An intense laser pulse in a plasma can accelerate electrons [1][2][3][4] to GeV energies in centimetres [5][6][7] . Transverse betatron motion 8,9 in the plasma wake results in X-ray photons with an energy that depends on the electron energy, oscillation amplitude and frequency of the betatron motion [10][11][12] . Betatron X-rays from laser-accelerator electrons have hitherto been limited to spectra peaking between 1 and 10 keV (ref. 13). Here we show that the betatron amplitude is resonantly enhanced when electrons interact with the rear of the laser pulse 14,15 . At high electron energy, resonance occurs when the laser frequency is a harmonic of the betatron frequency, leading to a significant increase in the photon energy. 10 X-ray pulses from synchrotron sources have become immensely useful tools for investigating the structure of matter 17 , which has led to a huge international effort to construct light sources for many different scientific and technological applications. Synchrotrons are usually based on radio-frequency (RF) accelerating cavities that are limited to fields of 10-100 MV m −1 because of electrical breakdown, which results in very large and expensive devices.High-power lasers, on the other hand, have led to the development of many new areas of science, as diverse as inertial confinement fusion and laboratory astrophysics to the study of warm dense matter. However, they now have the potential to transform accelerator and light source technology. In the late 1970s, Tajima and Dawson 1 proposed harnessing the ponderomotive force associated with intense laser fields to excite plasma waves and form wake-like structures 18 (as behind a boat) that travel with a velocity close to the speed of light, c. The electrostatic forces of these charge density structures can rapidly accelerate particles to very high energies 6 ; where momentum is gained analogous to a surfer riding an ocean wave. Recent progress in the development of laser wakefield accelerators (LWFAs) has enabled electron beams to be accelerated with unprecedented acceleration gradients 2-4 , three orders of magnitude higher than in RF cavities, thus reducing a 100 m long GeV accelerator to centimetres in length 6 . The LWFA can now produce high-quality electron beams with low emittance, ε n , of the order 1π mm mrad 19 , small energy spread 20 , δγ /γ 1%, where γ is the Lorenz factor, and high charge 4 , Q = 10-100 pC. At high laser intensities, in the so-called blowout regime 21 , the LWFA structure has an approximately spherical bubble shape with a radius of R ≈ 2 √ a 0 c/ω p , which is primarily determined by the normalized laser vector potential, a 0 = eA/m e c 2 and the plasma frequency, ω p = √ 4π n p e 2 /m e , where n p is the plasma density, e, the electron charge and m e , the electron mass 22 . The plasma wave is efficiently driven when the laser pulse duration is approximately a plasma period. Micrometre-long electron bunches that extend only a fraction of the plasma wavelength, λ p = 2πc/ω p , are self-injected and accelerate...

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

Many areas of the Earth’s crust deform by distributed extensional faulting and complex fault interactions are often observed. Geodetic data generally indicate a simpler picture of continuum deformation over decades but relating this behaviour to earthquake occurrence over centuries, given numerous potentially active faults, remains a global problem in hazard assessment. We address this challenge for an array of seismogenic faults in the central Italian Apennines, where crustal extension and devastating earthquakes occur in response to regional surface uplift. We constrain fault slip-rates since ~18 ka using variations in cosmogenic 36Cl measured on bedrock scarps, mapped using LiDAR and ground penetrating radar, and compare these rates to those inferred from geodesy. The 36Cl data reveal that individual faults typically accumulate meters of displacement relatively rapidly over several thousand years, separated by similar length time intervals when slip-rates are much lower, and activity shifts between faults across strike. Our rates agree with continuum deformation rates when averaged over long spatial or temporal scales (104 yr; 102 km) but over shorter timescales most of the deformation may be accommodated by <30% of the across-strike fault array. We attribute the shifts in activity to temporal variations in the mechanical work of faulting.

Progress in laser wakefield accelerators indicates their suitability as a driver of compact free-electron lasers (FELs). High brightness is defined by the normalized transverse emittance, which should be less than 1π mm mrad for an x-ray FEL. We report high-resolution measurements of the emittance of 125 MeV, monoenergetic beams from a wakefield accelerator. An emittance as low as 1.1±0.1π mm mrad is measured using a pepper-pot mask. This sets an upper limit on the emittance, which is comparable with conventional linear accelerators. A peak transverse brightness of 5×10¹⁵ A m⁻¹ rad⁻¹ makes it suitable for compact XUV FELs.

Changes in the ultrafast dynamics and terahertz Raman spectrum accompanying a helix-to-coil transition of a homo-polypeptide have been observed for the first time. Formation of the alpha-helix is associated with a shift to lower frequency of a broad Raman band attributable to solvent-peptide intermolecular hydrogen bonding. This band facilitates direct spectroscopic observation of so-called hydration water near a peptide and yields the first quantitative estimate of the time scale of the ultrafast dynamics in the solvation shell, which range from 0.18 to 0.33 ps (185-100 cm(-1)) depending on the secondary structure of the peptide. Such fast motions of solvent molecules have been referred to as the "lubricant of life" and are thought to play key roles in determining structure and activity of proteins.

Establishing the trajectory of thinning of the West Antarctic ice sheet (WAIS) since the last glacial maximum (LGM) is important for addressing questions concerning ice sheet (in)stability and changes in global sea level. Here we present detailed geomorphological and cosmogenic nuclide data from the southern Ellsworth Mountains in the heart of the Weddell Sea embayment that suggest the ice sheet, nourished by increased snowfall until the early Holocene, was close to its LGM thickness at 10 ka. A pulse of rapid thinning caused the ice elevation to fall ∼400 m to the present level at 6.5–3.5 ka, and could have contributed 1.4–2 m to global sea-level rise. These results imply that the Weddell Sea sector of the WAIS contributed little to late-glacial pulses in sea-level rise but was involved in mid-Holocene rises. The stepped decline is argued to reflect marine downdraw triggered by grounding line retreat into Hercules Inlet.

The laser-plasma wakefield accelerator is a compact source of high brightness, ultra-short duration electron bunches. Self-injection occurs when electrons from the background plasma gain sufficient momentum at the back of the bubble-shaped accelerating structure to experience sustained acceleration. The shortest duration and highest brightness electron bunches result from self-injection close to the threshold for injection. Here we show that in this case injection is due to the localized charge density build-up in the sheath crossing region at the rear of the bubble, which has the effect of increasing the accelerating potential to above a critical value. Bunch duration is determined by the dwell time above this critical value, which explains why single or multiple ultra-short electron bunches with little dark current are formed in the first bubble. We confirm experimentally, using coherent optical transition radiation measurements, that single or multiple bunches with femtosecond duration and peak currents of several kiloAmpere, and femtosecond intervals between bunches, emerge from the accelerator.

Very stable, high quality electron beams (current ~ 10 kA, energy spread < 1%, emittance ~ 1π mm mrad) have been generated in a laser-plasma accelerator driven by 25 TW femtosecond laser pulses.