NRL-built Argon Fluoride Laser marks breakthrough, sets new energy record

WASHINGTON – The U.S. Naval Research Laboratory research team set a new energy record on March 5 using an argon fluoride laser. This energy is twice the previous record.

It delivered a laser beam capable of applying more force to implode a laser fusion target than any other laser technology, which is the oomph needed for a nuclear fusion reaction.

The research team tested this capability by computer simulations with a small pellet about the size of a pea made of deuterium and tritium. Deuterium and tritium are isotopes of hydrogen that have additional neutrons in the nucleus. The chemical elements were frozen together and formed the inner skin of the hollow pellet.

The U.S. Naval Research Laboratory retrofitted one of its two gas laser facilities to use argon fluoride instead of krypton fluoride. Argon fluoride is a more efficient driver for laser fusion because of its short wavelength (λ= 193 nm) and broad bandwidth. (U.S. Navy photo)

“If the density and temperature are high enough, it ignites the nuclear fusion reaction and produces much more energy, 100 times more than the laser took to do all of this,” Andrew Schmitt, a physicist at NRL, said.

The NRL team wants to develop the science and technologies to a much higher energy scale between 500,000 to million joules to drive a higher performance fusion implosion.

To produce a higher energy laser it will require a facility specifically designed for argon fluoride.

NRL researchers already leverage the laser fusion technologies they developed for krypton fluoride on their argon fluoride experiments. They hope a new laser facility specifically designed for argon fluoride will further prove the viability of this gas as a cost-effective alternative to current laser fusion approaches.

Computer simulation of the impact progression to a deuterium-tritium pellet as a record-setting argon fluoride laser developed at the U.S. Naval Research Laboratory hits it. The top half of the graphic shows the reduction of the pellet size and the lower half shows the increase in temperature (red color equates to higher temperature). The simulation indicated that 109 times the incident laser energy is produced by the thermonuclear burn of the deuterium tritium fuel, enough energy gain for a power plant. (Graphics by U.S. Naval Research Laboratory)

Matthew Wolford, a research chemist, has investigated excimer laser technology for more than 18 years. An excimer laser usually contains a mixture of noble gas such as krypton or argon and a halogen gas such as fluorine. The gas mixture is irradiated by powerful electron beams and in response it emits a beam of deep ultraviolet light.

“We haven’t built an argon fluoride facility yet with the energy required to achieve laser fusion,” Wolford said. “We got 200 joules out of the NRL Electra facility. We need on the order of half a megajoule to compress enough hydrogen for fusion to occur. We have to go up three orders of magnitude. We would want to build a small facility and show that we can do the target physics.”

The energy output of 200 joules is enough energy to power a 20-watt LED light for 10 seconds. A half megajoule is enough energy to power the LED for 14 hours.

“Due to the higher argon fluoride laser efficiency we expect to build laser fusion facilities with lower cost, smaller size and lower electrical power requirements,” Wolford said. “It would provide a source of energy that would be cleaner than present day technologies such as fossil fuels. It’s the energy source of the future.”

Steve Obenschain, a physicist and head of the NRL Laser Plasma Branch, said argon fluoride laser is the shortest wavelength laser with the theoretical capability to deliver the high energies needed to drive laser fusion implosions to produce much more energy than the incident laser beams.

(Left) Photo of the Electra electron-beam pumped amplifier (before x-ray shielding was installed). (Right) The Electra diode’s vertical dimension was reduced from 30 cm to 10 cm to provide higher pump intensity for argon fluoride laser (ArF) operation. The specific pump rate for ArF has a lower small signal gain and higher saturation flux than krypton fluoride (KrF). The image shows a time resolved measurement of the emitted light produced by the reduced size electron beam interacting with the laser gas along the laser axis.

Laser fusion involves many laser beams to uniformly illuminate at high power hollow spherical targets to cause an implosion with speeds more than a 1000 times that of a jet airliner.

If done with sufficient precision, the deuterium and tritium fuel within the target will ignite and through a thermonuclear burn produce much more energy than needed to implode the pellet,” Obenschain said.

If successful, laser fusion has applications as a test bed for defense tests and would be an attractive future power source. The short wavelength and other attributes make the argon fluoride laser the ideal laser to obtain high performance fusion implosions. However, because of its extremely short wavelength and other technical challenges, high-energy argon fluoride laser lasers were thought to be much too difficult to build.

“NRL was already the world leader in the similar but longer wavelength krypton fluoride laser technology,” Obenschain said. “The team decided to explore the feasibility of employing argon fluoride laser as a fusion driver. In parallel, massive computer simulations investigated the advantages of utilizing the argon fluoride laser for fusion implosions. The results so far indicate argon fluoride laser could be a game changing driver for high performance laser fusion.”

About the U.S. Naval Research Laboratory NRL is a scientific and engineering command dedicated to research that drives innovative advances for the U.S. Navy and Marine Corps from the seafloor to space and in the information domain. NRL is located in Washington, D.C. with major field sites in Stennis Space Center, Mississippi; Key West, Florida; and Monterey, California, and employs approximately 2,500.