Rare earth-doped glass boosts lasers, tests experimental theories at National Ignition Facility

The Lawrence Livermore National Lab’s National Ignition Facility is “the world’s largest and most energetic laser facility ever built.”

Through the power of lasers, the facility can test extreme states of matter by focusing its lasers to create out-of-this-world intense temperatures and pressures.

The giant facility, spaning the width of three football fields and rising 10 stories tall, contains 192 laser beamlines used in a variety of experiments, including those directed towards the facility’s goals to achieve fusion ignition.

Construction of such a technologically advanced yet ginormous facility, officially dedicated in 2009, was no simple task—in addition to some serious engineering challenges, the facility also consumed 55,000 m3 of concrete, 7,600 tons of steel rebar, and 5,000 tons of structural steel.

But what’s inside NIF is even more impressive: 3,072 neodymium-doped phosphate glass slabs, each weighing 42 kg, that amplify the energy of the facility’s optical fiber-carried laser. (Each beamline houses 16 glass slabs.)

The facility’s numerous amplifiers, including those rare earth-doped glass slabs, enable the facility, as they collectively amplify a weak laser pulse into the incredible power capacity the facility provides.

The laser pulse starts out at a feeble ~1 billionth of a joule and is amplified through glass and mirrors throughout the huge facility, collecting energy all along the way, to boost its power upwards of 4 million joules.

“The amplifiers are designed to efficiently provide 99.99% of NIF’s power and energy,” the NIF website says. Read more about how NIF works here.

So why do NIF’s glass laser amplifiers contain rare earths?

Neodymium atoms in the doped glass (which provide the pink color) are excited with huge 6-foot-tall white light flashlamps. That energy is then released into the laser pulse as it passes through, boosting the power.

1015ctt rare earths NIF lo res

Each of the National Ignition Facility’s 192 beamlines contains two large amplifier sections. Laser amplifier glass is doped with rare earth ions, which, new research shows, prevent external heat and noise from affecting the laser transitions. Credit: LLNL

According to the NIF website, the laser glass was developed through partnerships with Schott North America and Hoya Corp. over a six-year R&D program. “This effort developed a revolutionary process for manufacturing meter-size slabs of laser glass that is 10 times faster, 5 times cheaper, and with better optical quality.”

Neodymium and other rare earths are useful elements here because of two reasons, LLNL scientist Michael Hohensee says in a LLNL news release. For one, the valence electrons of rare-earth atoms have high orbital angular momentum. And two, the rare earths’ valence electrons are shielded by other electrons, which provides a buffer from external heat and noise that would otherwise disrupt laser transitions.

Those properties are important for allowing scientists to perform complex experiments that test the very theories that govern physics.

“Thanks to both these properties, the electronic states of rare earths doped in a crystal make possible an electronic equivalent of the Michelson-Morley experiment that would be more sensitive than any other yet performed, helping to validate or rule out unified theories of gravity and particle physics,” Hohensee said.

The Michelson-Morley experiment, dating back to 1887, is a famous failed experiment that disproved the popular aether theory of the time and led to additional experiments that eventually spurred the theory of special relativity.

The paper detailing the LLNL team’s research, published in Physical Review Letters A, is “Effects of Lorentz-symmetry violation on the spectra of rare-earth ions in a crystal field” (DOI: 10.1103/PhysRevA.92.040101).

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