Perseverance, Ignition, Breakeven

Overcoming decades of enormous physics and engineering challenges, and despite persistent pessimism, skepticism, and criticism, the National Ignition Facility has achieved an historic milestone for controlled nuclear fusion, a target energy gain factor of Q > 1.

Last week, NIF focussed the world’s most powerful laser pulse on a small gold cylinder that converted the incident ultraviolet light into x-rays and caused an enclosed diamond-coated deuterium-tritium pellet to implode and convert some of its matter to energy: 2.05 MJ of energy went into the target, and 3.15 MJ came out.

I remember the fusion goals of my childhood as ignition and breakeven.  On 2021 August 8, NIF achieved ignition by surpassing the Lawson criterion (roughly, the triple product \text{density} \times \text{temperature} \times\text{confinement time}) to create a self-sustaining “burning plasma”, where fusion heating exceeded all cooling processes, but with a target energy gain of only Q \approx 0.72. Finally, on 2022 December 5, after 16 months of additional hard work, including increasing and balancing the laser power and thickening the pellet’s diamond coating, NIF achieved both ignition and breakeven with a record target energy gain of Q \approx 1.5.

Although nowadays the “ignition” and “breakeven” criteria are often conflated, as in this morning’s formal announcement, by any standard NIF appears to have at last achieved the grand but elusive goal embodied in its name. Like LIGO’s success in detecting gravitational waves, NIF’s success in achieving ignition and breakeven is a tribute to perseverance and diligence in the face of a daunting challenge.