Electric Double Layers

Double layers surround Earth, blocking high energy charged particles from space. Credit: NASA’s Scientific Visualization Studio.

September 9, 2020

Double layers can accelerate charged particles, as well.

“We have to learn again that science without contact with experiments is an enterprise which is likely to go completely astray into imaginary conjecture.”
— Hannes Alfvén

A recent press release from the European Space Agency (ESA) provides information about future technologies that will make space exploration more economical. The new ion thruster technology works by pulsing an electric arc between two electrodes, vaporizing the propellant, while at the same time creating a stream of charged particles (plasma), which is accelerated by an electromagnetic field.

A double layer forms in plasma when electric charge flows through it. Double layers are found in the plasma environment of Earth, as well as around the stars, creating phenomena like aurorae and electromagnetic radiation from pulsars.

Plasma pioneer and Nobel laureate, Irving Langmuir studied the behavior of charged particles moving through various gases. He was the first to coin the term “plasma” when referring to ionized gas, finding that they accelerated ions to supersonic velocities. Double layers are self-generating, so the effect has been incorporated into an efficient spacecraft thrust mechanism.

As mentioned, a double layer is a charge separation region that forms in plasmas. It consists of two oppositely charged parallel layers, resulting in a voltage drop and an electric field across the layer. These regions accelerate electrons and positive ions in opposite directions. Since moving electric charges generate electricity, there is an electric current present. If there are sufficiently large potential drops and layer separation, electrons might accelerate to relativistic velocities, producing synchrotron radiation.

Nobel prize winner Hannes Alfvén described a double layer as, “… a plasma formation by which a plasma—in the physical meaning of this word—protects itself from the environment. It is analogous to a cell wall by which a plasma—in the biological meaning of this word—protects itself from the environment.”

As written many times in the past, electricity can accelerate charged particles with energies of 10^20 electron volts or more. Since electricity requires a circuit for charge to flow, and an electric current forms a magnetic field, that field tends to constrict the current. Previous Picture of the Day articles identify that constricted channel as a Bennett pinch, or z-pinch. Laboratory experiments with particle accelerators confirm that the pinched filaments of electric current remain coherent over large distances.

At times, a double layer might interrupt charge flow in the circuit, causing a catastrophic rise in voltage across it. The powerful energy release of the exploding double layer is sometimes observed in power transmission switchyards when a circuit breaker is opened incorrectly, or in electrical experiments like the SAFIRE project, when millions of volts are suddenly produced.

Plasma physicist, Hannes Alfvén identified occurrences like that when he was contracted by the Swedish Power Company to investigate some serious accidents. A few of the mercury arc rectifiers used in the power transmission circuits had exploded for no apparent reason. Alfvén identified the cause as unstable double layers within the plasma flow.

He wrote:

“In Sweden the waterpower is located in the north, and the industry in the south. The transfer of power between these regions over a distance of about 1000 km was first done with a.c. When it was realized that d.c. transmission would be cheaper, mercury rectifiers were developed. It turned out that such a system normally worked well, but it happened now and then that the rectifiers produced enormous over-voltages so that fat electrical sparks filled the rectifying station and did considerable harm…An arc rectifier must have a very low pressure of mercury vapor in order to stand the high back voltages during half of the a.c. cycle. On the other hand, it must be able to carry large currents during the other half-cycle. It turned out that these two requirements were conflicting, because at a very low pressure the plasma could not carry enough current. If the current density is too high, an exploding double layer may be formed. This means that in the plasma a region of high vacuum is produced: the plasma refuses to carry any current at all. At the sudden interruption of the 1000 km inductance produces enormous over-voltages, which may be destructive.”

There are also double layers in space that emit radio waves over a broad band of frequencies. They can sort galactic material into regions of like composition and condense it. They can accelerate charged particles to cosmic ray energies. Double layers in space can explode for the same reason as Alfvén’s rectifiers, releasing more energy than is locally present. This effect can be seen in stellar flares or so-called “nova” outbursts.

Since plasma is composed of charged particles, their movement constitutes an electric current, which generates a magnetic field. Electrons spiral in the resulting magnetic field, creating synchrotron radiation that can shine in all high-energy frequencies, including extreme ultraviolet, X-rays, and gamma rays.

Cosmic plasmas and their activity can be replicated in the laboratory, allowing insights into the large-scale structures that populate the Universe. Since gravitational forces cannot be examined in the laboratory, consensus opinions about the gravity-only model of celestial objects suffer from a moribund condition.

Stephen Smith

The Thunderbolts Picture of the Day is generously supported by the Mainwaring Archive Foundation.

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