Space Propulsion (Plasma) Physics

Introduction

Wits plasma/electric propulsion research started about six years ago. The Aim is to design a small, cheap, novel space propulsion unit with applications on Micro/ Nano/ Pico satellites for orbital manoeuvres (increase satellite lifetime, formation flying), micro control (scientific applications) and controlled de-orbiting (now required)

There has been a recent increase in research activity in this area since

• There is a drive for general space component miniaturisation (cheaper, more versatile) which requires smaller propulsion units for smaller vehicles
• Electric propulsion is the most efficient in space (but is not scale invariant)

Laboratory facilities

Chamber

The 75cm diameter stainless steel vacuum chamber (in picture on right) is equipped with two diffusion pumps to achieve vacuums in excess of 10^-6mbar.This chamber simulates conditions very similar to the ones found in outer space.

For flow rates a micro-orifice flow meter is used, and high voltage is regulated via a precision power supply.

Over the past four years, the lab has received a number of additions which forces us to move to a bigger venue by the end of 2013.

Diagnostics

For diagnostics, there are a variety of tools available.

Spectrometer

A spectrometer mounted on a robotic arm is capable of determining plume plasma composition and temperature. Movement of the robotic arms is able to determine a temperature field within the plasma plume. The temperature has a bearing on the efficiency of the thruster.

An insulated plasma thermocouple is used for low temperature determination.

Visual and SEM microscopy

For erosion analysis, both visual and electron microscope images are analysed (the latter with an EDS analyser).This is an important tools used for lifetime determination of the thruster.

Micro-Thrust Measurement

Thust is measured using a micro-thrust balance capable of measuring in the micro-newton range. Sensor include a laser probe, a Hall sensor and recently a magnetic link balance.

Ion Flux measurement

Many measurements are computerised, such as the directional ion flux detector (See picture below).

Research

Corion Thruster

The Wits propulsion lab has developed a new type of thruster, called the corona ionization thruster (Corion). It has the following characteristics:

• Highly miniaturised (<1 gram)
• Efficiency of ~20%
• Low power (<1 Watt)
• Thrust of ~0.3 mN
• Any propellant
• Cheap
• Simple
• Stackable

On Right: The Corion thruster in operation.

Collaborations include the Laboratoire de Physique des Plasmas Ecole Polytechnique (France), the CSIR, Wits electrical engineering and the Cape Peninsula Technical University.

  The Vacuum arc thruster

The laboratory facilities are also used for a joint project with Wits Mechanical engineering in the testing and design characterisation of a vacuum arc thruster

 Left: Vacuum arc thruster firing. The automatically rotating rig measures the ion beam intensity using a shielded Faraday cup.

 Theoretical research

The plasma plume created by the Corion thruster needs to be described mathematically, to identify important design parameters and optimise performance. Also, the thrust mechanism is not entirely clear and our models do not capture the complete thrust behaviour. Work modelling the electric and thrust behaviour is still on-going.

Computer Simulations

In order to take more complicated phenomena such as a full plasma analysis into account, closed solutions for the equations describing the system can often not be found. In such cases computer simulations are useful in order to gain insight into important physical aspects of the thruster.

Simulations are also used to predict the orbital behaviour of a thruster mounted on a satellite in space.

Current programs

?Ecole polytechnique (Dr. T. Lafleur): plasma plume simulations

?CSIR (H. Liedberg): plume temperature and composition measurements

?Wits electrical engineering (Prof. Hofsajer): power supply for Corion

?CPUT (Prof. R. Van Zyl, F. Visser): inclusion of Corion system onto ZA-CUBE 2 for orbital testing.

?Wits mechanical engineering (Dr. C. Law/ J. Lun (SANSA)): on vacuum arc thruster

Some recent outputs

• ¡°Fluid Simulation for Corona ionization Thruster¡±, P. Ferrer, SAIP conference 2012, University of Pretoria
•  ¡°Rarefied gas ejection as a thrust mechanism for miniature electric space propulsion systems¡° M. Krommenhoek, Phil Ferrer, SAIP conference 2012, University of Pretoria
•  ¡°Fluid simulations of a spherical corona discharge applied to a hollow needle-to-plane system¡± by Ferrer and Lafleur to Journal of Physics D: Applied Physics (submitted)
• ¡°Direct thrust measurements of a miniature bipolar corona discharge thruster¡± by Ferrer and Lafleur, Applied Physics Letters (submitted),
• ¡°Miniaturization of electrostatic ion engines by ionization and acceleration coupling¡±, P. Ferrer and M. Tchonang, J. Phys. D: Appl. Phys. 44 (2011) 335204.
• ¡°Orbital simulations of a Cubesat with onboard propulsion system¡±, N. Daya, honours project (2012)
• ¡°Modeling the Corona Ionisation system¡± M. Tchonang, Masters Dissertation, Wits
• ¡°Miniaturization of ion propulsion through ionization/acceleration coupling - the corona model¡± P. Ferrer, 62nd International Astronautical Congress, Capetown (2011)
• ¡°Miniaturization of electrostatic space thrusters using ionization/acceleration coupling in discharge mode¡± P. Ferrer, South African Institute of Physics (SAIP) conference, (2011)
• ¡°Miniaturization of electrostatic space thrusters using ionization/acceleration coupling: Corona model¡± M. Tchonang, P. Ferrer,South African Institute of Physics (SAIP) conference, (2011)
• ¡°Modeling of the Corona ionization space propulsion thruster¡± by M Tchonang, P. Ferrer, South African Institute of Physics (SAIP) conference,(2010)
• ¡°Plasma propulsion¡±, K. Jefries, honours project
• ¡°Corona ionisation used in electric thrusters¡± by P. Ferrer, and Trevor LafleurPoster presented at the annual South African Institute of Physics (SAIP)

Active Researchers

Dr Phil Ferrer Current research interests include plasma propulsion for space applications, specifically micropropulsion. My work revolves around a miniaturised plasma engine, with coupled ionisation/acceleration mechanism of the propellant plume.