Ultrashort light pulses are powerful tools for time-resolved studies of molecular and atomic dynamics1. They arise in the visible and infrared range from femtosecond lasers2, and at shorter wavelengths, in the ultraviolet and X-ray range, from synchrotron sources3 and free-electron lasers4. Recent progress in laser wakefield accelerators has resulted in electron beams with energies from tens of mega-electron volts (refs 5,6,7) to more than 1 GeV within a few centimetres8, with pulse durations predicted to be several femtoseconds9. The enormous progress in improving beam quality and stability5, 6, 7, 8, 10 makes them serious candidates for driving the next generation of ultracompact light sources11. Here, we demonstrate the first successful combination of a laser-plasma wakefield accelerator, producing 55-75 MeV electron bunches, with an undulator to generate visible synchrotron radiation. By demonstrating the wavelength scaling with energy, and narrow-bandwidth spectra, we show the potential for ultracompact and versatile laser-based radiation sources from the infrared to X-ray energies. (Abstract from: http://www.nature.com/nphys/journal/v4/n2/abs/nphys811.html

A precise knowledge of the temperature and number of hot electrons generated in the interaction of short-pulse high-intensity lasers with solids is crucial for harnessing the energy of a laser pulse in applications such as laser-driven ion acceleration or fast ignition. Nevertheless, present scaling laws tend to overestimate the hot electron temperature when compared to experiment and simulations. We present a novel approach that is based on a weighted average of the kinetic energy of an ensemble of electrons. We find that the scaling of electron energy with laser intensity can be derived from a general Lorentz invariant electron distribution ansatz that does not rely on a specific model of energy absorption. The scaling derived is in perfect agreement with simulation results and clearly follows the trend seen in recent experiments, especially at high laser intensities where other scalings fail to describe the simulations accurately.

Plasma-based accelerators that impart energy gain as high as several GeV to electrons or positrons within a few centimeters have engendered a new class of diagnostic techniques very different from those used in connection with conventional radio-frequency (RF) accelerators. The need for new diagnostics stems from the micrometer scale and transient, dynamic structure of plasma accelerators, which contrasts with the meter scale and static structure of conventional accelerators. Because of this micrometer source size, plasma-accelerated electron bunches can emerge with smaller normalized transverse emittance (n < 0.1 mm mrad) and shorter duration (τ b ∼ 1 fs) than bunches from RF linacs. We review single-shot diagnostics that determine such small n and τ b non-invasively and with high resolution from wide-bandwdith spectral measurement of electromagnetic radiation the electrons emit: n from x-rays emitted as electrons interact with transverse internal fields of the plasma accelerator or with external optical fields or undulators; τ b from THz to optical coherent transition radiation emitted upon traversing interfaces. The duration of ∼ 1 fs bunches can also be measured by sampling individual cycles of a co-propagating optical pulse or by measuring the associated magnetic field using a transverse probe pulse. Because of their luminal velocity and micrometer size, the evolving structure of plasma accelerators, the key determinant of accelerator performance, is exceptionally challenging to visualize in the laboratory. Here we review a new generation of laboratory diagnostics that yield snapshots, or even movies, of laser-and particle-beam-generated plasma accelerator structures based on their phase modulation or deflection of femtosecond electromagnetic or electron probe pulses. We discuss spatiotemporal resolution limits of these imaging techniques, along with insights into plasma-based acceleration physics that have emerged from analyzing the images, and comparing them to simulated plasma structures. CONTENTS I.Introduction 1 II. Properties of plasma accelerator structures and beams 3 A. General properties of plasma electron accelerators 3 1. Frequency-domain holography 41 2. Longitudinal optical shadowgraphy 45 3. Transverse optical probing 46 4. Electron radiography 48 D. "Movies" of wake evolution 49 1. Multi-shot transverse probes 49 2. Single-shot frequency-domain streak camera 50 3. Single-shot imaging of meter-long wakes 52 E. Scaling of wake probes with plasma density 54 V. Conclusion 55 References 58

We present a particle-in-cell simulation of the relativistic Kelvin-Helmholtz Instability (KHI) that for the first time delivers angularly resolved radiation spectra of the particle dynamics during the formation of the KHI. This enables studying the formation of the KHI with unprecedented spatial, angular and spectral resolution. Our results are of great importance for understanding astrophysical jet formation and comparable plasma phenomena by relating the particle motion observed in the KHI to its radiation signature.

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