Uncovering the mysteries of the core of a thermonuclear fusion reactor through nuclear emission diagnostics.

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Posted by NewAdmin on 2025-01-17 12:51:26 |

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Researchers from Milan, Italy, are exploring the fundamental properties of matter at 150 million degrees by measuring its intrinsic radiation. Nuclear fusion, the process that powers the stars, releases immense energy when light nuclei fuse into heavier nuclei. The energy released is described by Einstein’s famous equation, E=Δmc², where Δm represents the mass difference between the light and heavy nuclei, and c is the speed of light. Each fusion reaction releases about one million times more energy than conventional fossil fuel combustion, making it the fundamental process driving the Universe.

On Earth, the most promising approach to achieving nuclear fusion for energy production is to confine a fully ionized gas, known as plasma, in a sophisticated magnetic containment device called a tokamak. Several challenging conditions must be met simultaneously to make nuclear fusion viable for energy production. The plasma core must reach about 150 million degrees, which is roughly ten times hotter than the Sun’s core. The plasma density must be high enough to ensure sufficient fuel undergoes fusion, and the energy produced must remain in the system long enough to maintain the fusion reaction with minimal external energy input.

Measuring the temperature and properties of the fusion reactor's core is crucial for making nuclear fusion a viable energy source on Earth. However, measuring an object at 150 million degrees requires a different approach, as conventional solid probes would be destroyed by the plasma.

The solution lies in the fact that fusion plasma is a powerful source of electromagnetic and nuclear radiation, such as neutrons and gamma rays. Neutrons, the energy carriers of the fusion process, are produced during fusion reactions, while gamma rays can be emitted by fusion or other nuclear reactions in the plasma core, or from the deceleration of fast electrons in certain off-normal scenarios.

The neutron and gamma-ray diagnostics group at the University of Milano-Bicocca and the Institute for Plasma Science and Technology, both located in Milan, are world leaders in developing instruments to measure neutron and gamma-ray radiation from magnetically confined fusion plasmas. Their research aims to unlock the secrets of the core of a thermonuclear fusion reactor.

Measuring neutron emission from the plasma core:The first generation of fusion reactors will use deuterium and tritium, hydrogen isotopes, as fuel. When a deuterium and tritium nucleus fuse, a neutron is released from the core, and its energy is determined by the temperature and abundance of the reacting nuclei.

Similar to the spectrum of light from a distant star, the energy spectrum of fusion-born neutrons acts as a fingerprint of the properties of the plasma fuel. However, measuring neutrons is challenging due to their lack of charge and their occasional interactions with matter, which can result in only a fraction of their energy being detected.

To measure neutrons effectively, it is essential to design and build proper spectrometers tailored to specific applications. Some neutron detectors are small and easy to integrate into the complex environment of a tokamak, such as inorganic scintillators or synthetic single-crystal diamond detectors. For applications requiring high sensitivity or detailed measurements, more complex instruments are needed, such as time-of-flight or magnetic proton recoil instruments used in the EAST and JET tokamaks.

Gamma rays and energetic particles:While most ions in a fusion reactor are at equilibrium temperatures of around 150 million degrees, a small fraction of particles have much higher energy. These fast ions, produced by fusion reactions or introduced by auxiliary heating systems, as well as runaway electrons in off-normal scenarios, must also be detected. If not mitigated, runaway electrons can cause significant damage to the reactor walls.

Energetic particles emit radiation, primarily in the form of gamma rays, which can result from nuclear reactions or bremsstrahlung radiation from runaway electrons. Detecting and analysing these gamma rays helps identify the properties of energetic particles responsible for their production. However, gamma-ray detection is less complex than neutron spectrometry and typically involves inorganic scintillators of medium size, but these instruments must still be customised for each reactor’s conditions.

Nuclear diagnostics in the burning plasma era:The next milestone in nuclear fusion is achieving a burning plasma, where the fusion reaction is primarily sustained by the heat it produces, rather than by external heating systems. Machines like ITER, SPARC, and BEST are being built to study this regime.

In the burning plasma state, the plasma will emit even more intense radiation, making neutron and gamma-ray diagnostics crucial for understanding the complex, nonlinear phenomena governing the behaviour of the self-organised burning plasma in fusion reactors.

The Milan-based neutron and gamma-ray diagnostics group is at the forefront of research in this new era, designing and developing neutron and gamma-ray spectrometers for key burning plasma devices. They are also training a new generation of scientists at the PhD and post-doctoral levels to pioneer the use of these instruments in the burning plasma environment, potentially contributing to the discovery of fundamental laws governing fusion reactors and advancing the future of energy production on Earth.

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