403 seconds! China's "artificial sun" has made a new breakthrough! How far are we from controlled nuclear fusion?

Produced by: Science Popularization China

Author: Wang Teng (Institute of Plasma Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences)

Producer: China Science Expo

On the evening of April 12, 2023, China's "artificial sun" EAST successfully achieved 403 seconds of steady-state high confinement operation mode (H-mode) plasma, setting a new record for the longest H-mode operation time after achieving 101 seconds of steady-state H-mode plasma in 2017.

What is H-mode plasma? What is its significance? Members of the EAST research team will tell you.

(Photo source: Xinhua News Agency)

Key points of controlled nuclear fusion research

To achieve nuclear fusion reactions, deuterium and tritium nuclei must be compressed into a very small nuclear force range. However, since the nuclei are positively charged, they must be at extremely high temperatures to obtain enough energy to overcome the Coulomb barrier between them. When the temperature reaches about 100 million degrees Celsius, the cross-section of deuterium and tritium nuclei for fusion reactions is the largest. However, no tangible container can confine such high-temperature plasma. Tokamaks, represented by EAST, use a circular spiral magnetic field to confine high-temperature plasma to achieve controlled nuclear fusion reactions.

Magnetic field configuration of a tokamak device

(Image source: Institute of Plasma Physics)

Therefore, the key to controlled nuclear fusion research is: first, to increase the temperature (T) and density (n) of the plasma as much as possible to improve the fusion reaction efficiency per unit volume and unit time; second, to confine the high-temperature, high-density plasma in a limited space for a sufficiently long time to slow down energy loss and further improve the fusion reaction efficiency. This confinement performance is generally measured by the energy confinement time ( τ****E ).

Three elements to achieve fusion reaction

(Image source: Institute of Plasma Physics)

According to the Lawson criterion, only the fusion triple product can produce effective fusion power output. Under classical conditions, the energy confinement time of H-mode plasma will increase by about 2 times, and the temperature and density of the plasma will also increase accordingly, and the fusion triple product will be greatly improved. Therefore, the International Thermonuclear Experimental Reactor (ITER) uses the H-mode energy confinement time calibration as the basis for designing the reactor.

Discovery and characteristics of high-constraint operation mode

The equilibrium configuration of the tokamak plasma needs to be maintained by a toroidal induced current, which also performs ohmic heating on the plasma. Early tokamaks mainly relied on ohmic heating to constrain the operating mode. However, theoretical and experimental studies have shown that unless very strong magnets (greater than 20 Tesla) can be developed, it is impossible to achieve fusion ignition conditions by relying solely on ohmic heating.

Fusion ignition requires further improvement of plasma energy. High-energy neutral particle beams and radio frequency waves can be used for auxiliary heating. The total heating power is generally several times higher than the ohmic heating power. However, experiments have found that under given operating conditions, the energy confinement time decreases with the increase of heating power. This confinement operation mode is called low confinement operation mode (L-mode). If the reactor is designed and operated according to the L-mode energy confinement time calibration, the device will be very large, which is difficult to accept in terms of difficulty and economy.

In 1982, German physicist Friedrich Wagner accidentally discovered the H-mode on the ASDEX tokamak device, which means that the energy confinement time under high-power heating is basically twice that of the previous low-confinement state. This extremely important event was a great encouragement to the controlled nuclear fusion community at the time. When the news reached the United States, some people even jumped on the table in excitement.

At the same heating power, the density and vertical component of the poloidal pressure of H-mode plasma rise to about twice that of L-mode.

(Photo credit: Tokamaks)

In the high confinement operation mode, as the auxiliary heating power is injected, the density and energy storage of the plasma increase over time, and the hydrogen (deuterium) α-line radiation signal decreases. At the same time, at the plasma boundary, its density and temperature gradients present a steeper step-shaped structure, accompanied by a strong spike oscillation of the α-line radiation signal at the boundary. These significant features of the H-mode indicate that the number and power of particles lost from the plasma to the wall are reduced, thereby improving the plasma confinement performance.

The changes in the radial distribution of density at five different times before and after the LH transition measured by ASDEX show that the H-mode has an obvious density distribution step formation.

(Photo credit: Tokamaks)

Behind the achievements is the hard-core strength of the operation team

A necessary condition for the formation of H-mode is that the auxiliary heating power must be greater than a certain critical value, called the threshold power. This threshold power is closely related to the parameters and operating state of the device, as well as the plasma parameters and quality.

First, the realization of H-mode places high demands on the control of impurities (i.e., non-hydrogen substances) in the plasma. EAST's perfect vacuum system and wall treatment technology provide guarantees for this. The wall is cleaned by discharge for a long time, so that the boundary particles hit the wall and recirculate very low, minimizing the sputtered impurities and keeping the plasma pure. At the same time, relying on EAST's advanced plasma control technology, the interaction between plasma and the wall and other components is reduced as much as possible, further controlling the increase of impurities.

EAST Vacuum and Wall Treatment System

(Image source: Institute of Plasma Physics)

Secondly, under the same experimental conditions, as the plasma density increases, the threshold power shows a trend of first decreasing and then increasing. Therefore, there is an optimal density at which the auxiliary heating power required to achieve H-mode is minimal, which is crucial for the realization of long-pulse steady-state H-mode. Focusing on this goal, the EAST operation team combined theoretical analysis with experimental research to find the optimal parameter range and device operation capability within a safe range. At the same time, good control over plasma density also provides an important condition for achieving the goal.

EAST achieved 403 seconds of steady-state H-mode plasma. From top to bottom: density, H factor and energy storage, specific pressure, heating power, lower divertor temperature

(Image source: Institute of Plasma Physics)

In addition, the long pulse operation capability of the EAST auxiliary heating system, precise electromagnetic measurement and plasma control, advanced plasma diagnosis and many other technologies provide strong guarantees. It can be seen that the realization of 403 seconds of steady-state H-mode plasma fully reflects the comprehensive level and high efficiency of the EAST operation team.

EAST auxiliary heating system

(Image source: Institute of Plasma Physics)

Setting records means creating the future

The development of fusion energy through H-mode can effectively reduce the scale and cost of the reactor. If ITER is designed according to the energy confinement time calibration of L-mode, the scale of the device will be very large and the estimated cost is about 10 billion US dollars; and later the energy confinement time calibration of H-mode was adopted to design ITER, and other modifications were added, the scale of the device was greatly reduced, and the construction cost was reduced to 5 billion euros.

EAST device

(Image source: Institute of Plasma Physics)

The realization of 403 seconds of H-mode plasma in the EAST device will maintain the leading level in the next five years, and further verify the feasibility of high-confinement steady-state operation. At the same time, it also provides necessary conditions for conducting in-depth research on the physical mechanism behind H-mode on a longer time scale, the influence of high-energy particles on plasma confinement performance under H-mode, the mitigation and control of boundary localized modes, and the verification and development of related theoretical models. With the exploration of high-confinement steady-state operation and the effective solution of related scientific problems, the development of fusion energy will be further accelerated, and the first light lit by nuclear fusion energy will be realized as soon as possible.

References:

[1] John Wesson, Tokamaks (4th edition), Oxford University Press, 2011.

[2] Qin Yunwen, Physical Basis of Tokamak Experiment, Atomic Energy Press, 2011.

[3] Dong Jiaqi, Tokamak high confinement operation mode and magnetic confinement controlled nuclear fusion[J], Physics, 2010, 39(06):400-405.

[4] Shi Bingren, Basics of Tokamak Physics, Zhejiang University Fusion Theory and Simulation Center (Special Lecture), 2007.

[5] Friedrich Wagner et al., Regime of Improved Confinement and High-Beta in Neutral-Beam-Heated Divertor Discharges of the Asdex Tokamak, Physical Review Letters, 1982, 49(19):1408-1412.

[6] EAST obtains steady-state high confinement mode plasma of the order of 100 seconds for the first time (Institute of Plasma Physics)

[7] Liu Wenbin, EAST experiment operation report (20230413), Institute of Plasma Physics