Jason M. Cole

Laser wakefield accelerators are bright, compact sources of synchrotronlike x-rays. By driving the wakefield using a 110 TW laser in a variable length gas cell, we are able to map the evolution of the electron beam and resulting x-ray emission. We find that using a laser pulse initially focused to larger than the matched spot size and extending the plasma length beyond the depletion length, dramatically increases the x-ray flux. Self-injected electrons are initially accelerated to > 2 GeV , emitting x-rays with critical energy E c > 20 keV . But, as these electrons dephase, a second high-charge electron bunch is produced and accelerated to ≈ 0.7 GeV . The x-rays produced by this bunch dominate the total emission with > 5 × 10 10 photons per shot with E c = 12 – 17 keV and a flux of 7 × 10 4 photons / mrad 2 / 0.1 % BW . Combining electron and x-ray measurements reveals that the increased emission results from the larger betatron amplitude of the secondary bunch. Simulations attribute this to rapid bubble expansion driven by laser compression over the depletion length, which leads to injection of a high-charge electron bunch with increased transverse momentum and thus stronger betatron radiation.

We report on a single-shot longitudinal phase-space reconstruction diagnostic for electron beams in a laser wakefield accelerator via the experimental observation of distinct periodic modulations in the angularly resolved spectra. Such modulated angular spectra arise as a result of the direct interaction between the ultrarelativistic electron beam and the laser driver in the presence of the wakefield. A constrained theoretical model for the coupled oscillator, assisted by a genetic algorithm, can recreate the experimental electron spectra and, thus, fully reconstructs the longitudinal phase-space distribution of the electron beam with a temporal resolution of approximately 1.3 fs. In particular, it reveals the slice energy spread of the electron beam, which is important to measure for applications such as x-ray free electron lasers. In our experiment, the root-mean-square energy spread retrieved is bounded at 9.9 MeV, corresponding to a 0.9%–3.0% relative spread, despite the overall GeV energy beam having approximately 100% relative energy spread.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Laser-wakefield accelerators (LWFAs) are high acceleration-gradient plasma-based particle accelerators capable of producing ultra-relativistic electron beams. Within the strong focusing fields of the wakefield, accelerated electrons undergo betatron oscillations, emitting a bright pulse of X-rays with a micrometer-scale source size that may be used for imaging applications. Non-destructive X-ray phase contrast imaging and tomography of heterogeneous materials can provide insight into their processing, structure, and performance. To demonstrate the imaging capability of X-rays from an LWFA we have examined an irregular eutectic in the aluminum-silicon (Al-Si) system. The lamellar spacing of the Al-Si eutectic microstructure is on the order of a few micrometers, thus requiring high spatial resolution. We present comparisons between the sharpness and spatial resolution in phase contrast images of this eutectic alloy obtainedviaX-ray phase contrast imaging at the Swiss Light Source (SLS) synchrotron and X-ray projection microscopyviaan LWFA source. An upper bound on the resolving power of 2.7 ± 0.3 μm of the LWFA source in this experiment was measured. These results indicate that betatron X-rays from laser wakefield acceleration can provide an alternative to conventional synchrotron sources for high resolution imaging of eutectics and, more broadly, complex microstructures.

We present a design for a pixelated scintillator based gamma-ray spectrometer for non-linear inverse Compton scattering experiments. By colliding a laser wakefield accelerated electron beam with a tightly focused, intense laser pulse, gamma-ray photons up to 100 MeV energies and with few femtosecond duration may be produced. To measure the energy spectrum and angular distribution, a 33 × 47 array of cesium-iodide crystals was oriented such that the 47 crystal length axis was parallel to the gamma-ray beam and the 33 crystal length axis was oriented in the vertical direction. Using an iterative deconvolution method similar to the YOGI code, modeling of the scintillator response using GEANT4 and fitting to a quantum Monte Carlo calculated photon spectrum, we are able to extract the gamma ray spectra generated by the inverse Compton interaction.

In the field of X-ray microcomputed tomography (μCT) there is a growing need to reduce acquisition times at high spatial resolution (approximate micrometers) to facilitate in vivo and high-throughput operations. The state of the art represented by synchrotron light sources is not practical for certain applications, and therefore the development of high-brightness laboratory-scale sources is crucial. We present here imaging of a fixed embryonic mouse sample using a compact laser–plasma-based X-ray light source and compare the results to images obtained using a commercial X-ray μCT scanner. The radiation is generated by the betatron motion of electrons inside a dilute and transient plasma, which circumvents the flux limitations imposed by the solid or liquid anodes used in conventional electron-impact X-ray tubes. This X-ray source is pulsed (duration 10¹⁰ photons per pulse), small (diameter 15 keV. Stable X-ray performance enabled tomographic imaging of equivalent quality to that of the μCT scanner, an important confirmation of the suitability of the laser-driven source for applications. The X-ray flux achievable with this approach scales with the laser repetition rate without compromising the source size, which will allow the recording of high-resolution μCT scans in minutes. Significance High-resolution microcomputed tomography with benchtop X-ray sources requires long scan times because of the heat load limitation on the anode. We present an alternative, high-brightness plasma-based X-ray source that does not suffer from this restriction. A demonstration of tomography of a centimeter-scale complex organism achieves equivalent quality to a commercial scanner. We will soon be able to record such scans in minutes, rather than the hours required by conventional X-ray tubes.