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SPACE PROPULSION 2022

ESTORIL, PORTUGAL | 09 – 13 MAY 2022

Testing the NANO AR³ FEEP cubesat electric propulsion system at ESA Propulsion Laboratory

Martin Eizinger (1), José Gonzalez del Amo (2), Luca Bianchi (2), Davina Di Cara (2),

Quirin Koch (3), David Krejci (3), Alexander Reissner (3), Kaarel Repän (3), Tony Schönherr (3)

(1)(2) European Space Agency (ESTEC), Noordwijk, The Netherlands, Email: martin.eizinger@esa.int

(3) ENPULSION GmbH, Wiener Neustadt, Austria, Email: tony.schoenherr@enpulsion.com

KEYWORDS: field emission electric propulsion,

thrust vectoring, characterisation, indium, Faraday

cup, ESA R&D, TDE

ABSTRACT:

Space propulsion systems undergo thorough

ground testing before being deployed in space. We

report the results of a functional verification and

performance characterisation test campaign of an

integrated electric propulsion system for cubesats

and microsats with purely electric thrust vectoring

capability and no moving parts.

Visualisations of the plume data obtained from

Faraday cup scans show a clear, corresponding

trend of the variation of the inclination and azimuth

angles of the thrust vector when these are

commanded.

The divergence angle computed from plasma

diagnostic data is 49°, independently of the

achieved inclination of the ion beam.

1. INTRODUCTION

This paper is based on an experimental verification

of the thrust vectoring capability of the DUT.

1.1. Thrust vectoring

Adjustability of the thrust vector of a space

propulsion system is a highly valuable feature.

Depending on the configuration of the propulsion

system on the spacecraft, it allows not only for more

advanced orbital manoeuvres but also for

introducing a torque. Furthermore, a propulsion

system can develop an undesired inclination of the

thrust vector over its lifetime or even present one at

beginning of life (BOL) despite strict manufacturing

tolerances, depending on the technology, e.g. [1],

[2]. This can be compensated only with the ability to

adjust the thrust vector.

Many approaches for thrust vectoring can be

identified. A very low-resolution example is the use

of multiple, spatially separated and selectively

activated thrusters. An asymmetry of the exhaust

plumes naturally results in an off-centred thrust [2].

If the bases of the thrusters are not coplanar, the

vector sum is also inclined with respect to the

spacecraft coordinate system.

A different approach is to mechanically change the

orientation of one or more thrusters, which naturally

brings about an inclined thrust vector. Due to the

complexity of bearings compliant with the

environment of outer space, intricate designs using

compliant mechanisms have been presented that

provide rigid rotation of nozzles around two axes [3].

Another possibility to achieve an inclined thrust

vector is by injecting a secondary flow that diverts

the main flow away from the geometric axis [4].

In contrast to the exhaust gas of chemical

propulsion systems, electric propulsion (EP)

systems have an additional mechanism for applying

forces or torques to the propellant, i.e. the electric

charge of the exhaust particles. In fact, that is how

most EP technologies produce thrust in the first

place, however commonly the resulting force of the

electric or magnetic field is collinear with the

geometrical axis of the thruster.

The device under test (DUT) for which data are

presented in this paper is categorised as an

electrostatic propulsion system. As such, the thrust

it produces is the reaction to the electrostatic force

applied to the ionised atoms in the exhaust plume.

By modifying the electric field to not be

axisymmetric, the ions experience an acceleration

component in radial direction, resulting in an overall

inclined thrust vector.

1.2. Device under test

The NANO AR³ is a fully integrated propulsion

system developed by ENPULSION in cooperation

with FOTEC in the frame of the ESA Technology

Development Element (TDE) project “Innovative

Propulsion systems for Cubesats and Microsats”. It

is a derivative of the flight-proven Indium FEEP

Multiemitter (IFM) NANO (now commercially called

“ENPULSION NANO”), which was first deployed in

space in 2018 [5][6] after over 20 years of

collaborative development between FOTEC and the

SP2022_266

2

European Space Agency (ESA) [7][8][9][10][11].

Since then, several new variants of the product

have been in development, including the NANO

AR³, whose primary attribute is its thrust vectoring

capability.

The NANO AR³, the device under test, is an indium

Field-Emission Electric Propulsion (FEEP) system

with a crown emitter and a segmented extractor, two

neutralisers and a Power Processing Unit (PPU).

The system provides controllable thrust between

100 and 350 µN, at a specific impulse greater than

2000 s and with a power consumption lower than

45 W. The underlying electric parameters are

entirely controlled by the on-board embedded

firmware through calibrated algorithms that take the

target angles as input.

1.2.1. Basic functionality

During operation and stand-by, the propellant is in

the liquid phase. Capillary forces transport it from

the reservoir to the expulsion area, which is a

circular configuration of 28 small needles, usually

referred to as the crown. The crown is part of the

emitter-subassembly, and entirely wetted with the

propellant, which is indium. It is furthermore

surrounded by the extractor ring, which in case of

the AR³ is split into three segments. Fig.1 shows this

arrangement from an external view of the final

assembly.

Figure 1: Close-up of emitter and extractor configuration of

the NANO AR³

During nominal operation, the extractors are at a

high negative potential, while the emitter is at a high

positive potential. The resulting electric field applies

a stress to the liquid, electrically conductive

propellant, which consequently deforms at the

needle tips into a shape known as a Taylor cone

[12]. At sufficiently high voltages, a jet of liquid starts

to emanate from the apex of the Taylor cone [12],

and the ions within it are accelerated by the electric

field. Varying the potential across the extractor

segments causes an asymmetric electric field and

consequently a radial component of acceleration.

Independent control of the extractor segments

requires additional electronics compared to the

model with a single extractor.

1.2.2. Propellant

Unlike the most common propellants, indium is not

a gas at room temperature. This, along with some

other differences to noble gases, makes it

convenient in many ways for use as a propellant.

However, it also leads to effects that need to be

considered in testing. For example, Mühlich et al.

developed an advanced design of Faraday cups for

ion current measurement, which is highly accurate

and specifically suitable for indium FEEP ion

sources [13].

1.3. Test objective

The objective of the test is to characterise the thrust

vectoring capability of the DUT. This is achieved by

commanding several combinations of inclination

and azimuth and qualitatively comparing

visualisations of the Faraday scans of these

operating points (OPs). The combinations of

inclination and azimuth of all inclined thrust vector

operating points are listed in Tab.1. The thrust value

varies based on inclination, namely 349 µN, 299 µN,

and 249 µN for 5°, 10°, and 12.5° respectively.

Table 1: Inclination and azimuth for all operating points

Incl

Az

Incl

Az

Incl

Az

5°

0°

10°

30°

12.5°

0°

5°

60°

10°

90°

12.5°

60°

5°

120°

10°

150°

12.5°

120°

5°

180°

10°

210°

12.5°

180°

5°

240°

10°

270°

12.5°

240°

5°

300°

10°

330°

12.5°

300°

2. METHODS

The main methods for obtaining test data are

plasma diagnostics, telemetry from the DUT, and

basic data from ground support equipment (GSE).

2.1. Test facility

The test is carried out in the SPF vacuum facility at

the ESA Propulsion Laboratory (EPL) of the

European Space Research and Technology Centre

(ESTEC). The main vacuum chamber has a

diameter of 2 m and a length of 2 m. High vacuum

is mainly achieved with a turbomolecular pump. The

cryogenic pumping systems of the facility are not

used, because of the propellant’s tendency of

sticking to surfaces already at room temperature.

2.2. Plasma diagnostics

In this test, all plasma data are obtained using

Faraday cups (FCs). Measurements are acquired at

multiple elevation angles (up and down relative to

the chamber axis) and sweep angles (left and right

relative to the chamber axis) to achieve a high

resolution of the ion beam. This is realised by a

semi-circular arm that holds FCs and can rotate

around a vertical axis (schematic see Fig.2).

3

Figure 2: Exemplary scheme of SPF’s diagnostic arm with

Faraday cups

In total, twelve FCs are used to resolve the elevation

from -72° to 72°, while the sweep angle reaches

from -70° to 70° with approximately 3° resolution.

The FCs are distributed asymmetrically to allow for

placement of one probe at 0° despite the use of an

even number of probes. The position of all probes is

summarised in Tab.2.

Table 2: Angular position of Faraday probes with respect to

the horizontal plane

ID

angle

supplier

ID

angle

supplier

1

-72

FOTEC

12

72

FOTEC

2

-54

FOTEC

11

54

FOTEC

3

-36

FOTEC

10

36

FOTEC

4

-24

FOTEC

9

24

FOTEC

5

-12

FOTEC

8

12

ALTA/

SITAEL

6

-6

FOTEC

7

0

FOTEC

Two different implementations of FCs are used.

More specifically, all but one probe are specifically

designed for use with indium (FOTEC probes). The

twelfth probe’s design is based on noble gases as

propellants (ALTA/SITAEL probe). This is to gain

information about the validity of the probe designs.

Comparability is enabled by placing the ALTA probe

at a location symmetric to one of the FOTEC probes

(specifically 12° and -12°, respectively). Fig.3 shows

one specimen of each probe.

Figure 3: Faraday probes (left: ALTA, right: FOTEC)

The FCs have a collector and a shield, biased at

-20 V, and +10 V respectively.

Data from these probes are acquired in sets called

“scans”, where one scan is one sweep of the arm.

Each scan is stored in a dedicated log file.

2.3. Telemetry

The DUT communicates with the electronic GSE

(EGSE) and transmits a wide range of data such as

housekeeping (e.g. bus voltage and current,

software fuses, temperature of critical components)

and operational quantities (e.g. internal voltages,

thrust derived from a mathematical model, thrust

vector angles).

The software on the EGSE computer makes these

data graphically available to the user and also

stores them in one continuously acquired log file,

split over time for different test segments.

2.4. Ground support equipment

Apart from the DUT-dedicated software, the EGSE

computer runs a software to acquire GSE-related

data and to supply the DUT with power. The DUT is

connected to a laboratory power supply that

simulates the spacecraft bus. In addition, the variant

tested in this campaign has an integrated relay that

serves as an on-off switch, allowing for switch-off of

the device without disabling the bus voltage. A

separate power supply provides the voltage for this

relay.

The GSE also measures the temperature at multiple

locations outside of the DUT, including the

temperature reference point (TRP) used for

simulation-based thermal analyses. In addition, the

pressure inside the facility measured with vacuum

gauges is recorded by the software. Similarly to the

telemetry, these data are acquired continuously, but

unlike the DUT telemetry, the data are not split into

test-specific segments.

3. DATA PROCESSING

Diagnostic data are processed in multiple steps

before analysis and presentation. Because these

steps are carried out independently by two parties

(ESA and ENPULSION), they are described

generically rather than with the explicit or

computational operations.

3.1. Axial offset correction

The location on the DUT from which ions are

extracted does not coincide with the centre of the

semi-circular arrangement of probes. Instead, an

offset in axial direction is present as a consequence

of facility-related technical limitations. This results in

multiple geometrical effects that need to be

corrected for.

3.1.1. Different distances of the probes

The effective distance from the location of ion

expulsion to each probe depends on both the

elevation angle of the probe and the sweep angle.

The offset can simply be added to the radius of the

arm if elevation and sweep angle are both equal to

4

zero (i.e. central probe directly in front of the DUT).

For all other sweep angles, a triangular summation

applies. This triangle only lies in the plane of the

sweep angle for the central probe; for all other

probes, both angles must be considered (see Fig.4).

Figure 4: Effect of a displacement “d” on the perceived

distances and angles of a probe

The distance between the diagnostic arm structure

and the probe collimators is assumed constant for

all probes. This approach neglects the slight

variation resulting from the non-radial orientation of

non-central probes because they are aimed at the

offset point. The final value of the distance is

obtained by taking the absolute of the position

vector of the probe after adding the displacement to

the z component as in Eq.1.

Eq.1

The components are calculated from the arm

radius, its position, and the mounting position of the

probe (see Fig.4):

Eq.2

Eq.3

Eq.4

3.1.2. Narrowing of angles

Both elevation and sweep angle are initially

measured with respect to the centre of the arm. To

convert them to the angles as seen from the location

of ion expulsion, the angles ’ and of the triangles

described in the previous paragraph are applicable.

Their computation follows from Eq.2, Eq.3, and Eq.4

using simple trigonometry.

3.1.3. Shadowing of the collimator

Another effect of the aforementioned displacement

is that the collimator of the probes appears as an

ellipse rather than as a circle from the point of view

of the location of expulsion. The vertical component

of this effect, i.e. the shadowing pertaining to the

elevation of the probes, is addressed by performing

a laser alignment procedure prior to acquisition.

However, because the offset also applies to the

rotational axis of the arm, this distortion of the

collimator also occurs for the sweep angle. This

results in a reduced intake area of the probe.

Knowing the area perpendicular to the direction of

motion of the ions is critical for correctly computing

the ion current density. Fig.5 shows three examples

of such a distortion (note that the vectors x, y, and z

are to be treated separately for each scenario, i.e.

even though z3 and z2 are drawn the same, they

would not have the same length for position 2 and 3

of any given probe).

Figure 5: Effect of a displacement “d” on the apparent shape

of the collimator (blue) illustrated on the example of three

different probe locations (1: central probe at 0°, 2: elevated

probe at 0°, 3: elevated probe at arbitrary nonzero angle)

Shadowing effects due to the thickness of the

collimator disc are neglected in this analysis. The

area relevant for the calculation of the current

density is the area of the ellipse, which is calculated

as , where is simply the radius of the

collimator and is equal to the radius shortened by

the cosine of the angle between the normal vector

of the collimator (p) and the position vector from the

expulsion point to the probe (r’). Because the

pointing of all probes to the offset point is performed

when the arm is at the centre, the normal vector p

can be computed from the ideal position vector r of

the probe (which results directly from the arm

radius, sweep angle, and elevation angle) by adding

the x and z components of the length d rotated by

the sweep angle. Finally taking the scalar product of

these two vectors yields the factor by which is

shorter than the radius. The area of the visible

ellipse is thus calculated as

Eq.5

with the pointing vector according to Eq.6 and the

position vector according to Eq.1.

Eq.6

3.2. Scaled current density

The aforementioned correction terms as well as

scaling based on distance and area are applied to

5

the raw currents according to Eq.7, eventually

yielding the current density scaled to a certain

distance from the DUT.

Eq.7

Note that the indexes “DUT” and “raw” imply the

transformation of angles as described.

This computation assumes that the trajectory of ions

from the point of expulsion to the FC collimator is a

straight line.

3.3. Averaging

Multiple Faraday scans over the whole range of

sweep and elevation angles are performed for each

operating point (OP), where an OP is characterised

by a unique combination of thrust magnitude, thrust

vector inclination, and thrust vector azimuth. The

data of these scans is averaged into one matrix per

operating point.

3.4. Interpolation

The matrix of scaled current densities for a single

OP is fed to a Clough-Tocher 2D interpolation

algorithm [14], which facilitates smoothly plotting the

data as well as integrating in a spherical coordinate

system. The domain of the resulting 2D field is given

by the extrema of the corrected sweep and probe

angles, i.e. two pairs of values.

3.5. Conversion to DUT coordinate system

From the point of view of the DUT, a spherical

coordinate system of zenith and azimuth is more

intuitive than a system of elevation and sweep

angle. This is particularly relevant for the

computation of the divergence angle. Fig.6 shows

the coordinate system into which the data are

transformed.

Figure 6: Spherical coordinate system centred at the

expulsion point of the device under test and aligned with the

device's Cartesian coordinate system

If the interpolated field is described using this

coordinate system, the definition of its domain

changes slightly: the two pairs of limits become, for

simplicity, one single limit describing the maximum

zenith angle where the field is defined at all

azimuthal angles. This maximum zenith angle

is the minimum of the absolute values of the four

limits of the original interpolated field.

The aforementioned coordinate transformation is

not performed for the presentation of the raw FC

measurements; instead, these are plotted as the

raw current over the (corrected) sweep angle.

3.6. Divergence angle

The divergence angle is computed based on the

total current and consequently relies on a

summation over the interpolation field. This is done

in spherical coordinates, where the limits of

integration are ideally such that a hemisphere is

covered. However, the zenith angle is limited to

approximately 60° as a result of the range of the

interpolation field.

The mathematical formulation for the integral of the

current density is shown in Eq.8, where the product

of the two terms in parentheses describes the area

of a surface element.

Eq.8

However, because the integration is carried out

numerically, discrete steps of and are taken,

causing a small error because the upper edge of the

surface element is shorter than the lower edge. To

minimise the impact of this error, the values for the

current density are taken at the centre of each

discrete step rather than at its edge. The first value

is therefore not taken at like the integral

indicates, but at

, where is the step size.

This integral is taken once for to

determine the total current (at least within the

domain of the data), and then once again with a

break-condition when 95% of the total current is

reached. The angle where this condition is reached

is taken as the divergence angle, equal to the half-

angle of the spherical sector whose cap accounts

for (at least) 95% of the total ion current.

3.7. Measurement offset correction

Performing a scan with the acquisition system on

but with the DUT off (i.e. with no plasma present)

shows a highly stable nonzero signal (see Fig.7).

Figure 7: Current measurements without plasma

6

This is likely a product of the measurement

electronics, caused by an input offset voltage or an

input bias current through the shunt. If this offset is

assumed to be independent of the amplitude of the

signal, it can simply be subtracted from the

measurement. Fig.8 shows data from one scan,

with and without the offset subtracted. More

specifically, the signals shown in Fig.7 are averaged

over the sweep angle, and these averages are

subtracted from the corresponding channel data.

Figure 8: Current density over sweep angle, plotted by

probe angle, for all probes, with idle offset (left) and with

offsets subtracted (right)

Previous Faraday cup measurements of the plume

of the IFM Nano – a comparable ion source – have

shown that the current density becomes zero

between 60 and 80 degrees (depending on the

operating point), and that it approaches this limit

linearly [15]. Accounting for the aforementioned

offset, both these characteristics can be seen in the

data of this test. Furthermore, the peak current

density is approximately three times higher than in

the aforementioned study, which correlates with the

three times higher emission current (approx. 3 mA

compared to 1 mA in [15]).

4. RESULTS

The presented results focus on information inferred

from plasma diagnostic data. For colour plots, the

lower limit is chosen slightly above zero, causing all

negative and zero values to appear white, providing

a stronger contrast along the edges.

4.1. Data examination and verification

Inspection of visualised data initially reveals two

artefacts. The first is a trough in the centre, along

the entire sweep. An example of this is given in

Fig.9.

Figure 9: Example case showing a trough at zero elevation

Due to the consistency of the location of this

observation across all operating points, in particular

its independence of the azimuth of the thrust vector,

it can be attributed to the measurement setup, most

likely either to the signal conditioning unit or to the

affected probe(s) themselves. Despite the artificial

nature of this observation, the related data are kept

in the presented results as they are.

Additionally, the data of some OPs show distinct

strands of unusually high or low signal compared to

data in the immediate proximity (example see

Fig.10).

Figure 10: Example case showing amplified or weakened

strands with a shape reminiscent of the diagnostic arm

From the shape of these strands, it is evident that

they correspond to one data point of sweep angle

measurements. They appear because scans may

differ in their sampling of the sweep angle, which,

when merging and averaging across multiple scans

of the same OP, may cause individual samples to

appear amplified or weakened relative to the rest of

the scan data.

4.2. Probe type comparison

The measurements of the subject probe (ALTA) is

qualitatively compared to a reference probe

(FOTEC) located at the same angle on the other

side of the mid-plane. This is done by plotting the

raw data of these two probes over the sweep angle.

Fig.11 shows that for an OP with an uninclined

commanded thrust vector, the ALTA probe acquires

a lower current than the FOTEC probe at a

comparable location.

Figure 11: Raw current measured across sweep angle for the

ALTA probe (blue) and a FOTEC probe placed on the

opposite side of the beam (orange)

7

This pattern of a lower amplitude emerges

consistently across OPs with an inclined thrust

vector, as can be seen in Fig.12.

Figure 12: Raw current measured across sweep angle for the

ALTA probe (blue) and a FOTEC probe placed on the

opposite side of the horizontal mid-plane (orange) for a

strongly inclined thrust vector at multiple azimuth angles

measured clockwise around the thruster axis starting at the

left horizontal, from top left to bottom right

Note here that in the first row of images, the beam

is actually inclined towards the subject, yet the

acquired current is lower than that of the reference,

albeit by less than for an uninclined thrust vector.

Meanwhile, the gap between the two increases

when the beam is inclined away from the subject

(ALTA probe).

4.3. Divergence angle

The divergence angle around the thrust vector is

approximately 49° for all OPs. Note however that

due to the limited field of view of the Faraday scans,

the computation of this angle is an underestimation

of the true divergence angle.

In comparison, Mühlich et al. [15] experimentally

found a divergence angle of 63° for the IFM Nano,

which is the precursor of the DUT. In a simulation of

that device, they found a divergence angle of 49°.

4.4. Thrust vectoring

An excerpt of significant OPs is presented in the

following. The colour scale is kept constant across

all figures, allowing for further comparison. However

this leads to noticeably less contrast for the lower

thrust OPs. Fig.13 shows colour plots of the post-

processed current density of OPs that have a thrust

vector inclination of 12.5°, which is the highest

inclination commanded during this in this campaign.

However, these OPs have the lowest commanded

thrust value, which is 249 µN.

Figure 13: Ion current density over x and y as seen from the

thruster, at 249 µN, 12.5° inclination, and various azimuth

angles measured clockwise around the thruster axis starting at

the left horizontal, from top left to bottom right (red dot marks

commanded thrust vector)

Fig.14 shows the OPs that have a slightly lower

thrust vector inclination (10°) while having an

increased thrust (299 µN).

Figure 14: Ion current density over x and y as seen from the

thruster, at 299 µN, 10° inclination, and various azimuth

angles measured clockwise around the thruster axis starting at

the left horizontal, from top left to bottom right (red dot marks

commanded thrust vector)

Fig.15 shows the OPs with the highest thrust

commanded during this test campaign (349 µN).

However, the thrust vector inclination is reduced to

5°, which makes the skewed current density

distribution less evident from the provided graphs.

Figure 15: Ion current density over x and y as seen from the

thruster, at 349 µN, 5° inclination, and various azimuth angles

measured clockwise around the thruster axis starting at the

left horizontal, from top left to bottom right (red dot marks

commanded thrust vector)

8

5. CONCLUSION

The qualitative characterisation of the NANO AR³

FEEP propulsion system was completed

successfully using the plasma diagnostics setup at

the EPL. Specifically, the inclination of the ion beam

without the use of moving parts was achieved and

verified by means of Faraday cup data. The beam

divergence is not measurably affected by the

inclination of the beam, and it is comparable to the

divergence angle obtained during a previous,

independent study on a precursor model.

The ALTA Faraday probe designed for use with

Xenon acquires a lower current than the FOTEC

probes, which are designed for use with indium.

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