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John O | March 2018

Researchers attempt to mimic space conditions in the lab

By Josh Perry, Editor


Researchers from Tohoku University (Japan) and the Australian National University (Canberra) have collaborated on experiments to discover how the energy state of plasma is influenced when the plasma interacts with magnetic and electrical fields, which will give a better understanding of magnetic nozzle plasma thrusters used to propel spacecraft.


Researchers recreated the conditions of outer space in the lab. (Wikimedia Commons)


According to a report from Tohoku University, the researchers performed an experiment that removed the electrons in the sytem that trap the electrical field, leaving only electrons that interact with the expanding magnetic field.


“The experimental results show the decreasing electron temperature along the expansion,” the report explained, “following a near perfect adiabatic expansion of an electron gas when electric fields are removed from the system.”


The laws of thermodynamics provide the necessary insight into the internal energy of a system and how that energy interacts with its surroundings. The report said, “Keeping in mind the first law of thermodynamics, there is presently no heat transfer but work must be done on the walls surrounding the system to lower its internal energy.”


In this experiment, the magnetic field represents the walls but because of its lack of physical boundary there is no heat transfer.


“When the electric fields within the plasma are removed, none of the electrons are trapped in the plasma system leaving the electrons free to interact with the confining magnetic wall: the plasma pressure force does work on the magnetic boundary,” according to the report. “This pressure force can also be understood as a Lorentz force generated to propel a spacecraft in a magnetic nozzle plasma thruster.”


The internal energy in the system is lowered by the electron gas working on the magnetic field. Researchers argue that this means that classical thermodynamic principles can be extended to the expansion of a collisionless electron gas in a magnetic nozzle.


The report concluded, “By removing the plasma-wall boundary in their laboratory plasma and thereby removing the corresponding electric field and electron trapping, the researchers have succeeded in reproducing conditions similar to those in space, i.e., with no boundary.”


The research was recently published in Physical Review Letters. The abstract read:


“A specially constructed experiment shows the near perfect adiabatic expansion of an ideal electron gas resulting in a polytropic index greater than 1.4, approaching the adiabatic value of 5/3, when removing electric fields from the system, while the polytropic index close to unity is observed when the electrons are trapped by the electric fields.


“The measurements were made on collisionless electrons in an argon plasma expanding in a magnetic nozzle. The collision lengths of all electron collision processes are greater than the scale length of the expansion, meaning the system cannot be in thermodynamic equilibrium, yet thermodynamic concepts can be used, with caution, in explaining the results.


“In particular, a Lorentz force, created by inhomogeneities in the radial plasma density, does work on the expanding magnetic field, reducing the internal energy of the electron gas that behaves as an adiabatically expanding ideal gas.”

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