Physicists at Rice University have simulated the kind of ultra-hot plasma found in the center of dead stars in a lab by creating laser-cooled plasma that is roughly fifty times colder than the ambient temperature in deep space.
The paradoxical work brings us the world’s first laser-cooled neutral plasma, which researchers hope will allow physicists to study some of the most exotic matter in the universe, such as the dense gases found in white dwarf stars, as well as make progress in fusion energy research.
Plasma is the fourth state of matter—an electrically conductive cloud of ultra-dense gas that is composed of ions and free electrons. Plasma is usually produced in extremely high temperature environments, such as the surface of the Sun, but in even more extreme environments (like at the center of an ultra-dense white dwarf star or Jupiter) plasma begins to behave in unusual ways that are difficult to replicate in a lab on Earth.
Simulating hot plasma in these extreme conditions can be accomplished in a lab, however, by making a plasma that is really, really cold.
As detailed in a paper published last week in Science, the physicists used an array of ten lasers to create the supercooled plasma. First, they vaporized strontium metal and suspended the vapor in an array of intersecting laser beams to allow it to cool. Next, the small cloud of cooled strontium vapor was ionized with a short pulse from another laser. The energy from this laser caused strips one electron from each strontium atoms to create a plasma of strontium ions and free electrons.
At the same time, this laser pulse causes the plasma to rapidly expand. The key breakthrough made by the physicists at Rice University was to use another laser array to blast the rapidly expanding plasma and cool it further. After this final laser pulse, the temperature of the plasma is only 50 millikelvins, or about -460 Fahrenheit, which is approximately 50 times colder than the vacuum of space.
According to the physicists, one of the main motivations for creating this ultra cold plasma was to study a phenomenon known as “strong coupling.”
When a strontium atom is ionized it loses an electron, which gives the atom a positive charge. Although these positively-charged ions repel each other in the plasma, this repulsive force is negligible compared to the amount of kinetic energy produced as heat.
“Repulsive forces are normally like a whisper at a rock concert,” Tom Killian, a physicist at Rice University and lead author of the research, said in a statement. “They’re drowned out by all the kinetic noise in the system.”
In extreme gravity environments, such as in the center of Jupiter or a white dwarf star, these positively-charged ions are forced so close together that the repulsive forces are stronger than the kinetic forces, even though the plasma is extremely hot. At this point the ions are all repelling one another and seek to find equilibrium, which means that they are repelled by all their neighboring ions equally. This repulsive balancing act is what is known as strong coupling.
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Although physicists are able to create extremely hot plasmas on Earth, replicating the extreme gravity conditions in the center of Jupiter to produce strong coupling in a lab isn’t possible. But if the goal is just to produce a plasma where the repulsive electric forces are stronger than the kinetic forces, this can be accomplished by going in the opposite direction.
In other words, Killian and his colleagues hope to simulate ultra-hot and ultra-dense plasmas by creating ultra-cold plasmas that are orders of magnitude less dense.
“We are just at the beginning of exploring the implications of strong coupling in ultracold plasmas,” Killian said. “I hope this will improve our models of exotic, strongly coupled astrophysical plasmas, but I am sure we will also make discoveries that we haven’t dreamt of yet.”