We report the generation of stable and tunable electron bunches with very low absolute energy spread (ÁE%5 MeV) accelerated in laser wakefields via injection and trapping at a sharp downward density jump produced by a shock front in a supersonic gas flow. The peak of the highly stable and reproducible electron energy spectrum was tuned over more than 1 order of magnitude, containing a charge of 1-100 pC and a charge per energy interval of more than 10 pC=MeV. Laser-plasma electron acceleration with Ti:sapphire lasers using this novel injection mechanism provides high-quality electron bunches tailored for applications.

We present an all-laser-driven, energy-tunable, and quasimonochromatic x-ray source based on Thomson scattering from laser-wakefield-accelerated electrons. One part of the laser beam was used to drive a few-fs bunch of quasimonoenergetic electrons, while the remainder was backscattered off the bunch at weakly relativistic intensity. When the electron energy was tuned from 17-50 MeV, narrow x-ray spectra peaking at 5-42 keV were recorded with high resolution, revealing nonlinear features. We present a large set of measurements showing the stability and practicality of our source.

Using quadrupole scan measurements we show laser-wakefield accelerated electrons to have a normalized transverse emittance of 0:21 þ0:01 À0:02 mm mrad at 245 MeV. We demonstrate a multishot and a single-shot method, the mean emittance values for both methods agree well. A simple model of the beam dynamics in the plasma density downramp at the accelerator exit matches the source size and divergence values inferred from the measurement. In the energy range of 245 to 300 MeV the normalized emittance remains constant.Laser-wakefield acceleration (LWFA) [1,2] can deliver ultrarelativistic electron beams in a compact setup with unique features [3][4][5][6]. It is receiving particular attention as a source or driver for ultrashort x-ray beams [7,8] and for its potential for realizing a tabletop free-electron laser (FEL) [9]. The electron bunch duration has recently been measured to be only a few femtoseconds long [10,11] which results in peak beam currents on the order of kiloamperes. An essential parameter for the performance of x-ray sources, FELs, or linear colliders is the transverse electron beam emittance. Previous emittance measurements of LWFA electron beams have used the pepperpot method [12][13][14] giving normalized emittances of $2:2 mm mrad with single shots down to the resolution limit of 1:1 mm mrad. As these measurements are not spectrally resolved, they rely on a low energy spread to give a meaningful normalized emittance. For LWFA beams which fluctuate in energy and energy spread, a simultaneous measurement of the spectrum is required. This technique is also limited to electron energies that can be sufficiently scattered by the pepper-pot mask; to date, measurements of a 508 MeV beam have been carried out [15]. Experiments characterizing the betatron radiation emitted by the electron beam while it is in the plasma suggest the beam size there to be & 1 m [16,17], which in combination with a divergence measurement give an estimated emittance of <0:5 mm mrad [18]. However, inferring the emittance from the electron beam size in the plasma and its downstream divergence in the vacuum can be unreliable as this neglects the plasma-vacuum density transition at the accelerator exit; here the decreasing strength of the plasma focusing forces result in an increase in beam size and decrease in divergence [13]. This publication reports on direct measurements of the emittance of LWFA electrons that are both energy resolved and that include the beam transport of the density downramp at the accelerator exit. This is achieved by analyzing their beam size around a focus using a quadrupole lens scan method [19].The transverse phase space of an electron beam is often specified using the Twiss parameters , , , and the natural emittance ". These parameters describe the volume and orientation of the particle distribution in phase space. The beam size at a particular position ðs 1 Þ is related to the Twiss parameters at s 0 by [20] ðs 1 Þ 2 ¼ M 2 11 ðs 0 Þ À2M 11 M 12 ðs 0 Þþ M 2 12 ðs 0 Þ: (1)Here M ij refers to the ij eleme...

X-ray phase-contrast imaging has recently led to a revolution in resolving power and tissue contrast in biomedical imaging, microscopy and materials science. The necessary high spatial coherence is currently provided by either large-scale synchrotron facilities with limited beamtime access or by microfocus X-ray tubes with rather limited flux. X-rays radiated by relativistic electrons driven by well-controlled high-power lasers offer a promising route to a proliferation of this powerful imaging technology. A laser-driven plasma wave accelerates and wiggles electrons, giving rise to a brilliant keV X-ray emission. This so-called betatron radiation is emitted in a collimated beam with excellent spatial coherence and remarkable spectral stability. Here we present a phase-contrast microtomogram of a biological sample using betatron X-rays. Comprehensive source characterization enables the reconstruction of absolute electron densities. Our results suggest that laser-based X-ray technology offers the potential for filling the large performance gap between synchrotron- and current X-ray tube-based sources.

Ion beams are relevant for radiobiological studies and for tumor therapy. In contrast to conventional accelerators, laser-driven ion acceleration offers a potentially more compact and cost-effective means of delivering ions for radiotherapy. Here, we show that by combining advanced acceleration using nanometer thin targets and beam transport, truly nanosecond quasi-monoenergetic proton bunches can be generated with a table-top laser system, delivering single shot doses up to 7 Gy to living cells. Although in their infancy, laser-ion accelerators allow studying fast radiobiological processes as demonstrated here by measurements of the relative biological effectiveness of nanosecond proton bunches in human tumor cells.

Due to their ultra-short duration and peak currents in the kA range 1,2 , laserwakefield accelerated electron bunches are promising drivers for ultrafast X-ray generation in compact free-electron-lasers (FELs), Thomson-scattering or betatron sources [3][4][5] . Here we present the first single-shot, high-resolution measurements of the longitudinal bunch profile obtained without prior assumptions about the bunch shape. Our method allows complex features, such as multi-bunch structures, to be detected. Varying the length of the gas target, and thus the acceleration length, enables an assessment of the bunch profile evolution during the acceleration process. We find a minimum bunch duration of 4.2 fs (full width at half maximum) with shot-to-shot fluctuation of 11% rms. Our results suggest that after depletion of the laser energy, a transition from a laser-driven to a particle-driven wakefield occurs, associated with the injection of a secondary bunch. The resulting double-bunch structure might act as an elegant approach for driver-witness type experiments, i.e. allowing a non-dephasinglimited acceleration of the secondary bunch in a plasma-afterburner stage 6,7 .Since the first demonstration of high-quality, quasi-monochromatic electron beams in 2004, laser wakefield acceleration (LWFA) has become a reliable scheme to accelerate electrons bunches to energies in the GeV range in plasma accelerator stages a few cm long 8-12 . The small scale of the acceleration structure, confining the bunch to a fraction of the plasma wavelength, implies bunch durations in the femtosecond range.Determining the detailed longitudinal profile of the generated bunches is important for understanding the accelerator dynamics, enabling accelerator control, and for determining their potential applications, such as driving compact FELs 13,14 . However, the limited temporal resolution of traditional methods, such as electro-optic sampling 15 , prevents their application to measuring the ultra-short bunches produced by an LWFA.Although recent experiments confirmed the ultra-short nature of LWFA electron beams 1,2 these relied on the assumption of a Gaussian longitudinal profile when determining the electron bunch duration. As in earlier work 2 , we determine the bunch profile from measurements of the spectrum of coherent transition radiation (CTR).However, our experiments advance prior work in several key aspects: (i) the bandwidth of the recorded spectrum covers a spectral range of more than 4 octaves at high resolution; (ii) the spectrum was recorded in a single-shot, preventing shot-to shot fluctuations in the electron bunch parameters distorting the measured spectrum; (iii) the

The application of phase-retrieval algorithms to the reconstruction of the longitudinal bunch profile of an electron bunch from the spectrum of the coherent transition radiation (CTR) it produces is considered. The development of a new algorithm for this application, the Bubblewrap algorithm, is described. Tests with synthetic data show successful reconstruction of the longitudinal profile of single and double electron bunches, provided that the CTR spectrum is known over a sufficiently wide range. The application of the Bubblewrap algorithm to the reconstruction of laser-accelerated electron bunch profiles from experimental data is demonstrated. The results are shown to be consistent with estimates of the bunch length obtained by other methods.

Few-fs electron bunches from laser wakefield acceleration (LWFA) can efficiently drive plasma wakefields (PWFs), as shown by their propagation through underdense plasma in two experiments. A strong and density-insensitive deceleration of the bunches has been observed in 2 mm of 10^{18}  cm^{-3} density plasma with 5.1  GV/m average gradient, which is attributed to a self-driven PWF. This observation implies that the physics of PWFs, usually relying on large-scale rf accelerators as drivers, can be studied by tabletop LWFA electron sources.