Photo: The Soviet Union’s T-3 tokamak attained record plasma temperature in the late 1960s. Credit: Getty Images/Gamma-Keystone.
It’s time for a closer look at the second of the six milestones every fusion energy company must pass to succeed. I sketched out these six milestones last year in an open letter to the fusion industry and anyone watching it — like investors, policymakers, journalists, analysts, energy experts, tech enthusiasts.
My goal was to provide some guidance to a field I know can be baffling. These are signals you can look for from any fusion company, including Commonwealth Fusion Systems.
Now and in the coming weeks, I’m digging deeper into each milestone in the letter to help explain this. Previously, I talked about the first fusion development milestone: showing a plasma — a cloud of particles heated up enough that the energy strips electrons off the atoms — that’s stable enough to do experiments on. Well, once you’ve done that, just what experiments would you want to do? It’s time to reach for the second milestone: heating up your plasma.
Can you heat your plasma to 10 million degrees Celsius?
The first step toward fusion is forming a plasma and keeping it stable. Next, you have to get the plasma a lot hotter — about 10 million degrees Celsius. In the scientific terms used in research papers, that energy level is called 1 kiloelectron volt, or keV. That’s hot enough that the world will take notice.
That 10 million degree temperature is most of the way toward the 15 million degree plasma at the center of the sun. It’s not hot enough to initiate fusion, the process that takes place when the plasma’s charged particles combine and release heavier elements and energy. But it shows you’ve got some real chops in confining the plasma well enough to be interesting for fusion. At these temperatures, the plasma really starts showing its true intentions — it’s not just a glow. The fact that it got hot means it’s insulated well enough to trap particles and become dense and confined.
The type of fusion machine we’re building at CFS, called a tokamak, uses powerful magnets to confine the plasma and generally keep it away from the machine’s interior walls. In fusion research history, the tokamak was the first device to reach this 1 keV temperature. Since then, several fusion approaches have also done so: stellarators, spherical tokamaks, shear-stabilized Z-pinches, field-reverse configurations, mirrors, and several types of inertial confinement.
Proof is in the plasma
The first actual machine to achieve the 1 keV threshold was a Russian tokamak called the T-3 shown in the photo above. It set off a global stampede of tokamak research — but only after British scientists verified the claim in 1969.
As with the first milestone, you’ll have to carefully measure your work so you can be sure of your success. One gold standard here is a laser-based diagnostic method called Thomson scattering. You fire a laser with a specific frequency of light into the plasma and then monitor the light that returns after the laser light bounces off the plasma’s hot electrons. The faster the electrons in the plasma are moving — in other words, the hotter they are — the more widely the returning laser light frequency will vary from the original beam’s frequency.
An aside: Technically, fusion occurs when hot ions fuse, but Thomson scattering measures the temperatures of electrons, not the ions. So you can be misled. But regimes with hot ions and relatively cold electrons don’t scale to fusion conditions, so measuring the electrons is usually conservative.
At CFS, Thomson scattering will be one method we use to gauge plasma temperature in SPARC, the tokamak we’re building now to demonstrate most of what we need to build a fusion power plant. Just about every single research-grade fusion machine — and there are hundreds of them — have used Thomson scattering to measure the plasma temperature. There are other ways to try to do this, but it’s easy to dupe yourself when trying to interpret things like ion spectra and X-rays.
Also like the first milestone, peer-reviewed research will provide the world with the assurance it needs that your result is real. In fact, the US Department of Energy funds teams of researchers to measure plasmas using Thomson scattering as a service harkening back to that first Russian claim. If people were willing to exchange this information at the height of the Cold War, it shows you how important it is to get it right.
Reaching this temperature is a big achievement. Every time it’s been done, the scientific community has been very excited (a shout-out to the most recent one to do it, Zap Energy!). However, it’s also only dipping your toes into the water: a fusion power plant will need to sustain plasma temperatures 10 times hotter yet, dense and well insulated enough for fusion reactions to really pick up. For details on that, stay tuned for the third milestone.
