For more than 3,000 days and nights, he turned the "beam of dreams" into the "light of reality"

Author: Zhang Shuanghu

At 3 a.m. on January 11, 2019, cheers came from the State Key Laboratory of High Field Laser Physics of the Shanghai Institute of Optics and Precision Mechanics, Chinese Academy of Sciences (hereinafter referred to as Shanghai Institute of Optics and Fine Mechanics).

The cheers mean that after more than 3,000 days and nights of hard work, the team led by Wang Wentao, a researcher at the institute, has turned the "beam of dreams" into the "light of reality."

If inspiration is a reward for hard work, then innovation can perhaps be seen as an encouragement to those who pursue their dreams.

Recently, after winning the honor of 2021 Chinese Academy of Sciences Annual Innovator, Wang Wentao calmly stated that the world-leading breakthrough "is not only the result of the team's hard work and overtime, but more importantly, it is the result of continuous experimentation, unremitting pursuit, and non-stop innovation based on deep exploration of physical mechanisms."

Innovation in daily work

On the LED display screen at the entrance of the National Key Laboratory of High-Field Laser Physics of the Shanghai Institute of Optics and Fine Mechanics, a slogan is particularly eye-catching: "Work overtime for 300 days and never return until we see light."

Wang Wentao told China Science Daily, "We made this pledge as soon as the laboratory was built in 2013."

In 2012, Wang Wentao led a young team to start research on laser tailfield electron acceleration and new desktop radiation sources.

From the beginning, he was clearly aware that conducting this research meant sitting on the "bench" for a long time - not only would he not be able to publish articles or obtain patents, but even the time it took to get promoted and gain recognition from his peers would be much longer.

At that time, the Shanghai Institute of Optics and Fine Mechanics was very supportive of this group of young people and tried every means to relieve their "worries". The team used this sonorous slogan to express their determination to go all out.

"'Three hundred days' is a figurative term," Wang Wentao said. "There are 365 days in a year, but the number of days we work overtime is actually much greater than 300 days."

Because the research on laser tailfield electron acceleration involves a delicate "dimming" process, and the quiet environment at night is more conducive to the experiment, overtime has become the norm, and they are basically busy until late at night every day.

Free electron laser is divided into three steps.

The first is to use laser drive. The laser group prepares and adjusts the laser every day, focusing it on the gas molecules to stimulate electrons and generate microwave fields.

The second is the quality of dimming.

The third is to stabilize the electron beam and achieve long-distance transmission to realize light emission.

"The microwave field is like the wake of a ship traveling at high speed on the sea, which will draw the electrons in and move forward together. Because the speed of light is very fast, it drives the electrons close to the speed of light, thus obtaining higher energy."

Wang Wentao explained, "It takes two hours for the laser group to adjust the light, and another four or five hours to stabilize and transmit it. This process is very boring and has to be repeated every day. In addition, any slight jitter or deviation in the front will prevent the electron beam from being transmitted to the back."

In this way, they spent five or six years "polishing" the work that they originally thought could be completed in two or three years, day after day.

“A lot of innovation has been done in this process.”

Wang Wentao believes that to complete this work, we must first overcome some traditional physical problems, such as the instability of laser plasma, the instability of the nonlinear effects of strong lasers, and the physical limitations of small-scale accelerators.

The team proposed and took the lead in verifying a variety of schemes such as "cascade tailfield", "gradient injection", "cooperative injection" and "chirp compensation", verified the cascade tailfield acceleration mechanism, and achieved many world-leading results such as laser-driven electron beams with the highest international quality (brightness close to the most advanced linear accelerator) and the lowest energy dispersion (0.2 thousandths), realizing the transition from following to leading in this field.

Digging deep into physical problems, starting from the principles, finding the right approach, proposing pioneering solutions, and implementing these solutions through clever experiments and technical means, this is how Wang Wentao's team has long understood the word "innovation".

"With the development of modern physics, innovations in basic physical principles and physical systems may not appear, but more innovations and improvements in experimental techniques and experimental plans will be made."

Wang Wentao said, "This may be called method innovation or technological innovation. The key is to find the right entry point."

The first rays of light appear

In 2004, scientists from the United States, France, Britain and other countries achieved a breakthrough in accelerating electrons in laser wakefields in experiments for the first time.

When this achievement was reported as a cover article in Nature, the title used was "dream beam".

"'Dream beam' has two meanings," said Wang Wentao. "One is the perfect acceleration in dreams, and the other is the accelerator that still exists in dreams but has not yet been realized. The scientific community believes that particle accelerators should be miniaturized in the future."

The laser wakefield makes it possible to miniaturize the accelerator.

Since then, miniaturized free electron lasers driven by laser wakefield accelerators, especially free electron lasers in the X-ray band, have become the common frontier pursued by scientists in this field.

In 2018, Wang Wentao's team successfully developed a stable desktop laser electron accelerator that produces a monoenergetic electron beam with a repetition rate of 100%.

Physicists believe that this shows that "benchtop particle accelerator", one of the five major breakthroughs in the laser field predicted in the "Vision 2020" special issue of Nature, has been achieved ahead of schedule. It also marks that my country has taken the lead in realizing the most critical transition from experiments to instruments in the field of benchtop particle acceleration.

In 2021, Wang Wentao's team significantly improved the quality of the electron beam accelerated by the laser wakefield, and combined it with an innovatively designed compact beam transmission and radiation system to achieve the first free electron laser amplified output based on a laser accelerator. The typical laser wavelength is 27 nanometers, the shortest laser wavelength can reach 10 nanojoules, and the single pulse energy can reach 100 nanojoules. Through methods such as orbital offset and spontaneous radiation calibration, it was proved that the energy gain in the last section of the undulator is as high as 100 times, which is the first time in the world to achieve spontaneous radiation amplified output in the extreme ultraviolet band based on a laser electron accelerator.

"We have already completed the proof of principle and achieved gain amplification. This is just the first step, and we cannot yet say that we have completed the free electron laser."

Wang Wentao said, "The 2004 article in Nature magazine was considered the 'dawn' of miniaturized accelerators. Our achievement is equivalent to the 'first ray of light', which is not the strongest light yet."

Approaching the dream of "miniaturization"

On July 22, 2021, the research results of Wang Wentao’s team were published on the cover of Nature.

The columns of Nature and Science published at the same time commented that "this achievement is another milestone in the field of laser wakefield since the report of 'Dream Beam' in 2004. It will have a significant impact on fellow researchers and is a major breakthrough." "This is a huge step forward!"

From large computers to various handheld terminals, computing power is increasing rapidly while size is shrinking.

Similarly, the scale of large accelerators, which can be tens of kilometers in size, makes them extremely "luxurious" scientific research equipment.

"Smalling is also for the purpose of becoming stronger. Just like the rapid development of integrated circuits, after people had personal computers and mobile phones, they still needed supercomputers. Only when the accelerator is reduced in size can it achieve more powerful functions in the possible space."

Wang Wentao said, "Some extremely high-energy electrons have been discovered in the universe. Such high-energy electrons cannot be achieved on Earth at present. In the future, we will use high acceleration gradient schemes to simulate such cosmic particles."

In addition to particle collisions and the study of basic physics problems, accelerating high-energy particles can also be used as a radiation source for a variety of applications in production and life.

At present, the use of laser wakefield electron accelerators has achieved a speed of ten meters. "The speed that originally required acceleration of dozens of kilometers can now be achieved with just a dozen meters."

"In the future, we will further improve the output power and photon energy of the free electron laser, and continue to work towards desktop free electron lasers while ensuring performance," said Wang Wentao.

China Science Daily (2022-02-25, Page 1, original title: "Wang Wentao: Finding the Right Entry Point for Dreams")

Editor | Zhao Lu

Typesetting | Guo Gang