Notable improvements on LWFA through precise laser wavefront tuning

Oumbarek Espinos D., Rondepierre A., Zhidkov A., et al. Notable improvements on LWFA through precise laser wavefront tuning. Scientific Reports 13, 18466 (2023); https://doi.org/10.1038/s41598-023-45737-5.Laser wakefield acceleration (LWFA) continues to grow and awaken interest worldwide, especially as in various applications it approaches performance comparable to classical accelerators. However, numerous challenges still exist until this can be a reality. The complex non-linear nature of the process of interaction between the laser and the induced plasma remains an obstacle to the widespread LWFA use as stable and reliable particle sources. It is commonly accepted that the best wavefront is a perfect Gaussian distribution. However, experimentally, this is not correct and more complicated ones can potentially give better results. in this work, the effects of tuning the laser wavefront via the controlled introduction of aberrations are explored for an LWFA accelerator using the shock injection configuration. Our experiments show the clear unique correlation between the generated beam transverse characteristics and the different input wavefronts. The electron beams stability, acceleration and injection are also significantly different. We found that in our case, the best beams were generated with a specific complex wavefront. A greater understanding of electron generation as function of the laser input is achieved thanks to this method and hopes towards a higher level of control on the electrons beams by LWFA is foreseen

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

Reaction-yield dependence of the (γ, γ′) reaction of 238 U on the target thickness

The dependence of the nuclear resonance fluorescence (NRF) yield on the target thickness was studied. To this end, an NRF experiment was performed on 238U using a laser Compton back-scattering (LCS) γ-ray beam at the High Intensity γ-ray Source facility at Duke University. Various thicknesses of depleted uranium targets were irradiated by an LCS γ-ray beam with an incident beam energy of ∼2.475 MeV. The scattering NRF γ-rays were measured using an High-purity Germanium (HPGe) detector array positioned at scattering angles of 90° relative to the incident γ-beam. An analytical model for the NRF reaction yield (NRF RY model) is introduced to interpret the experimental data. Additionally, a Monte Carlo simulation using GEANT4 was performed to simulate the NRF interaction for a wide range of target thicknesses of the 238U. The measured NRF yield shows the saturation behavior. The results of both of the simulation and the analytical model can reproduce the saturation curve of the scattering NRF yield of 238U against the target thickness. In addition, we propose a method to deduce the precise integral cross section of the NRF reaction by fitting the NRF yield dependency on the target thickness without any absolute measurements

Demonstration of tomographic imaging of isotope distribution by nuclear resonance fluorescence

Computed Tomography (CT) using X-ray attenuation by atomic effects is now widely used for medical diagnosis and industrial non-destructive inspection. In this study, we performed a tomographic imaging of isotope (208Pb) distribution by the Nuclear Resonance Fluorescence (NRF), i.e. isotope specific resonant absorption and scattering of gamma rays, using Laser Compton Scattering (LCS) gamma rays. The NRF-CT image which includes both effects of atomic attenuation and nuclear resonant attenuation was obtained. By accounting for the atomic attenuation measured by a conventional method at the same time, a clear 208Pb isotope CT image was obtained. The contrast degradation due to notch refilling caused by small-angle Compton scattering is discussed. This study clearly demonstrates the capability of the isotope-specific CT imaging based on nuclear resonant attenuation which will be a realistic technique when the next generation of extremely intense LCS gamma-ray sources will be available. The expected image acquisition time using these intense LCS gamma rays was discussed

Demonstration of tomographic imaging of isotope distribution by nuclear resonance fluorescence

Computed Tomography (CT) using X-ray attenuation by atomic effects has been used in various fields such as medical diagnosis. CT scanning of high density and high-Z industrial object using gamma-rays from radioisotopes, Bremsstrahlung gamma-rays in the MeV energy region, and quasi-monochromatic gamma-rays generated by laser Compton scattering (LCS) have been studied. However, even if such technologies are used, it is difficult to distinguish clearly a high-Z element from other high-Z elements by the atomic attenuation. Previous studies [4,5] have proposed a novel method to measure CT images for high density and high-Z objects using identification of a specific nuclide with Nuclear Resonance Fluorescence (NRF-CT). Here we show the demonstration of the isotope imaging with NRF-CT and a LCS gamma-ray beam. We measured a two-dimensional CT image of a lead isotope, 208Pb, inside a sample of an aluminum cylinder including an iron rod, a lead rod, and a hole (air). The preliminary NRF image includes both effects of atomic attenuation and nuclear resonant attenuation. By subtracting the atomic attenuation measured by a conventional method at the same time, a clear 208Pb isotope CT image was measured

Laser-plasma electron acceleration towards a compact XFEL in Japan

In Japan we are conducting a laser-plasma acceleration (LPA) project aiming a compact X-ray free-electron laser (XFEL) system and a compact carbon ion injector for cancer therapy including efficient laser driver development. In this talk we will focus on the laser electron acceleration part. Our LPA electron beam platform is located in the SPring-8 Center site at RIKEN, Japan, where the prototype XFEL of SACLA was tested. In the platform a Ti:sapphire laser system is installed and it can deliver three synchronized laser pulses (~20 – 30 fs) with pulse energies of 1 J, 2 J, and 10 J. Our strategy is staged electron acceleration using these pulses by manipulating electron phase space to obtain monoenergetic electron beams.In this talk we present our recent results on ~250 MeV, reproducible, quasi-monoenergetic electron production, undulator radiation test, etc.*This work was supported by JST-Mirai Program Grant Number JPMJMI17A1, Japan.IAS Program on High Energy Physics (HEP 2021