Particle acceleration based on high intensity laser systems (a process known as laser-plasma acceleration) has achieved high quality particle beams that compare favourably with conventional acceleration techniques in terms of emittance, brightness and pulse duration. A long-term difficulty associated with laser-plasma acceleration--the very broad, exponential energy spectrum of the emitted particles--has been overcome recently for electron beams. Here we report analogous results for ions, specifically the production of quasi-monoenergetic proton beams using laser-plasma accelerators. Reliable and reproducible laser-accelerated ion beams were achieved by intense laser irradiation of solid microstructured targets. This proof-of-principle experiment serves to illuminate the role of laser-generated plasmas as feasible particle sources. Scalability studies show that, owing to their compact size and reasonable cost, such table-top laser systems with high repetition rates could contribute to the development of new generations of particle injectors that may be suitable for medical proton therapy.

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

We present few-femtosecond shadowgraphic snapshots taken during the non-linear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma wave period is induced by a lengthening of the first plasma period. Three dimensional particle in cell simulations support our observations.Laser-wakefield accelerators (LWFA) operating in the 'bubble'-regime [1] can generate quasimonoenergetic multigigaelectronvolt electron beams [2,3] with femtosecond duration [4,5] and micrometer dimensions [6,7]. These beams are produced by accelerating electrons in laser-driven plasma waves over centimeter distances. They have the potential to be compact alternatives to conventional accelerators [8]. In a LWFA, the short driving laser pulse displaces plasma electrons from the stationary background ions. The generated space charge fields cause the electrons to oscillate and form a plasma wave in the laser's wake. This wave follows the laser at almost c, the speed of light; for low amplitude it has a wavelength ofwhere n e is the electron density of the plasma. At high amplitude, electrons from the background can be injected into the wake and accelerated, producing monoenergetic electron pulses [9][10][11]. Significant progress has been made regarding achievable peak energy [3], beam stability [12] and the generation of bright X-ray pulses [13][14][15]. Until now, most of our knowledge about the dynamics of the self-injection process has been derived from detailed particle-in-cell (PIC) simulations. These simulations show that self-focusing [16] and pulse compression [17] play a vital role in increasing the laser pulse intensity prior to injection. Furthermore, simulations indicate that self-injection of electrons is associated with a dynamic lengthening of the first plasma wave's period (the 'bubble'). This lengthening can be driven by changes of the electric field structure inside the plasma wave caused by the injected electrons [18]. In contrast, the lengthening may also be due to an intensity amplification of the laser pulse caused by the non-linear evolution of the plasma wave [19,20] or due to a local increase in intensity caused by two colliding pulses [21]. In these latter scenarios, injection is a consequence of the lengthening of the bubble. However, experimental insight into these processes is extremely challenging due to the small spatial and temporal scales of a LWFA.The plasma wave, a variation in the electron density, has an associated refractive index profile which can be detected using longitudinal [22][23][24] or transverse probes [5]. Longitudinal probes cannot measure the rapid and dynamic evolution of the plasma wave that occurs in nonlinear wakefield accelerators and suffer from the strong refraction caused by the steep refractive ...

Harmonic generation in the limit of ultra-steep density gradients is studied experimentally. Observations demonstrate that while the efficient generation of high order harmonics from relativistic surfaces requires steep plasma density scale-lengths (Lp/λ < 1) the absolute efficiency of the harmonics declines for the steepest plasma density scale-length Lp → 0, thus demonstrating that near-steplike density gradients can be achieved for interactions using high-contrast high-intensity laser pulses. Absolute photon yields are obtained using a calibrated detection system. The efficiency of harmonics reflected from the laser driven plasma surface via the Relativistic Oscillating Mirror (ROM) was estimated to be in the range of 10 −4 − 10 −6 of the laser pulse energy for photon energies ranging from 20 − 40 eV, with the best results being obtained for an intermediate density scale-length.PACS numbers: 52.59.Ye, 52.38.-r Keywords: surface high-harmonic generation, relativistic laser plasma interaction, attosecond pulse generation Ultrashort XUV pulses are a promising tool for a wide range of applications including attosecond laser physics and seeding of free-electron X-ray lasers. Typically, they are created by the nonlinear frequency up-conversion of an intense femtosecond driving laser field in a gaseous medium. Remarkable progress has been made to the present date with efficiencies reaching the level of 10 −4 at 20 nm wavelengths [1,2]. Such efficiencies are not yet available at shorter wavelengths or for attosecond pulse generation and the low intensities at which harmonic conversion takes place in gaseous media, makes harnessing the high peak power in the 0.1−1PW regime challenging. High-harmonic generation at a sharp plasma-vacuum interface via the Relativistically Oscillating Mirror (ROM) mechanism [3] is predicted to overcome these limitations and result in attosecond pulses of extreme peak power [4,5].While other mechanisms such as Coherent Wake Emission (CWE) can also emit XUV harmonics [6], the ROM mechanism is generally reported to dominate in the limit of highly relativistic intensities, where the normalized vector potential a 2 0 = Iλ 2 /(1.37 · 10 18 µm 2 W/cm 2 ) 1. The efficiency of ROM harmonics is predicted to converge to a power law for ultra-relativistic intensities [7], such that the conversion efficiency is given by η ≈ (ω/ω 0 ) −8/3 up to a threshold frequency ω t ∼ γ 3 , beyond which the spectrum decays exponentially. Here, γ is the maximum value of the Lorentz-factor associated with the reflection point of the ROM process. While these predictions correspond well with the observations made in experiments using pulse durations of the order of picoseconds in terms of highest photon energy up to keV [8,9] and the slope of the harmonic efficiency [10], no absolute efficiency measurements have been reported to date.The plasma density scale-length plays a critical role in determining the response of the plasma to the incident laser radiation. In the picosecond regime, the balance between the laser pre...