In the race to make fusion a practical energy source, there’s a milestone that captures a lot of attention and has been sparking a lot of discussion: Q>1.

In simple terms, “Q” compares how much power comes out of fusion reactions to how much power goes in to heat the plasma. When that ratio is greater than 1, the plasma is producing more power from fusion than the heating systems are putting in. This is called scientific breakeven, and it’s one of the most important proof points for any fusion concept.

But here’s where things get tricky: in the fusion world, not all Qs are the same. And if we don’t explain which one we mean, people can understandably get the wrong idea.

What We Mean by “Q” in This Context

When we at Proxima talk about Q>1 for our Alpha demonstration stellarator, we mean scientific (or physics) gain, sometimes written as Q_phys or Q_sci:

• P_fus = Total instantaneous fusion power output (from both alpha particles and neutrons).
• P_heat = Instantaneous auxiliary heating power input to the plasma.

It’s a measurement over a representative time period during steady plasma operation, not an average over the whole shot from startup to shutdown.

Note that Q values do not say anything about two more important elements of the story: the time duration of high Q operation, and the overall stability of the plasma – both of these being strengths of the QI stellarators that Proxima is working towards. We´ll come back to that in other posts, but for now… let’s be clear about Q itself.

What It’s Not

It’s not engineering gain (Q_eng), which would factor in all the power used by the entire facility, from magnets to cryogenics to cooling systems. Engineering gain is the measure that matters for a full power plant, like our Stellaris concept, which Alpha will be a key step toward. Scientific gain is the measure that tells us about the production of power from fusion reactions in a plasma, a critical element on the path to scaling up to power plants that produce electricity.

It’s also not just about neutron output: we count all fusion energy, including the energy carried by alpha particles, which is about 20% of the total.

Why Words Matter

Because the term “net energy” is often misunderstood, it’s important to use phrases that are clear, accurate, and hard to misinterpret:

• Scientific breakeven – well-known shorthand for Q=1 in physics terms.
• Net energy gain from fusion reactions – specifies the scope.
• Net power from fusion reactions – reinforces that we’re talking about instantaneous power, not integrated energy over time.

At Proxima, we avoid phrases like “more energy than it consumes” (which imply total plant breakeven) unless we are talking about total plant breakeven, which is a target of Stellaris.

How We Talk About It

When we talk about Alpha’s goal, we try our best to be technically precise and avoid common misconceptions. 

For example: “Alpha is designed to reach scientific breakeven (Q>1), meaning the fusion reactions will produce more power than is needed to heat the plasma – a critical step toward building the world’s first stellarator fusion power plant.”

Fusion is full of complex terms and nuanced definitions. We see it as our job to make sure that when we talk about milestones, everyone – from plasma physicists to policymakers to the general public – knows exactly what we mean. We believe that clarity builds trust, and trust is certain to be needed as we carry fusion to the grid.