Patent Application
20080271430
The present Application is based on International Application No. PCT/EP2006/069387, filed on Dec. 6, 2006, which in turn corresponds to French Application No. 05 12417 filed on Dec. 7, 2005, and priority is hereby claimed under 35 USC §119 based on these applications. Each of these applications are hereby incorporated by reference in their entirety into the present application.
The invention is situated in the field of plasma motors. These motors may, for example, be used in satellites or spacecraft, the propulsion of which necessitates low thrusts for long periods, as for example probes.
The propulsion of spacecraft in space (where terrestrial gravitation becomes negligible) requires low thrusts (low flow of ejected material), but high ejection speeds of “fuel” to minimize the on-board mass. In fact, the speed increase Δu of a spacecraft is linked to the gas ejection speed ue and to the initial and final masses, m0 and mf, of fuel by the following equation, called the “rocket equation”:
A high gas ejection speed is therefore imperative if it is desired to save fuel. Plasma motors allow these high ejection speeds to be attained. Two quantities are used to characterize a motor, the specific impulse:
expressed in seconds, where go is the gravity constant at the surface of the earth, and the thrust:
T={dot over (m)}ue
where {dot over (m)} is the mass flow rate.
The principle of plasma motors, described in the illustrated schema of
The various prototypes of plasma motors existing to date use, generally speaking, an ionization stage to generate a source of positively charged matter (positive ions), an acceleration stage and a neutralization structure. The ionization sources and the accelerating and neutralizing structures may be varied. But all the motors existing to date use only positively charged matter (positive ions) for propulsion, the negative charge (the electrons) serving solely for the ionization and the neutralization.
In this context, the main proposal in the present invention is to use a flow of positive ions and a flow of negative ions for the thrust. To do this, an electronegative gas (gas with a high electron affinity) is used as a fuel.
The thrust is therefore ensured by two types of ions, one of the types being positively charged, the other negatively. These ion beams neutralize each other (for example, by recombination) downstream to form a beam of fast neutral molecules, which allows a neutralization structure downstream of the acceleration to be dispensed with.
More precisely, the subject of the present invention is a plasma motor comprising the extraction of a positive ion flow, characterized in that it comprises:
The interest of the invention resides notably in the use of a single ionization stage and a single ionizable gas, allowing a negative ion flow and a positive ion flow of the same amplitude to be delivered.
Advantageously, the plasma motor according to the invention may furthermore comprise means for filtering the electrons freed in the ionization stage, during the ionization of the gas.
Advantageously, the plasma motor may comprise ion flow extraction means comprising at least one polarized grid.
Advantageously, the plasma motor may comprise means for creating an electric field comprising two conductor elements placed at the ends of the ionization stage to apply a voltage to said stage, or comprising a coil powered by a radiofrequency current.
The means for creating an electric field may also be of the helicon antenna type powered by a radiofrequency current.
According to a variant of the invention, the electronegative gas may be diiodine.
According to a variant of the invention, the electronegative gas may be oxygen.
According to a variant of the invention, the plasma motor may comprise means for creating an alternating field generating a pulsed plasma (alternation of on/off periods) allowing the extraction of ion flows during the off period, a period during which the electrons have disappeared (temporal filter of the electrons).
The plasma motor may advantageously comprise means for generating a static magnetic field within the ionization stage so as to filter the electrons in a steady state (spatial filter).
These means may be permanent magnets placed at the periphery of the ionization stage to create the magnetic field within said ionization stage.
According to a variant of the invention, the plasma motor may comprise means for extracting the negative and positive ion flows in a direction perpendicular to the direction of the magnetic field applied at the ionization stage. In this case, the plasma motor may advantageously comprise a cylinder constituting the ionization stage and at least one peripheral extraction stage mounted on said cylinder and equipped on the surface with polarized grids.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious aspects, all without departing from the invention. Accordingly, the drawings and description thereof are to be regarded as illustrative in nature, and not as restrictive.
The present invention is illustrated by way of example, and not by limitation, in the figures of the accompanying drawings, wherein elements having the same reference numeral designations represent like elements throughout and wherein:
In the example described below, the motor according to the invention comprises a structure supplied with electronegative gas as schematized in
A flow of electronegative gas A2 is introduced into the ionization stage 1. Under the action of a magnetic field, schematized by the arrow representing the electrical power Pe, the electronegative gas generates positive ions A+, negative ions A− and electrons e−. The ionization stage is connected to a stage 2 of filtering the electrons in such a way as to make available in the extraction stage 3 a plasma of positive ions and of negative ions lacking electrons due to the filtering means, which may, for example, be a static magnetic field. Extraction of the plasma is ensured in the case schematized here by two grids, polarized negatively 4 and positively 5.
The thrust is therefore ensured by the two types of ions (the negative charge and the positive charge). The neutralization downstream is no longer necessary because the beams of ions neutralize each other downstream (recombination) to form a beam of fast neutral molecules.
The ionization stage 1 may use any type of connection of the electrical energy to the plasma (for example, two plates continuously polarized at low frequency or at radiofrequency, a coil powered at radiofrequency for inductive coupling, or a microwave source).
The filtering stage 2 may be produced in at least two ways:
where me,i and ue,i are the mass and the speed respectively of the electrons or ions, e is the elementary charge, and B the amplitude of the magnetic field.
The extraction stage 3 may consist of accelerating grids, the dimensions of which are not necessarily similar to those of motors with a conventional grid, because the charge sheath properties of space are different in the absence of electrons.
The system comprises a horizontal cylinder: the ionization stage 1, where the dense plasma is generated by applying a radiofrequency voltage at 13.56 MHz to a helicon antenna, represented by the abbreviation RF. Helicon sources are known for producing very effective ionization. This cylinder furthermore comprises means 6 for introducing ionizable gas into the ionization stage. The diiodine I2 is used as fuel. This is a highly electronegative gas allowing the formation of a large quantity of heavy negative ions (the higher the mass, the greater the thrust; the mass of I2 is 254 AMU (Atomic Mass Units)). Furthermore, the ionization threshold of diiodine is low (10.5 eV to form I+), which favors the formation of positive ions at low energy cost. However, a priori any electronegative gas may be used (for example, oxygen). A static magnetic field B with an intensity of around 0.01-0.1 Tesla is applied in the source cylinder, allowing the electrons to be confined in the cylinder, as shown in
This magnetic field has two functions:
These stages may typically be equipped with polarized grids, as shown in
According to a variant of the invention, the extraction stages 3 illustrated in perspective in
Finally, the two extracted ion beams, with opposite signs, neutralize each other downstream (in space). Neutralization is therefore automatic and does not require an additional electron beam. The two beams may also recombine to form a beam of fast neutral molecules.
Typically, with a motor having an overall extraction area of around 500 cm2, an acceleration voltage of 1000 V (obtained by polarizing the extraction grids so as to optimize the ionic optics), it is possible to obtain an ionic current density of 10 mA/cm2, and hence a total extracted current of 5 A. Taking the mass of iodine, this current corresponds to a mass flow rate of ejected fuel of 6.5 mg/s. By considering an acceleration voltage of 1000 V, the ejection speed of the ions will be 40 km/s. Referring to the equations presented in the introduction, this mass flow rate and this ejection speed lead to the following performance: a thrust of 250 mN for a specific impulse of 4000 s.
It will be readily seen by one of ordinary skill in the art that the present invention fulfils all of the objects set forth above. After reading the foregoing specification, one of ordinary skill in the art will be able to affect various changes, substitutions of equivalents and various aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by definition contained in the appended claims and equivalents thereof.
| Number | Date | Country | Kind |
|---|---|---|---|
| 05 12417 | Dec 2005 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2006/069387 | 12/6/2006 | WO | 00 | 6/6/2008 |
