“Pulsed electric fields, such as the ones caused by lightning strikes, manifest themselves as voltage spikes, posing a destructive threat to electronic components and causing considerable damage. Scientists from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) have now discovered that such voltage spikes can also have useful properties. In the journal Physical Review Research (DOI: 10.1103/PhysRevResearch.3.033153), they report how nuclear fusion processes, for instance, can be significantly reinforced by extremely strong and fast-pulsed electric fields.
Nuclear fusions, such as those taking place in the sun, are enabled by the quantum mechanical tunneling effect. “The tunneling effect can help particles of like charge to overcome their mutual repulsion even if they don’t actually have enough energy to do so – at least not according to the laws of classical mechanics,” says Prof. Ralf Schützhold, head of the Theoretical Physics Department at HZDR: “We can observe this in the fusion of two light atomic nuclei, for example: The closer one nucleus gets to another, the greater the repulsion, like a mountain piling up in front of the nucleus. This is called the potential barrier. Instead of taking the more energy-consuming route over the top of this mountain, the laws of quantum mechanics allow the nucleus to penetrate, or ‘tunnel’, straight through the mountain – which requires far less energy – and to eventually fuse.”
Although the tunneling effect plays an important role in many areas of physics and was first described nearly a century ago, our understanding of the process is still incomplete today. “We were already familiar with various aspects of how electric fields influence tunneling processes. For example, electric fields can further accelerate particles, helping them gain more energy. They can also deform the potential barrier, thus increasing the probability of a tunneling effect,” says Dr. Christian Kohlfürst, outlining the state of research from which they started.
His colleague Dr. Friedemann Queisser sums up their results: “Our calculations are now showing a novel feature of pulsed, rapidly changing electric fields: They can cause the particles to be essentially pushed out of the potential barrier, making them tunnel more easily.” The HZDR team’s calculations illustrate this concretely in various examples, including a fusion reaction that is of potential interest for energy generation: the fusion of a proton with the isotope boron-11.
A fusion reaction with great advantages
This is interesting because of the fuel that is available relatively readily. Three alpha particles are produced in the process, each with a double positive charge. The remarkable aspect of this reaction is that the energy is released in the form of charged particles and not as neutron radiation, as is the case with the currently most considered fusion reactions. This has advantages: First, it significantly reduces problems associated with neutron flux, such as hazards posed by handling ionizing radiation. Second, the energy of charged particles can be converted into electricity directly, and thus far more easily.
However, the conditions required to harness the reaction are even more extreme than those needed for the deuterium-tritium fusion favored in the current ITER fusion reactor experiment. Triggering the proton-boron reaction is comparatively more difficult, and scientists are still searching for viable ways of doing it. Schützhold’s team is now demonstrating an option: “According to our calculations, a sufficiently fast and strong pulsed electric field can significantly reinforce not only deuterium-tritium fusion, but also the proton-boron reaction.”
Generating such fields is, however, very difficult. “In principle, think of it as in a thunderstorm, where the energy stored in huge cloud formations is discharged in the form of a lightning bolt, extremely fast and in a very confined space. Around the world, facilities are being built or planned to concentrate ever higher energies into ever shorter periods of time and ever smaller spaces,” says Schützhold. Unfortunately, the facilities available today are not yet quite capable of generating such fast and powerful “artificial lightning bolts”.
But there is a possible solution: The electric field of an alpha particle flying fast and, most importantly, close to the proton can act like such a pulsed electric field and impact it so hard that the proton can tunnel through the potential barrier of boron-11 and trigger the fusion reaction. Alpha particles with the necessary energy are actually produced in the proton-boron reaction, but they can also be injected from outside.
C. Kohlfürst, F. Queisser, R. Schützhold: Dynamically assisted tunneling in the impulse regime, in Physical Review Research, 2021 (DOI: 10.1103/PhysRevResearch.3.033153)”