Translation. Region: Russian Federation –
Source: Peter the Great St. Petersburg Polytechnic University –
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Russian scientists have achieved the first-ever precise determination of the origin of special waves in plasma—Alfvén oscillations. This discovery provides the key to solving one of the key problems in the safety and efficiency of controlled thermonuclear fusion, which is particularly relevant in the development of future energy sources. The measurement technique was developed at Peter the Great St. Petersburg Polytechnic University. The experiment was conducted on the unique Globus-M2 spherical tokamak at the A.F. Ioffe Physical-Technical Institute.
Alfvén oscillations are a special type of wave that occurs in plasma (an ionized gas) in the presence of a magnetic field. With a slight perturbation, the particles and the magnetic field itself begin to oscillate together, like a string carrying a wave. These oscillations propagate along magnetic fields and are observed both in laboratory setups and in space. For his theoretical description of these oscillations, Swedish physicist Hannes Alfvén received the Nobel Prize in Physics in 1970.
In laboratory settings, Alfvén oscillations are studied using toroidal (doughnut-shaped) magnetic plasma confinement devices, such as tokamaks. This design allows hot plasma, with temperatures up to 100 million degrees Celsius, to be confined using magnetic fields, preventing it from coming into contact with the walls. Tokamaks create conditions similar to those found inside the Sun, allowing energy to be generated through thermonuclear fusion. Alfvén oscillations inside tokamaks have a dual effect. While they facilitate energy and particle transfer, they can also lead to heat loss or instabilities, which can lead to plasma escaping the magnetic field and subsequent melting of the structure's walls. Therefore, studying the physical processes inside such devices is particularly important. Existing theoretical models and computer calculations have described how these oscillations should behave, but experimentally testing the theory under the challenging conditions of a real toroidal device has previously been elusive.
St. Petersburg scientists have achieved two important results for the first time in the world while studying Alfvén oscillations in the plasma of the Globus-M2 spherical tokamak at the Ioffe Institute.
"First, we experimentally determined where exactly Alfvén oscillations originate and exist within the toroidal setup. Measurements were conducted using microwave Doppler backscatter (DBS) diagnostics, developed by scientists at the Polytechnic University. This diagnostics allowed us to measure the electric field amplitude of Alfvén oscillations directly in the region of their development. Second, we discovered that different types of Alfvén oscillations and their harmonics can have different localizations," explained Alexander Yashin, PhD in Physics and Mathematics and head of the "High-Temperature Plasma Diagnostics" research laboratory at the Institute of Physics and Mechanics at St. Petersburg Polytechnic University.
Since the plasma temperature inside the tokamak is too high, the use of standard contact sensors for measurements is limited.
The Doppler backscatter method uses microwave radiation scattered by inhomogeneities in the plasma. This allows for remote and local measurement of key parameters. To ensure reliability, the Doppler backscatter data were compared with data from magnetic probes, which are traditionally used to study the dynamics of Alfvén oscillations but cannot provide information on their location or the local value of their amplitude. The comparison showed that the different methods yield consistent results, noted Arseny Tokarev, a research assistant at the Scientific Laboratory of Advanced Methods for Studying Spherical Tokamak Plasma at the Institute of Physics and Mechanics of St. Petersburg Polytechnic University.
Alfvén oscillations lead to significant losses of fast particles in the plasma. Their role in thermonuclear fusion is difficult to overestimate. Firstly, only they have sufficient energy to approach and interact, resulting in a thermonuclear fusion reaction. Secondly, they transfer part of their energy to slower particles, thereby increasing the plasma temperature. To achieve efficient and safe thermonuclear fusion, it is important to minimize the loss of high-energy particles. For example, according to calculations, the ITER experimental thermonuclear reactor, being built by an international research team in France, will withstand no more than a two percent loss of fast particles. Alfvén oscillations can cause much more significant losses. Therefore, the experimental data on the localization of Alfvén oscillations in plasma obtained by St. Petersburg scientists is a valuable contribution to the development of global thermonuclear energy.
The research was supported by the Ministry of Science and Higher Education of the Russian Federation under the state assignment in the field of science, project No. FSEG 2024 0005, using the Federal Center for Shared Use "Materials Science and Diagnostics in Advanced Technologies" of the A.F. Ioffe Physical-Technical Institute, which includes the unique scientific facility "Spherical Tokamak Globus-M."
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