Compact all-optical precision-tunable narrowband hard Compton X-ray source

www.nature.com/scientificreports OPEN Compact all‑optical precision‑tunable narrowband hard Compton X‑ray source T. Brümmer1, S. Bohlen1, F. Grüner2, J. Osterhoff1 & K. Põder1* Readily available bright X-ray beams with narrow bandwidth and tunable energy promise to unlock novel developments in a wide range of applications. Among emerging alternatives to large-scale and costly present-day radiation sources which severely restrict the availability of such beams, compact laser-plasma-accelerator-driven inverse Compton scattering sources show great potential. However, these sources are currently limited to tens of percent bandwidths, unacceptably large for many applications. Here, we show conceptually that using active plasma lenses to tailor the electron bunchphoton interaction, tunable X-ray and gamma beams with percent-level bandwidths can be produced. The central X-ray energy is tunable by varying the focusing strength of the lens, without changing electron bunch properties, allowing for precision-tuning the X-ray beam energy. This method is a key development towards laser-plasma-accelerator-driven narrowband, precision tunable femtosecond photon sources, enabling a paradigm shift and proliferation of compact X-ray applications. Since the demonstration of X-ray imaging by Röntgen in 18951, development of bright X-ray sources has allowed harnessing the power of X-rays in ever-evolving and broadening fields of research and applications, leading to enormous advances in topics as varied as advanced imaging2–5 and radiotherapy6–9 modalities as well as in crystallography applications10–13. Examples of medical breakthroughs relying on brilliant X-ray sources include K-edge subtraction (KES) imaging14, a potential alternative to digital subtraction angiography, as well as X-ray fluorescence imaging (XFI)15, enabling in-vivo pharmacokinetic studies. Such advanced diagnostic and treatment modalities place strict requirements on X-ray bandwidth: KES requires percent-level X-ray bandwidths with precision energy tunability to above and below a K-edge (80.7keV for gold), with smaller energy separations leading to lower dose16. Indeed many other applications, such as serial crystallography10,12, demand percent level bandwidths at tens of keVs. Additionally, femtosecond X-ray pulse duration and low jitter to short-pulse laser systems enable time-resolved pump-probe s tudies11 and the investigation of matter under extreme c onditions17. For these and many other use cases18,19, a compact X-ray source of precision-tunable narrowband radiation could trigger disruptive progress and catalyse their adoption into applications of high societal relevance. The final frequency ωx of a photon with initial frequency ωL Compton-scattered off an electron of energy γe me c 2, ignoring electron energy loss, is given by20 ωx = 2γe2 (1 − β cos θI ) ωL , 1 + γe2 θO2 + a2 /2 (1)  where θI and θO are the scattering and observation angles, respectively, β = 1 − γe−2 with electron Lorentz factor γe. a = qe A/me c is the normalised laser vector potential with peak value a0, where A is the vector potential of the laser field, c is the speed of light and qe and me are elementary charge and electron mass, respectively. For γe ≫ 1, θI ≃ π and a0 ≪ 1, the central frequency observed on the electron beam propagation axis is ωx ≃ 4γe2 ωL. The axial bandwidth of the scattered X-ray beam (see “Methods”) arises predominantly from the electron bunch energy spread δγe and divergence σθ21,22, along with a permille to percent level contribution from laser bandwidth δωL. Generation of narrow bandwidth X-ray beams thus requires an electron bunch with very low energy spread and divergence, leading to narrowband inverse Compton-scattering (ICS) sources previously being demonstrated with conventional a ccelerators23,24. Quasi-monoenergetic electron spectra from laser plasma acceleration (LPA) have allowed for the demonstration of extremely compact and tunable all-optical ICS sources with peaked 1 Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany. 2Universität Hamburg and Center for Free-Electron Laser Science, Luruper Chaussee 149, 22761 Hamburg, Germany. *email: Scientific Reports | (2022) 12:16017 | https://doi.org/10.1038/s41598-022-20283-8 1 Vol.:(0123456789) www.nature.com/scientificreports/ c LPA driver laser Active plasma lens a Electron beam d Scattering laser Plasma source b ICS interaction plane zf Dipole magnet X-ray beam Figure 1.  Schematic overview of the active plasma lens-tunable X-ray source. (a) A laser beam is focussed into a plasma source, generating an electron bunch. The electron bunch is captured and refocussed chromatically using an active plasma lens and interacts with a focussed scattering laser (potentially from a different, synchronised laser system) at a plane zf , generating X-rays via ICS. The electron bunch is deflected with a dipole magnet, leaving the X-ray beam. (b) Trajectories of electrons with different energies being focussed by an active plasma lens, highlighting the chromaticity of the focussing. (c) The energy-dependence of RMS electron spot size at zf . (d) The filtering response function Eq. (2) as a function of electron energy. The trajectories are calculated with lens current IAPL = 500 A. photon spectra25–27. However, with central energy and bandwidth of the X-ray beam directly derived from the electron bunch properties, milliradian divergence and  10 % relative energy spread results in demonstrated X-ray bandwidths of tens of p ercent25–28, unacceptably large for all applications discussed above. State-of-theart LPAs providing 2–3% FWHM energy spreads29,30 would result in X-ray pulses with at least 4–6% FWHM bandwidth, too large for KES. Results Here, we propose advanced control over the X-ray beam central energy and bandwidth by employing an active plasma lens (APL), a compact, high-strength radially symmetric focussing d evice31–33. The chromatic focussing of the APL allows tuning the central energy of the X-ray beam and reducing the effective energy spread and divergence of the electron bunch interacting with the scattering laser. Such advanced tailoring of the electron bunch allows LPA-driven X-ray beams with unprecedented percent-level X-ray bandwidths and precision energy tunability. The schematic setup of the proposed precision-tunable ICS X-ray source is depicted in Fig. 1a. Electrons in a plasma wakefield-accelerated femtosecond-duration bunch are focussed by an APL to longitudinal positions dependent on their energy, as depicted in Fig. 1b. Figure 1c shows the variation of electron bunch RMS size σ (γe ) at the Compton interaction plane zf . For interaction of a laser beam and electron bunch with gaussian profiles (see Supplementary Material S1), the response function N(γe )34, plotted in Fig. 1d, is given by N(γe ) ∝  1 2 2σ (γe ) w0 , +1 (2) showing that only energy slices focussed to a spot size similar (...truncated)