EP2433002B1 - Hall effect plasma thruster - Google Patents

Description

Field of the invention

The present invention relates to a Hall effect plasma thruster comprising a main annular ionization and acceleration channel having an open downstream end, at least one cathode, an annular anode concentric to the main annular channel, a pipe and a distributor. for supplying ionizable gas to the channel and a magnetic circuit for creating a magnetic field in said main annular channel.

The invention relates in particular to Hall effect plasma thrusters used for the electric propulsion of satellites.

Prior art

The life of Hall effect plasma thrusters is essentially determined by the erosion of the ceramic insulating channel under the effect of ion bombardment. Indeed, due to the topography of the electrical potential in the channel, part of the ions created is accelerated radially towards the walls.

The extension of the missions of telecommunication satellites and the increase in plasma ejection speeds required (in particular for so-called high specific impulse thrusters) impose longer and longer lifetimes than ceramics can no longer satisfy. based on boron nitride.

The high resistance to ion bombardment of certain electrically conductive or semi-conductive materials such as graphite makes them ideal candidates for the discharge channel of Hall effect thrusters.

The idea of using conductive materials and graphite in particular has been studied in the US by Y. Raitses et al (Princeton University). These studies have noted the advantage of graphite in terms of lifetime, but have not attempted to solve the problem of yield reduction related to the short-circuiting of the plasma.

The low yields found with conductive materials have so far prevented the widespread use of them in the construction of plasma thruster acceleration channels.

Thus, the discharge channels of the Hall effect thrusters currently consist of homogeneous insulating ceramic, usually based on boron nitride and silica (BN-SIO _{2 materials} ). Boron nitride ceramics allow Hall effect thrusters to achieve high performance in terms of efficiency, but exhibit high erosion rates under ion bombardment that limit the life of the propellants to about 10,000 hours as well. as their operation at higher specific impulses.
The document US 2002/008455 A1 describes an example of a Hall effect plasma thruster.

Definition and object of the invention

The present invention aims to overcome the aforementioned drawbacks and in particular to increase the life of the Hall effect plasma thrusters while maintaining a high energy efficiency.

These objects are achieved, according to the invention, by means of a Hall effect plasma thruster comprising a main annular ionization and acceleration channel having an open downstream end, at least one cathode, an annular anode concentric with the annular channel. main, a pipe and a distributor for supplying ionizable gas to the channel and a magnetic circuit for creating a magnetic field in said main annular channel, characterized in that the main annular channel comprises internal and external annular wall portions located at the adjacent said open end each comprising an assembly of juxtaposed conductive rings or semi-conductors in the form of lamellae separated by thin layers of insulation.

Advantageously, each conductive or semiconductor ring is divided into segments arranged in angular sectors and isolated from each other.

Preferably, the segments of each conductive or semiconductor ring are arranged in staggered relation to the segments of neighboring conductive or semiconductor rings.

According to a preferred feature of the invention, the thin insulating layers are disposed on all sides of a conductive or semiconductor ring with the exception of the face defining a portion of the inner wall of the main annular channel.

The assembly of conductive or semiconductor rings may extend over a length of the inner and outer annular walls less than the total length of the main annular channel.

According to a particular embodiment, the conductive or semiconductor rings are made of graphite whereas the thin insulating layers are made of dielectric material and in particular of pyrolytic boron nitride.

The thickness of the conductive or semiconductor rings is of the order of the electronic Larmor radius.

où r est le rayon de Larmor des électrons, ainsi qu'une condition qui détermine l'angle de découpage azimutal : $$R.\alpha <5\mathit{abs}\left(\frac{\mathit{Ez}}{\mathit{Et}}\right).r$$

avec :

• ° Ez, Et : champ électrique le long de l'axe et de l'azimut,
• ° R : rayon de bord de la portion d'anneau en contact avec le plasma,
• °α : angle de la portion d'anneau

Their maximum thickness a is estimated by the following expression: $\mathrm{at}<\frac{8}{3}r,$

where r is the Larmor radius of the electrons, as well as a condition that determines the azimuth angle of cut: $$R.\alpha <5\mathit{abs}\left(\frac{\mathit{ez}}{\mathit{And}}\right).r$$

with:

• ° Ez, And: electric field along the axis and the azimuth,
• ° R: edge radius of the ring portion in contact with the plasma,
• ° α: angle of the ring portion

According to one exemplary embodiment, the conductive or semiconductor rings have a thickness of between 0.7 and 0.9 mm while the thin insulating layers have a thickness of between 0.04 and 0.08 mm.

According to the invention, a pseudo-insulating discharge channel is made from a stack of rings or portions of rings made of a conductive or semiconductor material and covered with a thin layer of insulating ceramic.

This allows an increase in the lifespan of the thruster by a factor of 3 to 4 without possible loss of efficiency, since the structure allows to benefit from the advantages of low erosion rate of conductive materials without the disadvantages, and the channel may behave as an electrical insulator to the plasma with maximum limitation of the electronic currents created in the discharge channel.

The invention thus optimizes the structure of the discharge channels of the Hall effect plasma thrusters by implementing a partitioning of conducting or semiconducting walls into segments. isolated small dimensions which results in a sharp decrease in the short-circuit current which avoids a significant loss of efficiency.

The propulsion of telecommunication satellites is associated with strong economic stakes and the improvements that can be made to Hall effect plasma sources - currently recognized as the best performing for station keeping - are of great interest. The present invention responds directly to the trend of increased mission times required of geostationary satellites by improving the longevity of Hall effect plasma thrusters.

The present invention also makes it possible to operate thrusters with higher specific pulses (Isp) while maintaining a significant service life. It can therefore provide a significant competitive advantage of Hall effect plasma thruster propulsion.

Brief description of the drawings

• la figure 1 est une vue schématique en perspective avec arrachement d'un propulseur à plasma à effet Hall auquel est applicable l'invention,
• la figure 2 est une vue en perspective d'un quart d'un canal de décharge avec structure lamellée selon un exemple de réalisation de l'invention,
• la figure 3 montre une variante proposée et est une vue en perspective de l'ensemble de la structure lamellée d'un canal de décharge d'un propulseur à plasma à effet Hall selon l'invention,
• la figure 3A montre une variante proposée et est une vue de détail agrandie d'un segment en matériau conducteur ou semi-conducteur recouvert de dépôts isolants utilisé dans la structure lamellée de la figure 3, et
• la figure 3B est une section selon la ligne IIIB-IIIB de la figure 3A.

Other characteristics and advantages of the invention will emerge from the following description of particular embodiments, given by way of example, with reference to the appended drawings, in which:

• the figure 1 is a diagrammatic perspective view with cutaway of a Hall effect plasma thruster to which the invention is applicable,
• the figure 2 is a perspective view of a quarter of a discharge channel with laminated structure according to an exemplary embodiment of the invention,
• the figure 3 shows a proposed variant and is a perspective view of the assembly of the laminated structure of a discharge channel of a Hall effect plasma thruster according to the invention,
• the figure 3A shows a proposed variant and is an enlarged detail view of a segment of conductive or semiconductor material covered with insulating deposits used in the lamellar structure of the figure 3 , and
• the figure 3B is a section according to line IIIB-IIIB of the figure 3A .

Detailed description of preferred embodiments

We see on the figure 1 an example of Hall effect plasma thruster, also called stationary plasma thruster (PPS), which is applicable to the invention and which can be implemented in particular for the electric propulsion of satellites.

• un canal de décharge ou canal annulaire principal d'ionisation et d'accélération 120,
• une anode annulaire 125 concentrique au canal annulaire principal 120,
• une canalisation 126 et un distributeur associé à l'anode 125 et au canal annulaire principal 120 pour alimenter celui-ci en un gaz ionisable tel que le xénon,
• une cathode creuse 140,
• un circuit magnétique 131 à 136 de création d'un champ magnétique dans le canal annulaire principal.

Such a type of Hall effect thruster comprises the following main elements:

• a discharge channel or main annular channel of ionization and acceleration 120,
• an annular anode 125 concentric with the main annular channel 120,
• a pipe 126 and a distributor associated with the anode 125 and the main annular channel 120 to supply it with an ionizable gas such as xenon,
• a hollow cathode 140,
• a magnetic circuit 131 to 136 for creating a magnetic field in the main annular channel.

The anode 125 and the ionizable gas distributor can inject the fuel (such as xenon) into the propellant and collect the electrons from the plasma discharge.

The hollow cathode 140 has the function of generating the electrons which allow the creation of a plasma in the propellant and the neutralization of the jet of ions ejected by the propellant.

The magnetic circuit comprises an internal pole 134, an external pole 136, a magnetic yoke connecting the internal 134 and outer 136 poles, with a central ferromagnetic core 133 and peripheral ferromagnetic bars 135, one or more coils 131 arranged around the central core 133 and coils 132 disposed around peripheral bars 135.

The magnetic circuit allows the confinement of the plasma and the creation of a strong magnetic field E at the output of the thruster which allows the acceleration of ions up to speeds of the order of 20 km / s.

Different variants are possible for the realization of the magnetic circuit and the present invention is not limited to the embodiment described on the figure 1 .

The discharge channel 120 allows the confinement of the plasma and its composition determines the performance of the propellant.

Traditionally, the discharge channel 120 is ceramic. The thrust of the engine is ensured by the ejection of a jet of ions at high speed. However, this jet being slightly divergent, the collision of high energy ions with the channel wall leads to erosion of the ceramic output of the propellant.

For this reason, according to the invention, the discharge channel 120 comprises at least a portion 127 of the inner annular wall and at least a portion 128 of the outer annular wall, located in the vicinity of the open end 129 of the channel, which are not made of solid ceramic, but which each comprise an assembly of conductive rings or semi-conductors 150 juxtaposed in the form of lamellae separated by thin layers of insulator 152 (see figure 2 ).

The object of the invention is to significantly reduce the erosion of the thruster discharge channel. It also reduces energy losses and discharge instabilities that usually affect Hall effect thrusters using a discharge channel of electrically conductive or semiconductor material. While using materials such as graphite and carbides more resistant than ceramics with respect to ion bombardment, thanks to an assembly of conductive or semiconductor rings (for example in graphite) separated by thin layers of insulation (for example boron nitride), the invention makes it possible at the same time to reduce the erosion of the channel and to reduce the instabilities of discharge.

The discharge channel 120 of a plasma thruster according to the invention can thus comprise both a traditional upstream ceramic part with a bottom wall 123 and outer cylindrical walls 121 and internal 122 and a downstream part located between the part upstream and the opening 129 and comprising outer cylindrical walls 128 and internal 127 with a laminated structure composed of juxtaposed conductive or semi-conducting rings 150, which are insulated by thin layers of insulator 152 but have an uncoated surface 151 of insulation on the inner side facing the inner space 124 of the annular channel 120.

In order to eliminate any azimuth short-circuit currents induced by potential variations along the azimuth symmetry, azimuthal waves, ...), preferentially, the rings 150 are furthermore positioned in a plurality of isolated angular sections each extending over an angular sector Δθ ( Figures 3 and 3A ). For example, it is possible to have between 10 and 30 segments 150a, 150b in each ring 150.

Advantageously, the segments 150a of a conductive or semiconductor ring 150 are arranged in staggered relation to the segments 150b of the neighboring rings 150 ( Fig. 3 ).

As can be seen on the figure 3A the thin insulating layers 152, 153, 154, 155 are disposed on all sides of a segment of a conductive or semiconductor ring 150 with the exception of the face 151 defining a portion of the inner wall of the main annular channel 120.

By way of example, the assembly of conductive rings 150 extends over a length of the inner and outer annular walls of between 20 and 50% and preferably between 30 and 40% of the total length of the main annular channel 120. but this range of values is not limiting.

The sizing of the conductive or semiconductor rings 150 can be established from the calculation of the electronic currents received and emitted by the walls. As a first approximation, it can be shown that the short-circuit current flowing in the walls is proportional to the ionic current collected, which at constant electronic temperature and plasma density is approximately proportional to the conductive surface in contact with the plasma.

Moreover, for a given axial electric field, the potential difference seen by a conductive element is approximately proportional to its axial extent. As a result, for a channel of a given size, all losses by Joule effect by short circuit of the plasma is approximately proportional to the thickness of the rings. It can also be shown that the short-circuit current becomes negligible in the currents related to the secondary electronic emission (which are the only ones that exist in the case of an insulator) when the thickness of the rings is of the order of electronic Larmor radius. This defines the critical thickness of the rings to obtain a pseudo-insulating channel.

For example, the conductive rings 150, for example made of graphite with a low coefficient of expansion, may have a thickness of between 0.7 and 0.9 mm and typically of 0.8 mm.

The thin insulating layers 152 to 155, for example made of pyrolytic boron nitride, can have a thickness of between 0.04 and 0.08 mm, typically 0.05 mm, and can be deposited on the segments of conductive rings. 150 by a chemical vapor deposition process so as to cover each ring segment over its entire surface except at the edge 151 in contact with the plasma.

Claims (11)

1. A Hall effect plasma thruster having a main annular channel (120) for ionization and acceleration that presents an open downstream end (129) and comprising inner (127) and outer (129) annular walls portions situated in the vicinity of said open end (129), each of which comprising an assembly of conductive or semi-conductive rings (150), at least one cathode (140), an annular anode (125) concentric with the main annular channel (120), a pipe (126) and a manifold for feeding the channel (120) with ionizable gas, and a magnetic circuit (131 to 136) for creating a magnetic field in said main annular channel (120), the thruster being characterized in that in the main annular channel (120) each of said inner and outer annular wall portions (127, 128) situated in the vicinity of said open end (129), comprises an assembly of juxtaposed conductive or semi-conductive rings (150) in the form of laminations separated by fine layers of insulation (152), the thickness of which being comprised between 4 and 12% of that of said conductive or semi-conductive rings (150).
2. A plasma thruster according to claim 1, characterized in that each conductive or semi-conductive ring (150) is subdivided into segments arranged as angular sectors and insulated from one another.
3. A plasma thruster according to claim 2, characterized in that the segments of each conductive or semi-conductive ring (150) are arranged in a staggered configuration relative to the adjacent conductive or semi-conductive ring segments (150).
4. A plasma thruster according to any one of claims 1 to 3, characterized in that the fine layers of insulation are arranged on all of the faces of a conductive or semi-conductive ring (150) with the exception of the face (151) defining a portion of the inside wall of the main annular channel (120).
5. A plasma thruster according to any one of claims 1 to 4, characterized in that the assembly of conductive rings (150) extends over a length of the inner and outer annular walls (127, 128) lying in the range 20% to 50% of the total length of the main annular channel (120).
6. A plasma thruster according to any one of claims 1 to 5, characterized in that the conductive or semi-conductive rings (150) are made of graphite.
7. A plasma thruster according to any one of claims 1 to 6, characterized in that the fine layers of insulation (152) are made of pyrolytic boron nitride.
8. A plasma thruster according to any one of claims 1 to 7, characterized in that the thickness of the conductive or semi-conductive rings (150) is of the order of the electron Larmor radius.
9. A plasma thruster according to claim 6, characterized in that the conductive or semi-conductive rings (150) present thickness lying in the range 0.7 mm to 0.9 mm.
10. A plasma thruster according to claims 4 and 7, characterized in that the fine layers of insulation (152) present thickness lying in the range 0.04 mm to 0.08 mm.
11. A plasma thruster according to claims 4 and 7, characterized in that the fine layers of insulation (152) are deposited on the conductive or semi-conductive rings segments (150) through a vapor phase chemical deposition process in order to cover each ring segment on all of its surface except on the face (151) in contact with the plasma defining a part of the inner wall of the main annular channel (120).

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