[ Instrument Network Instrument Development ] In the Tokamak long pulse high-power operation, a large amount of heat and particles discharged from the central high-performance plasma will pass through the closed magnetic surface and eventually reach the divertor target along the magnetic field lines. The extremely high thermal load of the target plate and the strong interaction between the plasma and the wall are the severe challenges faced by the current magnetic confinement fusion devices and the future operation of the fusion reactor. Plasma off-target can effectively reduce the thermal deposition and material corrosion on the surface of the divertor target plate. It is the most promising divertor operating state in the future fusion reactor. Recently, the researchers of the EAST team of the Plasma Institute have continuously explored and realized the high-constrained H-mode off-target operation under the condition of ITER-like tungsten divertor, and revealed the related physical mechanism affecting the H-mode off-target through experiments and simulations. The related research results were published by Liu Jianbin, assistant researcher and associate researcher Liu Xiaoju, in the international fusion field journals Nuclear Fusion and Physics of Plasmas. Among them, Liu Xiaoju, associate researcher, the simulation results of the AS divertor off-target threshold were also edited by the PoP journal. Editor's Picks.
H-mode off-target discharge photo on EAST shot by fast CCD camera
In the off-target state of the divertor, the enhanced atomic and molecular processes lead to strong energy loss in the divertor region, and the surface of the target plate can achieve a lower plasma temperature, thereby alleviating the damage of the surface material of the divertor and the generation of impurities, but Deep off-targeting tends to cause particles in the divertor region, particularly impurities, to flow back into the main plasma. Some other mainstream tokamak devices in the world have been difficult to verify the stable off-target mode under long-pulse H-mode operation. For future fusion reactors, how to obtain this operation mode stably and reliably is an urgent problem to be solved. Researchers such as the Liang team of the EAST team, such as divertor physics, plasma control, operation, and diagnostics, have collaborated and used EAST's unique long-pulse operating capability to develop detailed physical mechanisms for high-power long-pulse stable off-target operation. Experimental and theoretical simulation studies. On the basis of the active feedback control of radiation power by Yuan Qiping project researcher in 2018, the divertor probe diagnostic data was recently used as a real-time feedback control signal to inject a small amount of impurity gas or increase the main plasma density through the divertor region on EAST. The stable and repeated H-mode off-target operation and its active control were realized respectively. Various experimental methods for active feedback control of H-mode off-target were developed. The compatibility of off-target and central high-performance plasma was verified on EAST.
In the experiment, it was found that the higher heating power, especially the radio frequency wave heating, increased the electron temperature of the plasma, thereby increasing the threshold of the off-target of the divertor. By comparing the experimental results of different longitudinal field directions, it is revealed that the boundary drift is a potential mechanism that affects the off-target asymmetry of the divertor. Combining the drift effect, optimizing the discharge shape of the divertor, and combining the pumping and wall suction of the cryopump, particle control and impurity removal are realized, and the plasma and divertor regions are improved by a very small amount of impurity injection and rapid response. The radiant power, while effectively reducing the heat flow and temperature of the divertor target plate, obtains a stable H-mode off-target operation. Plasma energy storage and confinement performance are always maintained at a high level, showing very good core and boundary compatibility, providing a potential new solution for future steady operation of fusion reactors. This is the first time that EAST has realized the off-target operation of H-mode divertor and its active feedback control, which has an important role in promoting the development of divertor physics and related control technologies.
The dependence of the divertor off-target density threshold on the scraping layer power (PSOL)
The theoretical simulator used SOLPS large-scale simulation program combined with experiments to carry out systematic simulation studies on the possible factors affecting the off-target threshold of the EAST divertor, and further understood the physical mechanism affecting the off-target of the divertor. The simulation found that improving the tightness of the divertor helps to localize the neutral particles in the divertor region, improve the divertor power dissipation, and reduce the off-target threshold. As the efficiency of the scraping layer energy flow (PSOL) increases, the off-target threshold of the divertor increases accordingly.
There is a linear relationship. The simulation study also pointed out that with the increase of carbon impurity concentration, the off-target threshold is significantly reduced, but also the carbon impurity concentration of the core is also increased, which may affect the binding performance of the core plasma, so it is necessary to be compatible with high performance in the future. For a steady-state fusion reactor, it is necessary to improve the tightness of the divertor and the shielding efficiency of impurities in the divertor zone. In addition, Associate Professor Yang Zhongshi also used SOLPS-ITER to conduct a systematic simulation study on the behavior of the radiation divertor implanted by helium and argon impurities used in the EAST experiment, and analyzed and compared the results of the physical experiment of the radiation divertor. The EAST physics experiment provides an important reference.
The above work has also received invitation reports from a number of domestic and international academic conferences. These research results provide important experimental and theoretical information for the future divertor plasma and wall interaction control of EAST's longer-pulse H-mode steady-state operation, which is the bias for the Chinese fusion engineering test reactor CFETR and the international thermonuclear experimental reactor ITER device. Filter design and operation provide an important scientific reference. Next, the researchers will focus on the off-target mechanism under higher power conditions and the stability study of the improved feedback control, and achieve active control of the divertor-target off-target compatible with the core at a longer time scale and higher power heating conditions.
H-mode off-target feedback control realized by impurity injection of EAST divertor
The above work is benefited from the collaboration of the EAST team and cooperation at home and abroad, especially with the United States General Atomic Energy Corporation. Relevant work is supported by national key research and development programs, the National Natural Science Foundation, the Chinese Academy of Sciences, and Anhui Province.
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