An ion thruster is a means of propulsion for spacecraft. The thrust is created by accelerating ions. The ions can be accelerated by electrostatic or electromagnetic force. Electrostatic thrusters use the Coulomb force, accelerating ions in the direction of the electric field. Electromagnetic thrusters utilise the Lorentz force. Both can be calculated using Opera.
- Controlling orientation and positioning of satellites
- Propulsion for deep space craft
Types of Electrostatic thrusters:
- Gridded electrostatic ion thrusters.
These commonly utilize xenon gas, ionized by bombarding it with energetic electrons. The extraction of positively charged ions is via an extraction system consisting of multi-aperture grids. After entering the multi-aperture grid system via the plasma sheath the positive ions are accelerated due to the potential difference between the grids, generating the thrust.
- Hall effect thrusters.
These accelerate ions with the use of an electric potential between a cylindrical anode and a cathode consisting of negatively charged plasma. Again, xenon is typically used as the bulk of the propellant, which is introduced near the anode, where it ionizes and flows toward the cathode. The positively charged ions accelerate towards and through the cathode, picking up electrons as they leave to neutralize the beam.
- Field-emission electric propulsion
FEEP thrusters typically use either caesium or indium as the propellant. Caesium and indium are used due to their high atomic weights, low ionization potentials and low melting points. A small propellant reservoir stores the liquid metal, which then flows through a narrow tube and into an accelerator (a ring or an elongated aperture in a metallic plate) about a millimeter past the tube end. Once the liquid metal reaches the end of the tube, an electric field applied between the emitter and the accelerator causes the liquid surface to deform. Utilising a high applied voltage can extract positive ions and the electric field created by the emitter and the accelerator then accelerates the ions. An external source of electrons neutralizes the positively charged ion stream to prevent charging of the spacecraft.
Types of Electromagnetic thrusters:
- Pulsed inductive thrusters
PITs consist of a large coil encircling a cone shaped tube. Ammonia is typically used, which is emitted from the tube. The coil is charged with pulses from capacitors attached to the coil. The current creates a magnetic field in the outward radial direction, which then creates a current in the gas in the opposite direction, ionizing the gas. The positively charged ions are accelerated away from the engine due by the Lorentz Force
- Lithium Lorentz force accelerator
A LiLFA thruster uses lithium vapor. Multiple cathode rods are packed into a hollow cathode tube. The gas enters the main chamber where it is ionized into plasma. This plasma then conducts electricity between the anode and the cathode. This current creates a magnetic field, which crosses with the electric field, thereby accelerating the plasma due to the Lorentz force.
Opera Charged Particle module:
The 3d Charged Particle Module of Opera calculates the interaction of charged particles in electrostatic and magnetostatic fields. It uses the Finite Element method to solve Maxwell’s equations for the steady-state case in a discretized model, and provides a self-consistent solution including the effects of space-charge, self-magnetic fields and relativistic motion. A comprehensive set of emitter models is provided, including thermionic and field effect emission from surfaces, secondary emission from surfaces and within volumes (used to model gas ionization), and models for unmagnetized and magnetized plasmas. It is possible to include multiple species of charged particles, each having user defined charge and mass.
Different types of particle emission models can be used to create the primary beams of charged particles. These include Child’s law and Langmuir/Fry relationships for the calculation of the space charge limited current from a thermionic emitter, field effect emission relationships, emission from the surfaces of plasmas and beams with defined current densities and initial energy. Several emitter surfaces can be specified in a model and any number of emission models can be added to each surface. The emission model defines the type of particle and emission characteristics that are required.
Secondary emission properties can be applied to labelled surfaces of the model. Collisions of the particle beamlets with these labelled surfaces are detected and secondary particles are introduced. These secondaries may also collide to produce further new secondary particles; the maximum number of generations of secondary particles can be limited. The space charge effects created by secondary particles can be excluded from the calculation.
The net power arriving at a secondary emitter surface (i.e the difference between the incident power and the power in secondary beams) is calculated and is available in the Post-Processor. The incident beam power can be determined by using a backscattered secondary with the energy loss factor set to a very small number
The volume plasma emitter uses a self-consistent plasma boundary method to model low density magnetised plasmas. The method is described as self-consistent because it determines the plasma boundary surface shape, together with the ion and electron currents that are consistent with the potential distribution caused by the space charge in the particle flows outside the plasma volume. Ions are not explicitly represented inside the plasma volume and the plasma is assumed to be quasi neutral.