Image: MIT’s Alcator C tokamak showed a high fusion performance score, called the triple product, in 1983. Credit: MIT Plasma Science and Fusion Center
At Commonwealth Fusion Systems (CFS), we’re focused on delivering fusion energy as soon as possible so the world can tap into this clean, safe, and effectively limitless new source of power. To do that, we — and all our competitors — must travel a six-step path to fusion energy.
I’m detailing each of these milestones in a series of posts, and this one is about the third: improving your fusion machine so you can approach conditions intense enough for fusion to take place, in a situation somewhat representative of what would be in a fusion power plant.
For those of you keeping an eye on all the companies striving to deliver fusion energy, this is the next achievement that helps separate the progress from the puffery.
To recap, milestone 1 is creating a stable plasma — a hot, hard-to-control cloud of electrically charged particles that’s the fuel for a fusion machine. Milestone 2 is heating that plasma to 10 million degrees Celsius, hot enough to see the complications a plasma throws at you.
For milestone 3, you’ve got to crank up your machine so its plasma is hot enough, dense enough, and well enough insulated — all at the same time.
That insulation requirement, called confinement, is the hardest part. With poor confinement, the plasma’s energy leaks out and you lose your chance at sustaining fusion at levels that would make more energy than it takes to heat up your plasma. Weak confinement is like heating your house with all the doors and windows open: You can do it, but you’ll have a big heating bill.
Note that confinement isn’t the same as how long your plasma is around; it’s how well it holds its heat in. Another analogy helps here: it’s not how long your coffee cup stays filled with coffee, but how long your coffee stays hot.
Multiplying temperature, density, and confinement time gets you a number called the triple product. If you’re going to pick one number to say how well your fusion machine works, the triple product is what you want. Basically, a good triple product shows you’re getting your plasma particles to be feisty enough, packed closely enough together, and staying hot on their own.
A good triple product is very important. Many fusion machines have faltered on the path: They get hot, but they don’t contain enough of the heat, and the hotter they get the more they leak the heat. It’s Sisyphean.
But if you can show a good triple product, it means that investors, press, policymakers, analysts, and other observers can have good reason to believe you’re a serious contender in the fusion energy race.
The measurement that matters: 1019 keV·s/m³
Now, buckle up for a minute. This section gets a bit technical, with three measurements, one moment of multiplication, and one big number.
The three measurements that describe plasma characteristics inside your fusion machine:
- Density (represented in equations by “n”): How many plasma fuel particles are packed into a given volume. Higher density means particles are more likely to collide. It’s measured in ions per cubic meter.
- Temperature (represented in equations by “T”): How hot those particles are. Higher temperature means particles in the plasma move around faster. It’s measured in kiloelectron volts, or keV.
- Confinement time (represented in equations by “τE”
or “tau”): How long the machine can hold onto the plasma’s energy. It’s measured in seconds.
In technical terms, you’ll see the triple product shown as nTτE. Numerically, the threshold of interest for the triple product is 1019 keV·s/m³ — a threshold the Department of Energy’s ARPA-E program uses. This number isn’t yet what a fusion power plant needs, but it’s in a regime that is pretty hard to get to and that’s dominated by all the vagaries you’re likely to see on the path to a higher triple product.
You have some room to maneuver to reach these three numbers. With a higher density, you can get away with a shorter confinement time, which is very important for the differences in magnetic confinement and inertial confinement. The temperature is less flexible since you don’t want to backslide from the second milestone of 1keV temperature.
But there’s no easy way to get this milestone’s triple product. Many fusion experiments have faltered on the way. There’s a graveyard of concepts that never got within a factor of 100 of the threshold I’m talking about here — Perhapsatron, I’m looking at you.
But if you do reach 1019 keV·s/m³, it means your fusion machine is reaching the conditions where fusion can start to take place in a way that could be a potential source of net energy. That’s where those nuclei smash into each other, forming heavier elements and releasing colossal amounts of energy, but doing so in a soup that doesn’t take too much energy to keep hot. And that’s the basis for a power plant, like the ARC machine CFS is designing for Chesterfield County, Virginia.
High triple product successes for fusion so far
The farther through the six steps we go, the fewer types of machines have reached those milestones.
For this triple product result, four types have made it — none of them yet from private companies. The first two approaches, tokamaks and stellarators, use powerful magnets to confine the plasma. The third, inertial confinement fusion, uses powerful lasers to trigger fusion in a pellet. The fourth, called MagLIF, combines laser energy and a strong magnetic field that an electrical current produces. You can see these results in peer-reviewed papers tracking fusion progress.
CFS is pursuing tokamaks, including the SPARC demonstration machine we’re building now at our Devens, Massachusetts, headquarters and the ARC fusion power plant we’re designing for Virginia. The tokamak I worked on at MIT’s Plasma Science and Fusion Center, Alcator C-Mod, is a descendent of Alcator A, which in 1978 became the first fusion machine to reach milestone 3.
In all cases, you have to use proven diagnostics technology to be sure you’ve really reached the milestone. If you read the post on the second milestone, you’ll remember Thomson scattering, a laser-based diagnostic technique that’s also important for milestone 3. It can also reveal plasma density — indeed, it’s the gold standard there.
You have to infer plasma performance, for example calculating confinement time from the amount of power we put into and how fast it leaks out. But decades of plasma operations have established reliable methods to figure out that performance from diagnostics data. As we operate SPARC, we’ll be sharing our results through peer-reviewed journals that present carefully checked, highly credible information. Trust is paramount.
The third milestone is crucial but not the last. Reaching it is very closely related to reaching the fourth milestone: net fusion energy, also called Q>1. Stay tuned to hear more about that milestone, which is the first big goal CFS has for SPARC.
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