Experiments were performed using the HERCULES laser system at the University of Michigan to study backward stimulated Raman scattering (BSRS) from a laser wakefield accelerator driven with a 30 fs pulse. 'The spectrum of backscattered light was found to be significantly broadened and red-shifted in cases where electrons were accelerated. BSRS broadening (red-shifting) was found to increase with respect to both plasma density and accelerated electron charge for laser powers exceeding 100 TW. Two-dimensional Particle-in-Cell simulations reveal temporal dynamics for the BSRS emission, which ceases as the wakefield bubble is evacuated of plasma electrons because of relativistic self-focusing. The intensity and duration of the BSRS signal was found to vary with plasma density and laser intensity. Both experimental and simulation results indicate that backward SRS is associated with plasma electron density within the wakefield bubble. This measurement can serve as a diagnostic of bubble dynamics, and is correlated with trapped electron charge in this regime.
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.
The electron injection process into a plasma-based laser wakefield accelerator can be influenced by modifying the parameters of the driver pulse.We present an experimental study on the combined effect of the laser pulse duration, pulse shape, and frequency chirp on the electron injection and acceleration process and the associated radiation emission for two different gas types-a 97.5%He and 2.5% N 2 mixture and pure He.In general, the shortest pulse duration with minimal frequency chirp produced the highest energy electrons and the most charge.Pulses on the positive chirp side sustained electron injection and produced higher charge, but lower peak energy electrons, compared with negatively chirped pulses.A similar trend was observed for the radiant energy.The relationship between the radiant energy and the electron charge remained linear over a threefold change in the electron density and was independent of the drive pulse characteristics.X-ray spectra showed that ionization injection of electrons into the wakefield generally produced more photons than self injection for all pulse durations/frequency chirp and had less of a spread in the number of photons around the peak X-ray energy.
Bright and ultrashort duration x-ray pulses can be produced by through betatron oscillations of electrons during laser wakefield acceleration (LWFA). Our experimental measurements using the Hercules laser system demonstrate a dramatic increase in x-ray flux for interaction distances beyond the depletion/dephasing lengths, where the initial electron bunch injected into the first wake bucket catches up with the laser pulse front and the laser pulse depletes. A transition from an LWFA regime to a beam-driven plasma wakefield acceleration regime consequently occurs. The drive electron bunch is susceptible to the electron-hose instability and rapidly develops large amplitude oscillations in its tail, which leads to greatly enhanced x-ray radiation emission. We measure the x-ray flux as a function of acceleration length using a variable length gas cell. 3D particle-in-cell simulations using a Monte Carlo synchrotron x-ray emission algorithm elucidate the time-dependent variations in the radiation emission processes.
High intensity, short pulse lasers can be used to accelerate electrons to ultra-relativistic energies via laser wakefield acceleration (LWFA) [T. Tajima and J. M. Dawson, Phys. Rev. Lett. 43, 267 (1979)]. Recently, it was shown that separating the injection and acceleration processes into two distinct stages could prove beneficial in obtaining stable, high energy electron beams [Gonsalves et al., Nat. Phys. 7, 862 (2011); Liu et al., Phys. Rev. Lett. 107, 035001 (2011); Pollock et al., Phys. Rev. Lett. 107, 045001 (2011)]. Here, we use a stereolithography based 3D printer to produce two-stage gas targets for LWFA experiments on the HERCULES laser system at the University of Michigan. We demonstrate substantial improvements to the divergence, pointing stability, and energy spread of a laser wakefield accelerated electron beam compared with a single-stage gas cell or gas jet target.
Betatron radiation from laser wakefield accelerators is an ultrashort pulsed source of hard, synchrotron-like x-ray radiation. It emanates from a centimetre scale plasma accelerator producing GeV level electron beams. In recent years betatron radiation has been developed as a unique source capable of producing high resolution x-ray images in compact geometries. However, until now, the short pulse nature of this radiation has not been exploited. This report details the first experiment to utilize betatron radiation to image a rapidly evolving phenomenon by using it to radiograph a laser driven shock wave in a silicon target. The spatial resolution of the image is comparable to what has been achieved in similar experiments at conventional synchrotron light sources. The intrinsic temporal resolution of betatron radiation is below 100 fs, indicating that significantly faster processes could be probed in future without compromising spatial resolution. Quantitative measurements of the shock velocity and material density were made from the radiographs recorded during shock compression and were consistent with the established shock response of silicon, as determined with traditional velocimetry approaches. This suggests that future compact betatron imaging beamlines could be useful in the imaging and diagnosis of high-energy-density physics experiments.
X-rays generated by the betatron oscillations of electrons in a nonlinear plasma wakefield have been proposed to be useful for a range of applications. In this study, the scaling of the x-ray fluence and energy with electron beam properties and laser power are studied and the application of the x-rays for imaging additive manufactured (3D printed) test objects is investigated. The scalings confirm a strong increase in x-ray fluence with laser power and the imaging shows strong fringe contrast due to the micron scale source size.
Annular quasimonoenergetic electron beams with a mean energy in the range 200-400 MeV and charge on the order of several picocoulombs were generated in a laser wakefield accelerator and subsequently accelerated using a plasma afterburner in a two-stage gas cell. Generation of these beams is associated with injection occurring on the density down ramp between the stages. This well-localized injection produces a bunch of electrons performing coherent betatron oscillations in the wakefield, resulting in a significant increase in the x-ray yield. Annular electron distributions are detected in 40% of shots under optimal conditions. Simultaneous control of the pulse duration and frequency chirp enables optimization of both the energy and the energy spread of the annular beam and boosts the radiant energy per unit charge by almost an order of magnitude. These well-defined annular distributions of electrons are a promising source of high-brightness laser plasma-based x rays.
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