WO2017115023A1 - Closed system for generating a plasma beam with electron drift and thruster comprising such a system - Google Patents

Abstract

The system (1) for generating a plasma beam comprises at least one device (3) for generating a magnetic barrier consisting of a first magnetic pole (5) and a second magnetic pole (6), at least one extraction and acceleration grid (4) and a device for alternating polarisation of the extraction and acceleration grid (4), said first and second magnetic poles (5, 6) of the device (3) for generating the magnetic barrier being arranged coaxially with respect to one another, in such a way as to create an intermediate space (7) between same, presenting a closed loop cross-section, said intermediate space (7) being intended for the circulation of the flow of particles that are used to form the plasma beam, said architecture of the system (1) allowing the generated electron drift to close in on itself, and thus preventing any interaction with the walls of the system (1).

Description

 A closed electron beam and propellant plasma beam generation system comprising such a system.

The present invention relates to a plasma beam generation system (highly electronegative plasma or ion-ion plasma) and a propeller, including space, comprising such a system.

 It is known that various techniques, such as space propulsion, but also the etching of semiconductor devices, magnetically confined fusion, biotechnology, implement electrically neutral plasma beams.

 The invention is described below in application to a thruster (including space).

 It is known that plasma beams can be generated by different methods.

 A first method consists in extracting and accelerating positive ions out of a plasma by means of positively polarized grids in order to create a potential drop with respect to the ambient medium and then to neutralize the flux of said positive ions thus obtained by a beam of auxiliary electrons to form an electrically neutral plasma beam. This neutralization is necessary to avoid an accumulation of negative charges that would oppose the accelerating field.

 A conventional space propeller which implements such a method, therefore comprises a neutralizer to obtain a generally electrically neutral particle jet and this after the acceleration of the particles. This neutralizer is a source of complexity, loss of reliability, reduction of the life of the propulsion unit (strong erosion), loss of efficiency (propellant flow dedicated to the neutraliser and clean electric consumption), and additional propellant .

Of course, this first method has the disadvantage of requiring such a neutralizer. To remedy this drawback, a second method is known, described for example in the documents WO 2007/065915 and WO 2010/060887. This second method consists in extracting and accelerating positive ions and negative ions out of a plasma by means of at least two grids respectively negatively polarized and positively, and combining the flow of said positive ions and the flow of said negative ions to to form an electrically neutral plasma beam. This second method has the advantage of an extremely fast recombination of positive ions and negative ions, especially if they come from the same source, but can be difficult to implement because of the simultaneous presence of grids. opposite polarizations.

 Document FR-2965697 (or US-13 / 825,913) discloses a method for forming a plasma beam by extracting and accelerating electrically charged particles from a plasma using at least one extraction and acceleration grid subjected to an electric bias, which overcomes this disadvantage. According to this method, the extraction and acceleration grid is biased with alternately positive and negative potentials so that the plasma beam is at least approximately electrically neutral. Thus, the grid extracts plasma and alternately accelerates the positive particles (that is to say the positive ions) and the negative particles (which may be negative ions or electrons), said positive and negative particles then combining to to form an electrically neutral plasma beam.

 A thruster implementing the latter characteristics therefore allows, via a successively positive and negative particle beam, to obtain a generally neutral propulsion jet and thus eliminates the need for a neutralizer.

The highly electronegative plasma thrusters of the latter type generally use a magnetic barrier to simultaneously filter the electrons from the electronegative discharge and increase the rate of production of the negative ions, so as to have a plasma having a very low electronic density upstream of the acceleration system. In a standard architecture, the magnetic field is generated perpendicular to the axis of the thruster with a maximum of intensity close to the acceleration zone.

 The combination of the magnetic field with the electric field related to the potential drop and / or with the pressure gradient, creates a drift of the electrons.

 The consequence of this drift, in the case of a standard thruster architecture, is the creation of an electron current directed towards the wall and a short circuit for the electronic fluid. Thus, the plasma becomes highly asymmetrical.

 This asymmetry due to electronic drift (which can not be removed) generates a drop in propeller efficiency, an asymmetric jet, and potentially other negative effects such as a decrease in propeller service life due to erosion. accelerated walls and a decrease in the density of negative ions that are destroyed by collision with electrons.

 The present invention aims to overcome this disadvantage. It relates to a plasma beam generation system having a particularly advantageous architecture that makes it possible to prevent the negative effects of electronic drift (which can not be suppressed).

 For this purpose, according to the invention, said system of the type comprising at least:

 a device for generating a magnetic barrier comprising a first magnetic pole and a second magnetic pole;

 at least one extraction and acceleration grid; and

 an alternative polarization device of the extraction and acceleration grid,

is remarkable in that said first and second magnetic poles of the device for generating the magnetic barrier are arranged coaxially with one another, so as to create between them a space intermediate having a closed loop in cross section, said intermediate space being intended for the circulation of particle streams which are intended to form the plasma beam.

 In the context of the present invention, said plasma beam is based on highly electronegative plasma or ion-ion plasma.

 Thus, thanks to the invention, by the particular arrangement of the two magnetic poles of the magnetic barrier, creates said intermediate space which forms a closed loop for the electronic current. The flow of electrons is thus no longer projected towards the wall but rotates in this intermediate space (or cavity). This allows to close the drift on itself, and thus to cancel the interaction with the walls (or surfaces). This new architecture of the plasma beam generation system eliminates, via the closure of the drift on itself, the aforementioned negative effects (yield reduction, asymmetric jet and erosion of the wall) of a standard standard architecture.

 Advantageously, the plasma beam generation system has at least partially a longitudinal axis, and the device for generating the magnetic barrier is configured so that the magnetic flux generated by said device is directed radially with respect to the longitudinal axis of said plasma beam generation system.

 Preferably:

 said first and second magnetic poles are arranged coaxially with said longitudinal axis; and

 said intermediate space has an annular shape in cross section.

 Furthermore, advantageously, the device for generating the magnetic barrier comprises at least one of the following sets:

 - a set of magnets;

 - a set of coils.

Preferably, said device for generating the magnetic barrier also comprises a magnetic circuit. In addition, to reduce a generated heat flow, at least one of said first and second magnetic poles, namely at least one that is arranged inwardly of the system, is provided with thermal reduction means, including thermal drains.

 Furthermore, advantageously, said extraction and acceleration grid has a general disk shape, and it is arranged orthogonally to the longitudinal axis, at a downstream end, covering at least said intermediate space.

 The present invention also relates to a propellant, in particular space, strongly negative plasma or ion-ion plasma, this thruster comprising a plasma beam generation system as described above.

 The present invention further relates to a plasma beam generation method. Said method, of the type comprising at least:

an electronegative plasma generation step;

 a step of generating a magnetic barrier, using a first magnetic pole and a second magnetic pole; and

 a step of extraction and acceleration,

is remarkable, according to the invention, in that, for the implementation of the step of generating the magnetic barrier, coaxially arranged the first and second magnetic poles for generating the magnetic barrier, one by relative to each other, so as to create between them an intermediate space having a closed loop in cross section, said intermediate space being intended for the circulation of particle flows intended to form the plasma beam.

 The appended figures will make it clear how the invention can be realized. In these figures, identical references designate similar elements.

Figure 1 is a partial view, in perspective, of a particular embodiment of a plasma beam generation system according to the invention and partially cut. Figures 2 and 3 are diagrams illustrating electronic drifts respectively open in the case of a standard architecture and closed in the case of an architecture according to the invention.

 The system (hereinafter "system 1") for illustrating the invention and diagrammatically shown in FIG. 1, is intended to generate a plasma beam.

 In the context of the present invention, the plasma beam is based on highly electronegative plasma or ion-ion plasma.

 More particularly, although not exclusively, the system 1 may be part of a (electric) thruster, in particular a space thruster (intended for the propulsion of a spacecraft).

 This system 1 is of the type comprising, in general:

 a device 2 for generating an electro-negative plasma (with a cylindrical insulating cavity 8 downstream);

a device 3 for generating a magnetic barrier (or field); and

at least one extraction and acceleration grid 4, to which is associated a device (not shown) for alternating polarization of said extraction and acceleration grid 4.

 The device 3 generates a magnetic barrier for both filtering the electrons from the electronegative discharge (device 2) and increasing the rate of production of negative ions, so as to obtain a plasma for which the electron density is very low upstream of the acceleration system (grid 4).

Usually, the extraction and acceleration grid 4 (or a set of grids) is biased alternately, positively and negatively, by the electric polarization device. This electric polarization device comprises, for example, a fast switch type MOSFET able to quickly switch two opposite electrical polarities, without significant potential overshoots. Thus, the extraction and acceleration grid 4 extracts plasma and alternately accelerates the positive particles (that is to say the positive ions) and the negative particles (which can negative ions or electrons), said positive and negative particles then combining to form an electrically neutral plasma beam outside the system 1.

 A satisfactory combination of positive and negative particles is obtained when the frequency of the alternation of positive and negative potentials is a radio frequency between a few kHz and a few MHz. The number of positive and negative particles extracted and accelerated depends on the duration and amplitude of the positive and negative potentials applied to the extraction and acceleration grid 4. As a result, by setting the duration and / or of the amplitude of the positive potentials and / or of the negative potentials, one can act on the quality of the electrical neutrality of the plasma beam. Such an adjustment can be performed in real time, for example by means of a grid current analysis or by means of an external probe observing the plasma beam.

 At the exit of the system 1, therefore, positive ions and negative ions appear alternately (but almost simultaneously), which combine and form the electrically neutral plasma beam (generated without the aid of a neutralizer), which is emitted in the opposite direction. illustrated by an arrow F in FIG.

 It is known that the combination of the magnetic field B with the electric field E linked to the potential drop and / or with the pressure gradient creates an electron drift - ExB or diamagnetic - at a speed Vd.

The drift, in the case of a standard (thruster) architecture, is illustrated in FIG. 2. In such a standard architecture, the space intended for the electron flow is delimited by walls P01 to P04 forming, in section cross-section, a rectangle or a square. In this case, the speed drift VdO created by the combination of the magnetic field BO (directed from the North pole N to the South pole S) and the electric field E0, is directed towards the wall P01. The plasma becomes highly asymmetrical and this asymmetry has negative repercussions on the overall performance of the system: the efficiency drops and the ion beam is inhomogeneous. To overcome these disadvantages (of a standard architecture), the system 1 has an advantageous architecture for closing the drift on itself in order to cancel the interaction with the surfaces, as specified below with reference to FIG. 3.

 More specifically, to do this, according to the invention, said magnetic poles 5 and 6 of the device 3, which are intended to form the magnetic barrier, are arranged coaxially with one another, so as to create between them an intermediate space 7 having a closed loop in cross section, as shown in Figure 1. This intermediate space 7 thus forming an annular-type cavity is intended for the flow of the particle stream used to form the plasma beam.

 As represented in FIG. 1, the system 1 has (at least partially) a longitudinal axis XX, and the device 3 is configured so that the magnetic barrier (or field) B1 that it generates is directed radially with respect to the XX longitudinal axis of the system 1, as shown in Figure 3.

 To do this, the two magnetic poles 5 and 6 are arranged coaxially with said longitudinal axis X-X, as shown in FIG.

 The magnetic pole 5 is arranged radially on the outside (with respect to the X-X axis), and the magnetic pole 6 is arranged radially in the center, along the X-X axis. Thus arranged, the two magnetic poles 5 and 6 create between them the intermediate space 7 which then has an annular shape in cross section in the example of Figure 3, the intermediate space 7 being delimited by two walls P1 and P2.

 Since the electrons mainly drift in the direction perpendicular to the fields B1 and E1, the system 1 has a magnetic topology configured to turn the electrons around the longitudinal axis XX of the annular system 1 (as illustrated by arrows Vd1 on the Figure 3).

In the context of the present invention, to obtain this closed electronic drift, it is sufficient that in cross section, the intermediate space 7 forms a closed loop. Preferably, as shown in FIG. 1 and described above, the intermediate space 7 is made in the form of a closed loop forming a ring with walls P1 and P2 which, in cross-section, form two concentric circles (with as center point representing in cross section the longitudinal axis).

 Thus, by the particular arrangement of the two magnetic poles 5 and 6 of the magnetic barrier, it creates, in the system 1 as described above, said intermediate space 7 which preferably represents an annular cavity. The flow of electrons is thus no longer projected towards the wall, but turns in this space (or cavity) intermediate 7. This allows to close the drift on itself, and thus to cancel the interaction with the walls ( or surfaces). This new architecture of the plasma beam generation system 1, thus eliminates, through the closure of the drift on itself, the negative effects (yield reduction, asymmetric jet and erosion of the wall) of a standard standard architecture.

 To form the magnetic poles 5 and 6, the device 3 for generating the magnetic barrier comprises at least one of the following sets:

- a set of magnets;

 - a set of coils.

 Preferably, said device 3 also comprises a magnetic circuit.

 In addition, to reduce a generated heat flow, the magnetic pole 6 which is arranged towards the inside of the system 1, is provided with means (not shown) of thermal reduction, preferably thermal drains.

Furthermore, as shown in FIG. 1, the extraction and acceleration grid 4 has a disc shape, and it is arranged orthogonal to the longitudinal axis XX, at a downstream end (with respect to the direction of the arrow). F), covering at least said intermediate space 7 (to be able to act on the entire flow of particles flowing in said intermediate space 7). FIG. 1 illustrates a possible application of the invention to the example of a highly electronegative or ion-ion plasma radiofrequency thruster 10 (with thus a closed electron drift). In that case :

 - The radial magnetic barrier B1 (directed along the spokes of the cylinder) is produced using magnets or coils with or without magnetic circuit.

The magnetic poles 5 and 6 are located, respectively, outside and inside the intermediate space 7;

 the ions are accelerated by means of a set of polarized grids 4 (alternatively), which is placed downstream of the maximum of the magnetic field; and the electronegative discharge is generated at the bottom of a cavity. This cavity has insulating walls (for example dielectric ceramic). The gas is injected either through orifices or using a porous wall. Plasma is produced via ICP flat antenna radiofrequency discharge. The plasma can be generated via a DC discharge (ie electron bombardment) or by microwave injection.

 The system 1, as described above, implements in particular the following steps E1 to E3:

 E1 / an electronegative plasma generation step, implemented in the usual manner by the device 2;

E2 / a step of generating a magnetic barrier, implemented by the device 3 comprising the magnetic poles 5 and 6, said magnetic poles 5 and 6 being arranged coaxially, with respect to each other, of so as to create between them the intermediate space 7 having a closed loop in cross section and for the circulation of particle streams which are used to form the plasma beam; and

 E3 / a step of extraction and acceleration, implemented at least by the extraction and acceleration grid 4, which is associated with the alternative biasing device of said grid.

 With the system 1, as described above, in addition to the aforementioned advantages:

the plasma and the beam are symmetrical in the azimuthal direction; and the confinement of the electrons is improved, and therefore the conversion efficiency of the molecules into negative ions is increased.

Claims

A plasma beam generation system, said system (1) comprising at least: a device (3) for generating a magnetic barrier comprising a first magnetic pole (5) and a second magnetic pole (6);  at least one extraction and acceleration grid (4); and  an alternative biasing device for the extraction and acceleration grid (4), characterized in that said first and second magnetic poles (5, 6) of the device (3) for generating the magnetic barrier are arranged coaxially with one another so as to create an intermediate space between them (7) having a closed loop in cross section, said intermediate space (7) being intended for the circulation of particle streams which are intended to form the plasma beam.  2. System according to claim 1, characterized in that it has at least partially a longitudinal axis (X-X), and in that the device (3) for generating the magnetic barrier is configured so that the magnetic flux generated by said generating device (3) the magnetic barrier is directed radially with respect to said longitudinal axis (XX).  3. System according to claim 2, characterized in that said first and second magnetic poles (5, 6) are arranged coaxially with said longitudinal axis (X-X).  4. System according to any one of the preceding claims, characterized in that said intermediate space (7) has an annular shape in cross section.  5. System according to any one of the preceding claims, characterized in that the device (3) for generating the magnetic barrier comprises at least one of the following sets: - a set of magnets; a set of coils;  6. System according to claim 5, characterized in that the device (3) for generating the magnetic barrier comprises a magnetic circuit.  7. System according to any one of the preceding claims, characterized in that at least one (6) of said first and second magnetic poles, which is arranged towards the inside of the system (1), is provided with means of thermal reduction.  8. System according to any one of the preceding claims, characterized in that said extraction and acceleration grid (4) has a general disk shape, and it is arranged orthogonally to a longitudinal axis (XX), to a downstream end, covering at least said intermediate space (7).  9. System according to any one of the preceding claims, characterized in that said plasma beam is based on highly electronegative plasma or ion-ion plasma.  10. Plasma thruster, characterized in that it comprises a system (1) for generating plasma beam, according to any one of claims 1 to 9.  11. A plasma beam generation method, said method comprising at least:  an electronegative plasma generation step;  a step of generating a magnetic barrier, using a first magnetic pole (5) and a second magnetic pole (6); and a step of extraction and acceleration; characterized in that, for the implementation of the step of generating the magnetic barrier, the first and second magnetic poles (5, 6) for generating the magnetic barrier are coaxially arranged, one with respect to the other, so as to create between them an intermediate space (7) having a closed loop in cross-section, said space intermediate (7) being intended for the circulation of particle streams which are intended to form the plasma beam.