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The objective of the Apollon 10 PW project is the generation of 10 PW peak power pulses of 15 fs at $1~\\text{shot}~\\text{min}^{-1}$. In this paper a brief update on the current status of the Apollon project is presented, followed by a more detailed presentation of our experimental and theoretical investigations of the temporal characteristics of the laser. More specifically the design considerations as well as the technological and physical limitations to achieve the intended pulse duration and contrast are discussed.

The objective of the Apollon project is the generation of 10 PW peak power pulses of 15 fs at 1 shot/minute. In this paper the Apollon facility design, the technological challenges and the current progress of the project will be presented.

The physical mechanism of the dynamics in laser–material interaction has been an important research area. In addition to theoretical analysis, direct imaging‐based observation of ultrafast dynamic processes is an important approach to understand many fundamental issues in laser–material interaction such as inertial confinement fusion (ICF), laser accelerator construction, and advanced laser production. In this review, the principles and applications of three types of commonly used ultrafast imaging methods are introduced, including the pump–probe, X‐ray diagnosis, and single‐shot optical burst imaging. We focus on the technical features such as the spatial and temporal resolution for each technique, and present several conventional applications.

While major progress has been made in the research of inertial confinement fusion, significant challenges remain in the pursuit of ignition. To tackle the challenges, we propose a double-cone ignition (DCI) scheme, in which two head-on gold cones are used to confine deuterium–tritium (DT) shells imploded by high-power laser pulses. The scheme is composed of four progressive controllable processes: quasi-isentropic compression, acceleration, head-on collision and fast heating of the compressed fuel. The quasi-isentropic compression is performed inside two head-on cones. At the later stage of the compression, the DT shells in the cones are accelerated to forward velocities of hundreds of km s –1 . The head-on collision of the compressed and accelerated fuels from the cone tips transfer the forward kinetic energy to the thermal energy of the colliding fuel with an increased density. The preheated high-density fuel can keep its status for a period of approximately 200 ps. Within this period, MeV electrons generated by ps heating laser pulses, guided by a ns laser-produced strong magnetic field further heat the fuel efficiently. Our simulations show that the implosion inside the head-on cones can greatly mitigate the energy requirement for compression; the collision can preheat the compressed fuel of approximately 300 g cm −3 to a temperature above keV. The fuel can then reach an ignition temperature of greater than 5 keV with magnetically assisted heating of MeV electrons generated by the heating laser pulses. Experimental campaigns to demonstrate the scheme have already begun. This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.

We report on the temporal contrast performance of the PHELIX facility in view of the requirements imposed by solid-target interaction experiments. The requirement analysis for the nanosecond and picosecond temporal contrast is derived from empirical data and simple theoretical modeling, while the realization shows that using an ultrafast optical parametric amplifier and plasma mirrors enables meeting this specification.

A differential B-dot probe is designed to diagnose the pulsed magnetic field on the Shenguang-II high-power laser facility. Differential B-dot pair is processed to cancel out common mode noise. On the Shenguang-II facility, a laser pulse with 0.5 kJ energy and the pulse width of 1.0 ns is shot onto a copper planar target, which generates plasma and a pulsed circular self-generated magnetic field. A negative exponential power law between the magnetic field and the distance away from targeting focus is confirmed by the B-dot pair. The negative exponent is – 1.75. Near the laser focus, a magnetic field approximating megagauss can be expected.

The influence of laser intensity on the resistive filamentation during intense electron beam transport in aluminum targets is investigated through theoretical analysis and three-dimensional hybrid simulations. Analysis of linear filamentation instabilities indicates that resistive filamentation is more likely to occur under high laser-intensity conditions. Our simulations reveal the underlying physical mechanisms: when the laser intensity is below a critical value (corresponding to the collimation parameter Πrc=1), the strong self-generated resistive field effectively suppresses electron beam filamentation, resulting in collimated transport. As the laser intensity increases, the enhanced current density, the larger divergence angle of the electron beam, and higher laser energy conversion efficiency counteract the pinching effect of the resistive magnetic field. This leads to a significant filamentation, with the current magnitude of each filament approaching the Alfvén current limit. Additionally, our study demonstrates that higher energy conversion efficiency increases the incident current density, thereby promoting filamentation instability, while larger initial divergence angles weaken the magnetic collimation effect, further enhancing filamentation.

Early hot electron can preheat the pellet fuel and thus lead to lower implosion performance. The properties of hot electrons in early stage of implosion experiments in Shenguang-100 kJ laser facility were investigated. It was shown that both the temperature and the energy of early hot electrons were very low. The upper limit of the temperature and the energy of early hot electrons in our experiments were only 7.7 keV and 0.35 J, respectively. Besides, the generation mechanisms of early hot electrons were also different from NIF experiments according to the results of the hard X-ray imager (HXI). In NIF experiments, two-plasmon decay and multi-beam stimulated Raman scattering (SRS) were dominate mechanisms that generate early hot electrons. However, SRS of the outer beams was our dominant mechanism. Spectrum of the scattered light of SRS was obtained by radiative hydrodynamic and ray-tracing simulations. The result showed that the spectrum was peaked at λs=482nm, which meant hot electrons with the temperature near 7keV can be generated. And from the result of HXI, hot electrons deposited onto the pellet were estimated to less than 6.8×10-3 J. Deeper analysis showed that, in the beam overlapping region, the plasma density was unsuitable for multi-beam SRS, so no hot electrons with larger temperature were generated.

The impressive progress in high-powered lasers has resulted in all-optical nonlinear inverse Compton scattering emerging as a potential method for generating ultra-short, brilliant γ ray in a remarkably compact setup. Nonetheless, the conversion efficiency and energy of currently implemented Compton γ-ray sources are still low. We present three-dimensional particle-in-cell simulations investigating the γ-ray emission resulting from the interaction of a femtosecond laser pulse (I=5×1021W/cm2) with a down-ramp density plasma. Our study reveals that a down-ramp density plasma affects the self-injection of electrons, resulting in a lower self-injection threshold. Consequently, more electrons can be trapped in the wakefield for acceleration. The simulation results demonstrate the production of high-energy γ ray with a maximum energy of Eγ,max = 148.18 MeV and a low emittance of θγ = 4.2 mm·mrad. Compared to the scheme without down-ramp density plasma, the conversion efficiency of laser energy to photons is improved from approximately 0.13 to 0.29%. With this scheme, we can avoid using high-power laser pulses and generate high-energy γ ray by using shaped-intensity laser pulses. This broadens the application range of all-optical Compton scattering.

We put forward a novel method to enhance and collimate terahertz waves by illuminating a tube target with a linearly polarized femtosecond laser. This target comprises a longitudinal channel, followed by a transverse flat target. The findings show that the interaction between the laser and the channel significantly reduces the transverse divergence of electrons while boosting their maximum cutoff energy. Consequently, the terahertz waves undergo effective collimation and enhanced intensity. Employing two-dimensional particle-in-cell simulations, we demonstrate that when the length and diameter of the channel are set to 90.0λ0 and 12.0λ0, the pointing angles of the terahertz waves are about - 5.7∘ and 6.6∘, respectively. Furthermore, the study delves into the individual impacts of laser and target parameters on the enhancement and collimation of terahertz waves. This method may serve as a novel pathway for enhancing and collimating terahertz radiation through the laser–solid interactions, offering a reliable and efficient solutions for terahertz radiation applications.