The Development Trend and Research Focus in the future
With the support of the major instrument project of the Ministry of Science and Technology, the research team of Peking University is building the first laser accelerator. In the future, it is expected to focus on the following directions to carry out application research and related industrialization exploration.
1. Application Research of laser proton accelerator
(1) Proton photography
With the development of laser technology, people began to use the proton source driven by ultrashort laser for photographic research. The MeV proton beam produced by laser acceleration has short cluster [PS (picosecond)] and long range. It is insensitive to the density fluctuation of small-scale laser plasma, sensitive to electromagnetic field, and it has high time resolution. Therefore, it can be used as a new diagnostic method to study the transient electromagnetic field in laser plasma. Because laser probe and X-ray backlight imaging are very sensitive to plasma density and temperature, it is impossible to diagnose this electromagnetic field directly. In addition, the proton source driven by laser has the advantage of strict synchronization with laser, which makes proton photography widely used in laser plasma physics.
At present, the study on imaging of biological samples by laser proton beam is still in the initial stage of research. The maximum energy of the wide spectrum proton used is low, and the proton energy stratification is less. Lack of in-depth follow-up analysis and processing of images, so less biological information is obtained. The present study of the imaging objects are also concentrated in a few organisms such as insects. Imaging studies on tissues and living bodies of larger organisms have not been reported. At present, there is no report on transmission imaging with laser accelerated single energy proton beam in the world.
(2) Fusion plasma diagnosis
Although Tokamak type magnetic confinement device has been selected to build experimental reactor (ITER) yet, some core physical problems have not been completely solved, such as the energy and particle confinement of fusion high-temperature plasma, various micro and macro instabilities, turbulence and transport are important aspects to realize the development and application of thermonuclear fusion energy.
Of course, the most important issue is the energy constraint, which is directly related to the scale, cost and future commercial competitiveness of the reactor. In the energy constraint problem, the main difficulty is the understanding of the so-called anomalous transport. The so-called anomalous transport refers to the transport phenomenon higher than expected by the classical and neoclassical (considering the annular configuration) theories. It involves complex physical processes such as plasma turbulence. Therefore, the study of plasma turbulence has become the most important frontier topic of Tokamak physics. Laser driven short pulse high current ion beam can play an important role.
(3)Proton acoustic imaging technology
Proton acoustic effect refers to the phenomenon that substances are irradiated by periodic high-intensity proton beams to produce ultrasonic signals. Sulak of Harvard University in the United States first irradiated the water simulator with a proton beam of more than 150mev in 1979. It was experimentally proved that the instantaneous thermal expansion caused by the proton beam can produce a strong enough detectable ultrasonic signal. In 1995, researchers at the University of Tsukuba in Japan detected the ultrasonic signal with hydrophone during the treatment of proton beam radiotherapy for a patient with liver cancer, and deduced the position of Bragg peak on this basis. However, the spatial resolution of ultrasonic signal is as low as 2 mm. If the resolution can be improved, it can become a unique treatment and detection method due to clinical application. Due to the short pulse length, high transient power, high acoustic signal intensity and short time, the ion beam generated by laser acceleration can obtain the real-time dose distribution in vivo with sub millimeter resolution.
（4）Study on the generation of high dose rate proton beam and biological irradiation effect
The laser accelerated proton beam has an ultrashort longitudinal length. Near the interaction point between laser and target, the beam pulse length is only picosecond (10 ^ - 12s). After the proton passes through the beam transport system and is far away from the action point, it is widened due to dispersion. But it is also on the nanosecond scale. Due to the ultrashort beam length, the instantaneous beam intensity of the laser proton beam can reach 10 ^ 2-10 ^ 3a, which is much higher than that of the traditional accelerator. When such a beam flows through the focus and enters the organism, it will produce an ultra-high radiation dose rate of up to 10 ^ 9gy. Through the special design of beam system, the wide energy spectrum of laser accelerated proton beam can also be used to simulate the space radiation field and study the mechanism of secondary cancer caused by radiotherapy.
(5) Laser driven warm and dense state generation
Compared with the traditional RF accelerator, laser driven proton beam has two advantages: high acceleration electric field gradient and high proton beam current density. The acceleration gradient of the former is more than three orders of magnitude higher than that of the latter, so it is expected to significantly reduce the size of large accelerators. At the same time, the proton beam density is ten orders higher than that of the traditional accelerator, and the proton beam density can be further improved by using the special back bending target structure design. The proton beam with such high beam density can quickly heat the solid material to a warm and dense state (~ 100eV). This scheme will be more flexible than the traditional nanosecond kilojoule laser shock loading solid materials to produce warm and dense materials. The former can cover a larger parameter space.
(6) Space radiation environment simulation
Compared with people's living environment on the ground, the outer space environment is very different. In addition to the low pressure and extremely low temperature similar to vacuum, there is also a special complex space ionizing radiation environment. This complex space radiation environment mainly includes: 1) high-energy electrons and protons captured by the earth's magnetic field, which form the earth's capture zone radiation; 2) Galactic cosmic rays (GCR) radiation coming from charged particles outside the solar system. The energy of particles ranges from tens of MeV to 1012mev, mainly composed of protons (85%), He ions (14%) and high-energy heavy ions; 3) Solar particle events, rare high-throughput charged particles during strong solar flares, mainly protons (90% - 95%) and He ions.
Sometimes there are albedo neutrons and protons in low earth orbit, which are secondary charged particles returned to space caused by the interaction between GCR and the earth's atmosphere. The harm of complex space environment to astronauts' health is the main factor restricting mankind's long-term stay in space and the development and utilization of space. It is necessary and urgent to study the phenomenon and mechanism of biological effect changes caused by the experimental model of life under the comprehensive and complex space environment, so as to fully understand the life and health problems that space travel may cause to astronauts, and develop feasible countermeasures suitable for China's national conditions and the quality of Chinese astronauts. The harm of high-energy protons to astronauts deserves more attention. The high-energy protons provided by laser ion accelerator can be used to study the damage and protection of irradiated organ like tissues.
2 Application of laser accelerator in proton radiotherapy
Compared with traditional radiotherapy, proton radiotherapy has at least four advantages: improving tumor irradiation level, improving local control rate, reducing complications and strengthening the effect of chemotherapy. All tumor patients suitable for radiotherapy are suitable for proton therapy. Especially for patients with early tumors, the five-year survival rate of proton therapy is more than 80%. Because children are more sensitive to radiation than adults, traditional radiotherapy will cause radiation damage to children's liver, kidney, spinal cord, ovary or testis. Proton therapy can protect important organs and tissues from damage through accurate "directional and fixed-point blasting" technology, so as to solve the problems in radiotherapy for children. Proton therapy also shows great advantages for tumors surrounded by important organs.
Proton radiotherapy may become the mainstream of tumor radiotherapy in the next 20-30 years. At present, the main accelerators used in proton radiotherapy equipment are traditional RF accelerators (including synchrotron and cyclotron). According to the statistics of the international particle therapy Cooperation Organization (ptcog), as of July 26, 2016, there were 64 proton heavy ion therapy centers in operation worldwide, mainly distributed in developed countries and regions such as the United States, Europe and Japan; Tens of thousands of people worldwide receive proton therapy every year.
The pioneering research of ion beam cancer treatment originated from Lawrence Berkeley Laboratory (LBNL) in the United States. In 1946, Wilson first proposed the application of proton beam in medicine. Since the 1980s, due to the gradual popularization of X-ray tomography, CT and MRI in technologically advanced countries, proton cancer treatment has made great progress. In 1992, Loma Linda University in the United States opened a special proton device for medicine, ushered in a new era of proton radiotherapy technology, officially announced that proton radiotherapy has entered the medical field and determined its position in application. In 1985, an international proton therapy Cooperative Oncology Group (ptcog) was established to conduct worldwide proton research. According to the statistics of PTCOG at the end of 2017, 66 proton therapy centers in the world were in normal operation by the end of 2017, including 27 in the United States, 16 in Europe, 13 in Japan, 3 in Russia, 2 in China, 2 in South Korea, 1 in Canada and 1 in South Africa. A total of 174512 tumor patients had received particle therapy, including 149345 tumor patients treated with proton, accounting for 85.58% of the total number of patients treated, and 21580 tumor patients treated with carbon particles, It accounts for 12.37% of the total number of patients treated. The efficacy of proton therapy has been widely recognized and fully affirmed.
According to incomplete statistics, there are more than 70 proton therapy center application projects submitted for approval in major cities in China. The national health and Family Planning Commission is cautious about them. A large part of the reason is that the domestic commercialized proton therapy system has not been successfully developed, and basically all equipment needs to be imported from foreign enterprises. Once more than 70 projects are approved, it is equivalent to tens of billions of dollars flowing overseas to further promote the technological upgrading of overseas enterprises, Consolidate their leading position in the industry. Therefore, from the perspective of national strategy, China urgently needs to independently develop domestic proton therapy equipment. Although the independent research and development of proton therapy equipment in China has been carried out, the gap between China and the international level is still very large, and the monopoly position of foreign proton therapy equipment cannot be shaken in a short time.
In the field of medical devices, it is common for foreign manufacturers to use the advantages of first mover technology to reduce the market share of domestic equipment by quickly iterating products, reducing costs and improving user experience. Taking linear accelerator X-ray radiotherapy equipment as an example, China has developed for more than 40 years, but this type of products still mainly relies on imports. Domestic products of 10 MV high-energy linear accelerator have low stability and almost no market. 6MV low-energy linear accelerator also lags far behind similar foreign products in terms of product performance. Products of Varian, Elekta and other companies sell for more than 20 million yuan, but occupy more than 90% of the market. Domestic equipment sells for 5 million yuan, but there is no market, that is, domestic equipment does not bring any market impact to imported equipment. From this point of view, it will be a very long process for China to catch up with foreign advanced technology and equipment that are at the absolute leading level, and even the gap may be widening.
The laser accelerator has the possibility of being used as the accelerator of the new generation proton radiotherapy system. Compared with large-scale traditional proton accelerators, laser accelerators are expected to have advantages in equipment demand space, installation difficulty, operation and maintenance cost, radiation protection difficulty, system complexity and so on. It can be expected that once the laser proton radiotherapy system based on the laser accelerator is successfully developed, China will have a great opportunity to occupy the commanding height of the laser proton radiotherapy equipment industry, change the long-standing mode of "leading abroad and following domestic" in the field of large-scale medical equipment, and realize "overtaking on corners". Therefore, it is of great industrial significance to develop the laser proton therapy system as soon as possible.
3. Development of high brightness gamma light source
Because of its high energy, γ-ray can interact with atomic nucleus to produce secondary particles / rays. It is a very unique probe to study the structure of atomic nucleus and identify different elements. Quasi single energy with high spectral peak brightness, controllable polarization and continuously adjustable energy γ Ray sources have broad application prospects in the fields of nuclear and astrophysics, nuclear safety, nuclear medicine, environmental science, energy science, material science and so on.
The existing common X / constant ray sources cannot produce high brightness quasi single energy gamma rays. Based on the inverse Compton scattering process between high-quality relativistic electrons and laser, it is the only way to produce continuous energy adjustable single energy gamma rays. Based on the existing electronic storage ring or Collider, a number of single energy gamma sources with energy up to 100MeV have been developed abroad, such as legs, Graal, Rokk, LEPs, newsubaru, AIST, etc.
Shanghai Laser Electron Gamma Source (SLEGS) is also under construction in China. This light source is one of the 16 line stations planned to be built by Shanghai light source phase II-line station. Its scientific goal is to carry out basic physical research in the fields of nuclear astrophysics and nuclear structure through photonuclear reaction; Recently, the Institute of High Energy Physics also plans to use the linear accelerator of Beijing Electron Positron Collider to generate gamma light source for frontier research of nuclear physics. These light sources are limited by the scale of the device and the performance of generating gamma rays, so they are limited in application. In recent years, a compact gamma source device based on the interaction of high brightness electron beam and terawatt laser has been proposed and made important progress. For example, LLNL laboratory in the United States proposed the T-Rex (Thomson radiated extreme X-ray) project, and carried out the nuclear resonance fluorescence verification experiment of elemental lithium by using the photons with hundreds of keV energy.
Subsequently, the MEGA-ray (mono energetic gamma rays) project was proposed to develop a compact, portable and high brightness quasi single energy laser based on X-band accelerator tube γ Ray source and development of applications based on nuclear resonance fluorescence are planned to be used for nuclear material supervision, nuclear waste detection and basic research of homeland security. Europe proposed the ELI-NP (Extreme Light Infrastructure—Nuclear Physics) project around 2010. It plans to use the high-quality electron beam with the highest energy of 720mev to interact with the laser with the wavelength of 512nm to produce a continuous energy adjustable energy of 0.2-19.5MeV γ Ray pulse, which is planned to be used in the fields of nuclear physics and astrophysical process simulation, advanced ray imaging, radioisotope production, positron generation and application. The project is now in the process of implementation.
Based on years of work accumulation, the Department of engineering and physics of Tsinghua University is building a 0.1-3MeV inverse Compton scattering single energy gamma ray source . At the same time, a compact gamma ray source scheme based on high gradient X-band acceleration structure is proposed to generate a quasi-single energy gamma ray source with adjustable continuous energy of 0.2-4MeV.
In the process of interaction with matter, femtosecond high-power laser can produce not only high-energy ions and electrons, but also high-energy γ photons. The current number of photons is limited by γ application of light source. If the all-optical gamma light source driven by laser is used, it will provide a new scheme for the generation of wide-spectrum gamma light source. The simulation shows that with the interaction of micro tube target and 10PW laser, the gamma photon yield is expected to reach the order of 10 ^ 14 / shot in theory, which may become a new scheme of MeV photon Collider in the future.
4. Development of broadband coherent light sources
Secondary photons in other energy regions can be produced by the interaction between super intense laser and matter. These secondary radiations have the characteristics of energy bandwidth (terahertz extreme ultraviolet), high brightness and short pulse time. It can be used in many fields, such as material characterization, ultrafast process research, biological imaging, high-resolution imaging and so on.
In the interaction between strong laser and solid target film target, high-energy MeV super hot electrons with good directivity will be produced, and the movement of super hot electrons will form a huge current (10-100ma, even GA). Some forward moving electrons will escape from the back of the target into the vacuum. When electrons pass through the target vacuum interface, transit radiation can occur due to the sudden change of dielectric properties. On the one hand, the length of ultrashort electron beam driven by femtosecond laser is often on the order of 10 microns or even shorter, which is less than the terahertz radiation wavelength. At this time, the transition radiation field of each electron can be coherently superimposed, and the total radiation energy is proportional to the square of the number of electrons, that is, coherent transition radiation.
On the other hand, the electric quantity of super-hot electron beam generated by strong laser solid target interaction can reach the order of nano Library (NC) or even higher, which is expected to produce terahertz radiation with energy of 100 micro joules or even mill joules. Two kinds of samples to be studied are pumped by ultra-strong terahertz radiation to realize the material nonlinear process under high-energy terahertz pumping. The specific detection methods include terahertz spectrum detection at room temperature and low temperature, single shot imaging and pump detection. More importantly, in order to study the ultrafast dynamic process of samples, ultrafast electron diffraction equipment will be developed. This is the first time in the world to combine intense laser driven terahertz radiation with ultrafast electron diffraction.
Extreme ultraviolet light source covers the wavelength range of about 100 nm to 3 nm in the electromagnetic spectrum. It is an important tool to study the structure and ultrafast dynamic process of various material systems such as atoms, molecules and condensates. In terms of wavelength characteristics, this spectral region covers the main resonance lines of atoms and the absorption edges of most elements with small and medium atomic numbers, which makes the use of extreme ultraviolet soft X-ray light source has a very unique advantage in the detection of various materials with element recognition; Moreover, due to the short wavelength, imaging with the radiation of this band can obtain higher spatial resolution, observe smaller structures, and lithography smaller patterns.
On the time scale, ultrashort pulses ranging from sub femtosecond to attosecond make it possible to track and detect the movement of electrons outside the atomic nucleus in real time, thus opening up a broad prospect of attosecond science and related applications. Coherent high brightness relativistic extreme ultraviolet pulses or isolated pulses can be generated by the interaction of high-intensity femtosecond pulse laser with gas molecules, solid surface and multilayer nano film targets. This pulse can be widely used in the research of material properties, ultrafast chemical process detection, ultrafast surface interface physics, high-resolution extreme ultraviolet imaging and other fields.
