Note: Descriptions are shown in the official language in which they were submitted.
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Field emission propulsion system and methods for calibrating and operating a
field emission
propulsion system
Technical field
The present invention relates to field emission propulsions for a spacecraft.
Furthermore, the
present invention relates to methods for operating a field emission
propulsion.
Technical background
A number of different propulsion technologies are known for a spacecraft, such
as chemical
propulsions, cold gas propulsions, gas ion propulsions, plasma propulsions and
the like. These
propulsion technologies have the disadvantage that they may not be
miniaturized satisfactorily
for smaller satellites due to physical or efficiency reasons. However, the
increasing use of very
small satellites requires the provision of suitable propulsion technologies
with high efficiency. In
particular, field emission propulsions are especially suitable for use in very
small satellites due to
their very high specific pulses of several 1,000 S.
For example, document AMR Propulsions Innovations, "IFM Nano Thruster", data
sheet,
July 26, 2017, http://www.propulsion.at, discloses a field emission propulsion
which uses a liquid
metal ion source with several liquid metal ion emitters. Since only one common
extractor
electrode is used for all liquid metal ion emitters, the individual emitters
may not be controlled
individually.
Also, due to manufacturing tolerances, the individual emitters do not ignite
simultaneously and
they ignite in an uncontrolled sequence. In addition, each of the liquid metal
ion emitters has an
individual emission behavior, such that the field arrangement of the liquid
metal ion emitters
usually produces an unpredictable thrust vector.
In addition, Bock, D., Tajmar, M., "Highly Miniaturized FEEP Propulsion System
(NanoFEEP) for
Attitude and Orbit Control of CubeSats", Proceedings of the 67th International
Astronautical
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Congress (IAC), IAC-16-C4.6.5, September 26 to 30, 2016, Guadalajara, Mexico,
have
published a field emission propulsion system for a very small satellite.
It is an object of the present invention to provide a field emission
propulsion system and a
method for its operation that is suitable for use in very small satellites,
achieving high efficiency
and operating with low losses. In addition, a variable thrust range of several
orders of magnitude
is to be achieved. It is a further object of the present invention to control
the ignition sequence
and to compensate for a varying thrust vector or to enable active control of
the thrust vector in
order to enable controlled operation of the propulsion system.
Summary
These objects are achieved by the method of operating a field emission
propulsion system as
claimed in claim 1, by a method of calibrating a field emission propulsion
system, and a method
of operating a field emission propulsion system according to the dependent
claims.
Further embodiments are specified in the dependent claims.
According to one embodiment a field emission propulsion system for a
spacecraft is provided,
comprising:
a control unit;
a propulsion assembly having a plurality of field emission propulsion units
comprising
an ion source having a plurality of ion emitters and extractor electrodes
associated
with the ion emitters and arranged in a field arrangement;
a plurality of extractor electrode voltage sources, which are each assigned to
the
extractor electrodes, in order to control them by the control unit with an
individual
extractor electrode voltage.
The above field emission propulsion system comprises a field arrangement of
several ion
emitters, each of which is assigned an extractor electrode. The ion emitter
may be assigned a
common emitter voltage or a common emitter voltage potential, while the
extractor electrodes
are electrically isolated from each other and may be controlled by means of
extractor electrode
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voltage sources with individually adjustable extractor electrode voltages and
with individually
adjustable extractor electrode voltage potentials, respectively.
In addition, the control unit may be configured to adjust the field strength
of an electric field
between the ion emitters and the associated extractor electrode to a specific
extractor electrode
voltage corresponding to a predetermined level of an ion current. The specific
extractor
electrode voltage for at least one specific propulsion unit is determined in a
calibration method
by measuring a current-voltage characteristic of the respective propulsion
unit by measuring an
emitter current through the ion emitter with the other propulsion units
deactivated at the same
time at different voltage differences between the extractor electrode and the
ion emitters, and by
adjusting the extractor electrode voltage or the extractor electrode voltage
potential in such a
way that an emitter current which corresponds to the predetermined level of
the ion current is
produced.
The above calibration method therefore provides to control the extractor
electrodes of the field
emission propulsion units individually with varying voltage differences
between the respective
extractor electrode and the respective ion emitters and simultaneously
measuring a current flow
from the emitter voltage source in order to measure a characteristic of the
corresponding ion
emitter. Thus, a voltage-dependent ion current may be determined for each ion
emitter, such
that a desired level of the ion current may be specifically set by adjusting
the respective extractor
electrode voltage or the respective extractor electrode voltage potential,
respectively. Thus, ion
emitters in a field arrangement may be assigned the same emitter voltage (with
the same emitter
voltage potential), and the extractor electrodes assigned to the ion emitters
may be individually
controlled to adjust the ion current of each individual ion emitter. Since the
ion emitters are at the
same voltage potential, they may be operated with the same emitter voltage
source or with a
common potential source, thereby reducing losses during high-voltage
generation and reducing
the installation space and mass of the overall system. Alternatively, the
extractor electrode
voltage sources for each of the extractor electrodes may be connected to each
other and to the
ion emitters with their positive potential terminal. The separate control of
the extractor electrodes
also allows for more precise adjustment of the thrust and direction of thrust
of the propulsion
assembly.
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Furthermore, a current measuring unit may be provided which is configured to
measure an
electric current flowing from one of the ion emitters, from several of the ion
emitters or from all
ion emitters and/or into the extractor electrode.
According to one embodiment, at least one of the extractor electrodes may be
formed with two,
three, four or more than four electrically isolated extractor electrode
segments, which together
form a particularly annular shaped extractor electrode, wherein the extractor
electrode voltage
source is configured to provide the extractor electrode segments with
individual segment
voltages, such that a predetermined direction of the emitted ion beam is
adjusted in operation,
and/or wherein separate segment voltage sources are provided for a plurality
of the extractor
electrode segments to provide the extractor electrode segments with individual
segment
voltages such that a predetermined direction of the ion beam is adjusted in
operation.
Preferably, the extractor electrodes are each composed of several electrically
isolated extractor
electrode segments, which in turn may be controlled with different segment
voltages. The levels
of the individual segment voltages are based on an extractor electrode voltage
to be applied or
an extractor electrode voltage potential to be applied.
In particular, the segment voltages may be adjusted by separate segment
voltage sources, by
voltage dividers, which generate segment voltages by dividing the extractor
electrode voltage
assigned to the respective extractor electrode, or by adjustable series
resistors of the extractor
electrode segments.
This allows for compensation of possible misalignment of a resulting thrust
beam during
operation due to component tolerances or the like. Due to the possibility of
selecting different
segment voltages, the requirements on the component tolerances for the
propulsion units may
be greatly reduced, since alignment errors of the resulting thrust beam or
geometrical alignment
errors of the ion emitters to the extractor electrodes may be actively
compensated.
By repeating the calibration process at regular intervals, undesired changes
in the ion emission
behavior of the individual ion emitters may be detected and, if necessary,
compensated during
long-term operation.
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Furthermore, a neutralizer may be provided to supply an electron current of
controllable
strength.
According to one embodiment, the ion source of the propulsion assembly may
comprise a fuel
tank for a liquid or liquefiable electrically conductive fuel, wherein the
fuel may be ejected for
field ionization at the tips of the ion emitters facing the respective
extractor electrode.
Preferably, the extractor electrodes are annular shaped with a central opening
arranged
concentrically with respect to an extension direction of the ion emitters.
According to one embodiment, the extractor electrodes may be supported by an
extraction plate
and electrically insulated from each other, wherein the extraction plate is
made of non-
conductive material, preferably.
Furthermore, the extractor electrode voltage sources may each have an
adjustable voltage
divider to provide an adjustable extractor electrode voltage.
Preferably, one, at least one, several or each of the extractor electrodes
comprises, along a full
or partial circumference, an electrically conductive first shielding structure
projecting in the
direction of the ion emitters, and/or one, at least one or each of the
extractor electrodes
comprises, along a full or partial circumference, an electrically conductive
second shielding
structure projecting in the direction facing away from the ion emitters.
The above described method is based on a field emission propulsion system with
a common
emitter electrode and separate extractor electrodes which may be controlled
separately with
individual extraction potentials.
According to a further aspect, a method for calibrating the above described
field emission
propulsion system is provided, wherein a field strength of an electric field
between the ion
emitters and the respective associated extractor electrode may be set for each
of the several
field emission propulsion units to an extractor electrode voltage
corresponding to a
predetermined ion current to be adjusted, which results from a current-voltage
characteristic of
the field emission propulsion units and the predetermined ion current to be
adjusted of a
respective one of the several propulsion units, comprising the following
steps:
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for each of the field emission propulsion units, measuring a current-voltage
characteristic by measuring an emitter current through the ion emitter of the
field
emission propulsion unit, with remaining field emission propulsion units
simultaneously deactivated or operated with constant current at different
extractor
electrode voltages;
setting the extractor electrode voltages for each of the field emission
propulsion units
depending respectively on the current-voltage characteristic and the
predetermined
ion current so as to produce an emitter current of the respective field
emission
propulsion units corresponding to the predetermined ion current to be
adjusted.
According to a further aspect, a method of operating the above described field
emission
propulsion system is provided, wherein a field strength of an electric field
between the ion
emitters and the respective associated extractor electrode is adjustable for
each of the plurality
of field emission propulsion units to an extractor electrode voltage
corresponding to a
predetermined ion current to be adjusted resulting from a current-voltage
characteristic and the
predetermined ion current to be adjusted of a respective one of the plurality
of field emission
propulsion units, wherein a predetermined thrust vector of the field emission
propulsion system
is set by controlling each of the field emission propulsion units with an
individual extractor
electrode voltage such that the predetermined thrust vector results as the sum
of the ion
currents from the field emission propulsion units.
Brief description of the drawings
The following drawings provide a more detailed explanation of the various
embodiments,
wherein:
Figure 1 illustrates a schematic representation of a field emission
propulsion system
comprising several propulsion units;
Figure 2 illustrates a cross-sectional representation of propulsion
units arranged side
by side;
Figure 3 illustrates a detailed cross-sectional representation of a
propulsion unit;
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Figure 4 illustrates a possible arrangement of the propulsion units of
the propulsion
system of Figure 1;
Figure 5 is a flow chart illustrating a method for calibrating the
propulsion units;
Figure 6 is an exemplary current-voltage characteristic of a propulsion
unit; and
Figures 7a to 7c illustrate different perspective views of modifications of
an extractor
electrode segmentation.
Description of embodiments
Figure 1 schematically illustrates the structure of a field emission
propulsion system 1
comprising a propulsion assembly 2, a neutralizer 3, and a control unit 4.
Figure 2 illustrates a
detailed view of a section of the propulsion assembly 2.
As shown in more detail in the cross-sectional view of Figure 2, propulsion
assembly 2
comprises a heater unit 21 for an ion source 22, which comprises a fuel tank
221 with fuel 223
and an ion emitter 222 electrically and fluidly connected to it. The heating
unit 21 serves to put
the fuel in the fuel tank 221 into a liquid state and to keep it liquid. The
heating unit 21 is
supplied with power by means of a heating controller 41 as part of the control
unit 4 and may be
temperature controlled by this.
The fuel tank 221 is made of an electrically conductive material such as
tantalum, rhenium,
tungsten, graphite or titanium. As shown in the more detailed cross-sectional
view of propulsion
units 23 in Figures 2 and 3, ion emitters 222 are configured with a tip, in
particular needle-
shaped, conical or pyramidal in shape, and comprise a device or configuration
to pump liquid
electrically conductive fuel 223 from fuel tank 221 for field ionization from
ion emitter 222. In
particular, a fluid line 224 running inside to the tip may be provided, which
discharges the liquid
electrically conductive fuel from the fuel tank 221 for field ionization from
the ion emitter 222, e.g.
supplied by the capillary effect. Alternatively, the ion emitters 222 may also
be porous with a
plurality of lines, wherein the fuel 223 may be supplied to the tip of the ion
emitter 222 due to the
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open porosity. Ion emitters 222 may be formed of tantalum, tungsten, rhenium,
titanium or other
refractory i.e. high-melting metals.
The fuel is passed through the fluid lines 224 of the ion emitters 222 by
means of a capillary
effect. An electrically conductive liquid or a low-melting metal, such as
gallium, indium, bismuth,
lead, gold or similar, may be considered as the material for the fuel.
Above the tip of each of the ion emitters 222 there is a respective extractor
electrode 24 having
a central opening 241 substantially coaxial with the tip of the ion emitter
222. Extractor
electrodes 24 are preferably supported by an extraction plate 25, and are
electrically isolated
from each other, e.g. by an extraction plate 25 formed of non-conductive
material.
The fuel tank 221 is electrically connected to the ion emitters 222 and
receives a high voltage
potential from an emitter voltage supply source 42. The emitter voltage supply
source 42 may be
adjustable and sets the emitter voltage or the emitter voltage potential to a
fixed value.
Extractor electrodes 24 are each individually connected to a controllable
extractor electrode
voltage source 43, which is part of control unit 4. The extractor electrode
voltage sources 43 are
individually adjustable in order to set an individual extractor electrode
voltage and thus an
individual electric field strength between the ion emitter 222 and the
extractor electrode 24 for
each of the propulsion units 23. As an alternative to separate extractor
electrode voltage sources
43 for each of the extractor electrodes 24, a common extractor electrode
voltage source 43 may
be provided, wherein the different voltages for the extractor electrodes 24
may be set by means
of correspondingly assigned voltage dividers. Other options for setting
individual extractor
electrode voltages for the extractor electrodes 24 are also conceivable.
The control unit 4 is especially configured to individually control the
extractor electrode voltage
or the extractor electrode potential of the extractor electrodes 24, such that
the timings of ignition
and the levels of ion emission may be controlled from the individual
correspondingly assigned
ion emitters 222. Thus, individual ion emitters 222 may be switched on or off
and different
emission currents may be controlled for each of the ion emitters 222. The
potential difference
between the emitter voltage potential and the extraction voltage potential is
usually several
+1000 volts.
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Since due to the emitter ion current from positively charged fuel ions, which
current is emitted
from the propulsion units 23 during operation, the propulsion system 1 is
charged negatively, an
electron current is usually generated and emitted by means of the neutralizer
3. Neutralizer 3
may, for example, be configured as a field emission electron source or thermal
electron source
in a manner known per se. For this reason, the control unit 4 comprises a
neutralizer control unit
45 which may control and supply power to the neutralizer 3 in a manner known
per se, e.g. to
keep the charge of the entire propulsion system 1 as neutral as possible.
Figure 4 illustrates an arrangement of extractor electrodes 24 in a plan view.
The extractor
electrodes 24, for example, are arranged around and concentric to the ion
emitter 222. In the
center of the extractor electrodes 24 there are substantially round openings
241 which are
arranged concentrically to the ion emitters 222 in order to emit the ion beam
from the ion emitter
222. The arrangement of the extractor electrodes 24 may be provided as a field
arrangement,
wherein the extractor electrodes 24 are arranged in rows and are offset from
each other to
achieve the highest possible arrangement density.
The extractor electrodes 24 are connected to each other on the extraction
plate 25, which
retains the extractor electrodes 24 in position. The extraction plate 25 may
be formed of
electrically non-conductive material, or the extractor electrodes 24 may be
mounted isolated on
the extraction plate 25. One, at least one or each of the extractor electrodes
24 comprises an
electrically conductive first shielding structure 242 projecting
circumferentially in the direction of
the ion emitter 222, which prevents, by the principle of shading, the
continuous coating of one
side of the extraction plate 25, which side is facing the ion emitters, with
accumulating fuel
material. This prevents the formation of an electrically conductive path
between the individual
extractor electrodes 24, and between the electrodes and the fuel tank 221
during operation,
which would result in an electrical short circuit.
Alternatively or additionally, one, at least one, several or each of the
extractor electrodes 24 may
comprise an electrically conductive second shielding structure 245 projecting
circumferentially in
the direction of the ion beam to be emitted, which prevents, by the principle
of shading, the
continuous deposition of one side of the extraction plate 25, which side is
facing away from the
ion emitters 222, with accumulating fuel material. The second shielding
structure 245 may be
formed torus-like. This prevents the formation of an electrically conductive
path between the
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individual extractor electrodes 24, and between the electrodes and the fuel
tank 221 during
operation, which would result in an electrical short circuit.
Furthermore, the extraction plate 25 may comprise, between the extractor
electrodes 24,
structures and/or recessed structures which are labyrinth-like or meander-
like, i.e. projecting
orthogonally to the surface direction of the extraction plate 25, which
structures extend along the
surface direction of the extraction plate 25, and thereby preventing, by the
principle of shading, a
continuous conductive coating during long-term operation by deposition of the
fuel material. For
example, a support between heating unit 21 and extraction plate 25 may have a
corresponding
labyrinth-like or meander-like form or steps which also prevent continuous
coating by shading.
In addition, an electrically conductive cover plate 27 may optionally be
mounted parallel to the
extraction plate 25 on the side of the extraction plate 25 facing away from
the ion emitters. In
particular, the cover plate 27 comprises circular openings 271 which are
located above the
extractor electrodes 24 in the direction in which the ion emitters 222 and the
extractor electrodes
25 are arranged and which, in particular, have the same or larger dimensions
(e.g. radii) than the
extractor electrodes 25 in the surface direction of the extraction plate 25.
The cover plate 27 may
be electrically isolated from the extractor electrodes 24. The electrical
insulation between the
cover plate 27 and the extractor electrodes 24 may be ensured by means of an
electrically
insulating spacer 28, which comprises labyrinth-like or meander-shaped
structures, in order to
protect the insulation as well in long-term operation against a continuous
conductive coating by
deposition of fuel. The provision of a cover plate 27 is advantageous, since
it is possible to
prevent particles in the environment from reaching the ion emitters 222 by
applying a voltage
potential. In addition, deposition of sputter particles or reflected fuel on
the upper side of the
extraction plate 25 may be prevented during prolonged operation.
During operation in space, the cover plate 27 may prevent the impact of a
local surrounding
plasma on the propulsion units 23. This prevents the attraction of e.g. free /
thermal electrons
from the surrounding plasma to the ion emitters 222, which electrons could
damage the ion
emitters. In addition, the voltage potential of the cover plate 27 prevents
false measurement of
emitter current by such a secondary electron current.
The control unit 4 also comprises a current measuring unit 44 to measure a
current flowing to
the extractor electrode voltage sources or from the neutralizer 3.
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For operation of the propulsion system 1, it is desirable to set equal or
defined thrust vectors of
the ion beam from ion emitters 222. Due to component and assembly tolerances,
different thrust
vectors occur when the same extractor electrode voltages are applied.
Therefore, a method is provided to control the strength of the ion beam in a
defined way. This is
performed by the defined individual adjustment of the field strength of the
electric field between
the ion emitters 222 and the respectively assigned extractor electrode 24 by
varying the
extractor electrode voltage or the extractor electrode voltage potential or
the voltage difference
between the extractor electrodes 24 and the associated ion emitters 222. To
set the extractor
electrode voltage, a method is provided as shown in the flow diagram of Figure
5.
In step S1, one of the propulsion units 23 is selected. In step S2, a current-
voltage characteristic
graph is measured for the selected propulsion unit 23. The current-voltage
characteristic
represents a characteristic of a current flow across a voltage difference
between the extractor
electrode voltage potential and the emitter voltage potential of the
respective propulsion unit 23,
which occurs at a field strength in the respective propulsion unit 23 set by
the extractor electrode
voltage. The measurement is performed with the other propulsion units 23
deactivated or
operating at constant (known) current (i.e. activated) and by means of the
current measuring unit
44, which in this case measures the level of the ion current of all activated
propulsion units 23.
The ion current level is measured by measuring the electric current flowing
from the emitter
voltage supply source 42 or the electric current flowing into the ion source.
The ion current of the
propulsion unit 23 to be measured corresponds substantially to the measured
electric current
flowing into the ion source minus the known ion currents of the other
propulsion units 23 (i.e.
with the other propulsion units 23 activated). In other words, if the
remaining propulsion units 23
are operated with a known current, the ion current of the respective
propulsion unit 23 may be
determined by subtracting the currents of the remaining propulsion units 23
from the detected
current. If only the propulsion unit 23 to be measured is active for each
measurement, the
measured electric current corresponds to the ion current at the applied field
strength or at the
applied voltage difference between the emitter voltage potential and the
extractor electrode
voltage potential. Thus, a current-voltage characteristic may be determined
for each of the
propulsion units 23. Figure 6 illustrates an example of such a current-voltage
characteristic
graph.
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Step S3 checks whether all propulsion units 23 have been measured. If this is
the case
(alternative: Yes), the method continues with step S4, otherwise the system
returns to step S1
and measures a next propulsion unit 23 that has not yet been measured. In this
way, a current-
voltage characteristic is recorded for each of the propulsion units 23.
In step S4, the extractor electrode voltages are controlled to set a field
strength corresponding to
a desired ion current strength for each of the propulsion units 23.
Further with reference to Figure 4 and in conjunction with the different views
of Figures 7a to 7c,
the extractor electrodes 24 may be segmented, with extractor electrode
segments 243 being
electrically isolated from each other e.g. by spacing, and forming the
circular extractor electrode
24 in the assembled state. It is possible to arrange the extractor electrode
segments 243
according to the embodiments of Figures 7a to 7c, wherein the extractor
electrodes 24 are
segmented into four identical extractor electrode segments 243 (see Figure
7a), into two
identical extractor electrode segments 243 (see Figure 7b) and into three
extractor electrode
segments 243 (see Figure 7c). By varying the segment voltages at the
individual extractor
electrode segments 243 of an extractor electrode 24, an asymmetry of the ion
beam emitted by
the ion emitter 222, i.e. an inclination of the path of the ion beam with
respect to the alignment
between the ion emitter 222 and the extractor electrode 24, may be
compensated. Such an
asymmetry results from component tolerances and manufacturing tolerances of
the propulsion
units 23.
If the extractor electrodes are segmented, the above described calibration
method may first be
performed by applying the extractor electrode voltages required for the
measurement to each of
the extractor electrode segments.
An asymmetry may be determined, for example, during the calibration method or
in a separate
procedure. For this purpose, each of the extractor electrode segments 243 may
be provided with
a separate current measurement facility. While each of the propulsion units 23
is measured one
after the other to determine the current-voltage characteristic such that an
ion beam is formed, a
parasitic current is measured through each of the extractor electrode segments
243 at one or
more specific extractor electrode voltages. For example, the extractor
electrode segment 243
through which the highest current flow is measured corresponds to the
extractor electrode
segment 243 which deflects the ion beam most strongly in its direction and
which is, accordingly,
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arranged closest to the ion beam. The individual segment voltages may now be
controlled based
on the desired extractor electrode voltage (or the desired field strength).
By varying a segment voltage applied to part of the individual 243 extractor
electrode segments
of the propulsion unit 23, the direction of the ion beam may be varied as
well, in addition to
applying the extractor electrode voltage to the other 243 extractor electrode
segments. For
example, by iterative control of the segment voltages at the part of the
extractor electrode
segments 243, the direction of the ion beam may be adjusted to a desired
direction, in particular
the direction parallel to the arrangement direction between the ion emitter
222 and the extractor
electrode 24. By iteratively controlling a part of the segment voltages based
on the previously
determined and set extractor electrode voltage, both the strength of the ion
beam may be
precisely controlled and the component and manufacturing tolerances of the
propulsion unit 23
may be compensated.
Alternatively, all segment voltages may be varied by the extractor electrode
voltage to be
adjusted such that the mean value of the individual segment voltages
corresponds
approximately to the extractor electrode voltage.
For example, the control of the individual segment voltages or the direction
of the ion beam may
be performed using voltage dividers in particular, wherein the respective
segment voltage is
generated from the extractor electrode voltage. Thus, segment voltages may be
generated by
voltage dividers, also by adjustable voltage dividers, by the extractor
electrode voltage source. A
separate control with individual voltage sources for each extractor electrode
segment is also
possible.
If, for example, in the embodiment of Figure 7a, a high current flow, compared
to the currents in
the other extractor electrode segments 243 through one of the extractor
electrode segments
243a, is measured, the corresponding segment voltage from the extractor
electrode voltage may
be reduced by setting an adjustable electrical series resistor or by setting
an adjustable voltage
divider in order to achieve a higher attraction of the fuel ions of the ion
beam through the other
extractor electrode segments 243. As a result, the ion beam is deflected away
from the
respective extractor electrode segment 243a, as it is attracted more by the
other extractor
electrode segments 243. By suitably calibrating the variable series resistors
assigned to the
extractor electrode segments 243 or the voltage dividers assigned to the
extractor electrode
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segments 243, the propulsion unit 23 may be calibrated accordingly. In this
way, component
tolerances of the extractor electrode 24 and alignment errors may be
compensated, and the
precision in the production and assembly of the extractor electrode segments
243 and the ion
emitter 222 may be reduced.
The above described field emission propulsion system may be operated by
separately
controlling the propulsion units 23. The ion currents of the individual
propulsion units 23 are
determined according to a thrust vector control by specifying a thrust vector.
The individual ion
currents are each controlled by specifying a corresponding extractor electrode
voltage resulting
from the current/voltage characteristic, such that in addition to a total
thrust strength resulting
from the sum of the ion beams, a predetermined moment is also applied to the
field emission
propulsion system, which results from the arrangement of the individual
propulsion units and the
respective thrust strengths resulting from the respective ion beams.
