Marriage of a 20keV superconducting XFEL with a 100PW laser
A new regime of science at exawatts and zeptoseconds
The Chinese national science and technology major infrastructure, Shanghai Coherent Light Facility (SCLF), organized an international review meeting for the Station of Extreme Light (SEL) in Shanghai on July 10, 2017. The reviewing committee members included experts in strong-field laser physics, high-energy-density physics, and theoretical physics from Germany, USA, UK, France, Japan, Canada; and China chaired by R. Sauerbrey and N .Wang. The working group, led by Ruxin Li of the Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS), has made a series of breakthroughs on high energy, high power, and high-repetition laser system development.
Reflecting on this, the Review Committee Report1 stated: "The architecture of the laser system of the Optical Parametric Chirped Pulse Amplification (OPCPA) and its interaction with the XFEL are well thought out. The proposed 1023 W/cm2 peak laser power is feasible. The working group has made a series of breakthroughs on high-power laser technologies in the past decades. Their constant effort has resulted in valuable experience, outstanding engineering skills, and international recognition for the group. Their strong track record has laid a strong foundation, which will provide the basis for successful construction of the 100 PW laser system."
Based on this, the Committee applauded the work, stating: "The Station of Extreme Light at Shanghai Coherent Light Facility is dedicated to cutting-edge research in strong field science and applications. This includes, for example, astrophysics, nuclear physics, cosmology, and matter under extreme conditions. The combination of the hard XFEL and the world-leading 100PW laser in SEL will initiate exploration of effects such as vacuum birefringence, one of the most prominent strong-field QED effects, acceleration mechanisms leading to ultra-high energy cosmic rays, simulation of black hole physics, and generation of new forms of matter."
The developments proposed are based on solid research carried out at SIOM (and other scientific organizations). In particular, the research and development of the OPCPA laser amplifier at the highest power level at SIOM. Shown in Figure 1 is SIOM's 10PW laser CPA device and the 10PW laser system. The 10PW laser system, Shanghai Superintense-Ultrafast Lasers Facility (SULF), is based on CPA technology and the diameter of the Ti-Sapphire used in the main amplifier is 235mm, which is the largest crystal for the laser manufactured by the scientists at SIOM.
Based on these developments, SIOM has launched a 100PW laser system, Station for Extreme Light (SEL). This system has two significant salient features. First, the level of its power will be an order of magnitude beyond the planned highest-powered laser, Extreme Light Infrastructure (ELI). Secondly, its design is a combination of the 100PW laser as part of the system in the SCLF's XFEL. This project received strong endorsement from the International Review Meeting that convened at SCLF of SIOM on July 10, 2017, and was approved by the Government of China. The overall funding level is approximately USD$1.3 Billion.
Extreme field regime
The parameters of SEL are well beyond what has so far been available. Table 1 shows typical principal physical parameters. The coherent x-ray energy from the SCLF ranges between 3 to 15 keV (hard x-rays) produced from the superconducting x-ray free electron laser (XFEL). The photon number per pulse of this XFEL is 1012. Its pulse focusability is 200nm with the energy resolution of 0.6eV. The x-ray's intensity at focus is as high as 1021W/cm2.
The parameters of the 100PW laser for optical photons are as follows: Its peak power is 100PW, while its focal intensity is as high as 1023W/cm2. (If we can managed to focus better than this, it could go toward 1025W/cm2). While this is a single shot performance, it could deliver the repetition rate of 1Hz of optical laser if the power is at 0.1 to 1PW.
These parameters by themselves are exciting. However, their coexistence and marriage as a combined unit shows a remarkable capability for future scienctific exploration. The combination of a synchrotron light source and an intense laser was first suggested and conducted in 1990s. Toshiki Tajima suggested that Professor Mamoru Fujiwara at Osaka University make use of the high-energy (8GeV) electrons of the SPRing-8 combined with an intense laser to make extremely high-energy gamma photons, which he did in his lab.2 Since then, the combination of these accelerator-based synchrotron light sources (or even more advanced XFEL with intense lasers) have come a long way. The present SCLF's marriage of these two will uncover a new regime of science and greatly impact various technologies and applications, such as nuclear photonics and nonlinear interferometry.
Table 1 shows the schematic layout of the SEL. The interaction of XFEL and the plasma chamber takes place in the experimental area. Figure 3 indicates the 100PW laser based on the OPCPA technology.
Figure 2: Schematic layout figure of SEL that couples the 100PW laser with the XFEL.
Figure 3: Details of the amplification stages of the 100PW laser based on OPCPA.
The scheme of this marriage is seen in the concept of the SEL at which the coherent high-energy x-rays photons are shone in the configuration shown in Figure 2. This way we will be able to observe the interaction of the high-energy x-ray photons and most intense lasers and their developed matter interaction. This will greatly increase the experimental probe of intense laser-matter interaction. The XFEL beam will provide ultra-short MHz x-ray beam with energy range of 3-5keV and significantly large photon number of 1012. Specific x-ray energy of 12.914keV will be used for QED experiments with very low energy spread of 0.6eV. The x-ray beam will collide head-on with the 100PW laser pulse in the experimental chamber. The 100PW laser system contains four beams and each beam reaches the peak power of 25 PW.
Figure 2 shows that the main laser system will occupy two floors and its power supply and control system are set at different floors. After the four-beam combination, the laser pulse will be sent to the experimental area on the bottom floor. There is a large-size vacuum chamber, where the 100PW laser pulse will be focused to 5μm and collide with the x-ray beam.
Details of the 100PW laser system are shown in Figure 3. At the core is the OPCPA system. The 100PW laser pulse starts at high temporal laser source, where its temporal synchronization signal comes from the XFEL beam. This source will generate high-quality seed pulses, which will go into the PW level repetition-rate OPCPA front-end. The laser energy will reach 25J and its spectrum width will support 15fs at PW level OPCPA front-end.
The main amplifier is based on OPCPA technology and it provides 99% energy gain of the whole laser system, which requires sufficient pump energy from a Nd Glass pump laser. The final optics assembly will compress the high-energy of 2500J 4ns laser pulse to 15fs. After the compression, the laser pulse will be sent into the experimental chamber with the peak intensity 1023 W/cm2. As shown in Figure 1, we developed and tested the performance of a high-intensity laser with CPA up to 10PW level.
High Field Science
The proposed SEL aims to achieve the ultimate in high field science3,4,5. Here, we describe a simple way to reach that goal.
The radiation dominance regime (1023 W/cm2) as described in Ref. 2 may be accessible and experimentally explored for the first time in sufficient details with the help of the coherent X-ray probe. As discussed in Sec. 1, if one can focus a bit narrowly, we may be able to enter the so-called QED Quantum regime (~1024 W/cm2)4,5.
The particle acceleration by laser will enter a new regime. The wakefield generation6 becomes so nonlinear that it enters what is sometimes called the bow-wake regime7. This may be relevant to the astrophysical extreme high-energy cosmic ray genesis by AGN (active galactic nuclei) jets 8. In this regime, the physics of wakefield acceleration and that of the radiation pressure acceleration begin to merge (1023W/cm2)9,10. Thus, the laser pulse should be able to pick up ions as well as electrons to become accelerated. Soon or later, the energy of ions begins to exceed that of electrons and their acceleration should become as coherent as the electron acceleration in this regime. Such acceleration will allow ion accelerators to be smaller. (A broader scope at this regime and slightly higher intensity regime than just mentioned has been reviewed9.)
However, it could go much further than that, since the invention of a new compression technique called "thin film compression11." With this technique, a laser may be compressed to even higher power and intensity such as EW and further by relativistic compression into the shortest possible pulses ever in zs12. We will thus see the continuous manifestation of the Intensity-Pulse Duration Theorem into the extension of EW and zs13. It will not only explore strong field QED physics14,15, but we will also see the emergence of new phenomena at play in a wider variety of fundamental physics, including: (1) possible search of the proposed "fifth force"16,17; (2) dark matter search by four wave mixing18; (3) x-ray wakefield in solid state matter19 and related x-ray and optical solid state plasmonics20; (4) possible testing of the energy dependence of gamma photon propagation speed in a vacuum to test the foundational assumption of the Theory of Special Theory of Relativity21; and (5) zeptosecond streaking of the QED process22.
Chen et al.23 suggested the exploration of general relativity using the equivalence principle of acceleration-gravity to test the Hawking-Unruh process.
Gamma-ray diagnosis and the marriage of XFEL and HFS
In the issues of high field science, we often enter into the physical processes in higher energies and shorter timescales, which may not be easily resolvable in optical diagnosis. Here, the powerful XFEL's resolution in time and space come in24. X-rays can be also signatures in high intensity experiments such as laser-driven acceleration experiments 25. A typical display of such interplay may be seen in the diagnostics of the physical processes in the problem of x-ray wakefield acceleration in solid-state matter. In this case, nanoscopic materials with a nanohole structure20 need to be observed and controlled. The surface of the nanotubes may be exhibiting surface plasmons and polaritons in nanometer size and zs temporal dynamics, best diagnosed by the XFEL. This is but an example of the marriage of a 20keV superconducting XFEL and a 100PW laser. In addition this technology will enhance studies in photon-induced nuclear physics26 and the treatment of nuclear materials27 (including nuclear waste), nuclear pharmacology, nuclear biochemistry, and medicine28,29.
Another example is to use gamma photons to mediate the vacuum nonlinearity caused by intense laser pulse to exploit zeptosecond streaking via the gamma photon mediation22. In this scheme the presence of intense laser pulse and x-ray photon play a crucial role. If this example elucidates a beginning of exploration of zeptosecond photometric and zeptosecond optics, it would be an achievement comparable of the opening of the femtosecond optics flowing by attosecond optics30.
One more example of exploring the proposition was recently made for the Fifth Force17. In the Hungarian nuclear experiment, a mysterious photon at the energy of 17MeV was observed. The paper5 suggested this emission of gamma photon may be due to the unknown force (the Fifth Force). It may be helpful if we can inject a large amount of monoenergetic photons at this energy to see if the reversal of this process of photon emission (i.e. injection of photon) can explore this process more quantitatively. We can check of the fifth force (17MeV gamma)16,17,31 with the process and an outcome of the following, utilizing the energy specific laser induced gamma photon interaction: e + 17MeV gamma → e + X.
Finally, there is a recent suggestion by Day and Fairbairn32 that XFEL laser pulses at 3.5keV may be used to investigate the astrophysically observed x-ray excess by fluorescent dark matter. Such an avenue may open up with this device. Such an effort along with the astrophysical observations may become an important interdisciplinary development.
In order to maximize the success of these implications, we recommend the formation of a broad international collaboration with the organizations and institutions that are engaging in related fields. Learning from these labs in their technologies, practice, and collaborative engagements should reduce risks and duplications and enhance learning and the scope of experience. Collaborations with a variety technology sectors are important both for the execution of experiments and their applications.
The authors are grateful for close discussions with all the committee members (Naiyan Wang, Roland Sauerbrey, Pisin Chen, See Leang Chin, Thomas Edward Cowan, Thomas Heinzl, Yongfeng Lu, Gerard Mourou, Edmond Turcu, Hitoki Yoneda, Lu Yu) of SEL. The discussions with Profs. T. Tait, K. Abazajian, T. Ebisuzaki, and K. Homma were also very useful. Prof. X. M. Zhang helped with our manuscript.
Toshiki Tajima is Norman Rostoker Chair Professor of the Department of Physics and Astronomy of University of California at Irvine and Chief Science Officer of Tri Alpha Energy, Inc. He served as Blumberg Professor of University of Texas at Austin, Director General of Kansai Photon Science Institute of JAEA, among other positions after his graduation from UCI with PhD in 1975. He is a Fellow of APS and Full Academician (foreign member) of Russian Academy of Science, and has been awarded with the Enrico Fermi Prize, Nishina Prize, Blaire Pascal Chair, Einstein Professorship of CAS, among other honors.
Ruxin Li is the Director of Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (SIOM, CAS), the Vice President of Shanghai Advanced Research Institute, CAS, and the Vice president of Chinese Optical Society. He is an Academician of CAS and a Fellow of OSA. His research fields include the laser technique of ultra-high peak power, laser driven particle acceleration and application, laser fusion, and generation and application of attosecond laser pulse, etc.
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32. F. Day and M. Fairbairn, submitted to J. High Energy Phys., 2017.
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