DE2053929B2 - High frequency ion thruster - Google Patents

Description

The invention relates to a high frequency ion thruster with a fuel delivery system, a High-frequency generator for ionizing the fuel, which is in gaseous form, by means of high-frequency energy in a gas discharge space, with an electrode for extraction and acceleration of the Ions from the gas discharge space and with a device for neutralizing the ion beam after passing through the extraction and acceleration electrode.

In such high-frequency ion thrusters used in spacecraft, Nculralgas is used in a discharge space consisting of dielectric material, for example quartz, by means of ionization Ionized inductively by adding a high-frequency general to the discharge space Induction coil is arranged, see H.W. Hole. State of the Art and Recent Developments of the Radio Frequency lonmotors, AIAA Paper 69 285. 1969. The ions graze by means of an acceleration

929 ₂

Electrostatically accelerated electrode and after passing the acceleration electrode through injector neutralized by electrons.

During the ionization of the neutral gas with high fre Frequency alternating fields are those that occur with conventional ion sources, the lifetime problems related to cathodes and anodes are eliminated, but on the other hand they are mechanical! Construction of the well-known high-frequency drive works as well as the electronic circuits zui Generation and regulation of the high frequency energy is complicated and time-consuming. This will do it again reduces the inherently long service life and operational safety of these ion thrusters.

Another, both in the known high-frequency engines and in the electrostatic The problem that arises from ion sources is that of both the radial and axial directions of the discharge more or less non-uniform plasma density distribution in the room, which, in addition to high wall and di (Tu sion losses also caused a low degree of effectiveness of these ion thrusters.

This is where the invention comes in. whose task e. <is an ion thruster of the type mentioned at the beginning to create the mechanically simple lsi and with a new design de; Discharge space one the efficiency of the ion engine improving plasma density is achieved

This object is achieved according to the invention in that the gas discharge space is in a / 4 high frequency resonator at its by one of the extraction and acceleration electrodes opposite Gende multi-pinhole closed end and the high frequency generator at the other, open end ir the high frequency resonator is arranged.

Such an ion thruster forms in contrast to the known high-frequency ion thrusters an organic unit, consists only of metal and has no electrical or electronic components such as resistors, capacitors, transistors, etc. their sound for faults and breakers is prone to. The mechanical and electrical construction of an ion engine according to the invention isi simple and robust.

It is already known in the case of ion sources, viii such as Helvetia Physica Acta. Vol. 30. 1957, HeM p. 292 to 296. that ions by means of a high frequency ionization is generated, the discharge space in which the ionization of the gaseous substances; is made in a /. 4-resonator laid is. Compared to an ion thruster according to the invention, it is such an ion source around separately excited resonators, but not around high-frequency generators, the one uniform design of the entire engine. Furthermore, the discharge space is at dei known ion sources not as in an ion engine according to the invention on the closed but arranged at the open end of the resonator At the open end of the resonator there is the loni sation rate higher than at the closed end, d; at the open end, i.e. in the area of the Schwin belly of the standing electric wave. that most of the energy is available for ionization however, by appropriately dimensioning the gas density in the discharge space and the amplitude dei standing wave also at the closed end de: high frequency generator a complete ionisaiioi

of gaseous fuel. Here, however, there is an additional significant advantage.

This advantage can be seen in the fact that the standing high-frequency electrical wave ·; by doing Discharge space Lorentz forces act on the ions and electrons of the gas discharge which how shown later, this towards the closed end of the high frequency generator, i.e. H. Accelerate in the direction of the multi-hole diaphragm opposite the acceleration electrode. Through this the plasma density increases in the vicinity of the multi-pinhole diaphragm; further is due to the high Concentration of the charge carriers and their axial velocity component at the multi-hole diaphragm, i.e. at the junction of the standing wave, a greater current density than in known ones Reached ion thrusters. In addition, the wall and recombination losses are lower because electrons and ions immediately after their formation by the high-frequency electrical wave in the direction of the Boundary electrode are accelerated.

This increases the efficiency of such an ion thruster compared to the known ones, so that z. B. at a certain acceleration voltage per unit of time more ions than at known ion thrusters extracted from the discharge space by the acceleration electrode and be accelerated; an increase in the thrust of the ion thruster can thus be achieved.

The remaining after the electrostatic acceleration of the ions within the discharge space, Electrons released during ionization are, as is known per se, via a neutralizing electrode injected into the accelerated ion beam.

In a further preferred embodiment of the invention, the high frequency resonator has two concentrically arranged hollow cylinders, which together form a coaxial /.4- Lecher line, see above that the device for neutralizing the accelerated ion beam and parts of the propellant delivery system arranged in the inner hollow cylinder who can η.

This arrangement brings no space savings also has the advantage that the device can be used for neutralization of the ion beam in the center of the bescr. downward ion beam lies, which has an unfavorable influence and uneven neutralization of the ion beam is avoided.

The device for neutralizing the ion beam can, as is known per se, a hollow cathode be that the electrons remaining in the discharge space after ionization via a means A plasma bridge generated by a gas discharge in the hollow cathode is injected into the ion beam.

It can, however, be simplified considerably, by means of a likewise arranged centrally tubular hollow body, the openings assigned to the discharge space on a cable and on its other side has openings facing the accelerated ion beam.

With this arrangement and design of the device for neutralizing the accelerated ion beam is from a location within the discharge space with high plasma pressure near the boundary electrode A plasma bridge to the accelerated via the hollow body guided through the acceleration electrode Ion beam formed without an additional gas discharge must be generated.

In this way, all that shorten the life of an engine are hollow or oxide cathodes avoided. Due to the failure of the heated hollow cathode and the associated energy supply and control, the engine is further simplified, which further increases operational reliability will.

The invention is described in more detail in two exemplary embodiments with reference to the drawing. It shows the

Fig. 1 shows a first embodiment of an ion thruster and the

F i g. 2 another embodiment.

An ion engine 1 consists of two metallic hollow cylinders 2 and 3 arranged concentrically to one another, both of which together form a coaxial /./4- Lecher line and the space 4 between them is closed on one side by an annular multi-hole diaphragm 5. A Farnsworth tube 6 is arranged on the side of the resonator formed by the two hollow cylinders 2 and 3 opposite the multi-hole diaphragm 5. It consists of two dynodes 61, 62 made of a material with a secondary electron emission greater than 1. z. B. nickel or silver, and arranged between the dynodes 61 and 62 cylinder hair dryer. Lattice-shaped anode 63. A voltage source 7 is connected between the multi-pinhole diaphragm 5 and anode 63 in such a way that the anode 63 has a positive potential with respect to the dynodes 61.

Between the multi-hole diaphragm 5 and the Farnswoi tli tube 6 is one made of dielectric material, preferably Quartz, existing wall 8 arranged, which is located between a multi-hole diaphragm 5 and wall 8 Discharge space 9 separates from the high-frequency generator 6.

Emits one of the dynodes 61 or 62 of the Farnsworth tube 6, e.g. through brief heating Electrons, they are accelerated towards the anode by the positive potential and braked after passing through it. The potential of the anode 63 is dimensioned in this way. that the electrons hit one of the dynodes with a certain residual speed after the braking process impact and trigger secondary electrons from this.

These in turn, like the reversing primary electrons, are now towards the anode 62 accelerates and strike additional secondary electrons from the opposite dynode when they strike out, so that the initially present primary electrons are multiplied like an avalanche. on In this way, very large current and voltage amplitudes can be generated.

In order to enable the function of this high-frequency generator 6, which is designed as a Farnsworth tube, must be the length of the connected between the two dynodes and anode, from the two concentric Hollow cylinders 2 and 3 existing /. 4-Resonatois are tuned in such a way that its natural period on the drum running line between the two dynodes 61 and 62 is coordinated. In the Rcsonatonaum 4 This creates a standing high-frequency electrical wave, the node of which is at the multi-pinhole diaphragm 5 and whose belly lies on the high-frequency generator 6.

The generation of the high-frequency wave is a matter of course also possible in other ways, e.g. B.

by means of avalanche diodes, commercially available disc triodes or by means of transistor switches.

In the inner hollow cylinder 3 is a relative to this by support elements 10, 11 and 12 electrical isolated fuel evaporator 13 arranged. Liquid mercury is evaporated in the evaporator 13, which reaches the discharge space 9 via several channels 14. A small part of the vapor Mercury enters a hollow cathode 16 provided with a heating coil 15, which cathode 16 passes through an opening in the middle of the multi-hole acceleration electrode 17 protrudes. Before the Hollow cathode 16, an ignition electrode 18 is arranged in an insulated manner. Between the multi-hole diaphragm 5 and the Accelerating electrode 17 is applied a DC voltage, the negative pole of which with the accelerating electrode connected is. The acceleration electrode 17 is connected to the outer hollow cylinder via retaining clips 19 and insulators 20 2 arranged flange 21 mechanically connected.

The mode of operation of the ion thruster 1 is now as follows:

In the discharge space 9 a gas discharge of the mercury is generated by means of an ignition electrode 22 at a pressure of 10 " ² to 10" ³ Torr, which is then maintained by the high-frequency alternating field so that the mercury is ionized.

The electric field E acting on the charge carriers generated during ionization is in cylindrical coordinates written

E (r, 2, f) = E ₀ (r, z) cos (wt) ,

(D

where r is the radial, ζ is the axial coordinate, based on the two hollow cylinders, and t is the time. The door ί = 0 existing field component E ₀ can be represented for a value L 'of the voltage source 7

E (r. Z. 0) =

)· In

cos

(2)

(3)

35

40

where T _{1 is} the inner radius of the outer hollow cylinder 2 and r, the outer radius of the inner hollow cylinder 3 and / = λ / 4 the length of the hollow cylinder 2, 3.

The magnetic field β induced by the high-frequency electrical wave is connected to the electrical field β via the 1st Maxwell's equation, which is calculated taking into account that only the radial component E ₁ . of the electric and angular component of the magnetic field is present in cylindrical coordinates too

simplified.

By substituting equations (1) and (2) into equation (3) and then integrating them in parts after the time one obtains for the magnetic field

1.-7 U. / .τ \. 1

, ·, -, -; : · Sin ι ^ j ζ} sin (uf = - B ".

η 2 / r In Ir ₂ T ₁ ) \ 2 1 J c ^θ

(4)
which is analogous to equation II) also as

ß _e (rr 1) = ß _{e <1} (r. ζ) sin (Mf) (4a)

can be written.

A comparison with equation (1) shows that B _e is phase-shifted by 90 ° with respect to the radial electric field £ _r.

However, the movements of the electrons and ions oscillating radially _{in the electric field r} are also phase-shifted by 90 ″ with respect to E _{r , so that they are in phase with the magnetic field ß e} , as shown below.

The equations of motion for the charged particles are as follows

m,

d (

at

= - e E _r ,

= e £ _r .

(51)

(52)

where the index c or i denotes a variable associated with an electron or an ion, and V _T denotes the respective radial velocity components.

Obtained from (51) and (52) taking into account the equation (1)

e
w ih "

(r, 2) -sin

E ₀ (^ z) ■ sin

(61)

(62)

Since both the magnetic field B ₉ and the radial velocities of the electrons and ions change sinusoidally, i.e. they are in phase, Lorentz forces act on the charged particles, which are represented by the product of the charge with the vector product of the speed of the charge carriers and the magnetic field permit:

F ₁ , = -

F ₁ = E (f _ri ß _e).

(71)

(72)

If we put the expressions for the velocities and the magnetic field (61) in (71) and (72). (62) and (4a) a. so will

= ■ —— in ir. Ζ) · B ₀ ir. Ζ) · sin ² (\ vt).

55 ^F ' ⁼

w m

(71a)

(72a)

It can be seen that the Lorentz force acting on the electrons and ions is always positive, i. H. to the junction the standing wave is directed.

Although the forces acting on the electrons and ions cause different speeds in the axial direction due to their different mass, these speed differences are eliminated by charge separation forces, so that electrons and ions move at the same speed V. in the r-direction. From the

Equation of equation for the plasma

'"' ■ ~ df ⁺ '"' ■ 'ι' ⁼ ~ ^{e (K} '- ' ^{Βθ) + C {V} " ^Βθ)

(8th)
one therefore obtains for K. ,, = K ₁₁ - = K

dK
('», +'«,.) Rf = -C (V _n . - K _n -) ß _e . (Shark

IO

Since K _{n is} very small compared to K _ri . and also in, very small versus hi, is. for a simplified see equation (8a)

W St.

e ² ,

£ ,, (/ ·. 2) ■ ß _o (r. R) siir («·

(f c.-τ / L V. / .τ \

χ cos

rJ

sin ² (ui)

(9)

It can be seen from this that the acceleration in

d V
r-direction --r- = between ζ = O and r = 1, ie up to

to the multi-hole diaphragm 5 is positive. Expressed differently this means that the ions and electrons to the multi-pinhole 5 are pressed, so that there is a high plasma and a high current density immediately in front of it trains.

The ions arriving at the multi- pinhole diaphragm 5 at the speed V are drawn in a known manner by the acceleration electrode 17 from the discharge space 9 and further accelerated electrostatically. The in the discharge space 9

Any remaining electrons are transferred to the ignition electrode after the hollow cathode 16 has been ignited 18 forming plasma bridge is injected into the accelerated ion beam so that it is neutralized is

As can be seen from equation (9). can, through the length of the discharge space, that of electrons and Ions at the multi-hole diaphragm 5 reached speed and thus also the plasma density present there can be set.

The plasma density increasing in the direction of the multi-hole diaphragm is a further embodiment the device for neutralizing the accelerated ion beam is possible, compare FIG. 2. The discharge space 9 has openings 31 on its side facing the multi-hole diaphragm 5. which the discharge space 9 connect to a tubular hollow body 32. The tubular hollow body 32. by an opening 33 of the accelerator rod 17 protrudes, is also provided with openings 34 on its side facing away from the engine. It will Plasma in the discharge space 9 pressed against the multi-hole diaphragm 5, so a small part of the Plasma through the openings 31 into the hollow body 32 and from this via the openings 34 to the accelerated Ion beam and thus form a plasma bridge. Those left in the discharge space Electrons are drawn into the accelerated ion beam via this plasma bridge, which is responsible for the Electrons represents a positive anode. The ion beam is thereby neutralized.

This second embodiment of the ion thruster has the additional advantage that none is thermally loaded electrodes can be used more. It goes without saying that this is the case in the second exemplary embodiment proposed design of the neutralizer also possible with other ion thrusters, as long as these have a sufficient plasma density in the area of the multi-hole diaphragm.

For this 2 sheets of drawings

Claims (5)

Claims: 20 1. High-frequency ion thruster with a fuel delivery system, a high-frequency generator for ionizing the fuel, which is present in gaseous form, by means of high-frequency energy in one Gas discharge space, with an electrode to extract and accelerate the ions from the Gas discharge space and with a device for neutralizing the ion beam the passage through the extraction and acceleration electrode, characterized in that that the gas discharge space (9) in a Λ / 4 high frequency resonator through it one of the extraction and acceleration electrode (17) opposite multi-hole diaphragm (5) closed End and the high frequency generator at the other, open end in the high frequency resonator is arranged. 2. High-frequency ion engine according to claim 1, characterized in that the high-frequency resonator has two coaxial metallic hollow cylinders (2, 3) that in the annular space (4) between the discharge space (9) is arranged in the hollow cylinders (2, 3) and that in the inner hollow cylinder (3) the device for neutralizing the fuel delivery system (13) is arranged. 3. High-frequency ion engine according to claim 2, characterized in that the device for neutralization consists of a tubular hollow body (32) which on one side openings (31) assigned to the discharge space (9) and the accelerated one on another side Has openings (34) facing the ion beam. 4. High-frequency ion engine according to claim 1, characterized in that the high-frequency generator (6) is a Farnsworth tube. 5. High-frequency ion engine according to claim 1, characterized in that the high-frequency generator (6) is a disc triode.