Scientists from KAIST (Korea Advanced Institute of Science and Technology) have proposed a new type of nuclear battery in which the energy of beta decay is first converted into electromagnetic waves and then into electricity. Unlike conventional radioisotope energy sources such as thermoelectric generators or beta-voltaic cells, the newly-developed radioisotope cyclotron generator does not have a strict physical efficiency limit. This allows it to potentially operate longer and more efficiently with the same resources, which is especially important for space and autonomous systems.
In conventional beta batteries, electrons emitted from a radioactive material enter a semiconductor and generate an electric current. However, the efficiency of this process has a limit of about 35%. The Korean researchers took a different approach: instead of slowing down the electrons in the material, they opted to harness their motion. The particles were placed in a magnetic field, where they began to spiral, emitting energy in the form of radio waves (so-called cyclotron radiation).
The greatest difficulty consisted in confining these electrons to the device. Simple magnetic traps were unsuitable for that purpose, as a significant portion of the particles escaped. This is why the scientists used a more advanced design, namely, a Penning-Malmberg trap. In this trap, a magnetic field inhibits the electrons radially, while an electric field inhibits them axially. The result is a nearly self-contained system, wherein the electrons remain for a long time, gradually releasing energy. The entire structure is placed in a resonator, a metal chamber tuned to a radiation frequency of approximately 28 GHz.
The researchers then created a computer model of their own. Conventional plasma programs do not take into account energy losses due to radiation, as they are negligible in most cases. Here, however, energy loss was a key effect. For that reason, the scientists developed a special computer code that allows them to track the movement of the particles and their radiation. They also simulated the resulting electromagnetic waves being collected and converted into electricity using an antenna and a circuit.
Calculations showed that the theoretical efficiency reaches 34.4% with optimal parameters (a voltage of about 90 kV and a resonator approximately 1 cm in size). This is on par with the best existing solutions, albeit with an important difference: it has no hard physical ceiling for further efficiency gains. The model also showed that the design can withstand such voltages: the electric field levels remain within acceptable limits for metals and insulators.
However, real-world applications are still a long way off. To use the device in real-world conditions, one has to learn how to make ultra-thin radioactive sources, maintain a stable vacuum and improve the efficiency of converting radio waves into electricity, which is currently much lower than in the ideal model. Nevertheless, the concept shows promise. It makes it possible to create compact energy sources without moving parts or high temperatures that will be able to operate for decades, including in space or underwater.
