A star in a shoebox: compact fusion reactors

Smaller reactors are gaining investor attention

Nuclear fusion power has long been “50 years away”, but the tide may be turning. In the shadow of major government projects such as ITER, a new class of prototype power plants is emerging.

Compact Fusion Reactors (CFRs) promise potentially low-carbon, baseload energy in a containerised format - provided that fuel cycle and materials issues can be resolved. In the next few years, expect to see CFRs shifting from pure research towards engineering-driven demonstration plants.

Miniaturising the Sun

CFRs pursues the same goal as the Sun: the fusion of light atomic nuclei (deuterium and tritium) to release gigantic amounts of energy. The key difference from traditional approaches lies in the form factor. Instead of building cathedral-sized facilities, CFR developers are using state-of-the-art materials such as high-temperature superconductors (HTS) to reduce the fusion chamber to the size of a lorry trailer or a small factory building.

SPARC, one of the first compact fusion reactors, was developed by engineers at Commonwealth Fusion Systems (CFS) in collaboration with researchers from MIT’s Plasma Science and Fusion Center. The reactor is currently under construction. According to CFS, Rare Earth Barium Copper Oxide (REBCO) high-temperature superconductors enable the generation of far stronger magnetic fields, meaning the extremely hot plasma can be stabilised within a much smaller space.

Magnetic vs inertial confinement

In the ‘compact’ fusion ecosystem, there is competition between magnetic confinement (tokamak and stellarator) and inertial fusion.

The tokamak is a ring-shaped design (doughnut shape) and the most thoroughly researched concept. Powerful HTS magnets compress the plasma so strongly that the facility can be significantly reduced in size.

A stellarator uses external 3D-shaped magnetic coils to create a plasma cage, designed to avoid the plasma flow typical of tokamaks. They are considered promising candidates for continuous operation - albeit at the cost of high complexity.

In inertial confinement (laser fusion), tiny fuel pellets are bombarded with high-energy laser pulses to ignite the fusion process for milliseconds.

As competition heats up, we expect to see a lot more prototypes emerging. Individual experiments have already demonstrated an energy surplus (Q>1, where Q = fusion power generated / power input) within narrowly defined limits, and there will be more to come.

Some notable CFR startups

Avalanche Fusion (USA) uses electrostatic fields to fuse ions in tiny orbits. Following a breakthrough in plasma stability (density increased by a factor of 40), the company secured fresh funding for its "Orbitron" in 2026 and a contract from the Defence Advanced Research Projects Agency (DARPA). The targeted applications are space propulsion, mobile defence systems and micro data centres.

Inertia (USA) is the direct commercial response to the historic ignition success of the US National Ignition Facility (NIF). With its “Thunderwall” system, the start-up aims to build the world’s most powerful laser. This is designed to fire at tiny fuel pellets to generate mini-explosions (fusion) at millisecond intervals. With a massive $450 million funding round (including from Google Ventures), Inertia is currently building the first automated factory for fusion fuel capsules. The target audience is energy suppliers. Inertia aims to have the first commercial gigawatt fusion power plant connected to the grid by 2030.

And there’s Germany, a major manufacturer of precision lasers and cooling systems, which is also incubating several CFR laboratories, particularly in the Munich-Greifswald region which is becoming known as ‘Fusion Valley’.

With its scientific excellence, government-funded research and a dynamic start-up scene, Germany is in a unique position globally.

The Max Planck Institute for Plasma Physics (IPP) in Greifswald operates Wendelstein 7-X, one of the largest and most powerful stellarators. W7-X provides global reference data for stellarator-based power plant design; in 2025, the facility achieved a record with fusion reactions sustained for 43 seconds.

Proxima Fusion is a spin-off from the IPP and is regarded as the German flagship. The start-up is using the findings from Greifswald to build a commercial stellarator using AI-supported design. Stellaris is set to supply energy on an industrial scale from the 2030s onwards.

According to Proxima Fusion, stellarators are the power stations of the future. The start-up is relying on cooperation with Germany’s strong optics and laser industry to achieve the precision required for fusion.

This article first appeared on Computing’s sister site Computing Deutschland. It is the first part of a two-part series on CMRs.