The History of Inertial Confinement Fusion: A Trip Down Memory Lane

Earlier this week, we covered the state of the art in inertial confinement fusion (ICF). Like any article that generates a lot of interest, we received a lot of comments from the fusion world. One of the most interesting results of this article was a call from Dr. Mike Campbell, who (interestingly enough) also lives in the San Diego area. Dr. Campbell is currently a Professor of Practice in Mechanical & Aerospace Engineering at the University of California San Diego (UCSD). More interestingly (at least to this article) was that Dr. Campbell was previously the director of the Laboratory for Laser Energetics (LLE) at the University of Rochester, and before that he was part of the group at Lawrence Livermore National Laboratory (LLNL), where he was one of the driving forces behind ICF and the establishment of the National Ignition Facility (NIF) at LLNL. As we discuss the history of inertial confinement in a highly abbreviated form (and don’t worry, we will do a similar piece on magnetic confinement and on hybrid approaches), I will make liberal use of some of the materials that Dr. Campbell provided!

Inertial Confinement Fusion – Inspired (Kind of) By The Hydrogen Bomb

Inertial confinement (particularly indirect drive ICF) is really the direct stepchild of the hydrogen bomb (shown below), just at a very, very much smaller scale. 

In the Teller-Ulam model of the thermonuclear weapon, x-rays from the “primary” (a small atomic bomb in the circle at the top) creates a storm of thermal X-rays. These x-rays bounce around inside the bomb’s radiation case (the outside container), and impinge on the “tamper” (the rectangular shape at the bottom which contains both lithium deuteride and a plutonium “spark plug”), causing it to ablate, compressing the lithium deuteride fuel. The compression from the x-rays also compresses the “spark plug”, making it go supercritical, further heating/compressing the lithium deuteride, and causing the lithium to break down into helium and tritium). This causes the deuterium and tritium to fuse; the fast neutrons from the fusion also cause the tamper (which is often made of uranium) to fission, increasing the overall weapon’s yield.

So how is inertial confinement (at least indirect-drive ICF) a very, very small version of a hydrogen bomb? For starters, laser ICF uses X-rays and the same ablation process that is used in a hydrogen bomb. The laser beams enter the hohlraum (a small gold-plated cylinder with two holes at the end, and which holds the fusion fuel), and hit the walls of the hohlraum to produce X-rays. Those X-rays heat and the fuel inside, causing it to compress, heat, and fuse.

The Evolution of Inertial Confinement Fusion Energy

Interestingly enough, the idea of using directed energy to cause fusion inside of a hohlraum was first proposed in 1960 by John Nuckolls of LLNL. Nuckolls is often referred to as the father of ICF by leading the efforts to mathematically characterize the scaling down of fusion. This led to the realization that by moving to “raw” deuterium and tritium as the fuel, it would be possible to utilize much lower power to cause fusion. This led to the creation of a laser ICF program at LLNL, which eventually morphed into the National Ignition Facility (NIF); the timeline for this is shown below.

The initial laser was known as “Janus”, which was a 10 joule, 1 watt laser. This was scaled into a number of different lasers, culminating in “Nova” (30 kilojoules, at 3 watt of power). Nova (like Shiva and the lasers before it) was a 20-beam infrared laser utilizing neodymium glass (Nd:glass) as the laser amplifier. Nova, which was built in 1984, was the first laser built to enable ignition; however, it failed due to Rayleigh-Taylor instability. This was realized by Nuckolls to be due to the infrared lasers not coupling well with the D-T targets. To solve this issue, LLNL moved from infrared lasers to ultraviolet lasers. This was achieved through frequency-shifting, which turned the infrared beam into an ultraviolet beam which, though weaker than the infrared beam, coupled better with the fusion targets and avoided the Rayleigh-Taylor instability situation. It was this technology which was eventually used in the National Ignition Facility (NIF).

Challenges to Achieving Ignition Energies in Laser ICF

Obviously, if you are going to use compounds like Nd:glass, you need to be able to make a LOT of it. The period between 1973 and 1977 was marked by large amounts of experimentation to find which types of glass worked best for laser-based ICF. One of the issues that was found was caused by the inclusion of platinum (Pt) in this glass as an impurity. The picture to the below shows damage that these platinum impurities caused within laser glass at a power of 5-7 joules/cm2. This problem, which was first seen between 1978 and 1987, became known as the “platinum damage catastrophe”, and nearly resulted in the cancellation of the Nova laser program.

Luckily, LLNL personnel and vendors worked together to find procedures to address this, resulting in an approach that “dissolved” these inclusions. This and other process improvements resulted in the ability to continuously melt glass by 2001, allowing laser glass vendors to produce tons of glass a year. The progress in laser glass-making is illustrated in the picture below, where even between 1973 and 1997 (not even mentioning the 1961 picture in the inset!).

Laser Beam and Pulse Shaping, and Scaling All of It to Q>1

Laser power and glass production weren’t the only issues that LLNL and other government laboratories encountered on the road to Q>1. Two of the more interesting (and interrelated) issues are beam shaping (both the beam diameter and the power continuity across the waveform) and pulse shaping. When most of us think of a laser beam, we think of something like a laser pointer – a continuous beam that looks pretty small (and pretty uniform) to our eyes. However, to a 2mm fuel pellet (about 1/12 of an inch) inside a hohlraum of 5.6mm in diameter and 9.6mm long (about a quarter of an inch wide by about a third of an inch long), a normal laser beam looks pretty big! Note that the hole that the laser beams need to go through to enter the hohlraum is roughly 2.6mm, or one-tenth of an inch. This requires a very small laser beam diameter, and NIF shrinks their beams (there are 192 of them) to smaller than 2mm in diameter.

These beams also need to be more than small – they need to have a fairly uniform energy across their wavefront. Even though these lasers do not directly heat the fusion target (remember, they cause the walls of the hohlraum to emit X-rays), any significant perturbations in wavefront energy density causes uneven x-ray emission, and hence uneven heating of the fuel. NIF uses a number of complex techniques to ensure wavefront uniformity, as well as to achieve extremely high surface smoothness inside the hohlraum (surface irregularities as large as 1 micron can cause uneven heating). Finally, the laser pulses at NIF have to be precisely synchronized – these beams are usually no more than a few nanoseconds in duration, with a synchronization precision of better than 20 picoseconds! This demands a number of cutting-edge optics technologies that have been developed during this century.

Conclusion: ICF Is A Technology That Has Taken Decades to Hit Q>1

Thanks again to Dr. Mike Campbell for the material that you provided for this article! When you think about it, the journey to Q>1 (achieved in 2022 by NIF) has spanned over six decades of experimentation by a lot of dedicated scientists, machinists, and vendors. Since commercial ICF developments (whether laser-powered or not) need to achieve similar levels of precision to be successful, ICF (and fusion in general) is really harder than rocket science!