Direct-Drive Laser Fusion Experiments Move Toward Target Ignition

Conditions capable of producing a direct drive laser fusion yield that is five times higher than the current record for laser fusion energy yield have been demonstrated. The work, which shows the use of direct-drive laser fusion to compress fuel to about half the pressure required to ignite it, represents an advance in a national research initiative to develop fusion as an energy source. In a direct-drive approach to inertial confinement fusion (ICF), researchers at the University of Rochester’s Laboratory of Laser Energetics (LLE) targeted 60 laser beams to strike a millimeter-sized pellet of fuel. At the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, researchers are using 192 laser beams in an indirect-drive approach to achieving laser fusion, in which the laser light is first converted into x-rays in a hohlraum.

In the direct-drive method of inertial confinement fusion, laser beams directly strike the fuel pellet. Courtesy of Michael Osadciw/University of Rochester.

LLE researchers reported that direct-drive implosions on the LLE’s OMEGA laser achieved core conditions that could lead to significant alpha heating at incident energies available on the NIF scale. The extrapolation of the experimental results from OMEGA to NIF energy assumes that the relative conditions produced at LLE are reproduced and scaled up at the NIF.

The LLE results indicate that the direct-drive approach may be a potential alternative to other fusion methods.

"We've shown that the direct-drive method is on par with other work being done in advancing nuclear fusion research," said LLE researcher Arijit Bose.

Researchers estimated that the current best-performing OMEGA implosion could produce approximately 125 kilojoules (kJ) of fusion energy. Similar levels of alpha heating have been observed in current highest performing indirect-drive NIF implosions.

One hundred kJ is roughly equivalent to 20 minutes’ output of a 100-watt light; but in a fusion experiment at NIF, that energy would be released in less than a billionth of a second, and would be enough to bring the fuel a step closer to conditions required for ignition. If ignited, thermonuclear fuel would unleash an amount of fusion energy much greater than the amount of input energy.

In the indirect-drive method of inertial confinement fusion, laser beams are converted to x-rays. Courtesy of Michael Osadwic/University of Rochester.

"In laser fusion, an ignited target is like a miniature star of about a 10th of a millimeter, which produces the energy equivalent of a few gallons of gasoline over a fraction of a billionth of a second. We are not there yet, but we are making progress" said professor Riccardo Betti.

E. Michael Campbell, deputy director of LLE, said the results were made possible because of a number of improvements in the direct method approach. One improvement involved the aiming of the 60 laser beams, which now strike the target more uniformly.

"It's like squeezing a balloon with your hands; there are always parts that pop out where your hands aren't," said Campbell. "If it were possible to squeeze a balloon from every spot on the surface, there would be a great deal more pressure inside. And that's what happens when the lasers strike a target more symmetrically."

"If we can improve the uniformity of the way we compress our targets, we will likely get very close to the conditions that would extrapolate to ignition on NIF. This is what we will be focusing on in the near future" said Valeri Goncharov, director of the LLE theory division.

"Arijit's work is very thorough and convincing. While much work remains to be done, this result shows significant progress in the direct drive approach, "said Betti.

Two additional enhancements were made at LLE: The quality of the target shell was improved to make it more easily compressed, and the diagnostics for measuring what's taking place within the shell were improved. Researchers are now able to capture x-ray images of the target's implosion with frame times of 40 trillionths of a second, giving them information on how to more precisely adjust the lasers.

"What we've done is show the advantages of a direct drive laser in the nuclear fusion process," said Campbell. "And that should lead to additional research opportunities, as well as continued progress in the field."

Bose says the next step is to develop theoretical estimates of what is taking place in the target shell as it is being hit by the laser, to garner information that could lead to further enhancements.

Two recent LLE papers report that OMEGA experiments match the current NIF record when extrapolated to NIF energies. Igniting a target is the main goal of the laser fusion effort in the United States.

The research was published in Physical Review E (doi: 10.1103/PhysRevE.94.011201) and in Physical Review Letters (doi: 10.1103/PhysRevLett.117.025001).