Axial Field Pulsed Thruster for CubeSat Propulsion earthweb.ess.washington.edu/space-propulsion

---------------------------------------------------------- ---------------------------------------------------------Advanced propulsion Laboratory Department of Earth & Space Sciences ------------------------------------------ -----------------------------------------Chayse Aubuchon, Ian Johnson, 2015 Undergraduate Research Robert Winglee Symposium Contact: chayseaa@uw.edu

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AbstractPulsed Plasma Thrusters (PPT) represent a simple, lightweight, cost effective

solution for in-space propulsion onboard small, power-limited spacecraft. PPTsoperate by electromagnetic and electrothermal acceleration of a solid propellant,creating a high velocity, low mass exhaust plume. A thruster’s impulse bit is theproduct of the ablated mass and its velocity. To increase the impulse bit, andtherefore the thrust efficiency (exhaust power / input power) of the PPT, an axialmagnetic field was added around the discharge chamber region. The addition of a0.1-1kG field to the thruster will increase the gyromotion of the electrons in thedischarge region, which in turn will increase the mass ablation and ionization rate ofthe thruster, both of which will increase the impulse bit. The field should betterconfine the expanding gas to prevent perpendicular thermal expansion. Initialperpendicular gyromotion will be converted to parallel axial velocity as the particlestravel downstream and into a weaker magnetic field. Current testing with Langmuirprobes and a high speed camera will compare varying magnetic field configurationsand determine if the applied field adds to the thrusters total thrust efficiency. Anelectromagnet was chosen over a permanent magnet for laboratory testing to allowfor variations to the fields strength and shape.

The ConceptPulsed Plasma Thruster (PPT) is hybrid electromagnetic (JrxBθ) andelectrothermal (pressure gradient) in-space propulsion thruster.• Historical operation: main discharge across propellant surfacebetween anode/cathode creates moderate Jr & Bθ and large ΔP• Previous research has added external Bθ with little success• We are adding external Bz to increase ΔP

-- analogous to magnetic nozzle-- Bz will impart a θ gyromotion increase thrust-- Increase e- in source increase ablation & ionization-- Axial fields decreased radial plasma movement-- Vperp to Vparallel as field weakens increased axial velocity

Previous Experiments

Current Experiment

Peak field strength of 780G

Dipole magnetic field configuration

Experiment built from apparatus of Solid Fuel High Power Helicon in collaboration withEagle Harbor Technologies, Inc.

- 1” diameter coaxial PPT with Teflon propellant- 60uF main discharge capacitor rated to 2kV (120J)- 780G base field from six electromagnets- PPT in the center of a dipole magnetic field

This testing of a high energy (100J+) laboratory thruster designed for interplanetary travelcalled the High Power Helicon.- New experiment to determine if results scale to CubeSAT size and power

Shown on the far left is thePPT (within plasma) placedthe within dipole magneticfield (6 concentric brownmagnets) and (left) thedownstream plasma plume.

Construction and Results to Date:

- Results show increased light emissions increased ablation- Typical PPT ablates ~2μg/J of Teflon- 32J PPT discharge (66μF capacitor charged to 980V)- Solenoid: 60 wraps, 1.2” diameter, 1” length, 133μH- 33μF capacitor charged to 570V- Results in 260A discharge creating ~5kG peak field

Acknowledgments

Future Improvements:

- Machine wound solenoid (uniform field)- Move PPT electrical connections, allowing solenoid to be closer to surface- Removable solenoid- Isolate RLC circuit to avoid magnetic interference with instruments- Optimize timing of field with respect to thruster discharge

I would like to thank the graduatestudents in the Advanced Propulsionlab for all there help. This would nothave been possible without theirguidance and advice. Special thanksalso to the people at the MechanicalEngineering Machine Shop for theirhelp in creating all the parts neededfor this experiment.

Traditional coaxial PPT Circuit (top), Telfon PPT (left & center), Sulfur PPT mounted within a CubeSAT (right).

Solenoid Magnetic Field

𝐵 =𝜇0𝑁𝐼

𝑙

Inductor Equation

𝑉 = −𝐿𝑑𝐼

𝑑𝑡

Field as function of V

𝐵 =𝜇0𝑁𝑉

4𝑙

𝐶

𝐿

Plume with external B-field Plume without external B-field