High brightness multi-MeV photon source driven by a petawatt-scale laser wakefield accelerator

Abstract

We present an experimental demonstration of a bright multi-MeV gamma source driven by a petawatt laser. The source generates on average $(1.2\pm0.6)\times10^9$ photons above 1 MeV per pulse, exceeding those of previous all-optical sources by a hundred times, and reached a peak spectral brightness of $(3.9 \pm 1.5)\times 10^{22}$ photons/mm$^2$/mrad$^2$/s/0.1%BW at $ε_γ\approx11$ MeV. The source was produced by inverse Compton scattering of a laser wakefield accelerated GeV electron beam and its back-reflected driving laser pulse. Its performance is well described by a simple model of the laser and electron properties at the collision point that allows quantitative predictions and identifies clear strategies to further enhance radiation efficiency. Our results highlight the promise of this source for fundamental physics studies, as well as for applications of nuclear resonance fluorescence and nuclear transmutation.

Figures (5)

• Figure 1: Left: Sketch of the experimental setup with key diagnostics. Right: Example diagnostic images: (a) scattering screen, (b) electron spectra (grey lines) and their average spectrum (standard deviation from mean in shaded), (c) gamma profiler and (d) spectrometer. Inset:(I) The laser pulse (red) drives a wake and accelerates an ultrarelativistic electron ($e^-$) bunch (blue). (II) The laser pulse ionises the surface of the kapton tape (orange) and forms a plasma mirror that reflects it. (III) The reflected laser pulse collides with the electron beam and emits radiation from inverse Compton scattering (purple).
• Figure 2: Average energy deposited in the gamma profiler $E_{dep}$ (error bars for standard deviation) for increasing distance $z$ between the tape and the end of the gas jet. The emitted power and $E_{dep}$ are expected to scale with the intensity at the interaction point, which decreases with expanding beam area, i.e. $E_{dep}(z) = A [w_0/w(z)]^2 + C$, where the Gaussian waist is $w(z) = w_0 \sqrt{1+[(z-z_0)/z_R]^2}$. The fitting parameters are the amplitude $A$, the constant offset $C$, the waist position $z_0$, and the Rayleigh length $z_R$. The fitting parameters for $z_0$ and $z_R$ are shown in the legend. The dashed line indicates $z_0$. Inset: Example gamma profiles at [2.5]mm (a), [5]mm (b) and [35]mm (c) distance, shown on the same scale. Counts on (c) were multiplied by 5 for visibility.
• Figure 3: Measured photon numbers above [1]MeV (systematic errors as errorbars) for 10 consecutive shots at the closest tape position along with their mean (dashed line) and standard deviation (shaded region), excluding the brightest shot (shot 4). The fitted critical energy is indicated by the marker colour.
• Figure 4: FBPIC simulation of the LWFA. a) and b) show the laser field, plasma electrons and trapped electron beam at the midpoint and exit of the accelerator respectively, where $\xi = z -ct$ and [$n_{e0}=1.75\times10^{18}$]cm$^{-3}$ is the electron density at the plateau. At the exit plane (corresponding to the closest mirror position in Fig. \ref{['fig:ICSYield']}) the laser has begun to diffract and has waist which is significantly larger than the electron beam. c) shows the plasma density profile and the evolution of the laser field amplitude during the simulation. d) shows the electron spectrum at [$z=25$]mm.
• Figure 5: Spectral brightness of our work (orange) compared to other all-optical radiation sources using single-beam TaPhuocNatPhoton2012TsaiPoP2015YuSciRep2016WuPPCF2019 (green circle) or two-beam ICS ChenPRL2013PowersNatPhoton2014SarriPRL2014KhrennikovPRL2015MirzaieNatPhoton2024 (blue triangle), and the SPring-8 synchrotron source (extracted from FletcherNatPhoton2015). Lines indicate spectral brightness and markers peak spectral brightness. Radiofrequency-based ICS sources provide high average flux of order $10^{14}$ photons/s at lower peak brightness $<10^{15}$DeitrickPRAB2018GuntherSynchrotronRad2020 up to MeV photon energies, at an average flux of [$10^{6} - 10^{10}$]photons/s at the MeV to GeV scale OhgakiNIMA2000KawaseNIMA2008WellerPPN2009WangNST2022