There are many architectures for high energy lasers. The National Ignition Facility (NIF) uses flash lamps to energize the laser beam. Other projects use gas-based amplifiers called excimer lasers. At Inertia, we’ve chosen a Diode Pumped, Solid State Laser (DPSSL) design. Our approach is to capitalize on the experience of decades of research in solid state lasers and developments in the semiconductor industry, which can now manufacture laser diodes, to replace flashlamps (similar to the transition from fluorescent lighting to LEDs).

Inertia is basing its power plant design on the proven approach to ignition and fusion gain at the NIF. Our laser needs to provide light that is as close as possible in character to the NIF (high average power/beam, precision pulse-shaping, consistent delivery to specifications), which delivered the first fusion ignition.

Inertia’s 10 megajoule (MJ) laser will be the most energetic ever built, with beamlines that are each a factor 10 or so more energy than any prior design. However, the most challenging part of building a diode-pumped laser at this scale is acquiring the semiconductor diodes. Today, the world doesn’t produce enough of the specific laser diodes that we need, so we’re working with industry partners to expand production by approximately two orders of magnitude. While this is challenging, it’s mostly a matter of investment in mass producing semiconductor diodes, which is a well understood industrial activity that doesn’t challenge basic science.

In fact, a similar scale-up just occurred for another variant of laser diode - the VCSEL. If your smart phone has a face unlock feature, it’s using hundreds of laser diodes to scan your face. Those diodes were produced at small scale a decade ago, but thanks to this new application, the laser industry scaled up production by orders of magnitude, and costs fell by orders of magnitude, over the last 5-7 years. This is nearly identical to the industrial scale up we are undertaking. The main alternative to solid-state lasers for fusion energy is gas-based excimer lasers, which are promising because they are fairly inexpensive to make, simple in construction, and about 10x as efficient as flash lamps. However, they carry their own risks. The largest excimer lasers built to date are several orders of magnitude less energetic than is needed for a fusion power plant, so they carry scientific risk in their scaling.

Additionally, the most frequently used gases are toxic, leading to potential public safety risks for large scale deployments. Although excimer lasers are 10x as efficient as flash lamps, their maximum efficiency is still in the single digits, approximately 7% efficient, about half as efficient as a solid-state laser. One of the upsides of an excimer laser is that they can natively produce ultra-violet light, eliminating the need for special optics for frequency conversion. However, the UV light is much shorter wavelength and carries different focusing characteristics than the NIF.

This wavelength is also much more damaging to the other laser components in the system, so an excimer laser would require substantially more durable optics for its entire optical chain than are available today. Finally, delivering high fidelity, temporal pulse shapes has not been demonstrated in excimer lasers. Delivering pulse shapes required for fusion ignition designs means leveraging new and unproven techniques at scale. Excimer lasers also have shorter coherence length in UV, relative to solid-state lasers. This presents many technical challenges with coherent beam combining of multiple, high-energy beams needed for a fusion power plant.

Therefore, Inertia’s strategy is to take the most direct, de-risked path from what is working today at NIF to commercial operations. Solid-state and DPSS lasers have decades and billions of dollars of government investment and are the most well understood in their ability to deliver high average power at high precision, a requirement for fusion ignition and high energy gain. Inertia believes that scaling up industrial production, even by quite a lot, is a safer path than the other options which carry more scientific risk.