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]