Prof. Prabhat Ranjan CEO and Co-founder of a nuclear fusion company called ASPL Fusion. He is former Vice Chancellor of D Y Patil International University, Akurdi, Pune
I began my research life on a magnetic mirror at Berkeley, then spent sixteen years making India’s first-generation tokamaks work. Here is why ASPL Fusion has chosen the magnetic mirror over the tokamak and the stellarator, and what the mirror still does not give us.
Fusion is a field of believers, and I have believed in more than one machine. I spent a large part of my working life building tokamaks. Today the company I lead is building magnetic mirrors. This post explains that shift, and why I think it is the right choice for ASPL Fusion and for India.
Tokamaks, stellarators and magnetic mirrors are all magnetic-confinement devices. They differ in one basic decision: how the magnetic field lines are closed. A tokamak bends the field into a doughnut and drives a large current through the plasma to complete the magnetic cage. A stellarator also uses a doughnut, but twists the field with intricate three-dimensional external coils, so no plasma current is needed. A magnetic mirror does not close the field at all. It is a straight tube of plasma held between two regions of stronger field, built from simple circular coils, which reflect most particles back towards the centre.
In his previous blog, Prof. Ranjan argued that America’s new fusion roadmap marks a change of posture — the state stepping back from building the first power plant and instead building the shared infrastructure that lets private companies build it. Here he asks the natural follow-up for India: not what our public institutions should build, but what they should open. A surprising share of what a private fusion industry needs already sits inside the Department of Atomic Energy.
In my last essay I argued that the deepest lesson of America’s new fusion roadmap is a change of posture — the state stepping back from building the first power plant, and instead building the shared infrastructure that lets private companies build it. That argument invites an obvious question for India: if our public institutions are to enable a private fusion industry, what exactly should they build?
After some months of looking closely, I think the more urgent question is what they should open. A great deal of what India needs already exists, scattered across the units of the Department of Atomic Energy. The fastest and cheapest way to accelerate fusion here is not a greenfield campaign of new institutes; it is to upgrade, repurpose, and — above all — open a defined slice of the capability the DAE has spent six decades building.
I do not write this from the outside. I carried out my own doctoral fusion research at Berkeley; spent nine years as a scientist at the Saha Institute of Nuclear Physics, where I worked on India’s first tokamak, commissioned in 1987; and later led the ADITYA tokamak at the Institute for Plasma Research, along with the operation and control group of its SST-1 superconducting tokamak. Several of the units I describe below I have worked inside, and I have watched this establishment’s plasma capability grow from a single small tokamak into a national programme. What follows is, in part, an argument that India underestimates what it already owns.
A platform, not a building
The American roadmap calls its shared infrastructure the Tritium-Blanket Development Platform — a distributed network of test stands and loops, public and private, that any developer can draw on because no single company can justify building them alone. India can assemble an equivalent almost entirely from assets it already holds. The right unit of thinking is not a new national laboratory but a platform — a coordinated set of shared user facilities, each anchored in an existing DAE unit and opened, under clear rules, to vetted private developers. Let me walk through fusion’s hardest gaps and where, in the DAE, each could be addressed.
At INEF 2026 I realised how little the fission and fusion communities still talk to each other. Yet a fusion device is, before it is ever a power plant, an intense source of fast neutrons — and that surplus can breed U-233 from India’s thorium, burn long-lived waste, and drive a sub-critical hybrid that never goes critical. Fission problems with a fusion answer.
The Neutron Bridge — Part 1 | Prof. Prabhat Ranjan
I spent two days at the India Nuclear Energy Forum at IIT Bombay this May, and I came away with one impression more strongly than any other. The people who build and run our fission plants — some of the most capable nuclear engineers anywhere — have, for entirely understandable reasons, not been watching what has happened in fusion over the last five or six years. And the fusion community, equally guilty, has rarely bothered to explain itself in terms a reactor physicist would find useful.
That is a shame, because the two fields are closer to needing each other than either side seems to realise. I want to use this note to close a little of that gap, written deliberately for colleagues who think in cross-sections and neutron budgets rather than press releases.
What changed in fusion while you were busy keeping the grid running
For most of our careers, “fusion” meant one thing: a very large tokamak, decades away, funded by governments. ITER is real and important, but it shaped a perception that fusion is monolithic and remote. That perception is now out of date in two specific ways.
First, the magnet problem has largely been solved by materials, not by scale. High-temperature superconductors — REBCO tapes in particular — let us reach the field strengths that used to require enormous low-temperature magnets, in machines a fraction of the size. A high field at a small radius changes the entire economic argument, because confinement and reaction rate scale steeply with field. This is why the credible new private programmes are an order of magnitude smaller than ITER and still expect meaningful performance.
Second, and more relevant to this audience, the field has rediscovered configurations the fission community would find refreshingly simple. I work on the axisymmetric magnetic mirror — the Gas Dynamic Trap lineage from Budker Institute in Novosibirsk, recently validated by the WHAM experiment at Wisconsin and its commercial successor. A linear mirror is not a closed toroidal device. It is, conceptually, a long straight magnetic bottle. It is easier to build, easier to maintain, and — crucially — it is an excellent neutron source even when its energy gain is modest. Hold that last point; it is the whole argument.
The deeper shift is one of intent. A generation of fusion programmes asked only one question: when do we put electricity on the grid? The newer programmes ask a different one first: what is fusion good for before it is a power plant? And the honest answer is that a fusion device is, first and foremost, a controllable, intense source of fast neutrons. That reframing is what should interest you.
Why a fusion neutron is different
A deuterium–tritium reaction releases 17.6 MeV, of which 14.1 MeV leaves as a neutron. Compare that with the roughly 2 MeV average of a fission neutron. The factor of seven in energy is not a curiosity — it opens reaction channels that are simply closed to a thermal or even a fast fission spectrum.
The fusion neutron is born about seven times more energetic than a fission neutron, clearing the thresholds for neutron multiplication and hard-spectrum fission that a reactor spectrum cannot.
At 14 MeV you get neutron multiplication almost for free. Inelastic and (n,2n) channels in beryllium and lead mean a blanket can return more neutrons than it receives. That surplus is the resource. In a critical reactor every neutron is spoken for — you live or die by the six-factor formula and you have almost no margin to spend neutrons on anything that does not sustain the chain. A fusion source hands you a neutron budget you do not have to balance against criticality.
What you choose to do with that surplus — breed fuel that has eluded our thorium programme for decades, burn the waste that fills our repositories, and generate power without the assembly ever going critical — is exactly where our two fields meet.
