A long-troubled laser megaproject is facing fresh hurdles.

A recent report concludes that although the $3.5 billion National Ignition Facility (NIF)—a Department of Energy (DOE) laser lab designed to heat and compress capsules of hydrogen isotopes until they fuse, releasing energy—is making technical progress, it is still a long way from its titular goal: ignition, or a fusion burn that sustains itself and produces more energy than it takes to spark it.

According to Physics Today magazine, the independent report, sponsored by DOE, suggests NIF-related research should shift from identifying the obstacles in the path to ignition, to whether ignition is even possible.

“Barring an unforeseen technical breakthrough and given today’s configuration of the NIF laser, achieving ignition on the NIF in the near term (one to two years) is unlikely and uncertain in the mid-term (five years),” the DOE report says. “The question is if the NIF will be able to reach ignition in its current configuration and not when it will occur.” The report recommends making better use of other facilities, not designed to achieve ignition, to better understand the underlying physics of the compressed fuel, known as high-energy density plasma. These include the Omega Laser Facility at the University of Rochester in New York, as well as the Z machine (an electric pulse generator) at DOE’s Sandia National Laboratories in Albuquerque, New Mexico.

NIF, based at DOE’s Lawrence Livermore National Laboratory in California, has long struggled to live up to its name. Even before its opening in 2009, many physicists were skeptical that the device, which focuses 192 powerful laser beams on a target, would achieve its ignition goal. DOE stuck with the project despite substantial construction delays and cost overruns, in part because they argued NIF would provide vital experimental results to nuclear weapons scientists responsible for maintaining the U.S. stockpile.

The National Ignition Campaign, a concerted effort to reach the fusion goal between 2010 and 2012, failed to deliver. Then began a 3-year effort to better understand the physics of what was happening in the fusion fuel as it was compressed by NIF’s 1.8 megajoule laser pulses. During that time, studies of ignition were interspersed with more research into the physics of nuclear weapons. The new report marks the end of that 3-year campaign.

The report states that there has been progress since 2012, including the first demonstration of “alpha heating,” when helium nuclei (produced by fusing hydrogen) stay trapped in the plasma and help heat it, thus helping to sustain the burn. Alpha heating is an essential process for ignition. With all the data gathered, researchers have made improvements to the computer models used to predict the outcomes of experiments, says Michael Campbell, a former NIF director who now works at the University of Rochester’s Laboratory for Laser Energetics. It was these models that led researchers to believe they would achieve ignition at NIF within a few years, and they continue to make overoptimistic predictions today. But Campbell says that NIF’s slow shot rate—only about 400 a year and only a fraction of them devoted to ignition—“slows down the rate of progress.”

Acknowledging the advances, the DOE report says “the present approach is too broad and diverse, and needs better focus.” After 2012, the research program into inertial confinement fusion, as this implosive compression version of fusion is known, was broadened out to include work at Omega, which uses an alternative “direct drive” approach in contrast to NIF’s indirect drive, and the Z machine, which compresses fuel with pulses of magnetic field. The report wants more coordinated research between the three facilities. “There is currently no published ‘roadmap’ to coordinate cross-[facility] activities,” the report notes.

Campbell says he always forecast that it would take “at least 10 years to figure it out.” He says he is “still hopeful it will work, but you can’t guarantee it.” Fusion, he says, “is just hard.”