In laser fusion, there are two main kinds of “fuel target” designs: direct and indirect drive. In a direct drive design, the ball of fuel is directly hit by lasers which compress the fuel. In indirect drive, the ball of fuel is held inside a tiny cylindrical can called a hohlraum. The laser beams enter the hohlraum and hit the inside wall, emitting x-rays which then illuminate the fuel from all sides.

In both cases, the absorbed energy (laser or x-ray) vaporizes the outer layer of the fuel capsule, causing it to blow outwards. Every action has an equal and opposite reaction, so the rest of the pellet implodes inwards. If you can do this symmetrically, then the fuel converges onto itself to create a tiny, super-dense and super-hot ball. Done right, this can lead to fusion of the atoms and the release of huge amounts of energy. The Sun and the stars use gravity rather than lasers to create similar conditions.

Inertia’s strategy is to take the most direct, lowest risk path from what is working today at the U.S. Department of Energy’s (DOE) Lawrence Livermore National Laboratory (LLNL) to commercial energy. All of LLNL’s ignition achievements use the indirect drive target approach. Igniting targets with a uniform bath of x-rays is proven, whereas igniting capsules directly by the laser is not. Indirect drive was chosen as the preferred path to ignition and energy gain by the US government many decades ago, and it has received $20 billion in funding over that period. Direct drive is less developed, has received a fraction of the support, and there are unresolved physics questions as to whether it will work. These physics issues could take years to resolve, whereas indirect drive works today.

Additionally, as a result of one of the unresolved physics challenges called “Laser Plasma Interactions” there is evidence to suggest that, in practice, coupling efficiency (how much laser energy actually interacts with the target) in direct drive systems may not be much better than indirect drive. Other remaining physics concerns include fuel pre-heating from high-temperature electrons, difficulties maintaining implosion symmetry, and more. Given that the yield from indirect drive targets is projected to be sufficient for a commercial power plant, these risks of direct drive are unnecessary for development of Inertia’s first plant. Once built, our high repetition rate laser could address these physics challenges more rapidly and also explore the possibility of directly driven systems.

The final point is that a power plant needs much more than just an ignited ball of fuel in order to work. The fuel needs to be protected from damage and melting when it is injected into the fusion chamber at high velocity, and the surrounding chamber needs to withstand the full force of the fusion output. A 5-year, $100M program run at the DOE’s LLNL by one of Inertia’s co-founders demonstrated (and patented) how indirect drive can solve these problems self-consistently.

However, one of the nice things about Laser Inertial Fusion Energy is its modularity: unlike other forms of fusion energy, our fuel targets, target chamber, laser driver, and power-generation systems are largely de-coupled from each other. This means we can swap out one part relatively easily without having to make major system-wide changes.

So, by de-coupling the driver system (laser) from the target, we can use the same laser to explore alternate fuel target design concepts over time. The potential benefit of direct drive targets is that more of the laser energy could be imparted on the target, if the scientific challenges with this approach can be controlled. This may result in a higher fusion energy output for the same laser-driver energy. As direct drive advances and demonstrates higher readiness, we’ll revisit it as an additional option. In the near term though, we’re focusing on the pathway with the strongest validation to accelerate delivery.