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.
We suggest an experiment to observe vacuum birefringence induced by intense laser fields. A high-intensity laser pulse is focused to ultra-relativistic intensity and polarizes the vacuum which then acts like a birefringent medium. The latter is probed by a linearly polarized x-ray pulse. We calculate the resulting ellipticity signal within strong-field QED assuming Gaussian beams. The laser technology required for detecting the signal will be available within the next three years. The interactions of light and matter are described by quantum electrodynamics (QED), at present the bestestablished theory in physics. The QED Lagrangian couples photons to charged Dirac particles in a gauge invariant way. At photon energies small compared to the electron mass, ω ≪ m e , electrons (and positrons) will generically not be produced as real particles. Nevertheless, as already stated by Heisenberg and Euler, "...even in situations where the [photon] energy is not sufficient for matter production, its virtual possibility will result in a 'polarization of the vacuum' and hence in an alteration of Maxwell's equations" [1]. These authors were the first to explicitly derive the nonlinear terms induced by QED for small photon energies but arbitrary intensities (see also [2]).The most spectacular process resulting from these modifications presumably is pair production in a constant electric field. This is an absorptive process as photons disappear by disintegration into matter pairs. It can occur for field strengths larger than the critical one given by [3,4] In this electric field an electron gains an energy m e upon travelling a distance equal to its Compton wavelength, λ e = 1/m e . The associated intensity is I c = E 2 c ≃ 4.4 × 10 29 W/cm 2 such that both field strength and intensity * Electronic address: theinzl@plymouth.ac.uk
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 the first detailed demonstrations of octave-spanning SC generation in all-normal dispersion photonic crystal fibers (ANDi PCF) in the visible and near-infrared spectral regions. The resulting spectral profiles are extremely flat without significant fine structure and with excellent stability and coherence properties. The key benefit of SC generation in ANDi PCF is the conservation of a single ultrashort pulse in the time domain with smooth and recompressible phase distribution. For the first time we confirm the exceptional temporal properties of the generated SC pulses experimentally and demonstrate their applicability in ultrafast transient absorption spectroscopy. The experimental results are in excellent agreement with numerical simulations, which are used to illustrate the SC generation dynamics by self-phase modulation and optical wave breaking. To our knowledge, we present the broadest spectra generated in the normal dispersion regime of an optical fiber.
We present the first observation of Thomson-backscattered light from laser-accelerated electrons. In a compact, all-optical setup, the "photon collider," a high-intensity laser pulse is focused into a pulsed He gas jet and accelerates electrons to relativistic energies. A counterpropagating laser probe pulse is scattered from these high-energy electrons, and the backscattered x-ray photons are spectrally analyzed. This experiment demonstrates a novel source of directed ultrashort x-ray pulses and additionally allows for time-resolved spectroscopy of the laser acceleration of electrons.
Modulated phases, commensurate or incommensurate with the host crystal lattice, are ubiquitous in solids. The transition between such phases involves formation and rearrangement of domain walls and is generally slow. Using ultrafast electron diffraction, we directly record the photoinduced transformation between a nearly commensurate and an incommensurate charge-density-wave phase in 1T-TaS(2). The transformation takes place on the picosecond time scale, orders of magnitude faster than previously observed for commensurate-to-incommensurate transitions. The transition speed and mechanism can be linked to the peculiar nanoscale structure of the photoexcited nearly commensurate phase.
We demonstrate a novel method to monitor the total angular distribution of the spectrum of hard x-ray emission from a plasma generated with femtosecond laser pulses with an intensity of 5 x 10(18) W/cm2 on a solid target. Measured and calculated angular distributions of x rays show a pronounced anisotropy for MeV photon energies. We complemented the spectral information by demonstrating a (gamma,n) nuclear reaction with a tabletop laser system.
Fusion neutrons from a heavy water droplet target irradiated with laser pulses of 3 x 10(19) W/cm(2) and from a deuterated secondary target are observed by a time-of-flight (TOF) neutron spectrometer. The observed TOF spectrum can be explained by fusion of deuterium ions simultaneously originating from two different sources: ion acceleration in the laser focus by ponderomotively induced charge separation and target-normal sheath acceleration off the target rear surface. The experimental findings agree well with 3D particle-in-cell simulations.
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