XUV interferometry using high-order harmonics: Application to plasma diagnostics

In this paper, we present and compare the two different XUV interferometric techniques using high-order harmonics that have been developed so far. The first scheme is based on the interference between two spatially separated phase-locked harmonic sources while the second uses two temporally separated harmonic sources. These techniques have been applied to plasma diagnostics in feasibility experiments where electron densities up to a few 1020 e[minus sign/cm3 have been measured with a temporal resolution of 200 fs. We present the main characteristics of each technique and discuss their respective potentials and limitations

Gaussian-Schell analysis of the transverse spatial properties of high-harmonic beams

High harmonic generation (HHG) is a compact source of coherent, ultrafast soft x-ray radiation. HHG is increasingly being used as a source to image biological and physical systems. However, many imaging techniques such as coherent diffractive imaging, and ptychography require coherent illumination. Characterization the spatial coherence of HHG sources is vital if these sources are to kind widespread applications. Here a new method for characterizing coherent radiation is used to investigate the near- and far- field spatial properties of high harmonic radiation generated in a gas cell. The intensity distribution, wavefront curvature, and complex coherence factor are measured for a range of harmonic orders, and the Gaussian-Schell model is used to determine the properties of the harmonic beam in the plane of generation. Our results show the measured spatial properties of the harmonic beam are consistent with the finite spatial coherence of the driving laser beam as well as variations of the atomic dipole phase. These findings are used to suggest new approaches for controlling and optimizing the spatial properties of light for imaging applications

Inefficient Magnetic-Field Amplification in Supersonic Laser-Plasma Turbulence

We report a laser-plasma experiment that was carried out at the LMJ-PETAL facility and realized the first magnetized, turbulent, supersonic (Maturb 2.5) plasma with a large magnetic Reynolds number (Rm 45) in the laboratory. Initial seed magnetic fields were amplified, but only moderately so, and did not become dynamically significant. A notable absence of magnetic energy at scales smaller than the outer scale of the turbulent cascade was also observed. Our results support the notion that moderately supersonic, low-magnetic-Prandtl-number plasma turbulence is inefficient at amplifying magnetic fields compared to its subsonic, incompressible counterpart

Experimental mitigation of fast magnetic reconnection in multiple interacting laser-produced plasmas

The meeting of astrophysical plasmas and their magnetic fields creates many reconnection sites. We experimentally compare the reconnection rate of laser-driven magnetic reconnection when it takes place at a single site and multiple sites. For a single site, where the ram pressure dominates the magnetic pressure, the measured reconnection rate exceeds the well-established rate of 0.1. However, in the case of multiple close-by sites, we observed a reduction of the reconnection rate. Hybrid-PIC simulations support this observation and suggest that the distortion of the Hall field as well as the concomitant obstruction of one of the outflows are detrimental to the magnetic reconnection rate

Bremsstrahlung cannon design for Shock Ignition relevant regime

International audienc

Bremsstrahlung cannon design for Shock Ignition relevant regime

Diagnostics Plasma pour l'installation PETAL sur le LMJInitiative d'excellence de l'Université de BordeauxEUROfusion - Implementation of activities described in the Roadmap to Fusion during Horizon 2020 through a Joint programme of the members of the EUROfusion consortiumNuméro CORDIS : 63305

Numerical investigation of spallation neutrons generated from petawatt-scale laserdriven proton beams

International audienceLaser-driven neutron sources could offer a promising alternative to those based on conventional accelerator technologies in delivering compact beams of high brightness and short duration. We examine this through particle-in-cell and Monte Carlo simulations, that model, respectively, the laser acceleration of protons from thin-foil targets and their subsequent conversion into neutrons in secondary lead targets. Laser parameters relevant to the 0.5 petawatt (PW) LMJ-PETAL and 0.6-6 PW Apollon systems are considered. Due to its high intensity, the 20-fs-duration 0.6 PW Apollon laser is expected to accelerate protons up to above 100 MeV, thereby unlocking efficient neutron generation via spallation reactions. As a result, despite a 30-fold lower pulse energy than the LMJ-PETAL laser, the 0.6 PW Apollon laser should perform comparably well both in terms of neutron yield and flux. Notably, we predict that very compact neutron sources, of ~ 10 ps duration and ~ 100 µm spot size, can be released provided the lead convertor target is thin enough (~ 100 µm). These sources are characterized by extreme fluxes, of the order of 10$^{23}$ n cm$^{-2}$ s$^{-1}$ , and even ten times higher when using the 6 PW Apollon laser. Such values surpass those currently achievable at large-scale accelerator-based neutron sources (~ 10$^{16}$ n cm$^{-2}$ s$^{-1}$), or reported from previous laser experiments using low-Z converters (~ 10$^{18}$ n cm$^{-2}$ s$^{-1}$). By showing that such laser systems can produce neutron pulses significantly brighter than existing sources, our findings open a path towards attractive novel applications, such as flash neutron radiography or laboratory studies of heavy-ion nucleosynthesis

Numerical investigation of spallation neutrons generated from petawatt-scale laser-driven proton beams

Due to their high cost of acquisition and operation, there are still a limited number of high-yield, high-flux neutron source facilities worldwide. In this context, laser-driven neutron sources offer a promising, cheaper alternative to those based on large-scale accelerators, with, in addition, the potential of generating compact neutron beams of high brightness and ultra-short duration. In particular, the predicted capability of next-generation petawatt (PW)-class lasers to accelerate protons beyond the 100 MeV range should unlock efficient neutron generation through spallation reactions. In this paper, this scenario is investigated numerically through particle-in-cell and Monte Carlo simulations, modeling, respectively, the laser acceleration of protons from thin-foil targets and their subsequent conversion into neutrons in secondary heavy-ion targets. Laser parameters relevant to the 1 PW LMJ-PETAL and 1-10 PW Apollon systems are considered. Under such conditions, neutron fluxes exceeding $10^{23}\,\rm n\,cm^{-2}\,s^{-1}$ are predicted, opening up attractive fundamental and applicative prospects

Inefficient magnetic-field amplification in supersonic laser-plasma turbulence

We report a laser-plasma experiment that was carried out at the LMJ-PETAL facility and realized the first magnetized, turbulent, supersonic plasma with a large magnetic Reynolds number ($\mathrm{Rm} \approx 45$) in the laboratory. Initial seed magnetic fields were amplified, but only moderately so, and did not become dynamically significant. A notable absence of magnetic energy at scales smaller than the outer scale of the turbulent cascade was also observed. Our results support the notion that moderately supersonic, low-magnetic-Prandtl-number plasma turbulence is inefficient at amplifying magnetic fields