Information
-
Patent Grant
-
6281622
-
Patent Number
6,281,622
-
Date Filed
Monday, August 23, 199924 years ago
-
Date Issued
Tuesday, August 28, 200122 years ago
-
Inventors
-
Original Assignees
-
Examiners
- Patel; Nimeshkumar D.
- Berck; Ken A
Agents
- Weingarten, Schurgin, Gagnebin & Hayes LLP
-
CPC
-
US Classifications
Field of Search
US
- 313 3621
- 313 161
- 315 11141
- 315 11121
- 315 11161
- 315 500
- 315 505
- 315 501
- 315 507
- 060 202
-
International Classifications
-
Abstract
The closed electron drift plasma thruster uses a magnetic circuit to create a magnetic field in a main annular channel for ionization and acceleration, said magnetic circuit comprises: an essentially radial first outer pole piece; a conical second outer pole piece; an essentially radial first inner pole piece; a conical second inner pole piece; a plurality of outer magnetic cores surrounded by outer coils to interconnect the first and second outer pole pieces; an axial magnetic core surrounded by a first inner coil and connected to the first inner pole piece; and a second inner coil placed upstream from the outer coils. The thruster also comprises a plurality of radial arms included in the magnetic circuit, and a structural base which is separate from the magnetic circuit and which serves, amongst other things, to cool the coils.
Description
FIELD OF THE INVENTION
The present invention relates to a closed electron drift plasma thruster adapted to high thermal loads, the thruster comprising a main annular channel for ionization and acceleration that is defined by parts made of insulating material and that is open at its downstream end, at least one hollow cathode disposed outside the main annular channel adjacent to the downstream portion thereof, an annular anode concentric with the main annular channel and disposed at a distance from the open downstream end, a pipe and a distribution manifold for feeding the annular anode with an ionizable gas, and a magnetic circuit for creating a magnetic field in the main annular channel.
PRIOR ART
Closed electron drift plasma thrusters having the structure shown in section in
FIG. 13
are already known, e.g. from document EP-A-0 541 309.
A thruster of that type comprises a cathode
2
, an anode-forming gas distribution manifold
1
, an annular acceleration channel (discharge chamber)
3
defined by inner and outer walls
3
a
and
3
b
, and a magnetic circuit comprising an outer pole
6
, an inner pole
7
, a central core
12
, a magnetic jacket
8
, an inner coil
9
, and an outer coil
10
.
The annular acceleration channel
3
is situated between an inner magnetic screen
4
and an outer magnetic screen
5
enabling the gradient of the radial magnetic field in the channel
3
to be increased. The channel
3
is connected to the outer pole piece
6
by a cylindrical metal part
17
.
From the thermal point of view, the channel
3
is surrounded not only by the magnetic screens
4
and
5
, but also by thermal screens
13
opposing radiation towards the axis and the central coil, and also to the outside. The only effective possibility for cooling by radiation is situated at the downstream end of the channel
3
which is open to space. As a result, the channel temperature is higher than it would be if the channel
3
could radiate through its outer lateral face.
Document WO 94/02738 discloses a closed electron drift plasma thruster
20
in which an acceleration channel
24
is connected upstream to a buffer or calming chamber
23
, as shown in
FIG. 14
which is an elevation view in half-axial section of such a structure.
The plasma thruster shown in
FIG. 14
comprises an annular main channel
24
for ionization and acceleration defined by parts
22
of insulating material and open at its downstream end
25
a
, at least one hollow cathode
40
, and an annular anode
25
concentric with the main channel
24
. Ionizable gas feed means
26
open out upstream of the anode
25
through an annular distribution manifold
27
. Means
31
to
33
and
34
to
38
for creating a magnetic field in the main channel
24
are adapted to produce a magnetic field in the main channel
24
that is essentially radial, having a gradient with maximum induction at the downstream end
25
a
of the channel
24
. These magnetic field creation means essentially comprise an outer coil
31
surrounded by magnetic shielding, outer and inner pole pieces
34
and
35
, a first axial core
33
, a second axial core
32
surrounded by magnetic shielding, and a magnetic yoke
36
.
The calming chamber
23
can radiate freely to space and thus contributes to cooling the channel
24
. However, the toroidal outer coil
31
opposes cooling of the channel
24
in its portion carrying the greatest heat load. In addition, the first inner coil
33
must provide a very high number of ampere-turns for the volume made available to it by the magnetic screen associated with the second axial coil
32
. This gives rise to a very high temperature.
Presently known closed electron drift plasma thrusters, which can also be referred to as stationary plasma thrusters, are used essentially for north-south control of geostationary satellites.
The structural characteristics of presently known closed electron drift plasma thrusters do not make it possible to optimize evacuation of the heat flux in operation. As a result, closed electron drift plasma thrusters cannot have a power level that is high enough to provide the primary propulsion of a mission such as transferring to geostationary orbit or an interplanetary mission, particularly since the ratio of area over dissipated power is smaller for a thruster that is large, which means that the temperature of a large plasma thruster of known type increases excessively, or that the mass of such a large known plasma thruster becomes excessive if heat flux is kept constant.
OBJECT AND BRIEF SUMMARY OF THE INVENTION
The invention seeks to remedy the above-specified drawbacks and to make it possible to optimize operation and heat flux evacuation in closed electron drift plasma thrusters in such a manner as to provide plasma thrusters of greater power than that of presently known closed electron drift plasma thrusters.
The invention thus seeks to propose a novel configuration for a closed electron drift thruster in which the thermal and structural design is improved compared with presently known plasma thrusters.
These objects are achieved by a closed electron drift plasma thruster adapted to high thermal loads, the thruster comprising a main annular channel for ionization and acceleration that is defined by parts made of insulating material and that is open at its downstream end, at least one hollow cathode disposed outside the main annular channel adjacent to the downstream portion thereof, an annular anode concentric with the main annular channel and disposed at a distance from the open downstream end, a pipe and a distribution manifold for feeding the annular anode with an ionizable gas, and a magnetic circuit for creating a magnetic field in the main annular channel,
wherein the magnetic circuit comprises:
an essentially radial first outer pole piece;
a conical second outer pole piece;
an essentially radial first inner pole piece;
a conical second inner pole piece;
a plurality of outer magnetic cores surrounded by outer coils to interconnect the first and second outer pole pieces;
an axial magnetic core surrounded by a first inner coil and connected to the first inner pole piece; and
a second inner coil placed upstream from the outer coils.
The presence of a plurality of outer magnetic cores interconnecting the first and second outer pole pieces allows a large portion of the radiation coming from the inner wall of the ceramic channel to pass between them. The conical shape of the second outer pole piece makes it possible to increase the volume available for the outer coils and to increase the solid angle over which radiation can take place. The conical shape of the second inner pole piece also makes it possible to increase the volume available to the first inner coil while still channelling the magnetic flux so as to perform a shielding function for the second inner coil.
Advantageously, the plasma thruster has a plurality of radial arms connecting the axial magnetic core to the upstream portion of the conical second inner pole piece, and a plurality of second radial arms extending the first radial arms and connected to said plurality of outer magnetic cores and to the upstream portion of the conical second outer pole piece.
The number of first radial arms and the number of second radial arms is equal to the number of outer magnetic cores.
A small gap is left between each first radial arm and the corresponding second radial arm, so as to add to the effect of the second inner coil.
In a remarkable aspect of the present invention, the plasma thruster includes a structural base of a material that is a good conductor of heat which constitutes a mechanical support of the thruster, which is distinct from the axial magnetic core, from the first and second outer pole pieces, and from the first and second inner pole pieces, and which serves to cool the first inner coil, the second inner coil, and the outer coils by conduction.
Advantageously, the structural base is covered on its lateral faces in an emissive coating.
Advantageously, the main annular channel has a section in an axial plane that is frustoconical in shape in its upstream portion and cylindrical in shape in its downstream portion, and the annular anode has a section in an axial plane that tapers in the form of a truncated cone.
According to a particular characteristic, the parts defining the main annular channel define an annular channel in the form of a single block, are connected to the base by a single support provided with expansion slots, and are secured to the single support by screw engagement.
In another particular embodiment, the annular main channel has a downstream end defined by two ring-shaped parts made of an insulating ceramic and each connected to the base via an individual support, and the upstream portion of the annular main channel is embodied by the walls of the anode which is electrically isolated from the supports by vacuum. The individual supports are coaxial.
By way of example, the ratio between the axial length of the parts made of insulating ceramic and the width of the channel lies in the range 0.25 to 0.5, and the distance between the walls of the anode and the support of the parts made of insulating ceramic lies in the range 0.8 mm to 5 mm.
The anode is fixed relative to the base by means of a solid column and by flexible blades.
Recesses can be milled in the base to receive the second radial arms, the ionizable gas feed pipe fitted with an isolator, a line for biasing the anode, and wires for powering the outer coils and the first and second inner coils.
Because of the presence of a structural base, the magnetic circuit can perform essentially the function of channelling the magnetic flux, while the solid base made of a material that is a good conductor of heat, e.g. a light alloy anodized on its lateral face, or of composite carbon-carbon material coated on its downstream face with a deposit of copper, serves simultaneously to cool the coils by conduction and to evacuate the heat losses by radiation, and also to provide the structural strength of the thruster.
The plasma thruster includes sheets of super-insulation material disposed upstream of the main annular channel, and sheets of super-insulation material interposed between the main annular channel and the first inner coil.
In a first possible configuration, the cone of the conical upstream second inner pole piece points downstream.
In another possible configuration, the cone of the conical upstream second inner pole piece points upstream.
According to another particular characteristic of the invention, the plasma thruster includes a common support for supporting the first inner coil, the conical second inner pole piece, and the second inner coil which are fixed to said common support by brazing or by diffusion welding, and said common support is assembled on the base by screw means with a thermally conductive sheet being interposed therebetween.
In a particular embodiment, in order to improve the cooling of the first inner coil which carries the greatest thermal loading, the first inner coil is cooled by a heat pipe connected to the inner portion of the common support and situated in a recess of the magnetic core.
In a variant, the first inner coil is cooled by a plurality of heat pipes connected to the upstream portion of the common support and passing through orifices formed in the second inner pole piece.
Preferably, the conical second outer pole piece has openings therein.
The first and second outer pole pieces are mechanically connected together by a non-magnetic structural link piece that has openings.
In a variant embodiment, the outer magnetic cores of the outer coils are inclined at an angle β relative to the axis of the thruster in such a manner that the axes of the outer magnetic cores are substantially perpendicular to the bisector of the angle formed by the generator lines of the cones of the first and second outer pole pieces.
According to another particular characteristic, the annular anode includes a manifold provided with internal baffles and having a plane downstream plate co-operating with the walls of the main channel to define two annular diaphragms, a rear plate fitted to the walls of the main channel to limit gas leakage in the upstream direction, and cylindrical walls provided with holes for injecting ionizable gas into the main channel.
BRIEF DESCRIPTION OF THE DRAWINGS
Other characteristics and advantages of the invention appear from the following description of particular embodiments given as examples and with reference to the accompanying drawings, in which:
FIG. 1
is an axial half-section view of a first particular embodiment of a closed electron drift plasma thruster of the invention;
FIG. 2
is a partially cutaway perspective view of the
FIG. 1
plasma thruster;
FIG. 3
is a perspective view of the central portion of a plasma thruster of the invention fitted with heat pipes;
FIG. 4
is a perspective and axial section view of an anode suitable for incorporation in a plasma thruster of the invention;
FIG. 5
is a fragmentary perspective and axial half-section view of another anode of simplified structure suitable for incorporation in a plasma thruster of the invention;
FIG. 6
is an elevation view in half-section of an annular channel support for a particular embodiment of a plasma thruster of the invention;
FIG. 7
is an exploded view of the central portion of a plasma thruster of the invention;
FIG. 8
is a section showing a heat pipe associated with a first inner coil of a plasma thruster of the invention;
FIG. 9
is a perspective view showing structural reinforcement between the outer pole pieces of a magnetic circuit of a plasma thruster of the invention;
FIG. 10
is a fragmentary diagrammatic view showing a particular embodiment of a plasma thruster fitted with inclined outer coils, in a variant embodiment of the invention;
FIG. 11
is a fragmentary view in axial half-section showing an anode forming a portion of the body of an acceleration channel in a particular embodiment of a plasma thruster of the invention;
FIG. 12
is an axial half-section view of another particular embodiment of a closed electron drift plasma thruster of the invention;
FIG. 13
is an axial half-section view of a first prior art closed electron drift plasma thruster; and
FIG. 14
is an elevation and axial half-section view of a second prior art closed electron drift plasma thruster.
DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS OF THE INVENTION
Reference is made initially to
FIGS. 1 and 2
showing a first example of a closed electron drift plasma thruster of the present invention.
The closed electron drift plasma thruster of
FIGS. 1 and 2
comprises a main annular channel
124
for ionization and acceleration which is defined by insulating walls
122
. The channel
124
is open at its downstream end
125
a
and in an axial plane its section is frustoconical in shape in its upstream portion and cylindrical in its downstream portion.
A hollow cathode
140
is disposed outside the main channel
124
and is advantageously at an angle α relative to the axis X′X of the thruster, where a lies in the range 15° to 45°.
In an axial plane, an annular anode
125
has a tapering section in the form of a truncated cone that is open in a downstream direction.
The anode
125
can have slots increasing its surface area in contact with the plasma. Holes
120
for injecting an ionizable gas coming from an ionizable gas distribution manifold
127
are formed through the wall of the anode
125
. The manifold
127
is fed with ionizable gas by a pipe
126
.
Particular examples of the anode
125
are described below with reference to
FIGS. 4 and 5
.
The discharge between the anode
125
and the cathode
140
is controlled by a magnetic field distribution that is determined by a magnetic circuit.
The magnetic circuit comprises a first outer pole piece
134
that is essentially radial. This outer pole piece
134
can be plane or slightly conical defining an angle e
1
lying in the range +15° to −15° relative to the outlet plane S (FIG.
1
).
The outer pole piece
134
is connected by a plurality of magnetic cores
137
surrounded by outer coils
138
to a second outer pole piece
311
of conical shape that is more marked than the possibly slightly conical shape of the first outer pole piece
134
. The half-angle e
2
of the cone of the outer pole piece
311
can lie in the range 25° to 60°. The outer pole piece
311
is advantageously open in register with the passages for the outer coils
131
so as to reduce radial size, and between the coils so as to improve cooling by radiation from the ceramic constituting the walls
122
of the channel
124
.
An essentially radial first inner pole piece
135
can be plane or can be slightly conical, defining an angle i
1
lying in the range −15° to +15° relative to the outlet plane S.
The first inner pole piece
135
is extended by a central axial magnetic core
138
surrounded by a first inner coil
133
. The axial magnetic core
138
is itself extended at the upstream portion of the thruster by a plurality of radial arms
352
connected to a second inner pole piece
135
that is upstream and conical, presenting a half-angle i
2
lying in the range 15° to 45° relative to the axis X′X of the thruster. In the embodiment of
FIGS. 1 and 2
, the cone of the second inner pole piece
351
points downstream. Throughout the present description, the term “downstream” means towards a zone close to the outlet plane S and the open end
125
a
of the channel
124
, while the term “upstream” means towards a zone remote from the outlet plane S going towards the closed portion of the annular channel
124
that is fitted with the anode
125
and the ionizable gas feed manifold
127
.
A second inner magnetic coil
132
is placed outside the upstream portion of the second inner pole piece
351
. The magnetic field of the second inner coil
132
is channelled by radial arms
136
placed in line with the radial arms
352
, and by the outer pole piece
311
and the inner pole piece
351
. A small gap, e.g. about 1 mm to 4 mm across can be left between the radial arms
352
and the radial arms
136
so as to complete the effect of the second inner coil
132
.
The axial magnetic core
138
is connected to the outer magnetic cores
137
by a plurality of magnetic arms
136
placed in line with the radial arms
352
. The number of radial arms
352
and the number of radial arms
136
is equal to the number of outer coils
131
placed on the outer magnetic cores
137
.
According to an important aspect of the present invention, the coils
133
,
131
, and
132
are cooled directly by conduction via a structural base
175
of heat-conductive material, said base
175
also serving as a mechanical support for the thruster. The base
175
is advantageously provided on its lateral faces with an emissive coating for improving the radiation of heat losses into space.
The base
175
can be made of light alloy, being anodized on its lateral face so as to increase its emissivity.
The base
175
can also be made of a carbon-carbon composite material coated on its downstream face with a deposit of a metal such as copper so as to maximize the emissivity of the lateral faces and minimize the absorptivity of the downstream face subject to radiation from the ceramic of the channel.
The presence of a massive base
175
acting both as a structural support and as means for cooling the coils
131
,
133
, and
132
by conduction makes it possible for the magnetic circuit proper to be lightened as much as possible.
In the example of
FIGS. 1 and 2
, the magnetic circuit has four outer coils
131
, two of which can be seen in FIG.
2
. Nevertheless, it would be possible to implement a number of outer coils
131
other than four.
The outer coils
131
and the associated magnetic cores
137
serve to create a magnetic field that is channelled in part by the downstream and upstream outer pole pieces
134
and
311
. The remainder of the magnetic field is taken up by the arms
136
grouped around the axial magnetic core
138
which is itself provided with the downstream inner pole piece
135
, the first axial coil
133
, the upstream conical second pole piece
351
, and the second coil
132
.
The magnetic flux supplied by the coil
132
is channelled by the pole piece
351
, the core
138
, the radial arms
136
, and the pole piece
311
, so that the coil
132
has no need for special magnetic shielding.
With reference to
FIG. 7
, it can be seen that the coil
133
, the pole piece
351
, and the coil
132
co-operate with a common support
332
to form an assembly which counts as a single block both mechanically and thermally speaking, this single block assembly being energetically cooled by conduction via the base
175
.
The coil
133
, the pole piece
351
, and the coil
132
can be secured to the common support
332
by brazing or by diffusion welding. The support
332
can itself be assembled to the base
175
by means of a screw. A conductive sheet is interposed between the base
175
and the support
332
so as to reduce the thermal resistance of the contact between them. The bore inside the pole piece is fitted to the axial magnetic core
138
so as to enable the two inner coils
133
and
132
and the pole piece
351
to be mounted together on the core
138
.
In traditional plasma thrusters, the structure
122
of ceramic material defining the annular channel
124
is held relative to the outer pole piece by a metal support.
In the present invention, as shown for example in
FIGS. 1
,
2
, and
6
, the structure
122
of ceramic material defining the channel
124
is fixed to the rear (i.e. upstream end) of the thruster by a metal support
162
so that the support does not constitute an obstacle to radiation from the downstream portion of the part
122
which is thus free to radiate into space.
Certain ceramics based on boron nitride are difficult to braze to metals. This problem can be eliminated if a mechanical fastening is used.
By way of example, it is possible to provide a thread of semicircular profile both in the part
122
made of ceramic material and in the support
162
. It is then possible to slide a wire
163
between the two parts
122
and
162
so as to hold them together. Such a disposition makes it possible to install the ceramic part
122
on the support
162
that has previously been mounted on the elements of the magnetic circuit.
The metal support
162
can be provided with a rib
165
and with notches
164
defining fingers making it possible to compensate differential expansion between the metal and the ceramic while also providing resilient clamping.
In a variant, it is also possible to use a mount in which the ceramic
122
is screwed into the support
162
, with the fixing stub of the support then being inverted, i.e. facing towards the inside of the cylindrical support
162
, and having openings to pass the wire
142
for biasing the anode and the pipe
126
for feeding the manifold
127
with ionizable gas.
FIG. 11
shows another variant embodiment of the channel
124
.
For a thruster that delivers high thrust, i.e. that is of large diameter, it is difficult to make a one-piece ceramic part to define the annular channel
124
. Under such circumstances, the part
122
that is made of ceramic material is subdivided into two distinct rings
122
a
and
122
b
that are mounted on distinct supports
162
a
and
162
b.
The ratio between the length of the ring-shaped ceramic supports
122
a
and
122
b
to the width of the channel
124
can typically lie in the range 0.25 to 0.5. The remainder of the channel
124
is embodied by the walls of the anode
125
. Electrical insulation between the anode
125
and the two supports
162
a
and
162
b
is provided by the vacuum. The distance between the walls of the anode
125
and the supports
162
a
and
162
b
constitutes a small amount of clearance lying in the range 0.8 mm to 5 mm.
The anode
125
shown in
FIG. 11
is supported by isolators such as
151
fixed on the solid base
175
which constitutes a natural electrostatic screen for the isolators such as
151
. The isolators
151
are extended by flexible blades
115
a
which protect them from differential expansion forces.
For a plasma thruster of large diameter, it can also be advantageous to implement an upstream inner pole piece
351
whose cone points upstream rather than downstream. The large diameter of the coil
133
in its downstream portion makes it possible to compensate the fact that the coil is of upstream section that is smaller than a large-based trapezium shape, which can make it easier to integrate ring supports
162
a
and
162
b
associated with separate rings
122
a
and
122
b.
Nevertheless, it should be observed that for plasma thrusters of diameter that is not too great, making the upstream inner pole piece
351
in the form of a cone pointing downstream makes it possible to increase the area of contact between the coil
133
of trapezium-shaped section and the base
175
(
FIG. 1
) while retaining a large volume for the downstream inner coil
133
without that making it necessary to act on the positions of the ends
111
and
112
of the pole pieces
351
and
135
which determine how the magnetic field is distributed.
The use of outer coils
131
(of which there may be three to eight, for example) fitted with magnetic cores
137
disposed between the outer pole pieces
134
and
311
allows a large portion of the radiation coming from the outer wall of the annular channel
124
to escape. The conical shape of the second outer pole piece
311
makes it possible to increase the volume available for the outer coils
131
and to increase the solid angle over which radiation takes place. The conical outer pole piece
311
is also advantageously provided with openings to increase the visible fraction of the ceramic parts
122
so as to obtain a magnetic circuit that is very compact and with a large amount of open space, thereby allowing radiation to take place from the entire lateral face of the channel
124
.
As already mentioned, the base
175
performs an essentially structural function. This solid base
175
has a resonant frequency that is high. The same must be true of the pole pieces. Unfortunately, if openings are made in the upstream outer pole piece
311
, then its resonant frequency becomes relatively low. Similarly, the essentially plane shape of the downstream outer pole piece
134
also gives rise to a resonant frequency that is not very high. To remedy this problem, it is possible to interpose a non-magnetic link piece
341
(
FIG. 9
) of essentially conical shape between the two pole pieces
311
and
134
. To allow radiation to take place, the piece
341
must itself be very open, however that does not harm its resonant frequency since the trellis-shaped elements of which it is constituted work essentially in traction and in compression.
In a variant embodiment, shown in
FIG. 10
, the ratio between the shape of the pole pieces
134
and
311
and the volume available for the outer coils is improved by inclining the axes of the coils. Thus, if the outer coils
131
form an angle β with the axis X′X of the thruster, such that the axis of an outer coil
131
is substantially perpendicular to the bisector of the angle u formed by the generator lines of the cones of the two pole pieces
134
and
311
, then an outer coil
131
can be of larger volume and the size of the base
175
can be reduced. As shown in
FIG. 10
, where the channel
124
, the coils
133
and
132
, and the pole piece
351
have been omitted for reasons of clarity, it is quite possible to combine the use of inclined outer coils
131
with an outer conical pole piece
311
having openings.
As already mentioned above, the base
175
plays an essential role in cooling by conduction of the common support
322
, the coils
133
and
132
, and the pole piece
351
which is itself advantageously provided with notches as shown in FIG.
2
.
However, cooling of the coil
133
which carries the greatest thermal load can be improved by using one or more heat pipes. Thus, in
FIG. 8
, there can be seen a heat pipe
433
organized in a recess
381
of the axial magnetic core
138
, but not coming into contact therewith. The heat pipe
433
can be welded or brazed to the inner face of the inner support
332
of the coil
133
, so as to make the support
332
isothermal.
FIG. 3
shows a coil
133
cooled by a plurality of heat pipes
433
a
,
433
b
connected to the upstream portion of the support for the coil
133
, and passing through orifices formed in the upstream inner pole piece
351
.
With reference again to
FIGS. 1 and 2
there can be seen sheets of super-insulating material forming a screen
130
placed upstream of the annular channel
124
, and sheets of super-insulating material
301
forming a screen which are interposed between the channel
124
and the first inner coil
133
. The super-insulating screens
130
and
301
thus eliminate the main part of the flux radiated by the channel
124
towards the inner coils
133
,
132
and the base
175
. In contrast, the parts
122
defining the channel
124
are free to radiate into space through the solid angle between the pole pieces
134
and
311
.
In the embodiment of
FIG. 11
, an electrostatic screen
302
is disposed upstream from the anode
125
to ensure that Paschen's law is complied with (insulation by vacuum) while contributing to holding the sheets of super-insulating material
130
in place. In addition, the outer face of the outer support
162
a
can receive an emissive coating to improve cooling of the ceramic of the parts
122
a
and
122
b.
FIG. 12
shows a particular embodiment of a plasma thruster of the invention in which the cone of the upstream second inner pole piece
351
points upstream. This disposition is more particularly adapted to thrusters of large diameter, but it can be used equally well with an acceleration channel
124
defined by a one-piece part
122
of ceramic material as shown in
FIG. 12
, or with an acceleration channel
124
defined by two distinct parts
122
a
and
122
b
of ceramic material, as described above with reference to FIG.
11
. In
FIG. 12
, the various elements functionally equivalent to elements described above with reference to the above-described figures, and in particular
FIGS. 1 and 2
, are given the same reference numerals, and they are not described again.
As can be seen in
FIG. 12
, recesses or milled portions
751
are formed in the base
175
to receive the second radial arms
136
, a line
145
for biasing the anode
125
, and wires
313
,
323
, and
333
for powering the outer coils
131
and the first and second inner coils
133
,
132
(FIGS.
7
and
12
). A recess can also be formed in the base
175
to receive the pipe
126
for feeding ionizable gas and provided with an isolator
300
(shown for example in FIG.
4
).
Advantageously, the outer coils
131
and the first and second inner coils
133
and
132
are made of shielded wire with mineral insulation. The wires of the various turns of the coils
131
,
132
, and
133
are secured by a brazing metal having high thermal conductivity.
The outer coils
131
and the first and second inner coils
133
and
132
are connected in series and are electrically connected to the cathode
140
and to a negative pole of the electrical power for anode-cathode discharge.
In prior art embodiments, such as the embodiment shown in
FIG. 14
, an annular buffer chamber
23
is implemented of size in the radial direction that is not less than the size of the main annular channel
24
and that extends upstream therefrom beyond the zone in which the annular anode
25
is placed.
In an embodiment of the invention of the kind shown in
FIG. 1
, a more compact disposition is obtained by using a main annular channel
124
which is of a section in an axial plane that is frustoconical in shape in its upstream portion, and cylindrical in shape in its downstream portion. Under such circumstances, the annular anode
125
has, in an axial plane, a tapering section in the form of a truncated cone.
It has been observed that a calming chamber effect can be obtained in the main channel
124
by increasing gas density locally, i.e. by reducing the gas flow section in the upstream direction, instead of increasing it.
FIG. 4
shows one possible embodiment of the annular anode
125
. A series of circular slots
117
formed in the solid portion
116
of the anode
125
makes it possible to provide protection against contamination. The ionizable gas is introduced via a rigid pipe
126
into a distribution chamber
127
which communicates with the circular slots
117
via injection holes
120
. An isolator
300
is interposed between the pipe
126
and the anode
125
which is connected by an electrical connection
145
to the positive pole of the electrical power supply for anode-cathode discharge.
It is also appropriate to be able to remedy problems of differential expansion between the anode
125
and the parts
122
that are made of ceramic material and that define the channel
124
.
For a solid anode fixed on three circular columns, it is possible to find an acceptable compromise between a high natural frequency of vibration as is obtained with columns that are short and acceptable thermomechanical stresses which require columns that are long.
One possible solution is shown in FIG.
4
. The anode
125
is supported both by a solid column
114
of circular section and by two columns
115
that have been thinned-down to form flexible blades, thereby achieving a compromise that is satisfactory from the point of view of differential thermal expansion.
FIG. 5
shows another possible embodiment of an anode
125
placed in the frustoconical portion of an acceleration channel
124
. In this case, the annular anode
125
has a manifold
127
fitted with internal baffles
271
and including a downstream plane plate
272
co-operating with the walls of the main channel
124
to define two annular diaphragms
273
. A rear plate
274
is fitted on the walls
122
of the main channel
124
to limit gas leakage in an upstream direction. Cylindrical walls with holes
120
enable the ionizable gas to be injected into the main channel
124
.
Claims
- 1. A closed electron drift plasma thruster adapted to high thermal loads, the thruster comprising a main annular channel for ionization and acceleration that is defined by parts made of insulating material and that is open at its downstream end, at least one hollow cathode disposed outside the main annular channel adjacent to the downstream portion thereof, an annular anode concentric with the main annular channel and disposed at a distance from the open downstream end, a pipe and a distribution manifold for feeding the annular anode with an ionizable gas, and a magnetic circuit for creating a magnetic field in the main annular channel,wherein the magnetic circuit comprises: an essentially radial first outer pole piece; a conical second outer pole piece; an essentially radial first inner pole piece; a conical second inner pole piece; a plurality of outer magnetic cores surrounded by outer coils to interconnect the first and second outer pole pieces; an axial magnetic core surrounded by a first inner coil and connected to the first inner pole piece; and a second inner coil placed upstream from the outer coils.
- 2. A plasma thruster according to claim 1, including a plurality of radial arms connecting the axial magnetic core to the upstream portion of the conical second inner pole piece, and a plurality of second radial arms extending the first radial arms and connected to said plurality of outer magnetic cores and to the upstream portion of the conical second outer pole piece.
- 3. A plasma thruster according to claim 2, wherein the number of first radial arms and the number of second radial arms is equal to the number of outer magnetic cores.
- 4. A plasma thruster according to claim 2, wherein a small gap is left between each first radial arm and the corresponding second radial arm.
- 5. A plasma thruster according to claim 1, wherein the main annular channel has a section in an axial plane that is frustoconical in shape in its upstream portion and cylindrical in shape in its downstream portion, and wherein the annular anode has a section in an axial plane that tapers in the form of a truncated cone.
- 6. A plasma thruster according to claim 1, including a structural base of a material that is a good conductor of heat which constitutes a mechanical support of the thruster, which is distinct from the axial magnetic core, from the first and second outer pole pieces, and from the first and second inner pole pieces, and which serves to cool the first inner coil, the second inner coil, and the outer coils by conduction.
- 7. A plasma thruster according to claim 6, wherein the structural base is covered on its lateral faces in an emissive coating.
- 8. A plasma thruster according to claim 6, wherein the parts defining the main annular channel define an annular channel in the form of a single block, are connected to the base by a single support provided with expansion slots, and are secured to the single support by screw engagement.
- 9. A plasma thruster according to claim 6, wherein the annular main channel has a downstream end defined by two ring-shaped parts made of an insulating ceramic and each connected to the base via an individual support, and wherein the upstream portion of the annular main channel is embodied by the walls of the anode which is electrically isolated from the supports by vacuum.
- 10. A plasma thruster according to claim 9, wherein the ratio between the axial length of the parts made of insulating ceramic and the width of the channel lies in the range 0.25 to 0.5, and wherein the distance between the walls of the anode and the support of the parts made of insulating ceramic lies in the range 0.8 mm to 5 mm.
- 11. A plasma thruster according to claim 9, wherein the anode is fixed relative to the base by means of a solid column and by flexible blade.
- 12. A plasma thruster according to claim 2, including a structural base of a material that is a good conductor of heat which constitutes a mechanical support of the thruster, which is distinct from the axial magnetic core, from the first and second outer pole pieces, and from the first and second inner pole pieces, and which serves to cool the first inner coil, the second inner coil, and the outer coils by conduction, wherein recesses are milled in the base to receive the second radial arms, the ionizable gas feed pipe fitted with an isolator, a line for biasing the anode, and wires for powering the outer coils and the first and second inner coils.
- 13. A plasma thruster according to claim 1, including sheets of super-insulation material disposed upstream of the main annular channel, and sheets of super-insulation material interposed between the main annular channel and he first inner coil.
- 14. A plasma thruster according to claim 1, wherein the cone of the conical upstream second inner pole piece points downstream.
- 15. A plasma thruster according to claim 1, wherein the cone of the conical upstream second inner pole piece points upstream.
- 16. A plasma thruster according to claim 6, including a common support for supporting the first inner coil, the conical second inner pole piece, and the second inner coil which are fixed to said common support by brazing or by diffusion welding, and wherein said common support is assembled on the base by screw means with a thermally conductive sheet being interposed therebetween.
- 17. A plasma thruster according to claim 16, wherein the first inner coil is cooled by a heat pipe connected to the inner portion of the common support and situated in a recess of the magnetic core.
- 18. A plasma thruster according to claim 16, wherein the first inner coil is cooled by a plurality of heat pipes connected to the upstream portion of the common support and passing through orifices formed in the second inner pole piece.
- 19. A plasma thruster according to claim 1, wherein the conical second outer pole piece has openings therein.
- 20. A plasma thruster according to claim 19, wherein the first and second outer pole pieces are mechanically connected together by a non-magnetic structural link piece that has openings.
- 21. A plasma thruster according to claim 1, wherein the outer magnetic cores of the outer coils are inclined at an angle β relative to the axis of the thruster in such a manner that the axes of the outer magnetic cores are substantially perpendicular to the bisector of the angle formed by the generator lines of the cones of the first and second outer pole pieces.
- 22. A plasma thruster according to claim 1, wherein the annular anode includes a manifold provided with internal baffles and having a plane downstream plate co-operating with the walls of the main channel to define two annular diaphragms, a rear plate fitted to the walls of the main channel to limit gas leakage in the upstream direction, and cylindrical walls provided with holes for injecting ionizable gas into the main channel.
- 23. A plasma thruster according to claim 6, wherein the base is made of light alloy that is anodized on its lateral face.
- 24. A plasma thruster according to claim 6, wherein the base is made of carbon-carbon composite material coated on its downstream face with a deposit of copper.
- 25. A plasma thruster according to claim 1, wherein the outer coils and the first and second inner coils are made of shielded mineral-insulated wire and wherein the wires of the various turns of the coils are held together by a brazing metal having high thermal conductivity.
- 26. A plasma thruster according to claim 1, wherein the outer coils and the first and second inner coils are connected in series and are electrically connected to the cathode and to a negative pole of the electrical power supply for anode-cathode discharge.
- 27. A plasma thruster according to claim 1, wherein the conical second outer pole piece has a cone half-angle lying in the range 25° to 60°.
- 28. A plasma thruster according to claim 1, wherein the conical second inner pole piece has a half-angle relative to the axis of the thruster lying in the range 15° to 45°.
Priority Claims (1)
Number |
Date |
Country |
Kind |
98 10674 |
Aug 1998 |
FR |
|
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Number |
Name |
Date |
Kind |
5763989 |
Kaufman |
Jun 1998 |
|
6158209 |
Latischev et al. |
Dec 2000 |
|
6208080 |
King et al. |
Mar 2001 |
|
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Number |
Date |
Country |
0541309 A1 |
May 1993 |
EP |
0781921 A1 |
Jul 1997 |
EP |
2693770 |
Jan 1994 |
FR |