Fusion Startup Builds Reactor With Magnetic Mirrors

A fusion start-up on a shoestring budget hopes to leapfrog more established companies by reviving a technology that’s been sidelined for half a century. The start-up, Terra Fusion in Baltimore, is developing a reactor that uses a relatively simple approach known as magnetic mirrors to confine hot plasma–the environment where fusion reactions can occur. The company is testing its strategy on an experimental reactor at the University of Maryland, built with parts from second-hand MRI scanners and re-purposed brewing equipment.

The goal is to eventually provide a route to commercial fusion at a fraction of the cost of competing systems, says Carlos Romero-Talamas, CEO at Terra Fusion. To that end, the company is developing a more powerful proof-of-concept device that aims to exceed scientific breakeven, where a reactor generates more power than is used to heat and confine the plasma inside it. It’s a key step towards the ultimate goal of generating useful amounts of carbon-free energy.

More than 50 fusion companies around the world are working toward that goal, and over the last year, have attracted a record $4.5 billion in new funding, surpassing government spending on the technology for the first time. Most of that money is going to large organisations working on technologies that appear closest to breakeven. These include giant toroidal tokamaks and pulsed laser systems.

But that doesn’t mean it’s too late to try a different approach, says Jason Cassibry, professor of mechanical and aerospace engineering at the University of Alabama in Huntsville, and an advisor to several fusion start-ups. “Magnetic mirrors have the advantage of being simple,” he says.

“If a nation or company wants to be first and have the highest power density, they might go with a tokamak. But if they want something that’s cheaper to build and easier to maintain, they might go with a mirror concept.” Terra Fusion is now in a race to demonstrate a bargain-basement reactor before its mainstream rivals.

How do magnetic mirrors confine plasma. Building a nuclear fusion reactor is an attempt to bottle the same reaction that powers the sun.

The tremendous gravitational force within stars propel hydrogen nuclei to fuse together into helium, releasing vast quantities of energy in the process. Scaling that down to a reactor on Earth requires either confining an incredibly hot plasma with magnetic fields or compressing and heating targets filled with fuel, in both cases usually deuterium and tritium.

Decades of fusion experiments pushing the limits of magnet, laser, and materials technologies have shown just how difficult that is. Magnetic mirrors are a particular configuration of natural or artificial magnetic fields that can trap energetic particles for long periods.

They can be found in the Earth’s Van Allen radiation belts, for example, which can capture electrons and protons from the sun for years, causing hazards to spacecraft and astronauts travelling between 1,000 and 60,000 kilometers in altitude. During the Cold War, researchers used traditional electromagnet coils at each end of a vacuum chamber to form magnetic mirrors on Earth.

The hope was that the mirrors would confine hydrogen nuclei within a plasma for long enough for them to be heated to the point of fusion. But those efforts failed.

The plasmas proved unstable and the mirrors were leaky, allowing energetic particles to escape. When the world’s largest and most expensive magnetic mirror reactor was mothballed at Lawrence Livermore National Laboratory the day after being completed in 1986, magnetic mirrors appeared to be a dead end.

Enormous donut-shaped tokamaks seemed to offer a smoother path to fusion breakeven. But some mirror research trickled on.

At the University of Maryland, engineers were intrigued by earlier experiments that suggested that the magnetic mirror strategy could work if the plasma was rotated at supersonic speeds. This could stabilize the plasma and reduce leaks.

From 2004 to 2010, Romero-Talamas, who was a postgrad student at the time, and other researchers operated this kind of reactor, called the Maryland Centrifugal Experiment, that gave some impressive results. “With only a few megawatts of power input, we were getting similar [temperature] results to the Livermore tokamak with hundreds of megawatts,” he says.