, vol. 337, no. 1-2, 11 Jan. 1999 pages 71-73, XP004197099, ISSN: 0040-6090, DOI: 10.1016/S0040-6090 (98) 01187-0 and the article by NISHIYAMA K ET AL.: “Microwave power absorption coefficient of an ECR Xenon ion thruster”, SURFACE AND COATINGS TECHNOLOGY, ELSEVIER, AMSTERDAM, NL, vol. 202, no. 22-23, 30 Aug. 2008 (2008-08-30), pages 5262-5265, XP025875510, ISSN: 0257-8972, DOI: 10.1016/J SURFCOAT.2008.06.069.
The category of Hall effect thrusters is characterized by a cylindrical anode and a negatively charged plasma. Hall effect thrusters use the drift of the charged particles in crossed magnetic and electric fields. Their drawbacks are, on the one hand, the presence of a continuous electric field which entails polarized electrodes and, on the other hand, the limitation in terms of plasma density which is linked to the formation of sheaths around these electrodes which oppose the penetration of the continuous electric field into the plasma, unlike the ultra high frequency field which easily penetrates into the ionized medium, hence the benefit of ultra high frequency (UHF) discharges. The document US 2006/290287 describes such a thruster.
The category of field effect thrusters is characterized by the ionization of a metallic liquid, its acceleration, then its electrical neutralization.
According to this second criterion, a second family is that of plasma thrusters known as “electromagnetic”. This family can be divided into six categories: pulsed inductive thrusters, magnetoplasmadynamic thrusters, electrodeless thrusters, electrothermal thrusters, helicon double layer thrusters and μgradB thrusters.
The category of pulsed inductive thrusters is characterized by an acceleration for discontinuous time intervals.
The category of magnetoplasmadynamic thrusters is characterized by electrodes which ionize the propellant gas and generate in it a current which, in turn, generates a magnetic field which accelerates the plasma via the Lorentz force.
The category of electrodeless thrusters is characterized by the absence of electrodes, which eliminates a weak point for the service life of plasma thrusters. The propellant gas therein is ionized in a first chamber by an EM wave, then transferred into a second chamber where the plasma is accelerated by inhomogeneous and oscillating electric and magnetic fields that generate a force known as ponderomotive. Patent U.S. Pat. No. 7,461,502 describes such a thruster. A drawback of this category of thrusters is their use of electromagnetic coils to generate the oscillating magnetic field, because their space requirement, their weight and their loss of energy by Joule effect, which are all relatively high, are ill-suited to space applications.
The category of electrothermal thrusters is characterized by the heating of the plasma to temperatures in the order of a million degrees, then the partial conversion of this temperature into axial speed. These thrusters require high-power electromagnetic coils to generate very intense magnetic fields in order to be able to confine a plasma, the electrons of which have very high speeds because of their temperature. Besides the space requirement and the weight of these coils, their heat dissipation by Joule effect noticeably degrades the energy yield of these thrusters. Patent U.S. Pat. No. 6,293,090 describes such a thruster, more precisely it relates to a radio frequency (RF) thruster using a lower hybrid resonance (absorption of energy by coupling of a very low frequency UHF wave via a combined oscillation of the ions and the electrons of the plasma) of the VASIMR (Variable Specific Impulse Magnetoplasma Rocket) type, where the plasma is not heated by resonance of its electrons, as is generally the case for thrusters of this category, but by excitation of its ions by a high-power EM wave.
The category of helicon double layer thrusters is characterized by the injection of the propellant gas into a tubular chamber around which an antenna is wound which emits an electromagnetic wave of a power high enough to ionize the gas then generate, in the plasma created in this way, a helicon wave which further increases the temperature of the plasma.
The category of “μgradB” thrusters, also called “space charge field” thrusters, is characterized by the diamagnetic nature of their force. Chapter 5.1 of the book “Physique des plasmas, cours et application” by J.-M. Rax thoroughly explains the theory of the movement of an electron excited by a UHF electromagnetic field in a static or slowly variable magnetic field. On page 152, in particular, the presence is described of a convergence or a divergence of the induced field lines and therefore of a force in the direction of this field, proportional to the μ magnetic moment and to the gradient of this magnetic field. This force is called “μgradB” or diamagnetic force. The thruster that forms the subject of the present patent application is effectively based on entirely “conventional” physical principles explained in the course of this chapter, the adiabaticity hypotheses mentioned on page 153 for the invariance of the μ magnetic moment being largely satisfied in the case of the invention. This book does not, however, disclose how to design a plasma thruster sustaining the plasma by ECR, the size of which can be reduced relative to the half-wavelength of the electromagnetic wave and the reliability of the ignition of which is improved even under conditions where the partial pressure of propellant gas is very low. The article by STALLARD B. W. ET AL.: “Whistler-driver, electron-cyclotron-resonance-heated thruster: experimental status”, JOURNAL OF PROPULSION AND POWER 1996 July-August AIAA, vol. 12, no. 4, July 1996 (1996-07), pages 814-816, XP008133752 describes a diamagnetic force thruster, the plasma of which is ignited and sustained by electron waves generated by an EM wave, with a frequency lower than the gyromagnetic frequency, emitted by two helically coiled antennas, and by a magnetic field, generated by electromagnetic coils, with an intensity greater than the ECR intensity. The propellant gas is injected into a zone where the magnetic field has decreased below the ECR intensity. It raises the problem of the incomplete ionization of the propellant gas of this thruster. In order to limit this incompleteness of this ionization, the gas chamber is segmented. Despite this precaution, although it is explained that the ionization becomes more complete when the gas flow rate decreases, it remains incomplete even for low flow rates. Nor is any disclosure made with respect to improving the reliability of the ignition for very low flow rates of propellant gas or means of reducing the size of this thruster.
None of the plasma thrusters of the state of the art combine, at the same time, the advantages of a reliable ignition (systematic and instantaneous ignition) and a complete ionization under all electromagnetic wave power and propellant gas flow rate operating conditions, in particular for very low flow rate and very low propellant gas partial pressure; the absence of a parasitic plasma jet directed upstream; a discharge chamber with a reduced size relative to the half-wavelength of the EM wave used to sustain the plasma; the ability to operate at magnetic field intensities that allow the use of permanent magnets, thus avoiding the space requirement, the weight and the losses by Joule effect of electromagnetic coils; making possible a controlled variation of the thrust and of the specific impulse; the ability to achieve an energy yield close to 1; accelerating a neutral plasma, thus not needing to be neutralized; and the service life of which is not limited by the wear of parts by the plasma nor by the depositing of propellant gas on the solar panels.
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