Electric propulsion: ion thrusters

The Tsiolkovsky rocket equation relates the velocity increment ΔV of the payload mass md the requiered mass of propellant mp , and its exit velocity Vex,

High exhaust propellant speeds exponentially decrease the ratio mp/md, and therefore the rocket fuel needs. Nevertheless, the nature of the chemical reaction limits the values of Vex in classical thrusters. Unlike conventional rockets, the propellant of an electric thruster is a plasma composed of charged particles that reach higher values of Vex accelerated by electromagnetic forces. Ion acceleration is an attractive idea because ions accelerated up to Eb = 1 eV (voltage drop of 1 volt) reach velocities in the order of Vex ≈ 105 m/s, higher than most conventional chemical engines.

The performances of rocket thrusters are characterized by the specific impulse, which measures the thrust T produced by unit of mass or weight of propellant. It is usually expressed as the weigth at the Earth surface,

and Isp has units of time in this case. Therefore, electric thrusters have higher specific impulse than chemical engines which rely on the electromagnetic energy transfer to the propellant. The different electric propulsion sytems could be roughly classified according to the charge acceleration mechanism [1, 2] as,

  • Electrothermal propulsion (Resistojet, Arcjet)
  • Electrostatic propulsion (Ion thruster, Hall thruster, Field emission electric propulsion thruster)
  • Electromagnetic propulsion (Pulsed Plasma Thruster, Magnetoplasmadynamic thruster)

The UPM gridded ion thrusters

The UPM PlasmaLab develops low power electrostatic ion thrusters also intended as laboratory plasma and ion sources. The first model was developed in 2008 and our improved design, now under evaluation in the laboratory, was avalilable since September 2012. Both are ring-cusp ion thrusters based on a system of grids for ion extraction and acceleration.

The basic elements of a gridded ion thruster are shown in the drawing. The primary plasma produced by a hollow cathode is injected into the so called discharge or ionization chamber where a fraction of the electrons are trapped along the magnetic field lines of the ring cusp magnetic configuration (green lines). These electrons ionize the additional flow of neutral gas (Xenon) increasing the plasma density whitin the ionization chamber. The ions are extracted and accelerated from this plasma by a set of grids and this ion beam is later neutralized by electrons from the outer hollow cathode. Otherwise, the extracted ion current becomes space charge limited. This neutralization process prevents the accumulation of positive charges at the exit section of the thruster and transforms the outgoing ion current into a plasma stream.

The thrust is produced by this plasma stream of accelerated ions and is usually low, in the order of 1-50 mN. Nevertheless, the large speeds of ions (in the order of 105 m/s) provide large values of the specific impulse compared with conventional chemical rockets.

Different models of gridded ion thrusters have been already used in space for orbit station keeping and also in deep space missions, but they usually require of electric powers over the kW. This fact bounds up today the electric propulsion to large satellites. The designs need to be reduced for microspacecrafts of 10-100 Kg in, either size and electric consumption, where the on board electric power available lies well below the kW. The Plasma Physics and engineering involved in the design and testing of low power ion thrusters constitute an active field of applied research. The effective downsizing requires in first place the replacement of the hollow cathodes employed for primary plasma production and ion beam neutralization.

    First model

The low power ion plasma thruster shown in the photograph was originally designed as an stable plasma source made of AISI 316 stainless steel. The primary plasma is produced by a low pressure electric discharge within the discharge chamber of 50 mm in diameter and 48.3 mm of lenght. The ring cusp magnetic configuration is produced by two alternate crowns of SmCo permanent magnets attached to the external surface of the discharge chamber not observed in the picture. This device was built in 2008 and operated up to September 2012.

The two acceleration and extraction stainless steel grids are fixed over a electrical insulator which looks partially blackened by the repeated use. The operation of this device requires of less than 200 W and the extracted ion current reaches 80 mA. The measurement of this flow of ions allows to estimate the trust shown in the figure. The throttle might be controlled because the magnitude of the extracted ion current relies on either, the extraction Vex and the ion acceleration voltages Vacc, giving specific impulses in the range Isp ~ 1400-2500 s-1.

The operation of this thruster highlighted the crucial role of the neutralization of the extracted ion flow. When the electron current from the neutralizer increments over a critical value the extracted ion current dramatically increases [3,4].

    Second design

The new project builds upon the useful lessons learned and the expertise gained with our first design. The new thruster is lightweight (made of alumininum) with a discharge chamber of 77 mm in lenght and outer diameter of 45 mm. The main characteristics are the improved magnetic configuration and its modular design, which allows the easy replacement or exchange of different parts. In particular the cathode handling which might be replaced or substituted using a separate cathode support. This will help to investigate the performaces of different cathodes for primary plasma production.

Although the ring cusp magnetic configuration is maintained, the new design makes use of three sets of crowns of magnets instead of the previous two. Each independent magnetic crown is thermally insulated from the discharge chamber wall and the position of the magnets is fixed by bolts, improving the mechanical stability.

This new design is now under evaluation and exibits lower power comsuption and an increased ion extraction current. The improved magnetic configuration resulted in the reduction of the discharge losses and also in a better confinement of charges within the discharge chamber.

[1] Physics of Electric Propulsion. R.G. Jahn. Dover Publications, New York (2006).
[2] Fundamentals of Electric Propulsion. D.M. Goebel and I. Katz. Wiley & Sons, New Jersey (2008).
[3] E. Criado et al., Phys. Plasmas 19, 023505 (2012).
[4] E. Roibás et al., Cont. Plasma Phys 53 (1) pp. 57–62 (2013).