21 Jan 2025
Ultra-bright high-energy beams will assist understanding of plasmas and dense matter.
The Lawrence Livermore National Laboratory (LLNL) has created the brightest X-ray source to date, by directing the laser of the National Ignition Facility (NIF) onto ultra-light metal foams.Reported in Physical Review E, the findings could now point to new ways of imaging and studying extremely dense matter.
"Your dentist's X-ray machine creates an electron beam that is crashed into a heavy metal plate; the electrons in the beam interact with the electrons bound to the metal atoms to create X-rays," said LLNL's Jeff Colvin.
"At NIF, we use the high-power laser beam instead of an electron beam to make the X-rays by crashing the beam into silver atoms and creating a plasma."
The choice of silver is important, explained Colvin, as the higher the atomic number of the metal atom, the higher the energy of the X-rays that it produces. The team used silver because they wanted to make X-rays with an energy greater than 20,000 electron volts.
The foam structure of the metal was also critical to achieving this goal, with the team manufacturing 4-millimeter-wide cylindrical targets using a mold and silver nanowires.
"We froze the nanowires suspended in solution in the mold, then applied a supercritical drying process to remove the solution, leaving the low-density porous metal foam," said LLNL researcher Tyler Fears.
NIF laser leads to new models of plasma behavior
NIF's laser involves 192 individual beams, combined to create the world's highest-energy laser system amplifier. A weak original pulse is split and carried to 48 preamplifiers that increase the pulse’s energy by a factor of 10 billion, with the 48 beams then further split into four beams each for injection into the 192 main laser amplifier beamlines.
Since achieving fusion ignition for the first time in December 2022 NIF has set progressive records for laser energy, and combined its core fusion research with study of potential commercial applications in industrial manufacturing.
In the new experiment, the NIF laser beams "deposited 1000 kJ of light into the target with a 400 TW, 2.5 nanosecond pulse," said the project in its published paper, heating up the target cylinder of silver foam in 1.5 billionths of a second.
In addition to creating the X-ray source, LLNL explored differing foam densities to determine which provided the maximum energy output. They also applied a new data analysis technique to attempt to understand the physics of the generated plasma.
That data revealed that these bright, hot metal plasmas are far from thermal equilibrium. Current models used to examine inertial confinement fusion at NIF typically assume plasmas are close to equilibrium, with electrons, ions and photons all having around the same temperature; but the models will now need to be adjusted.
“Going forward, this means we need to rethink our assumptions about heat transport and how we calculate it in these particular metal plasmas," said Jeff Colvin from LLNL.
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