Shooting The Way To Fusion Energy

In the simplest approach of so-called direct drive, a huge pulse of laser energy is focused from all sides onto a tiny spherical fuel pellet. The outer layer of the pellet is instantly transformed into an explosively-expanding plasma, creating a giant pressure wave that compresses the pellet down to a fraction of its original size.

At the center, the pressure reaches over 10 Mbar – more than 10 million times atmospheric pressure. The fusion fuel is compressed to 1,000 times the density of its normal solid form, and its temperature rises to over 100 million degrees. The fuel pellet explodes like a tiny hydrogen bomb.

In this form of direct drive laser fusion, there are no magnetic fields to keep the fuel confined, as in a tokamak, but only its own momentary inertia. Hence the name inertial confinement fusion (ICF).

First Light's“projectile fusion” method falls into the same category, inertial confinement fusion, but it uses a totally different method to compress the fuel.

Reaching super-high density is essential to all the many variants of ICF. Without super-compression not enough fusion reactions would occur, in the brief moment before the pellet flies apart, to release significant amounts of energy.

Future ICF power plants will necessarily operate in a pulsed mode, producing one or more micro-explosions per second. After each detonation, a new fuel-containing target must be introduced into the chamber.

Laser fusion has a long history, going back to the 1960s. Although enormous progress has been made, this approach has drawbacks, especially with regard to the prospects for future commercialization.

One of the biggest challenges is to achieve a perfectly spherical implosion of the fuel pellet. Without that, hydrodynamic instabilities arise that spoil the compression process. Enormous effort and ingenuity went into solving this problem.

In its milestone achievement of fusion ignition , the US National Ignition Facility (NIF), employed an alternative method called “indirect drive”: The fuel pellet is suspended in the center of a cylindrical enclosure called a”hohlraum”; the laser beams are directed through holes onto its walls, heating them to millions of degrees.

At that temperature matter emits X-rays. The hohlraum is instantly filled with a uniform field of intense X-ray radiation. Bombarded from all sides, the pellet implodes in a manner similar to what happens with direct drive.

While the results with“indirect drive” are encouraging, we should not forget that NIF, with its 192 laser beamlines, is the size of three football fields. Although cheaper and more efficient laser systems will no doubt become available, the basic fact remains that coherent laser light is an extremely“high quality” form of energy, inevitably more complicated and expensive to produce than energy in less coherent forms.

First Light's method, if it works out, would be an ideal alternative. The expensive,“high-quality” energy of a perfectly-timed burst of laser light is replaced by the far cheaper,“low-quality” kinetic energy of a flying projectile. And instead of having to focus energy onto a target from all sides simultaneously, we need only hit it on one side.

What makes this possible is a cube-shaped object that Hawker calls the“amplifier.” The amplifier transforms the shock wave, produced by the projectile's impact on the side of the cube into a vastly more powerful set of shock waves that converge onto the fuel pellet.

While details of its design are a proprietary secret, Hawker stresses that the underlying physical principles are well-known. These concern above all the nonlinear behavior of shock waves. Specifically, shock waves can be multiplied, intensified and focused by reflections on surfaces, passage through heterogeneous media and interactions between shock waves themselves.

As Hawker stresses, inventing the amplifier would have been impossible without the ability to accurately simulate these complex processes using powerful computers and sophisticated computer codes. Thanks to enormous progress it has become possible, in many cases, to substitute simulations for expensive and time-consuming experimental work. Thereby one can“play” with parameters and try out new ideas much more quickly.

I shall discuss the science behind the“amplifier” in the following article. Here I want to concentrate on the“easy” part: launching projectiles to super-high velocities.

It is estimated that to achieve fusion“breakeven” with First Light's projectile fusion method, projectiles must be accelerated to velocities on the order of 60 kilometers per second, equivalent to 180 times the speed of sound, or 216,000 kilometers per hour. No one has so far been able to accelerate a macroscopic object to such velocities.

First Light is proceeding stepwise toward that goal.

“The Big Friendly Gun ,” which I saw during my visit, achieves velocities of 7 km per second, or more than 25,000 km per hour. It is a two-stage cannon. The first stage uses exploding gunpowder to propel a piston down a tube. The piston compresses hydrogen gas in the second stage to 10,000 times atmospheric pressure, and the expanding gas accelerates the projectile.

This is the device First Fusion used in its first demonstration of fusion reactions by projectile fusion in November 2021.

It is worth noting that two-stage cannons of this sort have been built for a variety of purposes. NASA employs them, for example, to investigate the impact of micro-meteorites on spacecraft. They also provide a serious option for propelling payloads directly into space.

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Back in 1966 a two-stage cannon, built by the High Altitude Research Project (HARP) of the US and Canadian defense departments succeeded in shooting an 84 kg projectile up to the suborbital altitude of 179 km.

A follow-on US government program, the “Super High Altitude Research Project” (SHARP), a imed at shooting specially-designed satellites directly into Earth orbit. SHARP was discontinued for budgetary reasons, but the goal is being pursued today by a private company, Green Launch .

To reach higher velocities, First Light has built a device called“Machine 3,” which is one of the largest so-called pulsed power devices in the world. Electricity, stored in capacitor banks, is discharged in millionths of a second through two small electrode plates situated at the center of the device. The combination of the high current and the magnetic field they generate produces a gigantic force of repulsion (the so-called Lorenz force).

With a suitable setup this force can be used to accelerate a projectile – referred to in the business as a“flyer plate” – to astronomically high velocities. The acceleration is so tremendous that the maximum velocity is reached already within the first few millimeters of its motion.

Hawker (L) with First Light Fusion's 'Machine 3.' The blue boxes grouped around the target chamber are high-voltage capacitors. Photo: First Light Fusion

Machine 3 can produce velocities of about 20 km per second, three times the speed of the Big Friendly Gun.

The next step is“Machine 4,” which is designed to deliver velocities of 60 km per second, estimated as sufficient to achieve a net gain by fusion reactions. A projectile launched at that velocity from outside the Earth's atmosphere would reach the Moon in less than 2 hours.

Although Machine 4 will be the world's most powerful pulsed power device, the science and engineering principles involved are well-established. The Z-Machine at US Sandia National Laboratories, a giant, multi-purpose pulsed power facility, has already produced velocities of 45 km per second – the present world record.

Discharge of the Sandia Z-pinch machine in
Albuquerque, New Mexico, USA. Photo: Sandia

From a purely scientific point of view, all of this is the easy part. In the following article, I shall turn to the hard part of the projectile fusion problem and recount the fascinating story of how the study of shock waves produced by a small undersea creature, the pistol shrimp, contributed to the genesis of First Light Fusion's revolutionary“amplifier.”

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