Closed electron drift plasma thruster adapted to high...

Details Closed electron drift plasma thruster adapted to high... Closed electron drift plasma thruster adapted to high...

1999-08-23

2001-08-28

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

With positive or negative ion acceleration

Supplying ionizable material

C313S161000, C313S231010, C313S231510, C315S500000, C315S501000, C315S505000, C315S507000, C060S202000

Reexamination Certificate

active

06281622

ABSTRACT:

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

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