Recently, the Institute of Applied Physics of the Russian Academy of Sciences introduced the eXawatt Center for Extreme Light Study (XCELS), a research program for large scientific devices based on extremely high power lasers. The project includes the construction of a very high power laser based on optical parametric chirped pulse amplification technology in large aperture potassium dideuterium phosphate (DKDP, chemical formula KD2PO4) crystals, with an expected total output of 600 PW peak power pulses. This work provides important details and research findings about the XCELS project and its laser systems, describing applications and potential impacts related to ultra-strong light field interactions.

The XCELS program was proposed in 2011 with the initial goal of achieving a peak power laser pulse output of 200 PW, which is currently upgraded to 600 PW. Its laser system relies on three key technologies:
(1) Optical Parametric Chirped Pulse Amplification (OPCPA) technology is used instead of traditional Chirped Pulse Amplification (Chirped Pulse Amplification, OPCPA). CPA) technology;
(2) Using DKDP as the gain medium, ultra wideband phase matching is realized near 910 nm wavelength;
(3) A large aperture neodymium glass laser with a pulse energy of thousands of joules is used to pump a parametric amplifier.

Ultra-wideband phase matching is widely found in many crystals and is used in OPCPA femtosecond lasers. DKDP crystals are used because they are the only material found in practice that can be grown to tens of centimeters of aperture and at the same time have acceptable optical qualities to support the amplification of multi-PW power lasers. It is found that when the DKDP crystal is pumped by the double frequency light of the ND glass laser, if the carrier wavelength of the amplified pulse is 910 nm, the first three terms of the Taylor expansion of the wave vector mismatch are 0.

Figure 1 is a schematic layout of the XCELS laser system. The front end generated chirped femtosecond pulses with a central wavelength of 910 nm (1.3 in Figure 1) and 1054 nm nanosecond pulses injected into the OPCPA pumped laser (1.1 and 1.2 in Figure 1). The front end also ensures the synchronization of these pulses as well as the required energy and spatiotemporal parameters. An intermediate OPCPA operating at a higher repetition rate (1 Hz) amplifies the chirped pulse to tens of joules (2 in Figure 1). The pulse is further amplified by the Booster OPCPA into a single kilojoule beam and divided into 12 identical sub-beams (4 in Figure 1). In the final 12 OPCPA, each of the 12 chirped light pulses is amplified to the kilojoule level (5 in Figure 1) and then compressed by 12 compression gratings (GC of 6 in Figure 1). The acousto-optic programmable dispersion filter is used in the front end to precisely control group velocity dispersion and high order dispersion, so as to obtain the smallest possible pulse width. The pulse spectrum has a shape of nearly 12th-order supergauss, and the spectral bandwidth at 1% of the maximum value is 150 nm, corresponding to the Fourier transform limit pulse width of 17 fs. Considering the incomplete dispersion compensation and the difficulty of nonlinear phase compensation in parametric amplifiers, the expected pulse width is 20 fs.

The XCELS laser will employ two 8-channel UFL-2M neodymium glass laser frequency doubling modules (3 in Figure 1), of which 13 channels will be used to pump the Booster OPCPA and 12 final OPCPA. The remaining three channels will be used as independent nanosecond kilojoule pulsed laser sources for other experiments. Limited by the optical breakdown threshold of the DKDP crystals, the irradiation intensity of the pumped pulse is set to 1.5 GW/cm2 for each channel and the duration is 3.5 ns.

Each channel of the XCELS laser produces pulses with a power of 50 PW. A total of 12 channels provide a total output power of 600 PW. In the main target chamber, the maximum focusing intensity of each channel under ideal conditions is 0.44×1025 W/cm2, assuming that F/1 focusing elements are used for focusing. If the pulse of each channel is further compressed to 2.6 fs by post-compression technique, the corresponding output pulse power will be increased to 230 PW, corresponding to the light intensity of 2.0×1025 W/cm2.

To achieve greater light intensity, at 600 PW output, the light pulses in the 12 channels will be focused in the geometry of inverse dipole radiation, as shown in Figure 2. When the pulse phase in each channel is not locked, the focus intensity can reach 9×1025 W/cm2. If each pulse phase is locked and synchronized, the coherent resultant light intensity will be increased to 3.2×1026 W/cm2. In addition to the main target room, the XCELS project includes up to 10 user laboratories, each receiving one or more beams for experiments. Using this extremely strong light field, the XCELS project plans to carry out experiments in four categories: quantum electrodynamics processes in intense laser fields; The production and acceleration of particles; The generation of secondary electromagnetic radiation; Laboratory astrophysics, high energy density processes and diagnostic research.

FIG. 2 Focusing geometry in the main target chamber. For clarity, the parabolic mirror of beam 6 is set to transparent, and the input and output beams show only two channels 1 and 7

Figure 3 shows the spatial layout of each functional area of the XCELS laser system in the experimental building. Electricity, vacuum pumps, water treatment, purification and air conditioning are located in the basement. The total construction area is more than 24,000 m2. The total power consumption is about 7.5 MW. The experimental building consists of an internal hollow overall frame and an external section, each built on two decoupled foundations. The vacuum and other vibration-inducing systems are installed on the vibration-isolated foundation, so that the amplitude of the disturbance transmitted to the laser system through the foundation and support is reduced to less than 10-10 g2/Hz in the frequency range of 1-200 Hz. In addition, a network of geodesic reference markers is set up in the laser hall to systematically monitor the drift of the ground and equipment.

The XCELS project aims to create a large scientific research facility based on extremely high peak power lasers. One channel of the XCELS laser system may provide a focused light intensity several times higher than 1024 W/cm2, which can be further exceeded by 1025 W/cm2 with post-compression technology. By dipole-focusing pulses from 12 channels in the laser system, an intensity close to 1026 W/cm2 can be achieved even without post-compression and phase locking. If the phase synchronization between channels is locked, the light intensity will be several times higher. Using these record-breaking pulse intensities and the multi-channel beam layout, the future XCELS facility will be able to perform experiments with extremely high intensity, complex light field distributions, and diagnose interactions using multi-channel laser beams and secondary radiation. This will play a unique role in the field of super-strong electromagnetic field experimental