Michael A. Johnson: Preventing optical damage at the National Ignition Facility
The National Ignition Facility (NIF) was in trouble a dozen years ago. The Lawrence Livermore National Laboratory (LLNL) wanted the giant laser to show the feasibility of igniting a fusion chain reaction by heating and compressing tiny pellets of hydrogen fuel. The laser had reached the 1.8 megajoules (MJ) lab researchers expected would be needed, but the fusion yield was far short of ignition, the point where the fusion yield exceeded the energy delivered to the fuel pellet. The billion-dollar laser had reached a functional limit. Trying to crank up the power to ignite fusion would almost certainly wreck its optics, which were needed for a planned round of military experiments to assess the US nuclear weapons stockpile.
Today, LLNL has succeeded in a series of refinements that, step-by-step, have increased NIF’s output energy and enhanced its optical performance so that the 192-beam laser and its 18-inch optics can safely ignite fusion pellets. It’s still a long way from the dream of clean green laser-fusion energy, but NIF has reached a crucial milestone by producing more energy from fusion reactions than it needs to deliver to the fusion target to start the process. And that milestone was reached without destroying the laser’s optics in the process.
Controlling that optical damage was the key advance that pushed laser fusion across the ignition threshold. Light poses little threat to glass under ordinary conditions. But optical damage becomes a problem as laser pulses deliver more energy, squeezed into shorter bursts focused onto smaller areas of a laser’s optics. The higher the energy density, the more likely it is for a laser pulse to crack instrument optics, ablate their surfaces, or cause other damage. Once an optical surface is damaged, it becomes exponentially more vulnerable to further damage at an even lower energy-density threshold, unless it can be repaired so it regains its strength. High-energy lasers grew bigger once LLNL started scaling its laser fusion program in the 1970s, and the bigger the fusion lasers, the more experts were needed to study how to measure and control optical damage.
One of those damage experts is Michael A. Johnson, a luminary in optical heterodyning, a process that combines two optical signals very close in wavelength to generate the beat frequency between the two input wavelengths. Beat frequency can measure subwavelength features on an optical surface, making it invaluable for spotting tiny flaws. After he mastered the technique from Charles Townes at the University of California at Berkeley, Johnson says, “it became my life for the next forty years.”
LLNL hired Johnson in 1976, as the series of bigger lasers they were building for fusion research made controlling optical damage a more crucial issue. Mary Spaeth, now chief systems engineer for NIF, devised a strategy she called the “optics recycle loop.” She realized that some damage was inevitable, but that early detection of flaws in laser optics when they were small would allow repairs before more laser shots made the damage worse.
The recycle loop is preventative maintenance for NIF, the world’s biggest and most complex laser, and it has two fundamental elements. One is a measurement system able to spot tiny surface defects when they are small enough to repair. The other is a repair system capable of fixing nascent damage to an optic before it causes serious problems. The repairs don’t last forever, but they allow repeated reuse of individual optics, reducing the costs of operating the giant laser.
Delays in building NIF allowed Johnson to spend a few years on a LLNL project important to the electronics industry—developing optics for extreme-ultraviolet (EUV) photolithography. That diversion would prove fruitful when he returned to NIF.
Fused silica optics at the NIF are chemically treated to increase the optical damage threshold. Photo credit: NIF/LLNL.
As Johnson explains, the EUV wavelength was 11 nm, and the four mirrors in the stepping camera required 0.1-wave accuracy, to within just 1 nm. He developed a way to measure the EUV mirrors with green light, a wavelength 50 times longer than EUV. Another tool he found that could achieve that precision was the phase-shifting diffraction interferometer (PSDI), which splits a single laser beam into two parallel beams, focuses them into separate single-mode fibers, and combines their output to make an interference pattern. The beam from the single-mode fiber spreads perfectly spherically, allowing measurements down to 0.10 nm. When Johnson went back to LLNL laser group around 2004, he would need such nanometer precision to detect tiny flaws in its optics.
The Challenge of NIF
Johnson’s job at NIF was to develop sensitive optical measurement tools for Spaeth’s optics recycle loop. The technology he brought from the EUV lithography project gave him a head start in developing nanoscale measurements to examine the giant laser’s optics. However, the ambitious project was years behind schedule when it finally generated a 1.8-megajoule pulse, and the fusion yield was further below the target than NIF’s spending was above its budget.
Researchers tested a variety of ideas, including timing and shaping of pulses, improving control of frequency conversion and light management in the laser, and computer modeling of laser performance. Those tests brought some progress. However, what finally pushed NIF above the ignition threshold was improvement of the optics recycle loop.
“The real kicker was a 3D image of a flaw,” says Johnson. It was produced by aiming the two fibers in a PSDI at two adjacent areas, one flawed and one normal. It acts as two-beam holography to produce a 3D image of a defect in the optics, which, he says, “simplifies things very much.” It’s aided by software codes describing NIF optics that LLNL developed to replicate interferometer frequencies. Once the holograms are digitized, they can be run through the NIF code to see if the optic in question contains a flaw. If it does, the results showing the flaw can be run through the code to see if and how it could be repaired to continue being used in the laser. LLNL has a large group doing the measurements, and their computer results can be passed along to the Flaw Inspection Correction Facility, where a large and highly experienced staff do the repairs.
The repairs needed depend on the nature of the damage. If a reflective coating is chipped, it could be stripped off and replaced. Larger flaws require more elaborate repairs, such as drilling out a millimeter-scale cone in a damaged area with a carbon-dioxide laser and inserting a liquid crystal device into the beam path to electronically divert that part of the NIF beam away from the damaged area.
The first fusion ignition came on 4 December 2022 when the repaired NIF delivered 2.05 MJ of laser energy to a target, igniting a fusion reaction that yielded 3.15 MJ of energy, with minimal damage to the laser’s optics. Since then, small increases in laser power have yielded higher increases in yield. Tayyab Suratwala, LLNL program director for Optics and Materials Science & Technology, says NIF now can approach 400 shots a year, a big increase that allows many more experiments (on fusion, stockpile stewardship, and so on) than had been previously possible.
Johnson is a soft-spoken luminary who shares credit with the many colleagues who worked with him at NIF to control optical damage. His role is like that of the football player who carries the pigskin across the goal line. He may be the one who scored, but the whole team won.
Jeff Hecht is an SPIE member and freelancer who writes about science and technology.
