At ITER – the world’s largest experimental fusion reactor, currently under construction in France through international collaboration – the abrupt termination of magnetic confinement of high-temperature plasma through so-called “turbulence” poses a major open problem. As a countermeasure, turbulence mitigation techniques, which allow plasmas to be forcibly cooled when signs of plasma instability are detected, are the subject of intense research worldwide.
Now, a team of Japanese researchers from the National Institutes of Quantum Science and Technology (QST) and the National Institute of Fusion Sciences (NIFS) of the National Institute of National Science (NINS) have found that by adding approximately 5% neon to hydrogen ice grains, it is possible to cool plasmas more deeply. below their surface, and thus more effective than injecting pure hydrogen ice pellets.
Using theoretical models and experimental measurements with advanced diagnostics in the NIFS-proprietary large spiral device, the researchers elucidated the dynamics of the dense plasmoid that forms around ice grains and identified the physical mechanisms responsible for the successful optimization of forced cooling system performance, which is indispensable for conducting experiments at ITER. These findings will contribute to the creation of plasma control technologies for future fusion reactors. The team’s report is made available online at Physical review letters.
The world’s largest experimental fusion reactor, ITER, is being built in France through international cooperation. At ITER, experiments will be carried out to generate 500 megawatts of fusion power by maintaining the “burning state” of hydrogen isotope plasma at more than 100 million degrees. One of the main obstacles to the success of those experiments is a phenomenon called “turbulence” in which the composition of the magnetic field used to confine the plasma collapses due to magneto-hydrodynamic instability.
Turbulence causes high-temperature plasma to flow into the inner surface of the containing vessel, resulting in structural damage which may in turn cause experimental schedule delays and higher cost. Although the machine and operating conditions of ITER have been carefully designed to avoid any disruption, uncertainties remain and for a number of trials such that a dedicated machine protection strategy is required as a precaution.
A promising solution to this problem is a technique called “turbulence mitigation,” which forcibly cools the plasma at the stage when the first signs of instability that might cause turbulence are detected, thus preventing damage to the material components that encounter the plasma. As a primary strategy, the researchers are developing a method by using ice pellets of hydrogen frozen at temperatures below 10 K and injecting them into a high-temperature plasma.
The ice injected from the surface melts, vaporizes and ionizes due to heating by the ambient high temperature plasma, forming a layer of low-density and high-density plasma (hereinafter referred to as “plasmid”) around the ice. This low-temperature, high-density plasma mixes with the main plasma, whose temperature is lowered in the process. However, in recent experiments it has become apparent that when pure hydrogen ice is used, the plasma is ejected before it mixes with the target plasma, making it ineffective for cooling the high-temperature plasma deeper below the surface.
This repulsion was attributed to high plasmoid pressure. Qualitatively speaking, plasmas confined to a donut-shaped magnetic field tend to expand outward in proportion to the pressure. Plasmoids, which are formed by the melting and ionization of hydrogen ice, are cold but very dense. Since temperature equilibration is much faster than density equilibration, the pressure of the plasmid rises above the pressure of the hot target plasma. The result is that the plasmoid becomes polarized and experiences drift motion through the magnetic field, diffusing outwards before it is able to fully mix with the hot target plasma.
A solution to this problem was proposed from theoretical analysis: model calculations predicted that by mixing a small amount of neon with hydrogen, the plasmid pressure could be reduced. Neon freezes at a temperature of about 20 K and produces a strong radiation line in the plasmoid. Therefore, if neon is mixed with hydrogen ice before injection, part of the heating energy can be emitted as photon energy.
To demonstrate this beneficial effect of using a mixture of hydrogen and neon, a series of experiments were carried out in the large spiral device (LHD) located in Toki, Japan. For many years, LHD has operated a device called a “solid hydrogen pellet injector” with high reliability, which injects snow pellets about 3 mm in diameter at a speed of 1100 m/s. Due to the high reliability of the system, it is possible to inject hydrogen ice into the plasma with a temporal resolution of 1 ms, allowing measurement of the plasma temperature and density immediately after the injected ice melts.
Recently, the world’s highest Thomson Scattering (TS) temporal resolution of 20 kHz was achieved in an LHD system using a novel laser technology. Using this system, the research team was able to capture the evolution of plasmoids. They found that, as predicted by theoretical calculations, plasma ejection was suppressed when hydrogen ice was doped with approximately 5% neon, in stark contrast to the case in which pure hydrogen ice was injected. In addition, experiments confirmed that neon plays a beneficial role in the efficient cooling of plasmas.
The results of this study show for the first time that injecting hydrogen ice granules doped with a small amount of neon into a high-temperature plasma is beneficial to effectively cool the deep core region of the plasma by suppressing plasma ejection. The effect of neon doping is not only interesting as a new experimental phenomenon, but also supports the development of a baseline strategy to mitigate disturbances in ITER. A design review of the ITER disturbance mitigation system is scheduled for 2023, and the current findings will help improve system performance.
Matsuyama et al., Enhanced Absorption of Substances into Cyclic Plasma Using Mixed H + Ne Pellet Injection and Effects on ITER, Physical review letters (2022). DOI: 10.1103/PhysRevLett.129.255001
Provided by the National Institutes of Natural Sciences
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