US-based Commonwealth Fusion Systems (CFS) has received the second 48-ton half of the vacuum vessel for its SPARC fusion machine. This arrival marks the transition to the final assembly phase and brings the total project to 75% completion.

The completed 96-ton steel chamber is designed to house plasma heated to 100 million degrees Celsius. CFS plans to start operations in 2027, with the objective of achieving a net energy gain, known as Q>1, where the machine produces more energy than it consumes.

“Ultimately, the goal of SPARC is to get to Q>1 as fast as we can so that we can get to the next step — to build ARC, our fusion power plant,” said CFS Chief Science Officer and Co-founder Brandon Sorbom in a press release.

The project recently reached another milestone with the installation of the first two D-shaped toroidal field magnets. A total of 18 magnets will eventually surround the vacuum vessel to confine and compress the plasma. 

Each 24-ton magnet produces a 20-tesla magnetic field, a strength approximately 13 times that of a standard MRI machine. These components are necessary to hold the plasma in place using magnetic force so it does not touch the vessel walls during the fusion process.

Installing sensors and components with precision

Current assembly work involves installing diagnostic sensors and plasma-facing components with 200-micron precision. This tolerance is equivalent to the width of two human hairs. Engineers are also integrating mineral-insulated cabling throughout the vessel’s interior to monitor the machine’s performance. 

Once the two halves are equipped and welded together, vacuum pumps will remove the air to create an environment similar to outer space, providing the structural support necessary to withstand external air pressure.

The outfitting process begins with metrology, a measurement method used to confirm that the physical hardware matches the engineering designs. 

“Our team is already performing test installations with these components, which include mineral-insulated cabling that wends its way along the vacuum vessel’s interior walls and diagnostic sensors placed within some of SPARC’s ports — those large rectangular holes in the vacuum vessel’s exterior walls,” concluded the press release.

Proving the feasibility of fusion energy

The final interior step is the installation of components made from tungsten and tungsten alloys. This material is used because it has the highest melting point of all metals and can protect the diagnostic systems from the heat of the plasma.

The SPARC machine is a demonstration device intended to prove the feasibility of the CFS fusion energy approach. Once the vessel and magnets are fully integrated, the machine will use radio waves to heat fusion fuel into plasma. 

If the machine reaches the Q>1 threshold, it will confirm that the high-field magnet design can produce net energy from fusion.

This data will be used to design ARC, the company’s first commercial fusion power plant. According to CFS Chief Science Officer Brandon Sorbom, the immediate priority is reaching the net energy milestone to inform the construction of the ARC plant. 

The current work is focused on building the machine according to design specifications to ensure it can withstand the pressure and thermal loads required for commercial fusion.