PLASMA ACCELERATOR WITH MODULATED THRUST

Abstract

The invention relates to a plasma accelerator that produces and controls a plasma stream exhaust, in particular for space propulsion. The ions are produced inside the discharge chamber by working gas collisional ionization by electrons from a single electron source placed outside, also employed for ion beam neutralization. The ion motion is directed outwards through the exit side by the electric field between a cathode grid and the walls of the plasma chamber. The acceleration voltage imparts energy to the ion flux and an electrically biased control grid modulates the ion outflow from the discharge chamber and the electron inflow from the electron source. This allows electrical control of throttle and/or modulation of thrust delivered along the longitudinal direction of the thruster axis. Several plasma accelerators could be clustered together to provide controlled non-axial thrust using the individual control of throttle.

Claims

1. A plasma accelerator comprising: an electrically conductive discharge chamber with an open end, means for introducing ionizable propellant inside the discharge chamber, an active cathode configured to emit electrons for ionizing the propellant and neutralizing outflowing ions, the active cathode placed outside the discharge chamber, a cathode grid being a passive cathode placed after the open end of the discharge chamber, an electrically conductive control grid placed after the cathode grid, wherein the plasma accelerator further comprises: power supply means configured to apply: a potential (V.sub.CT) between the control grid and the active cathode for controlling thrust of outflowing plasma stream through the open end of the discharge chamber, a potential (V.sub.AC) between the active cathode and the discharge chamber for accelerating electrons into the open end of the chamber and ions towards the open end of the discharge chamber and, a potential (V.sub.DS) between the discharge chamber and the cathode grid for imparting an electric field between the discharge chamber and the cathode grid; wherein the active cathode, the cathode grid and the control grid are arranged so as to introduce electrons emitted from the active cathode into the discharge chamber through the control grid and the cathode grid.

2. The plasma accelerator according to claim 1, wherein it further comprising an inner cathode being a passive cathode electrically connected to the cathode grid, the inner cathode placed inside the discharge chamber.

3. The plasma accelerator according to claim 1, wherein the discharge chamber extends lengthwise along an axis of symmetry.

4. The plasma accelerator according to claim 1, wherein the cathode grid and the control grid have their open spaces aligned.

5. The plasma accelerator according to claim 1, further comprising a plurality of magnets configured to confine electrons in the discharge chamber.

6. The plasma accelerator according to claim 5, wherein the plurality of magnets is arranged concentrically around the discharge chamber with alternate magnetic poles.

7. The plasma accelerator according to claim 5, further comprising a casing for magnetically shielding the plurality of magnets.

8. The plasma accelerator according to claim 1, wherein the active cathode is a single one.

9. The plasma accelerator according to claim 1, wherein the ionizable propellant is a monatomic or molecular gas.

10. Space borne vehicle comprising at least one plasma accelerator according to claim 1.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0037] A series of drawings which aid in better understanding the invention and which are expressly related to an embodiment of said invention, presented as a non-limiting example thereof, are very briefly described below.

[0038] FIG. 1 is a cross-sectional plan scheme of the gridded plasma accelerator and its basic electrical connections in accordance with the preferred embodiment of present invention. The figure also shows the disposition of the crowns of permanent magnets with alternate polarities along the longitudinal direction of the discharge chamber.

[0039] FIG. 2 is a cross-sectional plan scheme of the electric field line distribution (dotted lines) between the two passive cathodes within the discharge chamber shown in FIG. 1.

[0040] FIG. 3 is a scheme of the crowns of permanent magnets 13, also indicated in FIG. 1, which shows the permanent magnets with alternate polarities disposed concentrically around the discharge chamber 11 and enclosed by the casing 20 for the magnetic field insulation of the external equipment.

[0041] FIG. 4 represents the ion current I.sub.B against the ion beam control voltage V.sub.CT for different acceleration voltages V.sub.AC indicated in FIG. 1 as measured in an embodiment of the present invention.

DESCRIPTION OF AT LEAST ONE EMBODIMENT OF THE INVENTION

[0042] FIG. 1 shows the cross sectional plan scheme along the axial direction of symmetry of the present invention with its electrical connections. The neutral gas employed as propellant is introduced through the pipe 10, into the discharge chamber 11. The pipe 10 is electrically insulated from the controlled gas leak system by the ceramic connector 12.

[0043] The plasma is essentially produced by the neutral gas atom collisional ionization by electrons from the active cathode 19 placed outside the discharge chamber 11. The electron source 19 can have different forms, such as a hollow cathode plasma discharge or thermionic electron emitter. This active cathode 19 provides electrons both along the direction of the control grid 18 and also in the opposite direction of the exiting ion beam indicated by the arrow in FIG. 1 for space charge neutralization. The block HC in FIGS. 1 and 2 represents control and heating and control system of the electron source 19.

[0044] A fraction of the electrons emitted from the cathode 19 enters into the discharge chamber 11 through the aligned open spaces of the control grid 18 and cathode grid 14. They are trapped inside the discharge chamber by the multiple magnetic-mirror fields produced by the crowns of permanent magnets 16 shown in FIGS. 1 and 3.

[0045] FIG. 1 shows two electrodes acting as passive cathodes; the central electrode 13 and the cathode grid 14, placed in front of the open end of the discharge chamber 11. The central cathode 13 is electrically insulated from the discharge chamber 11 by the ceramic housing 15, which places this electrode 13 over the longitudinal axis of symmetry of the system. The DC voltage V.sub.DS is applied between the conductive walls of the discharge chamber and the two electrically connected passive cathodes 13 and 14. The scheme of FIG. 2 shows the resulting electric field lines from this configuration of three electrodes with cylindrical symmetry around the longitudinal direction of the plasma thruster.

[0046] The conductive material of the discharge chamber 11 is also essentially transparent to the magnetic field produced by the permanent magnets 16 of FIGS. 1 and 3. The three crowns 16 are made of eight permanent magnets with alternate polarities shown in FIG. 3 and are placed concentrically to the discharge chamber 11. These crowns 16 of permanent magnets are also disposed as in FIG. 1 with alternate magnetic polarities along the longitudinal direction of the discharge chamber 11. Finally, a ring-shaped magnet 17 is located around the central cathode 13 placed at the closed end of the discharge chamber.

[0047] Such a configuration of permanent magnets produces a spatially periodic pattern of magnetic fields lines inside the discharge chamber 11, where the magnetic field lines connect the surfaces of the nearby magnets. The electrons perform a complex motion inside the discharge chamber where they are accelerated along the electric field lines indicated by the dotted lines in FIG. 2 and also confined by the multiple magnetic-bottle field lines (not shown in FIG. 2). This combination of electron trapping and acceleration reduces the collisional mean free path increasing ionizing collisions with neutral gas atoms. The ionization rate of the neutral gas therefore is greatly increased. The system is enclosed inside the casing 20 as illustrated in FIGS. 1 and 3. The casing 20 confines the magnetic field lines in order to protect the equipment nearby the plasma accelerator from the intense magnetic field produced by the permanent magnets 16 and 17.

[0048] The ions resulting from ionizing collisions of electrons are essentially driven along the electric field lines in FIG. 2 because they are more massive and therefore less affected by the local magnetic field. The positive charges are either attracted to the central cathode 13 or, alternatively, accelerated along the electric field lines towards the cathode grid 14.

[0049] The electric field lines of this configuration with two passive cathodes of FIG. 2 focus an important fraction of the positive ions created inside the discharge chamber towards the cathode grid 14. Consequently, a group of ions exits the discharge chamber moving along its axial direction and passing through the cathode grid 14 and the control grid 18, which have their open sections aligned.

[0050] This exiting ion outflow is accelerated downstream by the DC electric potential V.sub.AC imparted between the discharge chamber 11 and the electrical ground of the system as shows FIG. 1. The current I.sub.B through the power supply that delivers the acceleration voltage V.sub.AC is proportional to the flow of ions passing through the cathode grid 14. This electric field also accelerates upstream the electrons from the active cathode 19 passing through the grids 14 and 18 towards the discharge chamber. The energy of these ionizing electrons is also increased by the voltage V.sub.AC well over the ionization threshold of the neutral gas. This fact additionally increases the ionization rate inside the discharge chamber reducing the amount of neutral gas required to operate this plasma accelerator.

[0051] As in FIG. 1, the control grid 18 is biased to the DC electric potential V.sub.CT, which acts as a control potential. When the voltage V.sub.CT is null, the grid 18 permits the counter flow of electrons from the active cathode 19 and ions exiting the discharge chamber 11. When the voltage V.sub.CT is imparted, the control grid 18 repels the electrons from the active cathode 19 moving towards the cathode grid 14. Additionally, only ions with energies over a threshold can move outwards past the control grid

[0052] For low potentials V.sub.CT, the ion current passes through the control grid 18 and is later neutralized by electrons from the active cathode 19, and this plasma jet moves in the direction indicated by the arrow of FIG. 1. This plasma stream is accelerated by the potential V.sub.AC and modulated by the control voltage V.sub.CT as indicated in FIGS. 1 and 4 imparting momentum to the spacecraft in the direction of the arrow in FIG. 1.

[0053] Additionally, several plasma accelerators could be clustered together using the same acceleration voltage V.sub.AC but individual control voltages V.sub.CT as in FIG. 1. This cluster could deliver non-axial thrust using V.sub.CT to control the throttle of each different plasma accelerator allowing complex maneuvers in space.

[0054] The features of this plasma accelerator configuration are shown in FIG. 4, where the current I.sub.B indicated in FIG. 1 was measured in an embodiment of the present invention. The current is proportional to the counter flow of ions and electrons crossing the grids 14 and 18 in FIG. 1. The working gas pressure p=8.Math.10.sup.5 mB of Argon was low enough to neglect collisions between neutral atoms and charged particles. The current I.sub.B was measured for different acceleration voltages V.sub.AC as a function of the voltage V.sub.CT imparted to the control grid 18.

[0055] Control or modulation of the plasma stream by this plasma accelerator is shown in FIG. 4 through the decrement observed in the beam current I.sub.B as the control voltage V.sub.CT increases, holding acceleration potential V.sub.AC fixed. For low control voltages I.sub.B remains independent of the acceleration potential and essentially depends on the flow rates of neutral gas and ionizing electrons inside the discharge chamber. The abrupt decrement in beam current I.sub.B when V.sub.CT.sub.AC is caused by the development of a potential well between the cathode grid 14 and the active cathode 19. The voltage V.sub.CT imparted to the control grid 18 determines the depth of the potential well that precludes the ionizing electron inflow from the active cathode 19 as well as the ion outflow from the discharge chamber.

[0056] Additionally, it is advantageous for voltages V.sub.AC (300, 400 and 500 volts) and V.sub.CT (0-300 volts) in FIG. 4 for plasma acceleration and control to be well below those needed in the aforementioned gridded ion thrusters, in the order of a few kilovolts. These low voltages reduce the complexity of the electrical system, wear in the grids by ion bombardment, and avoid high voltage sparking. The overall electric power consumption also decreases typically below the range of 100 watts.

[0057] Although the invention has been explained in relation to its preferred embodiment(s) as mentioned above, it can be understood that many other modifications and variations can be made without departing from the scope of the present invention. It is therefore contemplated that the appended claim or claims will cover such modifications and variations that fall within the true scope of the invention.