Laser-plasma accelerated matter ejection system, particularly for ion propulsion of spacecraft

Abstract

A matter ejection system (2), particularly for the ion propulsion of a spacecraft, includes a target (10); an illumination system (12) for illuminating a front face (10A) of the target (10) by means of laser pulses; an ion source (15); particle acceleration means (16) for accelerating an ion beam received from the ion source (15); a primary ion guide (18) including focusing means (19) for focusing the ion beam onto a focal point defined on a rear face (10B) of the target (10);an ejection zone (14) extending from the rear face (10B) of the target for the ejection of an ionic flux torn from the ion beam as the latter passes through a laser acceleration zone (35) at the focal point, under the effect of an acceleration normal to the target (10) induced by an electric field resulting from an interaction between the target (10), the laser pulses produced by the illumination system (12) and the ion beam. Figure for the abstract: Figure 2;

Description

Title of the invention: Laser-plasma accelerated matter ejection system, particularly for ion propulsion of spacecraft technical field

[0001] The present invention relates to the field of matter ejection systems, particularly ion propulsion systems such as those intended for spacecraft propulsion, and specifically concerns such a system, as well as a matter ejection method, particularly for ion propulsion, which employs laser-plasma acceleration techniques. The invention is particularly relevant to large spacecraft requiring, on the one hand, a high thrust to rapidly reach a target speed and, on the other hand, capable of leveraging a high specific impulse to achieve a high maximum speed. Prior art

[0002] In the field of space propulsion, chemical propulsion systems generally offer a large thrust but their low specific impulse limits the maximum speed achievable.

[0003] Electric thrusters (ionic and plasma) on the other hand offer a high specific impulse but a thrust generally so low that their use is hardly conceivable in cases requiring a high thrust per unit of power.

[0004] The distinctive feature of ion thrusters is that they draw on an external source of electrical energy to accelerate the ions of a gas stored in a reservoir. The resulting thrust depends on the mass flow rate of the ejected gas multiplied by the velocity of the ejected particles. It is the very intense electric field applied to the ionized gas that generates the acceleration necessary to achieve the ion ejection velocity. The improvement in specific impulse compared to a chemical process results from a much lower mass flow rate and a significantly higher ejection velocity.

[0005] The various existing electric thrusters, such as grid thrusters, Hall current thrusters, field emission thrusters (“FEEP”), electromagnetic thrusters and plasma thrusters, are designed to offer a high specific impulse compared with the chemical thrusters they aim to replace.

[0006] A problem related to the use of electric thrusters lies in their low thrust F per unit power P, which is a corollary of the high specific impulse Isp (as suggested by the equation F / P = 2.iy(g0.Isp), where q is the efficiency and the constant g0 is the acceleration due to gravity at the equator). Generally speaking, the greater the specific impulse of a thruster, the lower its thrust for a given power. This problem is well illustrated by the FEEP (Field-Emission Electric Propulsion) thruster, whose ion ejection velocity of around 1 km / s is very high, but whose thrust does not exceed one millinewton. This limitation is partly due to a structural limitation on the maximum mass flow rate of ions and partly to the maximum energy acceptable to the system for accelerating the ions.

[0007] Moreover, in the particular cases based on the implementation of femtosecond lasers, the maximum repetition rate of such a laser limits the mass flow rate of the propellant and consequently the maximum thrust of the propellant.

[0008] Finally, conventional femtosecond laser heavy-ion acceleration techniques are optimized to provide maximum ion acceleration at the expense of the maximum mass of ions ejected with each laser pulse. This therefore limits the mass flow rate for a given repetition rate, and thus also the maximum thrust.

[0009] In this context, the inventors have set themselves the goal of taking advantage of recent discoveries in the field of high-intensity light and its applications in the field of interactions with matter.

[0010] Among these discoveries, one concerns the use of frequency drift amplification lasers, also called CPA lasers (from the English "Chirped power amplification"), for the acceleration of heavy ions using the TNSA (from the English "Target Normal Sheath Acceleration") and BOA (from the English "Break-Out Afterburner") techniques.

[0011] The TNSA technique is the best-known technique for laser ion acceleration. The more recent BOA technique relies on the use of even more powerful lasers and specially designed targets.

[0012] In general, TNSA and BOA techniques allow the atoms of a solid target with a thickness close to the skin thickness to be accelerated in ion form for the laser wavelength. To achieve this, a CPA-type laser with a pulse exceeding 10¹⁸ W.cm⁻² for TNSA and 10²¹ W.cm⁻² for BOA is generally used. These acceleration principles utilize the promoter force of a laser pulse to interact with matter at the atomic level. The shock wave produced by the pulse generates an extremely strong electric field over a very small-scale matter to ultimately propel ions at very high speeds behind the target. On heavy ions such as gold or lead, the speed, and therefore the energy transmitted to the ions by the laser, can be on the order of 10 MeV, which corresponds to a speed far greater than that of ions ejected by a state-of-the-art electric thruster.

[0013] Another technical advance that can be used in the context of the invention lies in the improvements made to lasers within the framework of the ICAN (International Coherent Amplification Network) project, which proposes a specific CPA laser architecture implementing optical fibers in the amplification stage before the optical compressor. This type of laser is characterized not only by high light power compatible with TNSA or BOA acceleration but also by very high contrast and a repetition rate of up to 15 kHz, whereas conventional CPA lasers are limited to a few Hertz.

[0014] In general, TNSA and BOA techniques were developed to advance the science of particle acceleration to very high energies, but these techniques suffer from a very low quantity of ejected ions, which makes their use completely unsuitable for ion propulsion. The low quantity of ions is due to the fact that a large amount of the laser energy is used to create an electric field on the back face of the target that is intense enough to, on the one hand, ionize the material on the back of the target (up to 3 ionizations per atom), and, on the other hand, to be able to remove the ions thus created from the back face of the target.

[0015] In this context, an objective of the present invention is to use the energy of lasers to eject more matter but at a lower speed, in order to achieve thrust levels higher than known types of ion thrusters.

[0016] The invention thus aims generally to increase the mass flow rate and maximum power in order to increase the maximum thrust achievable by an electric thruster. More specifically, the invention aims to use laser-plasma interaction to generate a considerably greater ionic thrust than that obtained with known ion thrusters while maintaining the excellent specific impulse generally afforded by ion thrusters. Description of the invention

[0017] The invention proposes for this purpose a matter ejection system, comprising: • a target adapted to the implementation of a laser-plasma acceleration technique such as the TNSA or BOA technique; • an illumination system configured to illuminate a front face of the target using laser pulses; • a source of ions; • particle acceleration means having a first input configured to receive an ion beam from the ion source and an output to deliver an accelerated ion beam from the ion beam received from the first input; • a primary ion guide having an inlet connected to the output of the particle acceleration means to receive the ion beam delivered by the latter, an output, and guidance means configured to guide the ion beam from the inlet to the output of the primary ion guide and including focusing means configured to focus the ion beam exiting the primary ion guide through the output onto a focal point defined on a rear face of the target; • an ejection zone, free of obstruction, extending from the rear face of the target to allow the ejection, along an ejection axis passing through said focal point, of an ionic flux torn from the ion beam when the latter passes through a laser acceleration zone defined at the level of said focal point in front of the rear face of the target, under the effect of an acceleration normal to the target induced by an electric field resulting from an interaction between the target, the laser pulses produced by the illumination system and the ion beam.

[0018] The use of an ion beam focused on a point at the rear of the target generally allows for the redistribution of the classical electron recirculation topology inherent in the TNSA technique and its derivatives, such as the BOA technique, so as to move the electron front outside the target beyond the thickness of the focused ion beam. This results in the application of an ionic acceleration electric field to the volume of ions between the rear face of the target and the electron front. On the one hand, this volume is considerably larger than in a conventional TNSA configuration. On the other hand, the gaseous ions involved are much more mobile than the solid ions of the conventional TNSA technique. This has the effect of distributing the energy of each laser pulse over a larger mass of ions.Consequently, there are more accelerated ions, but these ions have less energy than in a conventional TNSA configuration. The maximum thrust resulting from each laser pulse is thus improved. The invention therefore significantly pushes back the maximum thrust limits of the best known ion thrusters.

[0019] The advantages of the invention can of course also be used for applications other than space propulsion, in which it is also desirable to generate a large flow of ejected material.

[0020] In preferred embodiments of the invention, the particle acceleration means have a second input configured to receive a recirculating ion beam, and in which the output of the particle acceleration means is configured to deliver the accelerated ion beam from the ion beams respectively received from the first and second inputs.

[0021] In preferred embodiments of the invention, the system comprises a secondary ion guide having an inlet arranged opposite the outlet of the primary ion guide so as to receive at least partially the ion beam after it has passed in front of the rear face of the target, an outlet connected to the second inlet of the particle acceleration means to reinject the ion beam received from the inlet of the secondary ion guide as said recirculating ion beam, and guiding means configured to guide the ion beam from the inlet to the outlet of the secondary ion guide.

[0022] A loop circulation of ions (with the exception of those torn away by the electric field induced by the laser pulses and thus participating in the thrust) makes it possible to keep in the propulsion system the ions which have not (yet) contributed to the propulsion, and thus to optimize the specific impulse.

[0023] In preferred embodiments of the invention, the focusing means are configured to switch on command between a focused mode, in which the focusing means focus the ion beam exiting the primary ion guide onto the rear face of the target, and a recirculating mode, in which the focusing means deflect the ion beam so as to direct it towards the inlet of the secondary ion guide.

[0024] The recirculation mode thus makes it possible to maximize the ions captured by the secondary ion guide to recirculate in the particle acceleration means.

[0025] In preferred embodiments of the invention, the primary ion guide includes means for deflecting the ion beam from a plane inclined with respect to the ejection axis to a plane comprising the ejection axis.

[0026] The elements arranged upstream of the primary ion guide, such as the ion source and the particle acceleration means, can thus be arranged in a plane inclined with respect to the ejection axis, so as to limit the overall size of the system.

[0027] In preferred embodiments of the invention, the ion source includes means controlled to vary the ion flow rate of said ion beam from the ion source.

[0028] In preferred embodiments of the invention, the lighting system comprises several lighting blocks collectively configured to illuminate the front face of the target by each emitting a part of said laser pulses along distinct respective convergence axes.

[0029] In preferred embodiments of the invention, the illumination blocks are configured to emit said laser pulses in a phase-shifted manner and thus multiply the repetition rate of the laser pulses received by the target.

[0030] In preferred embodiments of the invention, the system comprises a force distribution plate, on which the front face of the target is applied, and comprising a through recess to allow the laser pulses to pass through said force distribution plate and reach a target region defined on the front face of the target.

[0031] The invention also relates to a spacecraft comprising a system of the type defined above, configured so that said ejection of the ionic flux generates a propulsive thrust, whereby said system constitutes an ion propulsion system.

[0032] The invention also relates to a method for implementing a system of the type defined above, comprising the steps of: • A) implement the ion source, particle acceleration means and primary and secondary ion guides so as to direct an ion beam in front of the rear face of the target; • B) implement the illumination system so as to direct repeated laser pulses onto the front face of the target; • C) eject a stream of ions torn from the ion beam as it passes through a laser acceleration zone defined at said focal point in front of the rear face of the target, under the effect of an electric field induced by a laser-plasma interaction.

[0033] In preferred embodiments of the invention, step A includes implementing the focusing means in the focused mode in a manner synchronized with each laser pulse, and implementing the focusing means in the recirculating mode, between two consecutive laser pulses.

[0034] In preferred embodiments of the invention, the ejection of the ion stream in step C generates a thrust used to propel a spacecraft. Brief description of the drawings

[0035] The invention will be better understood, and other details, advantages and features thereof will become apparent from the following description, given by way of non-limiting example and with reference to the accompanying drawings in which:

[0036] [Fig-1] is a very schematic perspective view of a spacecraft comprising a ionic propulsion system according to a preferred embodiment of the invention;

[0037] [Fig.2] is a schematic perspective view of the ion propulsion system;

[0038] [Fig.2A] is a larger scale view of part of [Fig.2], illustrating an ion circulation system which is part of the ion propulsion system;

[0039] [Fig.3] is a schematic side view of the ion propulsion system;

[0040] [Fig.4A] is a partial schematic view, projected onto an XY plane, of a primary magnetic deflector which is part of the ion circulation system;

[0041] [Fig.4B] is a partial schematic view, projected onto an XZ plane, of the primary magnetic deflector;

[0042] [Fig.5] is a larger scale view of part of [Fig.4B], illustrating an output of the primary magnetic deflector;

[0043] [Fig.6] is a schematic view in projection in an XY plane, of a part of the ion circulation system, illustrating a primary magnetic lens and a secondary magnetic lens;

[0044] [Fig.7] is a partial schematic view, in section in an XY plane, of the ion propulsion system, illustrating a target as well as the primary magnetic lens and the secondary magnetic lens respectively in focused and passive modes;

[0045] [Fig.8] is a view similar to [Fig.7], in which the primary magnetic lens and the secondary magnetic lens are respectively in recirculating and active modes;

[0046] [Fig.9] is a partial schematic view, in section in the XY plane, of the ion propulsion system, illustrating the target and its immediate environment. Detailed presentation of preferred embodiments

[0047] Fig. 1 illustrates very schematically a spacecraft 1, such as a space station or a space transport vehicle, comprising an ion propulsion system 2. Such a system generally enables the propulsion of the craft 1 by ejecting a flow of ions 4 at very high speed.

[0048] Figures 2-9 illustrate such an ion propulsion system 2 according to a preferred embodiment of the invention.

[0049] This system generally comprises a target 10 having a front face 10A and a rear face 10B (visible in particular on [Fig.9]), and adapted for the implementation of a laser-plasma acceleration technique such as the TNSA technique or the BOA technique, an illumination system 12 (Figures 2 and 3) configured to illuminate the front face 10A of the target by means of laser pulses, an ion circulation system to continuously bring ions to the vicinity of the rear face 10B of the target, and an ejection zone 14 ([Fig.3]).

[0050] The ion circulation system includes an ion source 15 ([Fig.2]), particle acceleration means 16 (Figures 2 and 3), a primary ion guide 18, and a secondary ion guide 20.

[0051] In general, particle acceleration means 16 exhibit ([Fig.2]): • a first 16A input configured to receive an ion beam from the ion source 15; • a second 16B inlet configured to receive a recirculating ion beam; and • an output 16C to deliver an accelerated ion beam from the ion beams received from the first and second inputs 16A and 16B.

[0052] The primary ion guide 18 features: • an 18A input connected to the 16C output of the particle acceleration means to receive the ion beam; • an 18B output arranged relatively close to target 10; and • 18C guidance means configured to guide the ion beam from the inlet 18A to the outlet 18B of this primary ion guide.

[0053] The primary ion guide 18 generally includes focusing means 19 configured to focus the ion beam exiting it onto a focal point on the rear face 10B of the target 10, as will become clearer in the following. In practice, the ion beam naturally occupies a certain volume around the focal point, resulting from the repulsive forces acting within the ion beam. The focusing means are designed, in any case, to concentrate the ion beam as much as possible onto the rear face 10B of the target, insofar as this maximizes the mass flow rate of ejected ions and consequently the maximum thrust per laser pulse.

[0054] In the preferred example illustrated, the primary ion guide 18 is configured to direct the ion beam towards the rear face 10B of the target at an angle substantially tangential to that rear face. A different orientation of the ion beam can nevertheless be envisaged without departing from the scope of the invention.

[0055] The secondary ion guide 20 features: • an input 20A arranged relatively close to the target 10, opposite the output 18B of the primary ion guide, so as to receive the ion beam after it has passed in front of the rear face 10B of the target, at least in the absence of illumination of the target by the illumination system; • an output 20B connected to the second input 16B of the particle acceleration means to reinject the ion beam received from the input 20A of the secondary ion guide, as the recirculating ion beam; and • 20C guiding means configured to guide the ion beam from the inlet 20A to the outlet 20B of this secondary ion guide

[0056] The ejection zone 14 ([Fig. 3]) is, generally, an unobstructed area extending from the rear face 10B of the target to allow the ejection of an ionic flux extracted from the ion beam as it passes in front of the rear face 10B of the target. This ejection is caused by an acceleration normal to the target 10 induced by an electric field resulting from an interaction between the target 10, the laser pulses generated by the illumination system 12, and the ion beam, thus generating a propulsive thrust. The ejection zone 14 generally has a divergent shape and rotates about an axis 21, hereinafter referred to as the ejection axis, which generally passes through a focal point of the laser pulses emitted by the illumination system 12 onto the front face 10A of the target. The ejection axis 21 is for example coincided with a central axis of the target 10.

[0057] Throughout this description, a Cartesian coordinate system is defined by an axial direction X corresponding to the direction of the ejection axis 21, and the transverse directions Y and vertical Z are defined orthogonally to the axis X and orthogonally to each other, without in any way prejudging the actual orientation of the propulsion system 2 in space.

[0058] Various aspects of the ion propulsion system 2 will now be described in more detail.

[0059] I. Ion circulation system and ion focusing on the target

[0060] The ion circulation system includes, for example, a gas reservoir 22 configured to supply the ion source 15 via a flow regulator 24. The ion flow rate at the outlet of the ion source 15 is controlled by a control law defined by a thrust computer, which may be of a conventional type and is not shown in the figures. This control law is a function of the thrust and specific impulse required from the propulsion system 2. Where applicable, this control law is defined in relation to the principles of magnetic field control and / or the principles of laser power control, which will be described below. Unlike a conventional ion thruster, the propulsion system 2 is thus capable of dynamically adjusting the thrust and specific impulse parameters in flight in order to adapt to all flight phases of a spacecraft.Indeed, by varying the ion injection rate in the particle acceleration devices 16, it is possible to dynamically vary the mass of ions ejected with each laser pulse, thus providing flexibility to the propulsion system. For example, by increasing the ion injection rate through the ion circulation system, it is possible to temporarily lower the specific impulse to achieve maximum thrust, which may be necessary in certain flight phases.

[0061] The ion source 15 is configured to inject the ion beam onto the central axis of a cyclotron which, in the preferred embodiment of the invention, constitutes the The aforementioned particle acceleration means 16. This injection of the ion beam from the ion source 15 is carried out via the first inlet 16A of the particle acceleration means 16, which forms a low-energy injector of the cyclotron. In the remainder of this description, the numerical reference 16 is used interchangeably to refer to the particle acceleration means in general and the cyclotron in particular.

[0062] Cyclotron 16 is configured to accelerate the ions until their maximum speed is reached, when their trajectory exceeds the maximum radius of the cyclotron.

[0063] The ion circulation system uses various magnetic fields to accelerate, guide, and focus the ion beam. In the preferred embodiment of the invention, all of these magnetic fields, including that of the cyclotron 16, are obtained by electromagnets. The use of electromagnets makes it possible to vary the magnetic fields and thus the focusing of the ion beam directed at the target 10, as will become clearer in the following. The variation of the magnetic fields, in combination with the action of the flow regulator 24, makes it possible to vary the specific impulse and thrust of the thruster, for a given amount of energy supplied to the target 10 by the illumination system 12. This property gives the propulsion system 2 a variability that allows it to adapt effectively to different flight phases of the spacecraft 1.

[0064] The cyclotron 16 and the elements of the ion circulation system provided upstream of the cyclotron, in this case the gas reservoir 22, the flow regulator 24 and the ion source 15, are generally arranged outside the ejection zone 14 so as not to hinder the ejection of the ion flux by the propulsion system 2. These elements are thus eccentric with respect to the ejection axis 21 and, in order to reduce the overall size of the system as much as possible, are oriented so that the ion beams circulate in general in a plane PI inclined with respect to a plane P2 parallel to the X and Y directions and including the ejection axis 21 ([Fig.2A]).

[0065] Therefore, in the illustrated example, with particular reference to [Fig.2A], the guidance means 18C which make up the primary ion guide 18 are configured to deflect the ion beam so as to give it an adequate orientation for its injection onto the rear face 10B of the target despite its initial circulation out of the plane P2.

[0066] For this purpose, the guiding means 18C include a magnetic deflector 26, hereinafter referred to as the primary magnetic deflector, having an inlet 26A connected to the cyclotron 16 (and constituting the inlet 18A of the primary ion guide), and an outlet 26B. The latter is, for example, oriented so as to direct the ion beam in the plane P2.

[0067] The primary magnetic deflector 26 consists of two ferromagnetic plates 28 ([Fig. 4A]) between which the ion beam 30 passes. These ions are generally positively charged so that they have charges of the same sign as the ions that may be removed from the target 10 itself due to the laser-plasma interaction. The magnetic field B generated in the air gap defined by the two plates 28, which is preferably of the same intensity as that used in the cyclotron 16, exerts a force Fi ([Fig. 4B]) on the moving ions. This force deflects the ion beam 30 over a radius of curvature identical to that of the maximum radius of the cyclotron 16, or slightly greater than it to take into account, where applicable, the higher energy of the ions exiting the cyclotron. The length of the primary magnetic deflector 26 is determined as a function of the angular deflection 0 ([Fig.3]), defined in the XZ plane, between the PI plane and the P2 plane, which is necessary to connect the input 26A to the output 26B. .

[0068] The use of magnetic fields of identical intensity in the cyclotron and in the various magnetic deflectors allows the use of the same electromagnet technology in all these components. Thus, the technological limitations are the same for the cyclotron and the deflectors. In particular, along the entire path defined by the ion circulation system, the maximum conceivable radius of curvature is the same and is imposed by the maximum magnetic field, itself determined by the electromagnet technology used. Furthermore, the use of the same electromagnet technology in all these components allows, by means of a unified control law, the deflectors to be controlled so that they dynamically adjust their magnetic fields according to the velocity of the ions exiting the cyclotron in order to maintain constant radii of curvature.This control law is designed to vary the magnetic fields and ion flow rate according to the thrust and specific impulse required at a given instant.

[0069] In a conventional magnetic deflector, the geometry of the ends is generally determined so that the ions travel the same distance under the field B regardless of their entry and exit points. This equidistant path corresponds to an exit symbolized by the dashed line 26B' ​​in [Fig. 5], sometimes referred to as the "neutral section".

[0070] Unlike such a conventional magnetic deflector, the deflector 26 advantageously has an asymmetry at its outlet 26B so as to focus the ion beam vertically (along the Z direction). It should be understood that without the additional focusing of the ion beam beyond the deflector 26, as will be described below, the vertical focusing induced by the deflector 26 would cause the ion beam to converge on a segment of focal length superimposed on the ejection axis 21. For this purpose, the outlet 26B (symbolized by a solid line in [Fig. 5]) is configured such that the ions travel a greater distance under the influence of the magnetic field B in the area 32A located on the outer side relative to the curvature of the deflector 26, and a shorter distance under the influence of the magnetic field B in the area 32B located on the inner side relative to the curvature of the deflector 26. This effect is achieved by tilting the outlet 26B relative to the neutral section 26B'. The degree of this tilt determines the degree of beam focusing.

[0071] At the output of the primary magnetic deflector 26 is another magnetic deflector, hereinafter referred to as the primary magnetic lens 34.

[0072] This primary magnetic lens 34 comprises a main section SI whose inlet 34A is connected to the magnetic deflector 26, and a section S3 hereafter referred to as the focusing section, arranged at the output of the main section SI and ending in an output 34B opposite a defined volume along the rear face 10B of the target and into which the ion beam is to be injected.

[0073] The main section SI is configured to perform a deflection according to the same principle as the deflector 26 but in the horizontal plane P2, for example at an angle of 90 degrees, as can be seen more clearly in Figures 2 and 6. Furthermore, as shown more clearly in [Fig. 7], the outlet 34C of the main section SI is configured to focus the ion beam horizontally (along the X direction). This means that, without the focusing induced upstream by the deflector 26, the horizontal focusing induced by the main section SI would cause the ion beam to converge on a vertical focal segment (parallel to the Z direction) located on the rear face 10B of the target 10 and passing through the ejection axis 21, and therefore, in this case, through the center of the rear face of the target 10B.

[0074] Thus, the combination of the focusing induced respectively by the primary magnetic deflector 26 and by the main section S1 of the primary magnetic lens 34 results in a convergence or focusing of the ion beam towards a focal point located at the intersection of the rear face of the target 10B and the ejection axis 21. The primary magnetic deflector 26 and the primary magnetic lens 34 thus jointly contribute to defining the aforementioned focusing means 19, in the illustrated embodiment.

[0075] The focusing section S3 is configured to operate on command in two modes: a focused mode, in which the focusing section S3 does not interact with the ion beam from the main section SI ([Fig. 7]), and an ion recirculation mode ([Fig. 8]), also called the "through" mode, in which the focusing section S3 increases the deflection of the ion beam, in line with the deflection produced by the main section SI, so that the ion beam exits of the primary magnetic lens 34 by being oriented parallel to the rear face 10B of the target, without being focused horizontally.

[0076] For this purpose, the focusing section S3 includes means for generating magnetic fields, such as an electromagnet (not visible in the figures), independently controlled and configured to produce a magnetic field similar to the field produced by the main section SI, up to the output 34B of the focusing section S3 which is defined so as to coincide with the neutral line.

[0077] The primary ion guide 18 thus allows, in focused mode, the deflection and focusing of the ion beam onto a theoretical focal point defined on the rear face 10B of the target 10, typically at the center of the rear face 10B. Electrostatic repulsive forces, on the one hand, and Van der Waals interaction forces, on the other, compete with the inertial force of the ions resulting from their velocity directed towards the focal point. The equilibrium resulting from this competition determines the volume and shape ([Fig. 9]) of the laser acceleration zone 35 extending from the rear face of the target 10B, over a distance (or thickness) d, and the ion density in this zone 35. In focused mode, this zone 35 corresponds to the region of ion acceleration by the electric field induced by the laser-plasma interaction during the illumination of the target 10.

[0078] The primary ion guide 18 allows, in the ion recirculation mode, to simply deflect the ion beam to allow its passage in front of the rear face 10B of the target and its recovery by the secondary ion guide 20.

[0079] The focused mode is intended to be active during each laser pulse produced by the illumination system 12 and in a manner synchronized with the start and end of the laser pulse.

[0080] The flow regulator 24 is preferably configured so that, when the focused mode is active and the flow regulator 24 is set to the maximum flow rate, the thickness d; of the ion beam focusing zone 35 does not exceed a limit thickness calculated as a function of a maximum thrust assigned to the ion propulsion system 2, which is mainly determined by the performance of the particle acceleration means 16 (in this case the cyclotron) and the various focusing means.

[0081] The concentration of ions on the target 10 is however achieved at the cost of significant friction of the ions on the rear face 10B of the target.

[0082] To limit the negative impact of such friction, the recirculation mode or "transparent mode" is designed to be active in the absence of a laser pulse. This mode thus makes it possible, on the one hand, to limit the heating of the target 10 and, on the other hand, to limit the energy loss of the ions that will not be ejected under the effect of the electric field induced by a laser pulse and which are therefore intended to recirculate, through the ion guide secondary 20, up to the means of accelerating particles 16, in this case in the cyclotron, to be subsequently reinjected towards the target 10.

[0083] The secondary ion guide 20 is, in general, analogous to the primary ion guide 18. It thus comprises a magnetic lens 36, hereinafter referred to as the secondary magnetic lens, analogous to the magnetic lens 34, and a magnetic deflector 38, hereinafter referred to as the secondary magnetic deflector, similar to the primary magnetic deflector 26.

[0084] The secondary magnetic lens 36 thus comprises a main section S2 configured to deflect the ion beam again in the horizontal plane P2, and, at the entrance of the main section S2, a switchable section S4 whose configuration is analogous to that of the focusing section S3 described above but which, due to the reversed arrangement of the primary 18 and secondary 20 ion guides, produces a reversed result.

[0085] Thus, the switchable section S4 is configured to operate on command in two modes: a passive mode, intended to be implemented simultaneously with the focused mode of the primary magnetic lens 34, and in which the switchable section S4 does not interact with the ion beam after the latter passes in front of the rear face 10B of the target, possibly with a rebound on it for part of the ions ([Fig.7]); and an active mode ([Fig.8]), intended to be implemented simultaneously with the ion recirculation mode of the primary magnetic lens 34, and in which the switchable section S4 initiates the rectification of the ion beam in continuity with the rectification carried out downstream of it by the main section S2.

[0086] For this purpose, the switchable section S4 includes magnetic field generation means, such as an electromagnet, controlled in a synchronized manner with the magnetic field generation means of the focusing section S3, and configured to produce a magnetic field similar to the field produced by the main section S2, from the input 36A of the switchable section S4 which is defined so as to coincide with the neutral line.

[0087] The magnetic deflector 38 is, for its part, configured to deflect the ion beam out of the P2 plane into the PI plane (in this case downwards in Figures 2 and 3).

[0088] The secondary magnetic lens 36 is preferably of smaller radius than that of the primary magnetic lens 34 to take into account the fact that the ion beam, after passing along the target 10, has lost some of its energy due to the focusing caused by the primary magnetic deflector 26.

[0089] The secondary magnetic deflector 38 preferably has the same radius as the primary magnetic deflector 26 in order to ensure mechanical symmetry with it. It should be noted, however, that the air gap is preferably larger in order to that the intensity of the magnetic field ensures the same deflection as in the primary magnetic deflector 26 despite a lower ion beam energy.

[0090] The reinjection of the ion beam from the secondary ion guide 20 into the particle acceleration means 16 is carried out via the second inlet 16B of the particle acceleration means 16, which forms a high-energy injector of the cyclotron. The position (in particular the injection radius) of this high-energy injector is determined to correspond to the energy level of the reinjected ions. In this way, the reinjected ions are accelerated again to be directed towards the target 10 by the primary ion guide 18, in the manner described above. A loop circulation of the ions (with the exception of those ejected by the electric field induced by the laser pulses and thus contributing to the thrust) makes it possible to retain in the propulsion system the ions that have not (yet) contributed to the propulsion, and thus to optimize the specific impulse. II. Lighting System

[0091] With reference to figures 2 and 3, the illumination system 12 consists of one - or, preferably, several - illumination block(s) 40, for example three in the illustrated embodiment.

[0092] Each illumination block 40 is arranged eccentrically with respect to the ejection axis 21. Where appropriate, the illumination blocks 40 are regularly distributed around the axis 21.

[0093] Each illumination block 40 includes an optical compressor 42 followed by one or more focusing lenses 44 configured to focus the corresponding laser beam towards the aforementioned focal center of the target 10 and along a convergence axis 46 ([Fig.3]).

[0094] The optical compressor 42 constitutes the final stage of a femtosecond CPA laser block which is not the subject of this disclosure. The optical compressor 42 comprises at least two diffraction gratings 48 parallel to each other and arranged so as to produce at the output a laser beam centered on the corresponding convergence axis 46.

[0095] This arrangement thus allows several illumination blocks 40 to contribute to the illumination of the same target 10.

[0096] In order to distribute the laser pulses over time, the lasers of the illumination blocks 40 are phase-shifted relative to each other so as to produce phase-shifted pulses of a duration T = 1 / (N * R), where N is the number of illumination blocks 40 and R is the repetition rate of each block. The overall repetition rate of the illumination system 12 thus corresponds to the repetition rate R of each illumination block 40 multiplied by the number N of illumination blocks 40.

[0097] The maximum thrust of the propulsion system can thus be increased by taking advantage of the fact that an incidence of the laser on the target in a direction not normal to the target is preferable in TNSA and similar techniques, to allow several lasers to collaborate on the same target. The resulting repetition rate, and therefore the maximum thrust of the propulsion system 2, is thus multiplied by the number of lasers used.

[0098] In addition, the ion propulsion system 2 is configured to allow the repetition rate of each illumination block 40 to be varied on command, so as to vary the thrust of the system independently of the specific impulse. III. Other components

[0099] Furthermore, the laser energy not transferred to the electrons results in heat accumulated on the target 10. Despite a high absorption rate (approximately 84% of the energy is transferred to the electrons), the amount of heat to be dissipated from the target 10 can be a limiting factor. In this case, this heat is advantageously dissipated via a load distribution plate 50 connected to one or more heat dissipation systems, depending on the density of the heat flux to be dissipated (Figures 2, 3, and 9). The means implemented for such heat dissipation can be of a conventional type and will not be described here.

[0100] Furthermore, the force distribution plate 50 acts as a mechanical support to allow the target 10 to transmit the mechanical load, resulting from the generated thrust, to a support (not visible in the figures) to which the plate 50 is rigidly fixed. The plate 50 is preferably made of a different material than that composing the target 10 so as to induce a change in medium conducive to the recirculation of electrons in the target. The plate 50 can be monolithic or composite and composed of any type of material with low thermal resistance.

[0101] A through-hole 52 in the stage 50 ([Fig. 9]) is formed so as to allow the laser pulses 54 to reach the center of the front face 10A of the target 10, or more generally a target region that can be defined on this front face 10A, despite the fact that this front face 10A is applied to the stage 50. In the illustrated embodiment, the through-hole 52 is conically flared in the direction from the rear face 10B to the front face 10A, and therefore has a larger diameter on the face of the stage exposed to the illumination system 12 and a considerably smaller diameter on the face of the stage in contact with the target 10. The angle of the cone is determined as a function of the convergence axis 46 of each illumination block 40. The diameter of the through-hole 52 on the side of the target 10 is of a dimension compatible with the diameter of laser pulse focusing.In compliance with this condition, the diameter is generally chosen to be the smallest. possible in order to allow the mechanical stress experienced by the target 10 to be transmitted as best as possible to the stage 50 while limiting the mechanical deformations of the target induced by the compression of the ionic flux at the rear of the target 10B.

[0102] Finally, the ion flux ejected into the ejection zone 14 along the X-axis from the laser acceleration zone 35 can be neutralized to conserve the overall charge of the system. Thus, a neutralization cathode, which may be of a conventional type and will not be described here, is placed in the ejection zone 14 so as to supply electrons and thereby neutralize the space charge in the accelerated ion jet. An accumulation of negative charges in the system can thus be avoided. Such a neutralization cathode could, for example, be a hollow cathode, of the type used in known ion and plasma thrusters. IV. Implementation Procedure

[0103] A method for implementing system 2 described above therefore generally comprises steps consisting of: • A) implement the ion source 15, the particle acceleration means 16 and the primary ion guides 18 and secondary 20 so as to direct an ion beam in front of the rear face 10B of the target 10, preferably continuously; • B) implement the illumination system 12 so as to direct repeated laser pulses onto the front face 10A of the target; and • C) eject a stream of ions torn from the ion beam as it passes through the focal point defined on the rear face 10B of the target, under the effect of an electric field induced by a laser-plasma interaction.

[0104] Unlike the TNSA technique, the majority of the ions constituting the ejected ion flux are not torn from the target but originate from the gaseous ion beam from the ion source 15. This remarkable feature of the invention means that, compared to the state of the art represented by the TNSA technique, the topology of the electron front and the electron recirculation scheme are strongly impacted by the presence of ions (in this case, positive charges) concentrated in the laser acceleration zone 35. The electron front is thus pushed outside the laser acceleration zone 35 parallel to the rear face 10B of the target 10. The extremely intense electric field along the ejection axis 21, resulting from the electron front and its counterpart inside the target, subjects the ions of the gaseous ion beam to a strong acceleration.On the one hand, the volume of ions subjected to acceleration can thus be considerably greater than in a conventional TNSA configuration. On the other hand, the gaseous ions involved are much more mobile than the solid ions of the conventional TNSA technique.

[0105] Where appropriate, step A includes the implementation of the focusing means 19 alternately in the focused mode, synchronized with each laser pulse, and in the recirculating mode, between two consecutive laser pulses

[0106] If applicable, step C generates thrust used to propel a spacecraft 1. V. Variants

[0107] In other embodiments, the "recirculating" and "active" modes are not provided for the primary ion guide 18 and the secondary ion guide 20 respectively, so that the ion beam is permanently focused on the rear face 10B of the target.

[0108] In other embodiments, a mode analogous to the recirculation mode is also implemented at the outlet of the primary magnetic deflector 26, and a mode analogous to the active mode is also implemented at the inlet of the secondary magnetic deflector 38. To this end, an electromagnet is activated at each of these locations when the illumination system 12 is not active, in order to prevent the vertical focusing of the ion beam by these deflectors 26 and 38 and thus maintain, as far as possible, the energy level of the ions not used for thrust and intended to constitute the recirculation beam. In such embodiments, the ions are thus reinjected with a higher energy level into the cyclotron via the high-energy injector 16B, which, in this case, is placed over a larger radius than in the embodiment described above.

[0109] Furthermore, in other embodiments, in which the "recirculating" and "active" modes are not provided for the primary ion guide 18 and the secondary ion guide 20 respectively, the various magnetic fields implemented in the system may be fixed and generated by permanent magnets. In still other embodiments, the magnetic fields may comprise a combination of fixed fields generated by permanent magnets and fixed or variable fields generated by electromagnets, at least for the implementation of the "recirculating" and "active" modes.

[0110] Furthermore, in other embodiments, the magnetic circuit enabling the focusing of the ions may include one or more magnetic elements of any shape, the geometry and arrangement of which enable the following three functions to be performed: focusing the ion beam on the rear face 10B of the target, positioning the particle acceleration means 16 outside the ejection zone 14 of the ionic flux accelerated by the propulsion system, and reinjecting, into the particle acceleration means 16, the ions not accelerated by the electric field induced by the illumination system 12.

[0111] Furthermore, in other embodiments, the cyclotron constituting the particle acceleration means 16 can be replaced by any device enabling ionic acceleration, such as a synchrotron, a tandem accelerator, a linear accelerator (also called a Linac), a rectifier accelerator, or a static accelerator. In other embodiments, the particle acceleration means 16 may comprise several such accelerators in combination, for example, a combination of a cyclotron with one or more of the aforementioned accelerators.

[0112] Furthermore, in other embodiments, the illumination system 12 may comprise only one illumination block, the laser block of which has an emission axis coinciding with the ejection axis 21.

[0113] Furthermore, in other embodiments, instead of CPA lasers, each illumination block 40 may comprise another type of laser. In all cases, obtaining adequate laser-plasma interaction generally requires that the intensity of each laser beam exceed 10¹⁸ W.cm⁻² on the target.

[0114] Furthermore, the action of the laser pulses on the front face 10A of the target and the action of the focused ion beam on the rear face 10B of the target can generate erosion on both faces of the target. In this regard, in some embodiments, the force distribution plate 50 may include mechanical means for moving the target 10 transversely in front of the through recess 52 to modify the region of the target subjected to illumination and the ion beam, or may include means for replacing the target 10 with another target when the first target reaches a certain degree of wear, so as to increase the lifespan of the propulsion system.

[0115] Furthermore, in other embodiments, the gas reservoir 22, the flow regulator 24 and the ion source 15 can be replaced by a device chosen according to the type of liquid or solid material used as an ion source as an alternative to the gaseous source used in the embodiment described above.

[0116] Furthermore, the geometry of the magnetic elements implemented within the ion guides 18 and 20 depends on the ionization number of the ions circulating in the circuit. In other embodiments, one or more magnetic deflectors located at the outlet of the ion source 15 may be provided so as to allow only ions whose ionization number is compatible with the geometry of the ion guides and, more generally, of the propulsion system, to pass to the low-energy injector 16A.

[0117] Furthermore, in cases where a larger system footprint is acceptable, the particle acceleration means 16 and the elements of the ion circulation system provided upstream of these particle acceleration means 16, in this case the gas reservoir 22, the flow regulator 24 and the ion source 15, can be arranged so that the ion beams circulate globally in the plane P2 parallel to the X and Y directions and including the ejection axis 21 (while nevertheless remaining eccentric from the ejection axis 21). In this case, the primary magnetic deflectors 26 and secondary magnetic deflectors 38 can be replaced by magnetic focusing devices that do not induce deflection of the ion beam, to constitute the aforementioned focusing means 19.

Claims

Demands

1. A matter ejection system (2), comprising: • a target (10) adapted for implementing a laser-plasma acceleration technique such as the TNSA or BOA technique; • an illumination system (12) configured to illuminate a front face (10A) of the target (10) by means of laser pulses; • an ion source (15); • particle acceleration means (16) having a first inlet (16A) configured to receive an ion beam from the ion source (15) and an output (16C) for delivering an accelerated ion beam from the ion beam received from the first inlet (16A);• a primary ion guide (18) having an inlet (18A) connected to the output (16C) of the particle acceleration means (16) to receive the ion beam delivered by the latter, an output (18B), and guiding means (18C) configured to guide the ion beam from the inlet (18A) to the output (18B) of the primary ion guide and comprising focusing means (19) configured to focus the ion beam exiting the primary ion guide (18) through the output (18B) onto a focal point defined on a rear face (10B) of the target (10);• an ejection zone (14), free of obstruction, extending from the rear face (10B) of the target to allow the ejection, along an ejection axis (21) passing through said focal point, of an ionic flux torn from the ion beam when the latter passes through a laser acceleration zone (35) defined at the level of said focal point in front of the rear face (10B) of the target, under the effect of an acceleration normal to the target (10) induced by an electric field resulting from an interaction between the target (10), the laser pulses produced by the illumination system (12) and the ion beam.;

2. System according to claim 1, wherein the particle acceleration means (16) have a second input (16B) configured to receive a recirculating ion beam, and wherein the output (16C) of the particle acceleration means (16) is configured to deliver the accelerated ion beam from the ion beams respectively received from the first and second inlets (16A, 16B), the system comprising a secondary ion guide (20) having an inlet (20A) arranged opposite the output (18B) of the primary ion guide (18) so as to receive at least partially the ion beam after it passes in front of the rear face (10B) of the target, an output (20B) connected to the second inlet (16B) of the particle acceleration means (16) for reinjecting the ion beam received from the inlet (20A) of the secondary ion guide as said recirculating ion beam, and guiding means (20C) configured to guide the ion beam from the inlet (20A) to the output (20B) of the secondary ion guide.

3. System according to claim 2, wherein the focusing means (19) are configured to switch on command between a focused mode, in which the focusing means focus the ion beam exiting the primary ion guide (18) onto the rear face (10B) of the target, and a recirculating mode, in which the focusing means (19) deflects the ion beam so as to direct it towards the inlet (20A) of the secondary ion guide (20).

4. System according to any one of claims 1 to 3, wherein the primary ion guide (18) comprises means (26) for deflecting the ion beam from a plane (PI) inclined with respect to the ejection axis (21) to a plane (P2) comprising the ejection axis (21).

5. System according to any one of claims 1 to 4, wherein the ion source (15) comprises controlled means (24) for varying the ion flow rate of said ion beam from the ion source (15).

6. System according to any one of claims 1 to 5, wherein the illumination system (12) comprises several illumination blocks (40) collectively configured to illuminate the front face (10A) of the target by each emitting a portion of said laser pulses along distinct respective convergence axes (46).

7. System according to claim 6, wherein the illumination blocks (40) are configured to emit said laser pulses in a phase-shifted manner and thus multiply a repetition rate of the laser pulses received by the target (10).

8. System according to any one of claims 1 to 7, comprising a force distribution plate (50), on which the front face (10A) of the target is applied, and comprising a through recess (52) to allow laser pulses to pass through said force distribution plate (50) and reach a target region defined on the front face (10A) of the target.

9. Spacecraft (1) comprising a system according to any one of claims 1 to 8, configured such that said ejection of the ionic flux generates a propulsive thrust, thereby said system constitutes an ion propulsion system (2).

10. A method for implementing a system (2) according to any one of claims 1 to 8, comprising the steps of: • A) implementing the ion source (15), the particle acceleration means (16) and the primary (18) and secondary (20) ion guides so as to direct an ion beam in front of the rear face (10B) of the target (10); • B) implementing the illumination system (12) so as to direct repeated laser pulses onto the front face (10A) of the target; • C) ejecting a stream of ions stripped from the ion beam as it passes through a laser acceleration zone (35) defined at said focal point in front of the rear face (10B) of the target, under the effect of an electric field induced by a laser-plasma interaction.

11. A method according to claim 10, wherein the system is a system according to claim 3, and wherein step A comprises implementing the focusing means (19) in the focused mode in a manner synchronized with each laser pulse, and implementing the focusing means (19) in the recirculating mode, between two consecutive laser pulses. 24

12. A method according to claim 10 or 11, wherein the ejection of the ion stream in step C generates thrust used to propel a spacecraft (1).

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