Focusing on the specifics of nuclear fusion

We need to adopt a more sophisticated picture of what fusion reactions actually are.

Much nuclear fusion research today—whether in academic groups or in startups—adheres to a simplistic picture of what a fusion reaction actually is. This simplistic picture has its roots in mid-20th century physics and has not been significantly updated since.

In this traditional picture, a fusion reaction is seen as an event that occurs with a specific probability, when two nuclei collide at certain velocities. What that probability is for the respective velocities can be looked up for each combination of colliding nuclei from nuclear databases such as Nudat. For historical reasons, these fusion reaction probabilities are called “cross sections.“

That is pretty much all there is to how the fusion reaction itself is treated in the dominant view. Almost all other efforts towards improving the performance of fusion systems go into arranging for (a) high collision velocities; and (b) frequent occurrences of collisions.

From this picture derives the famous triple product, which describes how the fusion rate is expected to depend primarily on the kinetic energy of the particles, the density of the particles (which determines the collision frequency), and the confinement time (how long there is a chance of high-velocity collisions e.g. the duration of a pulse that heats the nuclei so as to get high velocities among them).

What is remarkable is that this “collision + evaluation of the cross section” picture can be almost described as semi-classical. Yes, some basic quantum considerations play a role in the way cross sections are often determined based on quantum tunneling calculations—but even here, researchers tend to work with approximations such as the WKB approximation and the neglect of nuclear potentials that throw out a lot of the interesting and intriguing details of the involved nuclei.

This is where our work comes in. We treat atomic nuclei as full blown quantum systems with all the bells and whistles that one is used to from the way full blown quantum systems are treated in atomic and molecular physics. In our picture, a much more comprehensive one, each nucleus is modeled based on the wave function that describes the occupation probability of all of its nucleons (i.e., its constituents). And nuclear reactions such as fusion reactions are seen as the reconfiguration of nucleon occupation probability from one state of the collective wave function to another.

This closes the historically grown gap between nuclear engineering and quantum engineering and opens up new possibilities that nuclear engineers to date could only have dreamed of.