X-ray harmonic comb from relativistic electron spikes

X-ray devices are far superior to optical ones for providing nanometre spatial and attosecond temporal resolutions. Such resolution is indispensable in biology, medicine, physics, material sciences, and their applications. A bright ultrafast coherent X-ray source is highly desirable, for example, for the diffractive imaging of individual large molecules, viruses, or cells. Here we demonstrate experimentally a new compact X-ray source involving high-order harmonics produced by a relativistic-irradiance femtosecond laser in a gas target. In our first implementation using a 9 Terawatt laser, coherent soft X-rays are emitted with a comb-like spectrum reaching the 'water window' range. The generation mechanism is robust being based on phenomena inherent in relativistic laser plasmas: self-focusing, nonlinear wave generation accompanied by electron density singularities, and collective radiation by a compact electric charge. The formation of singularities (electron density spikes) is described by the elegant mathematical catastrophe theory, which explains sudden changes in various complex systems, from physics to social sciences. The new X-ray source has advantageous scalings, as the maximum harmonic order is proportional to the cube of the laser amplitude enhanced by relativistic self-focusing in plasma. This allows straightforward extension of the coherent X-ray generation to the keV and tens of keV spectral regions. The implemented X-ray source is remarkably easily accessible: the requirements for the laser can be met in a university-scale laboratory, the gas jet is a replenishable debris-free target, and the harmonics emanate directly from the gas jet without additional devices. Our results open the way to a compact coherent ultrashort brilliant X-ray source with single shot and high-repetition rate capabilities, suitable for numerous applications and diagnostics in many research fields

generation and transport of high-current relativistic electron beams in high intensity laser-solid interactions

In this thesis, the generation and transport of ultra-high intensity laser-driven relativistic electron beams in overdense plasma is investigated experimentally and numerically. The fast electron beam is experimentally diagnosed by means of a 2D Cu Kα imager and the TNSA-generated proton beam. Analytical models together with a 3D hybrid-PIC code are employed to simulate the beam properties in solids. The effects of the self-generated fields on the fast electron beam transport, the effect of the preplasma density scale length on the laser energy coupling to fast electrons and the influence of the laser spot size on the fast electron beam generation and transport, and on the subsequent proton beam, are reported. Fast electron injection and transport in metal foils irradiated at laser intensity up to 4 x 10²⁰ W/cm², is investigated . The beam transport is simulated over a wide range of beam source conditions and with or without inclusion of selfgenerated magnetic fields. The resulting hot electron beam properties are used in rear-surface plasma expansion calculations to compare with measurements of the beam of accelerated protons. An injection half-angle of ~ 50° - 70° is inferred, which is larger than that derived from previous experiments under similar conditions. The influence of laser spot size on laser energy coupling to electrons, and subsequently to the TNSA-generated protons, in foil targets is reported. Proton acceleration is characterized for laser intensities ranging from 2 x 10¹⁸ - 6 x 10²⁰ W/cm², by variation of the laser energy for a fixed spot size, and by variation of the spot size for a fixed energy. At a given laser pulse intensity, the maximum proton energy is higher under defocus illumination compared to tight focus. The results are explained in terms of higher laser pulse energy and geometrical changes to the hot electron injection. The laser-to-electron energy conversion efficiency is investigated in metal foils over a wide range of preplasma density scale lengths. A hybrid-PIC code is employed to model the fast electron beam transport in the solid, for a given hot electron source. The resulting fast electron density is used to infer the maximum proton energy for comparison with experimental results. It is shown, in agreement with previous published work, that some preplasma density scale length leads to an enhancement of the energy coupling efficiency of laser light to fast electrons.In this thesis, the generation and transport of ultra-high intensity laser-driven relativistic electron beams in overdense plasma is investigated experimentally and numerically. The fast electron beam is experimentally diagnosed by means of a 2D Cu Kα imager and the TNSA-generated proton beam. Analytical models together with a 3D hybrid-PIC code are employed to simulate the beam properties in solids. The effects of the self-generated fields on the fast electron beam transport, the effect of the preplasma density scale length on the laser energy coupling to fast electrons and the influence of the laser spot size on the fast electron beam generation and transport, and on the subsequent proton beam, are reported. Fast electron injection and transport in metal foils irradiated at laser intensity up to 4 x 10²⁰ W/cm², is investigated . The beam transport is simulated over a wide range of beam source conditions and with or without inclusion of selfgenerated magnetic fields. The resulting hot electron beam properties are used in rear-surface plasma expansion calculations to compare with measurements of the beam of accelerated protons. An injection half-angle of ~ 50° - 70° is inferred, which is larger than that derived from previous experiments under similar conditions. The influence of laser spot size on laser energy coupling to electrons, and subsequently to the TNSA-generated protons, in foil targets is reported. Proton acceleration is characterized for laser intensities ranging from 2 x 10¹⁸ - 6 x 10²⁰ W/cm², by variation of the laser energy for a fixed spot size, and by variation of the spot size for a fixed energy. At a given laser pulse intensity, the maximum proton energy is higher under defocus illumination compared to tight focus. The results are explained in terms of higher laser pulse energy and geometrical changes to the hot electron injection. The laser-to-electron energy conversion efficiency is investigated in metal foils over a wide range of preplasma density scale lengths. A hybrid-PIC code is employed to model the fast electron beam transport in the solid, for a given hot electron source. The resulting fast electron density is used to infer the maximum proton energy for comparison with experimental results. It is shown, in agreement with previous published work, that some preplasma density scale length leads to an enhancement of the energy coupling efficiency of laser light to fast electrons

Unraveling resistive versus collisional contributions to relativistic electron beam stopping power in cold-solid and in warm-dense plasmas

We present results on laser-driven relativistic electron beam propagation through aluminum samples, which are either solid and cold or compressed and heated by laser-induced shock. A full numerical description of fast electron generation and transport is found to reproduce the experimental absolute Kα yield and spot size measurements for varying target thicknesses, and to sequentially quantify the collisional and resistive electron stopping powers. The results demonstrate that both stopping mechanisms are enhanced in compressed Al samples and are attributed to the increase in the medium density and resistivity, respectively. For the achieved time- and space-averaged electronic current density, ⟨jh⟩∼8×1010 A/cm2 in the samples, the collisional and resistive stopping powers in warm and compressed Al are estimated to be 1.5 keV/μm and 0.8 keV/μm , respectively. By contrast, for cold and solid Al, the corresponding estimated values are 1.1 keV/μm and 0.6 keV/μm . Prospective numerical simulations involving higher jh show that the resistive stopping power can reach the same level as the collisional one. In addition to the effects of compression, the effect of the transient behavior of the resistivity of Al during relativistic electron beam transport becomes progressively more dominant, and for a significantly high current density, jh∼1012 A/cm2 , cancels the difference in the electron resistive stopping power (or the total stopping power in units of areal density) between solid and compressed samples. Analytical calculations extend the analysis up to jh=1014 A/cm2 (representative of the full-scale fast ignition scenario of inertial confinement fusion), where a very rapid transition to the Spitzer resistivity regime saturates the resistive stopping power, averaged over the electron beam duration, to values of ∼1 keV/μm