Note: Descriptions are shown in the official language in which they were submitted.
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A HALL EFFECT PLASMA ACCELERATOR
This invention relates to a Hall effect plasma accelerator, sometimes
known as a closed electron drift accelerator. The invention arose when
considering the design of such accelerators for use as thrusters on satellites
or
other spacecraft. However, it is also applicable to accelerators intended for
other
uses, for example plasma etching and machining workpieces in a vacuum.
A conventional Hall effect accelerator comprises an annular accelerating
IS channel extending circumferentially around an axis of the accelerator and
also
extending in an axial direction from a closed end to an open end. An anode is
located, usually at the closed end of the channel, and a cathode is positioned
outside the channel close to its open end. Means is provided for introducing a
propellant, for example xenon gas, into the channel and this is often done
through passages formed in the anode itself or close to the anode. A magnetic
system applies a magnetic field in the radial direction across the channel and
this
causes electrons emitted from the cathode to move circumferentially around the
channel. Some but not all of the electrons emitted from the cathode pass into
the
channel and are attracted towards the anode. The radial magnetic field
deflects
the electrons in a circumferential direction so that they move in a spiral
trajectory, accumulating energy as they gradually drift towards the anode. In
a
region close to the anode the electrons collide with atoms of the propellant,
causing ionization. The resulting positively charged ions are accelerated by
the
electric field towards the open end of the channel, from which they are
expelled
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at great velocity, thereby producing the desired thrust. Because the ions have
a
much greater mass than the electrons, they are not so readily influenced by
the
magnetic field and their direction of acceleration is therefore primarily
axial
rather than circumferential with respect to the they are neutralized by those
electrons from the cathode that do not pass into the channel.
In this specification the terms "upstream" and "downstream" will be used
for convenience to describe directions with reference to the movement of ions
in
the channel.
Conventionally, the required radial magnetic field has been applied across
the channel using an electromagnet having a yoke of magnetic material which
defines poles on opposite sides of the channel, i. e. one radially inwardly
with
respect to the channel and the other radially outwardly with respect to the
channel. An example is shown in European patent specification 0 463 408 which
shows a magnetic yoke having a single cylindrical portion passing through the
middle of the annular channel and carrying a single magnetizing coil; and a
number of outer cylindrical members spaced around the outside of the
accelerating channel and carrying their own outer coils. The inner and outer
cylindrical members are bolted to a magnetic back plate so as to form a single
magnetic yoke. Another similar arrangement is shown in European patent
specification 0 541 309.
It is well known that it is important to achieve a well defined distribution
of the magnetic field within the channel and various arrangements of coils and
magnetic bodies have been proposed in the past for this purpose. For example,
Russian Patent Specification 2022167 describes arrangements of up to sixteen
coils and magnetic screens.
Such attempts to achieve the optimum distribution of magnetic field
within the channel can conflict with the need to keep the weight and
complexity
of the accelerator to a minimum when it is designed for use as a satellite
thruster.
Also an important factor, recognized in Russian Patent Specification 2022167,
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that needs to be taken into consideration is that temperatures inside the
channel
are very high and the coils need to be isolated from such high temperatures to
prevent damage.
According to a first aspect, this invention provides a Hall effect plasma
accelerator comprising a substantially annular accelerating channel having
closed
and open ends and a source of magnetic field positioned behind the closed end
of the channel and having an axis extending in the same direction as the axis
of
the channel.
According to a second aspect, this invention provides a Hall effect plasma
accelerator comprising a substantially annular accelerating channel having
closed
and open ends and a source of magnetic field positioned behind the closed end
of the channel and extending around the axis of the channel.
By using the invention it is possible to provide a Hall effect accelerator
with an optimum distribution of magnetic field inside the acceleration channel
by
means of a simpler and less heavy arrangement using a single source of
magnetic
field, such as a single coil or permanent magnet. The simpler design which
results is considered particularly suitable for relatively small accelerators
and
allows the source of magnetic field to be positioned away from the
accelerating
channel thereby reducing the heating effect in the coil resulting from heat
transferhed from the channel. The position of the source of magnetic field
behind
the accelerating channel can also provide improved cooling of the source of
magnetic field in operation, thereby further reducing the chance of damage
through excessive heat. In pais aligned with an outer wall of the accelerator
offers considerable heat advantages.
The shape of the substantially annular accelerating channel is not limited
to a circular cross-section but could have an elongated, polygonal or
irregular
form. The source of magnetic field (which might be a permanent magnet or an
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electromagnet depending upon the requirements) has an axis extending in the
same direction as the axis of the channel, that is to say at least a component
of
the axis of the source of magnetic field extends in the direction of the axis
of the
channel. The axis of the source of magnetic field does not have to be parallel
to
the axis of the channel.
The accelerator preferably includes a first magnetic body, which defines
magnetic poles radially inwardly and outwardly of the channel. This first
magnetic body may substantially or partially enclose or merely be in proximity
to the source of magnetic field, depending on the specific application. For
example, a more open stnlcture without the coil being substantially enclosed
may
be preferred where the cooling of the coil is critical.
The first magnetic body preferably includes two main inner and outer
walls which may be generally cylindrical in form or of another suitable form
as
appropriate to the shape of the accelerating channel being used and which
extend
from respective poles close to the open end of the channel, respectively
inside
and outside of the channel, to positions behind the closed end of the channel.
A
linking part of the first magnetic body behind the closed end of the channel
may
enclose the space between those walls to a fuller or lesser extent depending
on
the requirements for the magnetic field and for the reduction of heat levels
in the
particular application. This linking part preferably defines, possibky in co-
operation with the inner and/or outer, main wall (or an extension thereof) an
annular space, co-axial with the axis of the channel. This annular space
houses
the source of magnetic field and in a preferred arrangement, its outer wall is
defined by an upstream extension of the main outer wall so that the source of
magnetic field is located as far as reasonably possible from the source of
heat and
so that the surface area available for radiation of heat is maximized. The
actual
shape of the linking part will depend on the form of magnetic source used and
may comprise a single or a number of straight or curved sections.
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A prefen;ed feature of the invention is the inclusion of a second magnetic
body magnetically separate from and enclosed within the first magnetic body.
This second magnetic body is preferably in the shape of a circle of "U" shaped
cross-section arranged so that its "U" shape encloses the closed end of the
5 channel thereby acting as a screen to reduce the magnetic field in the
region of
the anode.
Two embodiments of the invention will now be described by way of
example with reference to the drawings in which:
Figure 1 shows an axial cross-section through a first embodiment of the
invention, showing only one half of the cross-section, on one side of the
axis, the
other half on the other side of the axis being a mirror image;
Figure 2 shows in cross-sectional form equivalent to that of Figure 1, a
second, preferred embodiment of the invention and illustrates lines of
magnetic
1 S field; and
Figure 3 shows the second embodiment in a perspective view cut in half
though its axis and with its ceramic accelerating channel removed to reveal
features of internal construction.
Referring to Figure 1 the accelerator is generally symmetrical about an
axis X-X. It comprises an annular accelerating channel 1 defined by a ceramic
insert la extending from a closed, upstream end (the lower end as shown in
Figure 1) to an open, downstream end. At the upstream end of the channel there
is located a substantially circular anode 2 and a collector 3 which delivers
propellant gas, typically xenon, to the channel in the vicinity of the anode
2. A
cathode 4 is mounted outside the channel, close to the downstream end and is
supplied with a negative potential by a power supply 5. A first hollow annular
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magnetic body 6 encloses all but the open, downstream end of the accelerating
channel 1 and comprises a main outer cylindrical wall 7 radially external with
respect to the annular channel. This wall 7 is associated with a radially
inwardly
extending pole-piece 7a. The magnetic body 6 also has a second main inner
cylindrical wall 8 radially internal with respect to the channel and an
associated
radially outwardly extending pole-piece 8a. A linking part 9 joins the two
walls
7 and 8 together at one end of the magnetic body 6 behind the closed end of
the
accelerating channel 1.
A second hollow annular magnetic body 10 is of U-shaped cross-section.
It encloses the closed end of the channel 1 and is itself totally enclosed
within
the first magnetic body 6. A source of magnetic field 11, in the form of an
electromagnet coil having its physical and magnetic axes coincident with the
axis
X-X, is situated behind (i.e. axially upstream of) the closed end of channel 1
and
is enclosed by the first magnetic body 6. In an alternative construction the
coil
11 could be replaced by an annular permanent magnet of equivalent magnetic
effect. The second magnetic body 10 is supported by supports 14 to the first
magnetic body 6. The supports 14 are made of non-magnetic material i.e. a
material which does not influence the magnetic field or, expressed another
way,
having a relative permeability close to unity. This ensures that the supports
do
not distort the distribution of the magnetic field in the channel 1.
The pole-pieces 7a and 8a create an optimal magnetic field radially across
a region close to the open end of the accelerating channel 1 whilst the second
magnetic body 10 serves to reduce or eliminate any magnetic field in the
region
of the anode 2. Dissipation of heat from the channel is encouraged by slots 12
provided in first wall 7.
In Figures 2 and 3 similar features to Figure 1 have been given the same
reference numerals for clarity, however, some of the detail of Figure 1 has
been
omitted for simplicity. Figures 2 and 3 show a second, preferred embodiment
of the invention including, in Figure 2, magnetic field lines 13. These lines
13
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illustrate the radial nature of magnetic field created across the accelerating
channel 1. In Figure 2 the lines of magnetic field 13 have been omitted where
they pass inside the magnetic bodies 6 and 10 and would be too close together
to
show clearly. It will be noted that in the embodiment of Figures 2 and 3 the
coil
is further away from the accelerating channel than its counterpart in Figure
1.
This reduces heating of the coil still further.
The outer wall 7 of the first magnetic body 6 has a part 7b extending
behind the closed end of the channel 1. This is linked to the inner wall 8 by
linking part 9 comprising sections 9a, 9b and 9c. Section 9a extends radially
inward with respect to the axis of the annular channel 1 before meeting
section
9b which extends axially downstream towards the closed end of the channel thus
defining a cavity for the magnetic coil 11. The link between the outer and
inner
walls 7 and 8 is completed by section 9c of the linking part which extends
from
9b to the end of the inner wall 8 situated behind the closed end of the
channel 1.
Because section 9b is substantially longer than 9a and because section 9c
increases the diameter of the coil, its surface area is large, this assisting
heat
dissipation.
