Scientists have almost completed the development of the High Resolution Neutron Spectrometer (HRNS). The system will be used to measure both the number and energies of neutrons emitted by plasma across the full range of fusion power expected for the ITER reactor.
The research team revealed that the HRNS is one of the essential plasma diagnostics of ITER, whose operational role is the neutronic measurement of the nt/nd ratio in a plasma core. Coexisting with the other ITER diagnostics makes HRNS a powerful tool for efficient and precise plasma diagnostics.
Scientists also pointed out that the complexity of the ITER tokamak brings with it many variables that had not been considered of primary importance until now, such as the magnetic field or high temperature in the detector region.
HRNS provides information about the proportions of deuterium, tritium
“HRNS provides us with information about the proportions of deuterium and tritium, hydrogen isotopes that combine inside the reaction chamber,” says Dr. Jan Dankowski (IFJ PAN), the first author of the article describing the spectrometer.
“Measuring the fast neutron population from the two dominant reactions in the plasma is a direct indicator of fuel composition, ion temperature, and combustion quality. In ITER and future reactors, this will be a key tool for controlling and optimizing reactor operation.”
Dankowski highlighted that the lack of this information would effectively mean losing one of the most important plasma diagnostic tools, significantly hampering scientific research at ITER and the safe operation of future power reactors.
The spectrometer design is a joint effort by physicists and engineers from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Kraków, the University of Uppsala, and the Istituto per la Scienza e Tecnologia dei Plasmi in Milan, developed in close cooperation with the ITER Organization.
Scientists revealed that the nuclei of hydrogen isotopes form plasma, which, being electrically charged, can be held in isolation from the walls by a magnetic field inside the toroidal vacuum chamber of the reactor (these sorts of reactors are called tokamaks).
Researchers also pointed out that this plasma must be additionally heated to reach a temperature of 150 million Kelvin, which guarantees the proper course of the reaction. The high-energy neutrons produced during fusion, being electrically neutral, escape towards the walls of the tokamak, allowing most of the energy produced to be recovered (and ultimately creating tritium in collisions with lithium).
HRNS spectrometer divided into four sub-assemblies
The research team revealed that in order to ensure the operation of the HRNS spectrometer under the full range of conditions anticipated in the ITER reactor, it had to be divided into four independent sub-assemblies. Each of these is a separate spectrometer, operating on different principles and designed for a different range of neutron flux intensities.
Physicists from the IFJ PAN are developing the first subassembly, called TPR (Thin-foil Proton Recoil). Here, neutrons knock protons out of a thin polyethylene foil – and their scattering angles depend on the energies of the neutrons. Nearly 100 silicon detectors are responsible for the detection of the protons. The second subassembly is the NDD (Neutron Diamond Detector) spectrometer, where neutrons are recorded by over a dozen diamond detectors, according to a press release.
The last two subassemblies, FTOF (Forward Time-of-Flight) and BTOF (Backscattering Time-of-Flight), measure the flight times of neutrons and estimate their kinetic energy based on the velocities determined in this way, with FTOF analysing neutrons that maintain a direction of motion similar to the original one, and BTOF analysing those scattered at large angles, as per the research team.
Physicists revealed that the HRNS spectrometer will be installed behind a thick concrete protective wall surrounding the fusion chamber, near an opening several centimetres in diameter, to be able to detect neutrons produced in the very center of the plasma. Depending on the reactor’s power, its flux will vary dramatically, reaching hundreds of millions of particles per square centimetre per second.