Measuring conditions in volatile clouds of superheated gases known as plasmas are central to pursuing greater scientific understanding of how stars, nuclear detonations and fusion energy work. For decades, scientists have relied on a technique called Thomson scattering, which uses a single laser beam to scatter from plasma waves as a way to measure critical information such as plasma temperature, density and flow.
Now, however, a multidisciplinary team of Lawrence Livermore National Laboratory (LLNL) researchers has successfully demonstrated a potentially simpler, more accurate way to measure plasma conditions with two laser beams that cross paths, creating a data signal that is about a billion times stronger than what is available from the Thomson scattering method.
This breakthrough could give physicists working on complex high energy density science and inertial confinement fusion (ICF) research at facilities like LLNL's National Ignition Facility (NIF) an innovative new tool.
"The proof of principle worked beautifully and now we're exploring how we can take this to the next level," said LLNL experimental physicist Andrew Longman, the lead author of a paper on the research recently published in the science journal Physical Review Letters.
Plasma, a fast‑moving, superheated mix of free electrons and ions, is sometimes called the "fourth state of matter" and behaves differently from familiar solids, liquids and gases.
For decades, physicists have typically used Thomson scattering, with instruments like spectrometers detecting tiny amounts of light that scatter off energized plasma particles. The data gleaned from this scattered light can be used to characterize the density, temperature and velocity of those electrons.
"When we're doing implosions at NIF, knowing the conditions where all these lasers cross is really important because that tells us how energy is transferred from one to another," Longman said. "It affects the implosion symmetry. NIF and other facilities have struggled to measure these conditions."
Improvements such as NIF's sophisticated Optical Thomson Scattering Laser System helped refine ICF experiments at NIF, the world's most energetic laser system and the only lab on earth where fusion ignition has been achieved. Ignition, which produces more fusion energy than the amount of laser energy delivered to the NIF target, has provided more opportunities to understand the environments found inside stars or nuclear explosions.
However, the weak Thomson scattering signals that come back remain a challenge.
"Basically, the signal that you measure is really like enhanced noise from the plasma, but it's very tiny," said LLNL physicist Pierre Michel. "A lot of background noise gets added to it."
