The Paper's reporter Zhang Jing
The laser is one of the greatest inventions of mankind and is considered to be the "fastest knife", "the most accurate ruler", and "the brightest light". The microscopic material world under the focus of this light is so fascinating that Zhang Jie has studied it for more than 30 years.
The 2021 Future Science Awards are awarded to Academician Zhang Jie of Shanghai Jiao Tong University/Institute of Physics of the Chinese Academy of Sciences, rewarding him and his team for producing precise and controllable ultra-short pulse high-energy electron beams by regulating the interaction between lasers and matter, and applying them to the fast-fire study of laser nuclear fusion and the realization of ultra-high-space-resolution high-energy electron diffraction imaging.
What is the rapid fire research of laser nuclear fusion? What does ultra-high space-time resolution high-energy electron diffraction imaging mean? A few days ago, Zhang Jie took time out of his busy scientific research work to accept an exclusive interview with the surging news (www.thepaper.cn) to interpret the scientific significance of the award-winning results and make suggestions for the future development of young researchers.
Academician Zhang Jie
Zhang Jie said that there are two types of scientific problems that physicists like to explore, the first is the bottleneck problem encountered in the development of human society, such as the biggest problem that the current development of human society urgently needs to solve - the ultimate energy problem, and the second is the most difficult mystery in nature to understand, such as the structure and function of the microscopic world.
Lasers have excellent directivity, coherence, and polarization, so laser-matter interactions can produce precisely controllable ultra-short pulsed high-energy electron beams. On the one hand, high-energy electron beams can precisely transport the energy they carry into pre-compressed fusion fuels, achieving rapid heating and triggering nuclear fusion reactions. Nuclear fusion energy is regarded as the "ultimate energy" of the future society because its fuel comes from seawater, its efficiency is ten million times that of fossil energy, there is no long-term nuclear waste, and there is no carbon emissions.
On the other hand, ultra-short pulse high-energy electron beams can also be used as extremely sensitive probes, providing ultra-high space-time resolution research methods for detecting ultrafast dynamic processes in the microscopic world. The high-energy electron diffraction and imaging device developed by Zhang Jie's team has achieved ultra-high spatial resolution at the subaeration level and ultra-high time resolution of 50 femtoseconds.
1 femtosecond is equal to 1000 trillionths of a second, and before Zhang Jie's team, the best international level of time resolution was 150 femtoseconds. "There are many important ultrafast processes in the microscopic material world whose time scales happen to be around 100 femtoseconds, so when our device reaches a time-resolution capability of 50 femtoseconds, it gives humans the ability to directly observe these ultrafast processes in the microscopic world for the first time." Zhang Jie made an analogy, just like the photography of high-speed moving objects, only the speed of the camera "shutter" is faster than the speed of motion, can be clearly imaged.
Since the 1990s, the continuous exploration and discovery of the microcosm frontier under the laser focus has fascinated Zhang Jie for more than 30 years. On September 12, at the moment of receiving the announcement of the material science prize of the Future Science Prize, he was meeting with team members to summarize the summer experiments that had just ended.
"I'm not in favor of comparing learning and scientific research to the ascetic culture of 'learning the sea without end'. The fundamental driving force of scientific exploration is human curiosity, which is one of the instincts that human beings have been able to evolve for a long time, so the process of learning and scientific exploration is actually very happy. We must learn to enjoy the joy of learning and the process of scientific exploration itself. For young researchers and students, Zhang Jie gave his advice.
Many times, we overemphasize the boring nature of scientific research, but in fact, the curiosity to explore the mysteries of nature and the satisfaction of solving difficult problems are the greatest motivation for scientists to explore. "The reward of our scientific exploration is the joy of discovery and the satisfaction of curiosity, which we think is much stronger than the dopamine produced by food, games or other recreational activities." Zhang Jie joked.
The "artificial sun" in the laboratory: two research paths for controlled nuclear fusion have come to the threshold
The interior of the Sun and many stars is warmer than tens of millions of degrees Celsius, and violent nuclear fusion reactions are taking place every moment. Zhang Jie introduced that the energy released by the sun per second is about 3.9 × 10 ^26 joules, although it reaches the surface of the earth only one billionth of the energy released by the sun per second, but this is also a huge energy, and it is this energy that makes all life activities on the earth possible.
Nuclear fusion reactions are a universal phenomenon in the universe and are the source of energy for stars, such as the Sun. Fusion energy is also at the forefront of energy development around the world, and if humans can control this energy, they can get rid of the current energy and environmental crisis on the earth.
The feedstocks required for controlled nuclear fusion are the two isotopes of hydrogen, deuterium and tritium. Deuterium can be extracted from seawater, and tritium can be produced from the very rich reserves of lithium on Earth. It is estimated that if 1 liter of deuterium extracted from seawater is fully involved in the fusion reaction, the energy released is equivalent to the energy released by the combustion of 300 liters of gasoline. Tritium, also known as superheavy hydrogen, has a half-life of 12 years, and its fusion reaction with deuterium is relatively easy.
Zhang Jie introduced that the energy produced by the fusion reaction of deuterium contained in a cubic kilometer of seawater is equivalent to the total energy generated by all oil reserves on the earth, so the development of fusion energy will solve the energy needs of human beings "once and for all". But if humanity wants to successfully achieve controlled thermonuclear fusion reactions on Earth and thus obtain enormous energy, it must create the following three necessary conditions.
One is extremely high temperatures, so that deuterium-tritium fuel becomes a hot plasma of more than 100 million degrees Celsius; the other is extremely high density, so that the probability of quantum tunneling of deuterium-tritium nuclei increases, and it is convenient to leave the alpha particle energy produced by fusion to continue to participate in nuclear fusion reactions; the third is that plasma is constrained in a limited space for a long enough time.
So far, human research on controlled nuclear fusion has been divided into two main categories. The first is magnetically constrained nuclear fusion, typical experimental devices such as the all-superconducting tokamak nuclear fusion experimental device (EAST) of the Hefei Institute of Physical Sciences of the Chinese Academy of Sciences and the FRENCH ITER experimental device.
All-superconducting tokamak nuclear fusion experimental device (EAST) from Hefei Institute of Physical Sciences, Chinese Academy of Sciences
ITER experimental setup in France
The second is laser nuclear fusion, typical experimental devices such as China's Divine Light Laser Device and the United States' National Ignition Device (NIF). Nifs, which cover the size of three football fields, use the traditional central ignition laser nuclear fusion scheme. NIF began its official ignition experiments in 2010, and in an August 8 episode of this year, it was close to the equilibrium point of the output energy and input energy of the fusion reaction.
China's divine light laser device
Laser nuclear fusion consists of two stages: fuel compression and heating. Zhang Jie told the surging news (www.thepaper.cn) that the traditional central ignition laser nuclear fusion scheme requires the simultaneous compression and ignition of tritium-tritium fuel using a laser device with huge energy, and the synchronous compression and ignition process will involve extremely complex nonlinear physical processes. He believes that this scheme can be used as a research scheme for the controlled laser nuclear fusion process, but due to the low efficiency, other ignition schemes need to be explored in the future as the generation of nuclear fusion energy.
In addition to the United States, there are other laser fusion ignition schemes in the world that are being studied. For example, Zhang Jie's team is currently exploring another ignition scheme, using a specially designed laser waveform and target configuration, separating the compression process from the ignition process, and rapidly igniting the compressed fuel through a precisely regulated ultra-short pulse high-energy electron beam, reducing physical instability and improving the efficiency of laser energy to ignition energy.
Among them, the precise regulation of ultra-short pulse high-energy electron beams is the key to ignition. Since the 1990s, Zhang Jie's team has undergone a large number of experimental and theoretical studies to achieve precise regulation of the emission direction and energy of high-energy electron beams, and to realize the guidance and focus of surface autogenetic electromagnetic fields on high-energy electron beams.
"Our laser fusion experimental research is mainly using the Shenguang-2 laser device of the Shanghai Institute of Optics and Mechanics of the Chinese Academy of Sciences, and at present, our scheme has completed 6 rounds of experiments and has made great progress."
Shenguang-2 upgraded laser device is a large-scale laser device independently developed by China. Zhang Jie told the surging news (www.thepaper.cn) that the team will do 12 rounds of experiments while further upgrading the Shenguang-2 laser device, and their goal is to verify the self-heating of alpha particles in 2026, providing a solid experimental foundation for the realization of the fast-fire scheme.
One of the two major products of nuclear fusion is the alpha particle, "each alpha particle carries 3.5 MeV (mechatron volts) of energy, in the experiment we will find a way to leave this energy behind in order to continue to heat the tritium tritium plasma, to achieve self-sustaining combustion." “
"Humans have been working for decades to ignite magnetically constrained nuclear fusion and laser fusion reactions." Zhang Jie said that now both kinds of nuclear fusion research paths have "reached the threshold": the energy output of nuclear fusion and the energy of input have reached a balance point, and the next step is to continue to work towards the milestone goal of output energy being greater than the input energy by a hundred times.
When the output energy of the fusion reaction is greater than 100 times the input energy, it is possible to explore the establishment of commercial power stations. If fusion energy can be realized at an early date, it will be the fundamental guarantee for the sustainable development of human society. "There's a saying that we who do nuclear fusion will always say that it's 50 years before we achieve fusion, but I don't think this time our time will go back because we've really reached the threshold." Zhang Jie said.
Ultrashort Pulse Electron Diffraction and Imaging: Entering the World of 50 Femtosecond Ultrafast "Suba-Es" Atoms
Physical scientists mainly explore two types of problems, the first is the bottleneck problem that restricts the further development of human society, and the second is the most difficult mystery in nature to understand, such as the structure and function of the microscopic world.
If most of the major scientific discoveries in the last century are related to the spatial structure of the material microscopic world, in this century, human beings hope to deeply understand the function of the material microscopic world on the basis of understanding the microscopic structure of matter, that is, the dynamic process of rapid changes in the microscopic structure of matter with time, which requires both ultra-high spatial resolution and ultra-high time resolution.
Electron microscopes have ultra-high spatial resolution, but no ultra-high time resolution, so they can only probe the spatial structure of the static microscopic world of matter.
"The atoms of any matter are actually moving rapidly, so the observation of microscopic time protons needs to be added to the ultra-high spatial resolution ability." What we do is combine the ultra-high spatial resolution of electron microscopy with the ultra-high time resolution of ultrafast optics, so that we can study the ultrafast dynamic processes of the microscopic world that we previously thought were stationary, but have been evolving in time. Zhang Jie told the surging news (www.thepaper.cn).
The ultra-short pulsed meerochron volt electron diffraction and imaging device they developed has achieved subaeration-level spatial resolution capabilities. Angstroms are units of length, 1 angstrom is equal to one billionth of a meter, and aeton is a smaller scale than angstroms. They also increased the device's time resolution to a record 50 femtoseconds, while 1 femtosecond equals 1000 trillionths of a second, compared with the previous best international level of 150 femtoseconds.
"Many of the ultrafast physical and chemical processes in the microscopic material world occur on a timescale of around 100 femtoseconds, so when our devices have both subaerget-level spatial resolution and 50 femtoseconds of time resolution, it means that only we can see these ultrafast kinetic processes at the atomic scale." Zhang Jie made an analogy, just like the photography of high-speed moving objects, no matter how fast the movement speed, as long as the speed of the camera shutter is faster, it can be clearly imaged.
High-performance time-resolved angle-resolved photoelectron spectrometer and megavolt ultrafast electron diffraction device
Using this device, Zhang Jie's team and collaborators successfully realized the regulation of ultrafast light fields on the dimensions of quantum materials, observed transient photogeneral novel states, and realized the first observation of new phase transitions induced by light and important physical and chemical ultrafast processes such as single-molecule imaging.
"In terms of observing ultrafast dynamic processes in the microscopic material world, one of the dreams of human beings is to be able to make films of single-molecule motion." For example, although we can write the chemical formula of the carbon dioxide molecule, we have not directly observed what the carbon dioxide single molecule looks like and how the carbon dioxide molecule moves.
Zhang Jie said that the responsibility of physicists is to directly see the single molecule image and take the image of the single molecule movement for in-depth study. "We first use a string of femtosecond laser pulses to line up the carbon dioxide molecules, and then use the ultra-short pulse high-energy electron beam to observe the carbon dioxide molecules in the queue, you can see the position and structure of the carbon dioxide molecules at different times, and then arrange the images at different moments to form a single-molecule movie."
For the future, in terms of ultra-high space-time resolution electron diffraction and imaging, Zhang Jie said that the team's next effort is to achieve 1 femtosecond time resolution capability, which will be another very important threshold. 50 femtosecond time resolution capabilities can allow us to see the motion process of atoms clearly, if the realization of 1 femtosecond magnitude of time resolution, human beings can see the movement process of electrons, thus making an important breakthrough in the understanding of the microstructure and function of matter.
"The responsibility of the physicist is not only to see, but also to gain a regular understanding of how single molecules are composed and why they move like this." At this time, Zhang Jie's eyes flashed with excitement.
Editor-in-Charge: Li Yuequn
Proofreader: Liu Wei