Focusing on fusion: Debbie Callahan talks commercial laser fusion

With the world’s energy demands increasing, and our impact on the climate becoming ever clearer, the search is on for greener, cleaner energy production. That’s why research into fusion energy is undergoing something of a renaissance.
Construction of the International Thermonuclear Experimental Reactor (ITER) in France – the world’s largest fusion experiment – is currently under way, while there are numerous other large-scale facilities and academic research projects too. There has also been a rise in the number of smaller commercial companies joining the race.
One person at the forefront of fusion research is Debbie Callahan – a plasma physicist who spent 35 years working at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in the US. She is now chief strategy officer at Focused Energy, a laser-fusion firm based in Germany and California, which is trying to generate energy from the laser-driven fusion of hydrogen isotopes.
Callahan recently talked to Physics World online editor Hamish Johnston about working in the fusion sector, Focused Energy’s research and technology, and the career opportunities available. The following is an edited extract of their conversation, which you can hear in full on the Physics World Weekly podcast.
How does NIF’s approach to fusion differ from that taken by magnetic confinement facilities such as ITER?
To get fusion to happen, you need three elements that we sometimes call the triple product. You need a certain amount of density in your plasma, you need temperature, and you need time. The product of those has to be over a certain value.
Magnetic fusion and inertial fusion are kind of the opposite of each other. In a magnetic fusion system like ITER, you have a low-density plasma, but you hold it for a long time. You do that by using magnetic fields that trap the plasma and keep it from escaping.
In inertial fusion – like at NIF – it’s the opposite. You don’t hold the plasma together at all, it’s only held by its own inertia, and you have a very high density for a short time. In both cases, you can make fusion happen.
What is the current state of the art at NIF, in terms of how much energy you have to put in to achieve fusion versus how much you get out?
To date, the best shot at NIF – by which I mean an individual, high-energy laser bombardment of the target capsule – occurred during an experiment in April 2025, which had a target gain of about 4.1. That means that they got out 4.1 times the amount of energy that they put in. The incident laser energy for those shots is around two megajoules, so they got out about eight megajoules.
This is a tremendous accomplishment that’s taken decades to get to. But to make inertial fusion energy successful and use it in a power plant, we need significantly higher gains of more like 50 to 100.

Can you explain Focused Energy’s approach to fusion?
Focused Energy was founded in July 2021, and has offices in the US and Germany. Just a month later, we achieved fusion ignition, which is when the fusion fuel becomes hot enough for the reactions to sustain themselves through their own internal heating (it is not the same as gain).
At NIF lasers are fired into a small cylinder of gold or depleted uranium and the energy is converted into X-rays, which then drive the capsule. It’s what’s called laser indirect drive. At Focused Energy, however, we’re directly driving the capsule. The laser energy is put directly on the capsule, with no intermediate X-rays.
The advantage of this approach is that converting laser energy to X-rays is not very efficient. It makes it much harder to get the high target gains that we need. At Focused Energy, we believe that direct drive is the best option for fusion energy to get us to a gain of over 50.
So is boosting efficiency one of your key goals to make fusion practical at an industrial level?
Yes, exactly. You have to remember that NIF was funded for national security purposes, not for fusion energy. It wasn’t designed to be a power plant – the goal was just to generate fusion energy for the first time.
In particular, the laser at NIF is less than 1% efficient but we believe that for fusion power generation, the laser needs to be about 10% efficient.
So one of the big thrusts for our company is to develop more efficient lasers that are driven by diodes – called diode pump solid state lasers.
Can you tell us about Focused Energy’s two technologies called LightHouse and Pearl Fuel?
LightHouse is our fusion pilot plant. When operational, it will be the first power plant to produce engineering gain greater than one, meaning it will produce more energy than it took to drive it. In other words, we’ll be producing net electricity.
For NIF, in contrast, gain is the amount of energy out relative to the amount of laser energy in. But the laser is very inefficient, so the amount of electricity they had to put in to produce that eight megajoules of fusion energy is a lot.
Meanwhile, Pearl is the capsule the laser is aimed at in our direct drive system. It’s filled with deuterium–tritium fuel derived from sea water and lithium.

How do you develop the capsule to absorb the laser energy and give as much of it to the fuel as possible?
The development of the capsule for a fusion power plant is quite complicated. First, we need it to be a perfect sphere so it compresses spherically. The materials also need to efficiently absorb the laser light so you can minimize the size of that laser.
You have to be able to cheaply and quickly mass produce these targets too. While NIF does 400 shots per year, we will need to do about 900,000 shots a day – about 10 per second. We’ll also have to efficiently remove the exploded target material from the reactor chamber so that it can be cleared for the next shot.
It’s a very complicated design that needs to bring together all the pieces of the power plant in a consistent way.
When you are designing these elements, what plays a bigger role – computer simulations or experiments?
Computer simulations play a large part in developing these designs. But one of the lessons that I learned from NIF was that, although the simulation codes are state of the art, you need very precise answers, and the codes are not quite good enough – experimental data play a huge role in optimizing the design. I expect the same will be true at Focused Energy.
A third factor that’s developing is artificial intelligence (AI) and machine learning. In fact, at Livermore, a project working on AI contributed to achieving gain for the first time in December 2022. I only see AI’s role in fusion getting bigger, especially once we are able to do higher repetition rate experiments, which will provide more training data.
What intellectual property (IP) does Focused Energy have in addition to that for the design of the Pearl target and the LightHouse plant?
We also have IP in the design of the lasers – they are not the same lasers as used at NIF. And I think there’ll be a lot of IP around how we fabricate the targets. After all, it’s pretty complicated to figure out how to build 900,000 targets a day at a reasonable cost.
We’ll see a lot of IP coming out of this project in those areas, but there’s also the act of putting it all together. How we integrate these things in order to make a successful plant is important.
What are the challenges of working with deuterium and tritium as materials for fusion?
We chose deuterium and tritium because they are the easiest elements to fuse, and have been successfully demonstrated as fusion fuel by NIF.
Deuterium can be found naturally in sea water, but getting tritium – which is radioactive – is more complicated. We breed it from lithium. Our reactor designs have lithium in them, and the neutrons from the fusion reactions breed the tritium.
Making sure that we have enough tritium, and figuring out how to extract that material to use it for future shots, is a big task. We have to be able to breed enough tritium to keep the plant going.
To work on this, we have a collaboration funded by the US Department of Energy to work with Savannah River National Lab in South Carolina. They have a lot of expertise in designing these tritium-extraction systems.
How will you capture the heat from the deuterium–tritium fusion reaction?
We will use a conventional steam cycle to convert the heat into electricity. It’s funny – we’ll have this very hi-tech way of producing heat, but at the end of the day, we will use a traditional system to produce the electricity from that heat.
So what’s the timeline on development?
Our plan is to have a pilot plant up by the end of the 2030s. It’s a fairly aggressive timeline given the things that we have to do. But that’s part of being a start-up – we have to take some risks and try to move quickly to achieve our goal.
To help that we have, in my view, a superpower – we have one foot in Europe and one foot in the US. There are a lot of opportunities between the two continents to partner with other companies, universities and governments. I think that makes us strong because we have access to some of the best talent from around the world.
How does working at Focused Energy compare with life as an academic at Lawrence Livermore?
There are a lot of similarities. My role now is to bring the knowledge and skills I learned at NIF to Focused Energy, so it’s been a natural transition.
In fact, there was a lot of pressure working at NIF. We were trying to move very quickly, so it’s actually very similar to working in a start-up like Focused Energy.
One of the big differences is the level of bureaucracy. Working for a government-funded lab meant there were lots of rules and paperwork, which takes up your time and you don’t always see the value in it.
In contrast, working for a small start-up means we can move more quickly because we don’t have as many of those kinds of constraints. Personally, I find that great because it leaves more time for the fun and interesting things – like trying to get fusion on the grid.
Are you still involved in academic research in any way?
As a firm, we are still out there collaborating with academics. Last year, for example, we gave four separate presentations at the American Physical Society Division of Plasma Physics meeting.

I feel very strongly about peer review. Of course, publishing isn’t our number one priority, but we need feedback from others. We’re trying to do something that no-one’s done before, so it’s important to have our colleagues give us feedback on what we’re doing, point out mistakes we’re making or things we’re forgetting.
Working with universities and national labs in both Europe and the US is vital. Communicating with others in the field is important for us to get to where we want to go.
And of course, being an active part of the fusion community is good for recruitment too. We regularly give presentations at conferences that students attend. We meet those students and they learn about our work – and they might be future employees for our company.
What’s your advice for early-career physicists keen on joining the fusion industry?
There are so many opportunities right now, especially compared to the start of my career when the work was mainly just at universities or national labs. Nowadays, there are a lot of companies in the sector. Not all of them will survive because there’s only so much money, but there are still lots of opportunities. If you’re interested in fusion energy, go for it.
The field is always developing. There’s new stuff happening every day – and new problems. So if you like problem-solving, it’s great, especially if you want to do something good for the world.
There are also opportunities for people who are not plasma physicists. At Focused Energy we have people across so many fields – those who work on lasers, others who work on reactor design, some developing the AI and machine learning, and those who work on target physics, like me. To achieve fusion energy, we need physicists, engineers, mathematicians and computer scientists. We need researchers, technicians and operators. There’s going to be tremendous growth in this sector.
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