A pocketful of Sun in Livermore

The Sun and other stars are powered by nuclear fusion. 2 atoms of hydrogen come to a violent union to produce a helium atom, a neutron and some energy. The temperatures and pressures in the centre of the Sun were produced, if only for a few nano-seconds at the National Ignition Facility (NIF) at Lawrence Livermore National Labs (not too far from my home) on the 5th of December, 2022. It had been about 70 years in the making, and the net energy gain achieved was hailed by some as a breakthrough as revolutionary as landing on the Moon. 

Solar temperatures and pressures are required to overcome the Coulombic or electro-static repulsion of the hydrogen atoms (really hydrogen isotopes deuterium and tritium - and their fusion is called D-T-fusion) and get their strong nuclear forces to take over and bind the constituent D-T protons and neutrons into one resulting Helium atom. 

There are several ways to achieve this temperature and pressure. The first one, is inside a thermo-nuclear weapon, a hydrogen bomb. The hydrogen bomb has a primary fission device that explodes and compresses the fusion (D-T) fuel, to produce a secondary fusion reaction.

A Tokamak nuclear fusion reactor design. A toroidal solenoid produces the magnetic field that confines, and subsequently fuses the D-T nuclear fuel.

The second way to do it, is in a controlled fusion reaction, in a lab. Several designs have been proposed, including a Tokamak - a toroidal plasma chamber where D-T atoms are compressed and heated to fusion conditions. This is called magnetic confinement. Then, there is the so-called inertial confinement, which was the method used to achieve fusion at NIF.

The sun at the tip of a pencil: Technicians load a DT-pellet, inside a cylindrical enclosure called the hohlraum at the tip of the pencil-shaped rod in the Target chamber.  The 192 lasers come in through the holes in the target chamber and hit the hohlraum.

Over at NIF, on that fateful day in December 2022, scientists, technicians and engineers cleared the oscillator room, the Target chamber and surrounding areas, and counted down rocket-launch-style from 10 to 0 to the 'Shot'. At the press of a button, 192 high-energy laser beams raced across a triple-football-field sized building and focused their 2 mega-joules of energy at a DT-pellet the size of a pepper-corn.

Let's try and put this in context. 2 mega-joules of energy is the energy consumed by a 60 watt bulb in about 9 hours. Furthermore, the energy required to generate those 192 laser beams is about 800 mega-joules - so about 400 bulbs burning for those 9 hours - the energy requirements of a small city block.

In the NIF, all this energy is focused on the DT-pellet in the space of about 10 nanoseconds. For a brief period, the peak power (energy per unit time) value at the DT-pellet was 200 terawatts (TW). The peak power requirement for generating the lasers was a mind-boggling 80,000 TW. Contrast this to the 800 GW of peak electricity demand for the US National electricity grid. So, for a brief moment, NIF used 100,000 times more power than than the US grid. 

The hohlraum, a high-Z metal enclosure which holds the nuclear fuel, the D-T pellet.

This particular method to generate a fusion reaction is called indirect drive inertial fusion because the laser beams don't directly hit the D-T pellet. Instead, they are focused on a small cylindrical enclosure made of a high-Z (or high atomic number) metal that surrounds the pellet. This cylindrical enclosure, called a hohlraum (literally hollow-cavity in German) absorbs the laser beams and re-emits this energy as X-rays, creating a uniformly high temperature environment. This X-ray radiation compresses the cryogenically cooled 50-50 D-T fuel pellet inward at 400 km/second, vaporizing it and achieving a peak temperature of about 300 eV. This 300 eV or electron-volt, another unit of energy, this time the jargon of high-energy physics, corresponds to about 3.5 million degrees Celsius and this is not too far from the temperature inside the sun. The reaction produces about 3.1 mega-joules of energy, but for the briefest instant. The fuel in the D-T pellet is quickly used up and the reaction is over in nanoseconds.

The key difference between the 'shot' on December 5 and all prior attempts (the NIF facility had been running for more than 10 years), was a Target Gain larger than 1. This means that the energy output from the fusion reaction was more than the energy input into it. G_target on that day was 1.5, corresponding to an output yield of 3.1 mega-joules, 1.5 times more than the input laser energy of 2.05 megajoules. It took 11 years for the team to achieve this, and over this period, they improved target gain by three orders of magnitude.

Keep in mind that the total device output to input ratio or efficiency is still much lower than 1 (0.004 or about 0.4 % efficiency). This is because of the tremendous amounts of energy required to generate the lasers. 

The lasers at NIF. 

The facility at NIF houses the world's most powerful laser.

Actually, it is a bank of 192 lasers that takes 3 football field lengths of piping (and tremendously sophisticated optics and electronics) to generate.

The lasers start off as a single laser pulse that is split and amplified multiple (more than a quadrillion) times to create the 192 beams with a combined energy of 2 mega-joules that converge at the centre of the Target Chamber.

For context, if you were to take your 5 mW pocket-laser-pointer and use mirrors to focus multiple of them at a single point, you would need 40,000 trillion of them to match the power of the NIF lasers. So, it's safe to say that this experiment is beyond the capacity of the hobbyist at home or even most university labs.

So - what is it good for? The initial news-hype around the experiment was mainly about nuclear fusion based power generation, but actually, this is not the most efficient way to generate power - given the 0.4% efficiency figure mentioned earlier. The NIF shot used up much more power compared to what was produced by the fusion reaction, even if the output of the reaction itself was more than the power of the laser beams. This comes down to the efficiency - or lack thereof - of generating these high-energy laser beams. 

The NIF was built to ensure that America's nukes keep working in the 21st century. The thermonuclear stockpile is now multiple decades old, and absent underground nuclear tests (the last of which was conducted in the early 90s), this is the only way to study the conditions in the aftermath of a thermonuclear explosion. While the efficiency of laser-driven fusion is low, the experiment's success - decades in the making - represents a critical step towards achieving controlled nuclear fusion.

The 1250-ton Tokamak at the ITER fusion reactor in France. 30 metres tall and the same wide, it has taken 30 years to plan and 10 years to build. Newer, smaller tokamaks built with high temperature superconducting magnets are expected to be a fraction of this size.

Theoretically, it might some day be possible to massively improve the efficiency of these lasers, and increase the amount of D-T fuel so that the reaction sustains for a longer time, re-using part of the energy yield to power the lasers, but that is a longer shot (pardon the pun) compared to the Tokamak reactor design discussed earlier in this post. New higher temperature superconducting magnets are making Tokamaks smaller - about the size of a large house - compared to the previous designs that took up a small city block. Commonwealth Fusion Systems (a spinoff from MIT) expects to have their first functioning room-sized Tokamak ready in the next year or so.

So - we've come a long way, in the 70 years since the first hydrogen bomb, in making thermo-nuclear reactions in the lab. We are going to maybe have to wait a few more years - hopefully 10-20 at the most - in seeing nuclear fusion reactors make up a significant part of our power grid.