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Seminar

The 68th KPSI Seminar

Lecture by ELI-BL theory group

One of the most promising techniques to enhance focused laser intensities is to tightly focus the beam using an ellipsoidal plasma mirror. It has been successfully employed to obtain sub-μm spot size [1] resulting in an order of magnitude increase in focused intensity in comparison with a typical spot size of about 4 μm obtained with f/3 off axis parabolic mirror. The tightly focused laser beam has some important properties, which modify the laser target interaction process. First of all, it is associated with a large curvature of the wavefront of the laser beam out of focus, which has an influence on the propagation angle of accelerated particles. Secondly, the field is not purely transverse but it also includes non-negligible longitudinal field components. In particular the longitudinal electric field is very important as its amplitude is ~0.14λ₀/w₀ of the transverse field at the periphery of the focal spot, where l0 is the laser wavelength and w₀ the Gaussian beam waist.

The electron acceleration process under tight focusing is studied using PIC simulations with the code EPOCH [2]. The electrons are first pulled from the target surface by the longitudinal component of the laser electric field. In the next phase, the electrons are accelerated by the transverse laser electric field component. Depending on their distance from the target surface, they are accelerated either towards the center of the focal spot (electrons close to the target surface) or in the opposite direction as the transverse component of the laser electric field has a different phase in these two regions. Finally, the v×B Lorentz force accelerates these electrons into the target or into the vacuum away from the target and they propagate further ballistically. As the longitudinal field has an opposite phase on both sides of the focal spot, the bunches originating from this process are phase shifted by half a laser period and they propagate in different directions because of the wavefront curvature. This opens the possibility to diagnose the absorption process with coherent transition radiation (CTR) at the rear side of the target, where the bunches arrive at a rate given by the laser frequency (ω₀) in contrast with the simple v×B heating process where the rate is given by 2ω₀.

This work is supported by Czech Science Foundation project 18-09560S and European Regional Development Fund - Project "CAAS" (No. CZ.02.1.01/0.0/0.0/16_019/0000778).

Our work is supported by projects High Field Initiative (CZ.02.1.01/0.0/0.0/15 003/0000449) and Extreme Light Infrastructure Tools for Advanced Simulation (CZ.02.1.01/0.0/0.0/16_013/0001793) from the European Regional Development Fund and by Czech Science Foundation (18-09560S).

References:

[1] M. Nakatsutsumi, A. Kon, S. Buffechoux et al., Opt. Lett. 35, 2314–2316 (2010).
[2] T. D. Arber, K. Bennett, C. S. Brady et al., Plasma Phys. Control. Fusion 57, 1–26 (2015).

The laser-plasma interactions are dominated by the QED regime since intensities of the forthcoming laser facilities are approaching 10^23-24 W/cm². Here we present the high brightness γ- photon emission and e⁺e^- pair creation accompanied with the high harmonic generation. Relativistic oscillating mirror reflects the incident intense laser field and generates the focused attosecond pulse with enhanced intensity. A large number of high energy photons are emitted by the collisions between the radiation trapped electrons and the high harmonic pulses. The corresponding photons are counter-propagating through the strong laser field which provides a large cross section for pair creation. Relativistic positron bunches are generated and further accelerated in the reflected laser field. The peak intensity of the γ-ray reaches 0.74 PW with the brilliance of 2×10²⁴ s⁻¹mm⁻² mrad⁻² (0.1%BW)⁻¹ (at 58 MeV). A GeV positron beam is obtained with density of 4×10²¹ cm⁻³ and a particle number of 5.6 × 10⁹.

This work was supported by the project ELITAS (CZ.02.1.01/0.0/0.0/16_013/0001793) and by the project High Field Initiative (CZ.02.1.01/0.0/0.0/15_003/0000449) both from European Regional Development Fund.

References:

[1] Y. J. Gu, O. Klimo, S. V. Bulanov, S. Weber, Brilliant gamma-ray beam and electron–positron pair production by enhanced attosecond pulses, Communications Physics, 1, 93 (2018)
[2] Y. J. Gu, S. Weber, Intense, directional and tunable γ-ray emission via relativistic oscillating plasma mirror, Opt. Express, 26, 19932 (2018).

With the advent of multi-petawatt laser systems like the ELI-Beamlines (Czech Republic), APOLLON (France) and SEL (China) the laser-driven ion accelerators will enter the acceleration regimes dominated by radiation pressure [1]. High quality ion beams with low emittance and narrow energy spectrum will be generated when these lasers irradiate tailored targets.

Below we present the results of studying the effects of the interface modulations in double layer targets. The numerical particle-in-cell simulations with the code EPOCH [2] are used. We show that the pre-modulated targets can undergo relativistic Rayleigh-Taylor [3] and Richtmyer- Meshkov instabilities. Their use can improve the properties of generated ion beams [4].

It is shown that small perturbations originated from the interface modulation grow during the laser-target interaction. This leads to the formation of low-density regions and high-density ion bunches between them at the positions determined by the pre-modulation geometry. The ion bunches are then accelerated by the laser radiation pressure. The collimated central bunch of proton beam has the average energy in the multi-GeV range with narrow energy spread. The laser accelerated ion beams from composite targets will find applications in nuclear physics research [5].

Our work is supported by projects High Field Initiative (CZ.02.1.01/0.0/0.0/15 003/0000449) and Extreme Light Infrastructure Tools for Advanced Simulation (CZ.02.1.01/0.0/0.0/16_013/0001793) from the European Regional Development Fund and by Czech Science Foundation (18-09560S).

References:

[1] T. Esirkepov, M. Borghesi, S. V. Bulanov et al., Phys. Rev. Lett. 92, 175003 (2004).
[2] T. D. Arber, K. Bennett, C. S. Brady et al., Plasma Phys. Control. Fusion 57, 113001 (2015).
[3] F. Pegoraro and S. V. Bulanov, Phys. Rev. Lett. 99, 065002 (2007).
[4] S. V. Bulanov, E. Y. Echkina, T. Z. Esirkepov et al, Phys. Rev. Lett. 104, 135003 (2010).
[5] M. Nishiuchi, H. Sakaki, T. Z. Esirkepov et al, Plasma Phys. Rep. 42, 327 (2016).

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