Atoms Hollowed by Laser Form Novel Plasma

A highly unusual plasma composed of hollow atoms has been created using one of the world’s most powerful lasers. The surprising result shows that a little-explored region of physics is now accessible through the unprecedented intensities being reached at some laser facilities.

A hollow atom occurs when an electron buried in an atom is removed, usually by being hit by another electron, creating a hole while leaving all the other electrons attached. This process creates plasma, a form of ionized gas. An x-ray is released when the hole is filled.

Normally, the process involves removing electrons from the outer shells of atoms first and working inward, but the new technique empties atoms from the inside out. The work, led by scientists from the University of York and the Russian Academy of Sciences’ Joint Institute for High Temperatures, shows that it is possible to remove the two most deeply bound electrons from atoms.

A highly unusual plasma composed of hollow atoms has been created using one of the world’s most powerful lasers — the Vulcan petawatt laser. The surprising results show that a little-explored region of physics is now accessible through the unprecedented intensities being reached at some laser facilities. Images courtesy of the Central Laser Facility.

The experiment, carried out using the Vulcan petawatt laser at the Central Laser Facility at the Science and Technology Facilities Council’s Rutherford Appleton Laboratory, could further understanding of fusion energy generation, which employs plasmas that are hotter than the core of the sun.

“At such extraordinary intensities, electrons move at close to the speed of light and as they move, they create perhaps the most intense x-rays ever observed on Earth,” said principal investigator Dr. Nigel Woolsey from the York Plasma Institute, Department of Physics, at the University of York. “These x-rays empty the atoms from the inside out; a most extraordinary observation and one that suggests the physics of these interactions is likely to change as lasers become more powerful.”

Analysis and theoretical work, led by Los Alamos National Laboratory (LANL) in the US and Osaka University in Japan, showed the mechanism for hollow atom generation was not because of the collision of electrons or driven by the laser photons, but rather was driven by the resulting radiation field from the interaction.

“This experiment has demonstrated a situation where x-ray radiation dominates the atomic physics in a laser-plasma interaction; this indicates the importance of x-ray radiation generation in our physics description,” said co-author Dr. Alexei Zhidkov of Osaka University. “Future experiments are likely to show yet more dramatic effects, which will have substantial implications for diverse fields such as laboratory-based astrophysics.”

If the scientific and technological challenges can be overcome, fusion offers the potential for an effectively limitless supply of safe, environmentally friendly energy. The experimental work was designed to further scientists’ understanding of how intense lasers can create electron beams with speeds close to the speed of light, then use these beams to heat fusion fuel to thermonuclear temperatures.

The front end of the Vulcan laser.

“The measurements, simulations and developing physics picture are consistent with a scenario in which high-intensity laser technology can be used to generate extremely intense x-ray fields,” said Dr. Sergey Pikuz of the Joint Institute for High Temperatures. “This demonstrates the potential to study properties of matter under the impact of intense x-ray radiation.”

“This was a very dynamic experiment which led to an unexpected outcome and new physics,” said Rachel Dance, a University of York doctoral physics student. “The hollow atom diagnostic was set to measure the hot electron beam current generated by the laser, and the results that came out of this in the end showed us that the mechanism for hollow atom generation was not collisional or driven by the laser photons, but by the resulting radiation field from the interaction.”

The research — with funding from the Science and Technology Facilities Council, the Engineering and Physical Sciences Research Council and the Royal Society — was reported in Physical Review Letters (doi: 10.1103/PhysRevLett.110.125001).

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