Hall drive

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Selection of Russian hall drives
American Hall drive in operation (Xenon)

A Hall drive or Hall effect drive ( English Hall-Effect Thruster , Hall Thruster ) is an ion thruster in which a magnetic field increases the efficiency by preventing the electrons from reaching the anode . With this type of ion source, high thrust efficiencies and a long service life are possible even with high powers up to the 100 kW range. For the thrusters previously used on spacecraft , however, only a few 100 to 1000  W were available, with which thrust forces of 10 to 100  mN result.

As with other ion thrusters, xenon is typically used as a supporting mass , the positive ions of which are accelerated to speeds between 10 and 80 km / s by an electric field .

history

Research and development of ion drives go back to the 1960s, particularly in the USA and the Soviet Union. While experiments were being carried out with lattice ion sources in the USA , the Kaliningrad-based company FAKEL brought the Hall-effect drive to the point where it was ready for flight. Since the successful first use in 1971 on the METEOR satellite , more than 50 satellites have been equipped with FAKEL drives.

During the Cold War, but especially after the opening of the Iron Curtain , Hall drive technology was exported to the western world and developments in France ( SNECMA ), Italy ( Sitael , formerly Alta) and in the USA ( Busek , Aerojet , JPL , NASA and US Air Force Research Laboratories ) made it to flight application and commercial marketing. With SMART-1 , the first European Hall drive (Snecma) was successfully used for a flight mission in 2003. The first test flight of an American Hall drive (Busek) took place in 2006, the first American flight application with such a drive (Aerojet) in 2010. In German-speaking countries, research on Hall drives was carried out at DLR Stuttgart in the 1960s and 1970s, but these are current no known research and development activities.

Research and development have also been carried out on Hall drives in East Asia, particularly in Japan, since the 1980s. In 2012, China tested a drive on the Shijian 9A technology satellite , and in 2013 South Korea followed with a test drive on STSAT 3 and DubaiSat 2 .

The NASA funded the development of high performance Hall effect thrusters at Aerojet Rocketdyne from 2016 to 2019 with 67 million US dollars. The aim is to have operational engines for deep space missions by 2020.

Layout and function

At the beginning, different research groups experimented with similar designs, for which different names have been established:

  • Drive with wide acceleration channel: engl. Stationary Plasma Thruster (SPT), Russian стационарный плазменный двигатель (СПД). Alternative names are French Propulsion Plasmique Stationaire (PPS) or English. Magnetic-layer type (German magnetic layer drive )
  • Drive with narrow acceleration channel: engl. Thruster with Anode Layer (TAL), Russian двигатель с анодным слоем (ДАС)
Cross section through an SPT hall drive

Both types have in common an annular gap that is open on one side, which in the TAL forms a completely metallic hollow anode. In the SPT, the anode is limited to the channel base, while the side walls are ceramic, e.g. B. from boron nitride . The choice of material is decisive for the service life of the engine. The gas serving as a supporting mass is metered in at the bottom of the channel. The channel is surrounded concentrically by a magnet system, which is often formed by coils, but permanent magnets are also occasionally used. The magnetic field penetrates the channel approximately in a radial direction.

Electrons are emitted from an externally attached cathode . Due to the space charge, they largely follow the ion beam and neutralize it. A smaller part is drawn towards the anode by the accelerating voltage. The magnetic field directs them on circular paths in front of and in the channel, whereby the orbital speed of the electrons adjusts itself so that the electrostatic and Lorentz forces just compensate each other (as with the Hall effect , hence the name of the engine). The electric field exists between the anode and the space charge of the circulating electrons. By impact ionization additional free electrons and ions. After a short fall in the direction of the anode, the secondary electrons have orbital speed, and the energy losses of the impacting electrons are also compensated for by a drift in the direction of the anode. The fact that the drift current is relatively low is important for the energy efficiency of the engine. The much higher ring current is important for the most complete possible ionization of the supporting mass, because when operating in a vacuum, the gas density is too low for a few ions to be able to carry away the neutral gas through collisions.

The electric field accelerates the ions axially out of the gap. Due to their mass thousands of times higher, their speed is much slower than that of the electrons, so that they hardly affect the magnetic field. Nevertheless, the exit speed of 10 to 80 km / s is far higher than with conventional chemical engines.

Many years of optimization have resulted in flight models with thrust efficiencies over 50%, which is why the use of these engines is so attractive. Efficiency levels of up to 75% have already been achieved in experimental models.

literature

  • Dan M. Goebel et al .: Fundamentals of electric propulsion - Ion and Hall thrusters. Wiley, Hoboken 2008, ISBN 978-0-470-42927-3 .

Web links

Individual evidence

  1. ^ NASA Works to Improve Solar Electric Propulsion for Deep Space Exploration. NASA, April 19, 2016, accessed April 27, 2016 .