Feasibility of a down-scaled HEMP-Thruster [0.5ex] as...
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Feasibility of a down-scaled HEMP-Thruster as possible μN-propulsion system for LISA Andreas Keller 1,2 , Peter K¨ ohler 2 , Waldemar G¨ artner 2 , Franz Georg Hey 1 , Marcel Berger 1 , Claus Braxmaier 3 , Davar Feili 2 , Dennis Weise 1 , and Ulrich Johann 1 1 Astrium GmbH - Satellites, 88039 Friedrichshafen, Germany 2 University of Giessen, I. Physikalisches Institut, 35392 Giessen, Germany 3 University of Applied Sciences Konstanz, Institute for Optical Systems, 78462 Konstanz, Germany Introduction Candidate propulsion systems for NGO are currently FEEP, Cold Gas and μRIT Alternative could be a down-scaled HEMP thruster due to its simplicity Experimental feasibility study on down-scaling HEMP thrusters in order to gain a deeper understanding of the influence of design parameters Goal is to comply with LISA requirements in terms of thrust level (0.1 - 150 μN) and thrust noise (0.1 μN/ √ Hz) in LISA measurement band (10 −4 - 1 Hz) Operation Principle Static electric field used to ionise the gas via electron bombardement as well as to accel- erate the ions Cusped static magnetic field increases ion- isation probability, reduces erosion of the walls and focusses the ion beam Simple system consisting of a high voltage power supply and a gas feed a + - - Design of Thrusters FEM simulation of static magnetic field for optimisa- tion SmCo ring magnets (higher operation temperature than NdFeB) Alumina discharge chamber Different housing materials (ceramics, aluminium and steel) which differs in mag- netic and electric properties Density Plot: |B|, Tesla 6.650e-001 : >7.000e-001 6.300e-001 : 6.650e-001 5.950e-001 : 6.300e-001 5.600e-001 : 5.950e-001 5.250e-001 : 5.600e-001 4.900e-001 : 5.250e-001 4.550e-001 : 4.900e-001 4.200e-001 : 4.550e-001 3.850e-001 : 4.200e-001 3.500e-001 : 3.850e-001 3.150e-001 : 3.500e-001 2.800e-001 : 3.150e-001 2.450e-001 : 2.800e-001 2.100e-001 : 2.450e-001 1.750e-001 : 2.100e-001 1.400e-001 : 1.750e-001 1.050e-001 : 1.400e-001 7.000e-002 : 1.050e-001 3.500e-002 : 7.000e-002 <0.000e+000 : 3.500e-002 Operation Test facility T-shaped vacuum chamber Length 1 m Diameter 0.5 m Volume 300 l Turbo molecular pump with 700l/s throughput Placed on a damped optical table Operation space (minimal values) for different housing materials Space for stable thruster operation dependent on housing material Thrust values calculated with measured divergence efficiencies Lowest calculated thrust of 70 μN with ceramics housing, corresponding specific impulse 125 s For higher thrust values (150 - 430 μN) specific impuls is between 300 - 580 s 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 1 2 3 4 5 6 7 8 9 Mass flow, sccm Power, W 113 192 69 196 150 218 174 130 263 242 221 66 436 375 320 264 335 223 283 405 355 298 238 210 292 319 387 325 269 317 330 293 359 278 221 141 122 Ceramics Aluminum Steel 3 113 19 69 196 150 218 174 130 242 66 2 436 375 335 283 405 355 387 92 2 221 298 238 210 292 3 3 221 141 122 263 320 264 223 319 7 325 5 269 9 317 5 5 323 330 293 359 278 306 Numbers on the right side of data point denotes the calculated thrust (μN) Characterisation thruster thrust neutraliser neutraliser actuator of detector arm Faraday cups RPA Test facility Length 1 m Diameter 1.6 m Turbo molecular and cryopumps with 25000 l/s throughput in total Array of Faraday Cups and Retarding Po- tential Analyser can be rotated around thruster Plume geometry Faraday Cups measure the angular de- pendent ion flux Ceramics and aluminum thruster shows side lobes Steel thruster shows central peak Plume geometry is independent of elec- tric and dependent on magnetic proper- ties of housing Angle, degree Current density, μA/mm Faraday Cup measurements Ion acceleration voltage Retarding Potential Analyzer measures the angular dependent ion acceleration voltage Charge state of ion unknown which is needed for energy determination Ions at 60 ◦ passed full potential differ- ence (probably created in upstream cusp) while ions at 0 ◦ passed only a fraction of potential difference (downstream cusp) High acceleration voltages points to a high acceleration efficiencies 0 0.2 0.4 0.6 0.8 1 1.2 0 1 2 3 4 5 6 x 10 μHEMP Ceramics 300V 0.8sccm Relative voltage U/U a Derivative of current, μA/V Thruster Voltage (V) Mass flow (sccm) Divergence efficiency η div Angle of aperture ν 90 ( ◦ ) Thrust (μN) I sp (s) Ceramics 300 0.8 0.518 78.0 200 280 Steel 170 0.9 0.573 79.4 280 350 Aluminum 350 0.7 0.537 74.5 170 270 Thrust Measurement Direct thrust measurement with a pendu- lum to determine thrust and thrust noise and compare with models Goal: Sensitivity 0.1 - 1800 μN Highly symmetric setup with a reference pendulum for common mode suppression of seismic noise Electrostatic actuator enables closed loop operation (constant deflection of pendu- lum reduces error sources originating from changing positions) and spring tuning (neg- ative spring constant, wider measurement range possible) Electric connection via springs (no cables to the balance which may change spring con- stant) Optical readout with picometer heterodyne interferometer Conclusion and Outlook Principal feasibility of down-scaled HEMP thruster demonstrated Further optimisation necessary in order to comply with LISA requirements Systematic thruster test campaign planned with variation of all relevant design parameters Thrust balance has to be characterised without thruster operation and calibrated in open loop mode Astrium GmbH [email protected]
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Feasibility of a down-scaled HEMP-Thrusteras possible μN-propulsion system for LISAAndreas Keller1,2, Peter Kohler2, Waldemar Gartner2, Franz Georg Hey1, MarcelBerger1, Claus Braxmaier3, Davar Feili2, Dennis Weise1, and Ulrich Johann1
1Astrium GmbH - Satellites, 88039 Friedrichshafen, Germany2University of Giessen, I. Physikalisches Institut, 35392 Giessen, Germany3University of Applied Sciences Konstanz, Institute for Optical Systems, 78462 Konstanz, Germany
Introduction
� Candidate propulsion systems for NGO are currently FEEP, Cold Gas and μRIT� Alternative could be a down-scaled HEMP thruster due to its simplicity� Experimental feasibility study on down-scaling HEMP thrusters in order to gain a deeper
understanding of the influence of design parameters� Goal is to comply with LISA requirements in terms of thrust level (0.1 - 150 μN) and thrust
noise (0.1 μN/√
Hz) in LISA measurement band (10−4 - 1 Hz)
Operation Principle
� Static electric field used to ionise the gas viaelectron bombardement as well as to accel-erate the ions
� Cusped static magnetic field increases ion-isation probability, reduces erosion of thewalls and focusses the ion beam
� Simple system consisting of a high voltagepower supply and a gas feed
a
+
-
-
Design of Thrusters
� FEM simulation of staticmagnetic field for optimisa-tion
� SmCo ring magnets (higheroperation temperature thanNdFeB)
� Alumina discharge chamber� Different housing materials
(ceramics, aluminium andsteel) which differs in mag-netic and electric properties
Density Plot: |B|, Tesla
6.650e-001 : >7.000e-0016.300e-001 : 6.650e-0015.950e-001 : 6.300e-0015.600e-001 : 5.950e-0015.250e-001 : 5.600e-0014.900e-001 : 5.250e-0014.550e-001 : 4.900e-0014.200e-001 : 4.550e-0013.850e-001 : 4.200e-0013.500e-001 : 3.850e-0013.150e-001 : 3.500e-0012.800e-001 : 3.150e-0012.450e-001 : 2.800e-0012.100e-001 : 2.450e-0011.750e-001 : 2.100e-0011.400e-001 : 1.750e-0011.050e-001 : 1.400e-0017.000e-002 : 1.050e-0013.500e-002 : 7.000e-002<0.000e+000 : 3.500e-002
Operation
� Test facility� T-shaped vacuum chamber� Length 1 m� Diameter 0.5 m� Volume 300 l� Turbo molecular pump with 700 l/s throughput� Placed on a damped optical table
� Operation space (minimal values) for different housing materials� Space for stable thruster operation dependent on housing material� Thrust values calculated with measured divergence efficiencies� Lowest calculated thrust of 70 μN with ceramics housing, corresponding specific impulse
125 s� For higher thrust values (150 - 430 μN) specific impuls is between 300 - 580 s
0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.91
2
3
4
5
6
7
8
9
Mass flow, sccm
Pow
er, W
113
192
69
196
150
218
174
130
263
242
221
66
436
375
320
264
335
223
283
405
355
298
238
210
292 319
387
325
269
317 323
330
293
359
278
221
141 122
306
CeramicsAluminumSteel
3 113
19
69
196
150
218
174
130
242
66
2
436
375
335
283
405
355
387
92
2
221
298
238
210
292
3
3
221
141 122
263
320
264
223
319
7
325 5
2699
317 55
323 330
293
359
278
306
Numbers on the right side of data point denotes the calculated thrust (μN)
Characterisation
thrusterthrust
neutraliser neutraliser
actuator of detector arm
Faraday cupsRPA
� Test facility� Length 1 m� Diameter 1.6 m� Turbo molecular and cryopumps with
25000 l/s throughput in total� Array of Faraday Cups and Retarding Po-
tential Analyser can be rotated aroundthruster
� Plume geometry� Faraday Cups measure the angular de-
pendent ion flux� Ceramics and aluminum thruster shows
side lobes� Steel thruster shows central peak� Plume geometry is independent of elec-
tric and dependent on magnetic proper-ties of housing ���� ��� ��� ��� ��� � �� �� �� �� ���
�
���
���
���
���
���
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Angle, degree
Cur
rent
den
sity
, μA
/mm
�
Faraday Cup measurements
� ��������������������������������������� �������������
� Ion acceleration voltage� Retarding Potential Analyzer measures
the angular dependent ion accelerationvoltage
� Charge state of ion unknown which isneeded for energy determination
� Ions at 60◦ passed full potential differ-ence (probably created in upstream cusp)while ions at 0◦ passed only a fraction ofpotential difference (downstream cusp)
� High acceleration voltages points to ahigh acceleration efficiencies
0 0.2 0.4 0.6 0.8 1 1.20
1
2
3
4
5
6 x 10�� μHEMP Ceramics 300V 0.8sccm
Relative voltage U/Ua
Der
ivat
ive
of c
urre
nt, μ
A/V
���������
��
Thruster Voltage (V) Mass flow (sccm) Divergenceefficiency ηdiv
Angle of aperture ν90 (◦) Thrust (μN) Isp (s)
Ceramics 300 0.8 0.518 78.0 200 280Steel 170 0.9 0.573 79.4 280 350Aluminum 350 0.7 0.537 74.5 170 270
Thrust Measurement
� Direct thrust measurement with a pendu-lum to determine thrust and thrust noise andcompare with models
� Goal: Sensitivity 0.1 - 1800 μN� Highly symmetric setup with a reference
pendulum for common mode suppression ofseismic noise
� Electrostatic actuator enables closed loopoperation (constant deflection of pendu-lum reduces error sources originating fromchanging positions) and spring tuning (neg-ative spring constant, wider measurementrange possible)
� Electric connection via springs (no cables tothe balance which may change spring con-stant)
� Optical readout with picometer heterodyneinterferometer
Conclusion and Outlook
� Principal feasibility of down-scaled HEMP thruster demonstrated� Further optimisation necessary in order to comply with LISA requirements� Systematic thruster test campaign planned with variation of all relevant design parameters� Thrust balance has to be characterised without thruster operation and calibrated in open loop
mode
Astrium GmbH [email protected]
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