When liquid metal turns to plasma, the physics get weird

Observing the temperature where a liquid under high-density conditions crosses over to a plasma state.

Erupting plasma loops are seen above the surface of the sun. Plasma is the most abundant form of matter in the universe, and Rochester scientists are finding new ways to observe and create plasmas. (NASA/SDO photo)
Erupting plasma loops are seen above the surface of the sun. Plasma is the most abundant form of matter in the universe, and Rochester scientists are finding new ways to observe and create plasmas. (NASA/SDO photo)

For the first time, analysts at the University of Rochester’s Laboratory for Laser Energetics (LLE) have figured out how to transform a liquid metal into a plasma and to observe the temperature where a liquid under high-density conditions traverses to a plasma state. This discovery could lead to better understanding of stars and planets. It may also aid in the realization of controlled nuclear fusion—a promising alternative energy source whose realization has eluded scientists for decades.

Plasmas comprise of a hot soup of free moving electrons and particles—molecules that have lost their electrons—that effectively leads electricity. Despite the fact that plasmas are not common on Earth, they involve the vast majority of the matter in the observable universe, for example, the outside of the sun. Researchers can produce counterfeit plasmas here on Earth, normally by warming gas to thousands of degrees Fahrenheit, which strips the atoms of their electrons.

As Mohamed Zaghoo, a research associate at the LLE, and his colleagues observed, however, there is another way to create a plasma: under high-density conditions, heating a liquid metal to very high temperatures will also produce a dense plasma.

One of the unique aspects of this observation is that liquid metals at high densities exhibit quantum properties; however, if they are allowed to cross over to the plasma state at high densities, they will exhibit classical properties.

LLE scientists transformed liquid metals into plasmas under high-density conditions. Increasing the density to extreme conditions made the liquid enter a state where it exhibited quantum properties. The bottom panel shows the quantum distribution of electrons in a dense liquid metal, where only two electrons can share the same state. However, when the temperature is increased to 0.4. Fermi temperature (approximately 90,000 degrees Fahrenheit), the electrons rearrange themselves in a random way that resembles a hot soup of plasma and the electrons loose their quantum nature and behave classically (top panel). (Laboratory for Laser Energetics / Heather Palmer)
LLE scientists transformed liquid metals into plasmas under high-density conditions. Increasing the density to extreme conditions made the liquid enter a state where it exhibited quantum properties. The bottom panel shows the quantum distribution of electrons in a dense liquid metal, where only two electrons can share the same state. However, when the temperature is increased to 0.4. Fermi temperature (approximately 90,000 degrees Fahrenheit), the electrons rearrange themselves in a random way that resembles a hot soup of plasma and the electrons loose their quantum nature and behave classically (top panel). (Laboratory for Laser Energetics / Heather Palmer)

So as to observe a liquid metal traverse to a plasma, the LLE specialists began off with the fluid metal deuterium, which showed the traditional properties of a liquid. To expand the thickness of the deuterium, they cooled it to 21 degrees Kelvin (- 422 degrees Fahrenheit). The analysts at that point utilized the LLE’s OMEGA lasers to set off a solid shockwave through the ultracool fluid deuterium.

The shockwave compacted the deuterium to weights up to five million times more prominent than atmospheric pressure, while likewise expanding its temperatures to very nearly 180,000 degrees Fahrenheit. The example began off totally straightforward, yet as the weight rose, it changed into a sparkly metal with high optical reflectivity.

Zaghoo said, “By monitoring the reflectance of the sample as a function of its temperature, we were able to observe the precise conditions where this simple lustrous liquid metal transformed into a dense plasma.”

“We observed that the liquid metal initially exhibited the quantum properties of electrons that would be expected at extreme temperatures and densities. However, at about 90,000 degrees Fahrenheit, the reflectance of the metallic deuterium started rising with a slope that is expected if the electrons in the system are no longer quantum but classical. This means that the metal had become a plasma.”

Suxing Hu, a senior scientist at LLE and a co-author on the study said, “Increasing the density to extreme conditions made the liquid enter a state where it exhibited quantum properties. Raising the temperature even further made it turn into a plasma, at which point it exhibited classical properties, yet was still under high-density conditions.”

“What is remarkable is that the conditions at which this crossover between quantum and classical occurs is different from what most people expected based on plasma textbooks. Furthermore, this behavior could be universal to all other metals.”

Zaghoo said, “Understanding these fundamentals of liquids and plasmas allows researchers to develop new models to describe how materials at high densities conduct electricity and heat and can help explain the matter in the extremes of the solar system, as well as help in attaining fusion energy.”

“This work is not just a laboratory curiosity. Plasmas comprise the vast interiors of astrophysical bodies like brown dwarfs and also represent the states of matter needed to achieve thermonuclear fusion. These models are essential in our understanding of how to better design experiments to achieve fusion.”

The observations are published in Physical Review Letters.