T abletop plasma accelerators can now produce GeV-range electron beams 1-5 and femtosecond X-ray pulses 6 , providing compact radiation sources for medicine, nuclear engineering, materials science and high-energy physics 7 . In these accelerators, electrons surf on electric fields exceeding 100 GeV m −1 , which is more than 1,000 times stronger than achievable in conventional accelerators. These fields are generated within plasma structures (such as Langmuir waves 8 or electron density 'bubbles' 9 ) propagating near light speed behind laser 2-4 or charged-particle 5 driving pulses. Here, we demonstrate single-shot visualization of laser-wakefield accelerator structures for the first time. Our 'snapshots' capture the evolution of multiple wake periods, detect structure variations as laser-plasma parameters change, and resolve wavefront curvature; features never previously observed.These previously invisible features underlie wave breaking, electron injection and focusing within the wake, the key determinants of charge, energy, energy spread and collimation of the accelerated beam. Because of their microscopic size and luminal velocity, these critical structures previously eluded direct singleshot observation, inhibiting progress in producing high-quality beams and in correlating beam properties with wake structure. Here, in contrast, we reconstruct wake morphology in real-time, enabling rapid feedback and optimization.Recent advances in laser-wakefield accelerators dramatically illustrated the link between beam quality and plasma structure 1-4 . Earlier laser-wakefield accelerators produced electron beams with large divergence and energy spread, but by introducing a plasma channel guide 1,2 or by carefully adjusting the laser-plasma conditions to produce an electron density cavity behind the driving pulse 3,4 , collimated, nearly mono-energetic beams from 80 MeV to 1 GeV were demonstrated. Nevertheless, the plasma structures themselves remained invisible. Previous direct measurements of laser wakes with spatial resolution better than a plasma wavelength 10-13 (l p ) used frequency-domain interferometry 14 , in which a focused femtosecond probe pulse measured local electron density n e (ζ) at only a single time delay ζ behind the driving pulse within the co-propagating wake for each laser shot. Wake structure was then accumulated painstakingly by probing a different ζ on each subsequent shot. However, multi-shot techniques average over

The dynamics of plasma electrons in the focus of a petawatt laser beam are studied via measurements of their x-ray synchrotron radiation. With increasing laser intensity, a forward directed beam of x-rays extending to 50 keV is observed. The measured x-rays are well described in the synchrotron asymptotic limit of electrons oscillating in a plasma channel. The critical energy of the measured synchrotron spectrum is found to scale as the maxwellian temperature of the simultaneously measured electron spectra. At low laser intensity transverse oscillations are negligible as the electrons are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are directly accelerated by the laser and enter a highly radiative regime with up to 5% of their energy turned into x-rays. PACS numbers: Valid PACS appear hereThe advent of high power lasers has led to rapid progress in the field of plasma based particle acceleration [1]. In particular, the measurement of monoenergetic electron beams from wakefields generated by short lasers [2] has stimulated great interest in producing such beams and understanding their dynamics. One potential use for these compact sources of energetic particles is as a driver for novel light sources. Laser-accelerated electrons could be injected into a magnetic undulator realizing a compact tunable-energy femtosecond x-ray source synchronized to the laser. A laser-based x-ray source could be downsized further, using the self-generated magnetic and electrostatic fields of the plasma channel as a miniature undulator [3]. For electron beams of sufficiently high quality, an ion channel laser analogous to conventional free electron lasers may be feasible [4]. X-rays can also be produced in intense laser-plasma interactions by nonlinear Thomson scattering [5].Relativistic electron beams have also been measured from interactions at very high laser intensities, where electrons gain energy directly from the laser [6]. At high intensity, the ponderomotive force of the laser can expel plasma electrons leaving a positively charged ion channel. Electrons inside the channel experience a net focusing force due to the space charge and undergo oscillation at the betatron frequency ω β = ω p / √ 2γ z0 , where ω p is the plasma frequency and γ z0 is the Lorentz factor associated with the electrons motion along the plasma channel. Electrons resonant with the laser frequency can gain energy from the transverse electric field of the laser, which can be directed into longitudinal momentum through the v × B force [7]. Accelerating charges radiate electromagnetic radiation. For small betatron strength parameters a β = γ z0 r β ω β /c 1 (undulator limit), the spectrum of the radiation will be narrowly peaked about the resonant fre-is the Doppler factor and α is the angle between the direction of observation and the direction of γ z0 [8]. This highlights the interdependency of spectral and angular distributions. As a β → 1, emitted radiation also appears at harmonics of the resonant...

We consider the effect of laser beam shaping on proton acceleration in the interaction of a tightly focused pulse with ultrathin double-layer solid targets in the regime of directed Coulomb explosion. In this regime, the heavy ions of the front layer are forced by the laser to expand predominantly in the direction of the pulse propagation, forming a moving longitudinal charge separation electric field, thus increasing the effectiveness of acceleration of second-layer protons. The utilization of beam shaping, namely, the use of flat-top beams, leads to more efficient proton acceleration due to the increase of the longitudinal field.