Laser-plasma accelerators (LPAs) are capable of accelerating charged particles to very high energies in very compact structures. In theory, therefore, they offer advantages over conventional, large-scale particle accelerators. However, the energy gain in a single-stage LPA can be limited by laser diffraction, dephasing, electron-beam loading and laser-energy depletion. The problem of laser diffraction can be addressed by using laser-pulse guiding and preformed plasma waveguides to maintain the required laser intensity over distances of many Rayleigh lengths; dephasing can be mitigated by longitudinal tailoring of the plasma density; and beam loading can be controlled by proper shaping of the electron beam. To increase the beam energy further, it is necessary to tackle the problem of the depletion of laser energy, by sequencing the accelerator into stages, each powered by a separate laser pulse. Here, we present results from an experiment that demonstrates such staging. Two LPA stages were coupled over a short distance (as is needed to preserve the average acceleration gradient) by a plasma mirror. Stable electron beams from a first LPA were focused to a twenty-micrometre radius--by a discharge capillary-based active plasma lens--into a second LPA, such that the beams interacted with the wakefield excited by a separate laser. Staged acceleration by the wakefield of the second stage is detected via an energy gain of 100 megaelectronvolts for a subset of the electron beam. Changing the arrival time of the electron beam with respect to the second-stage laser pulse allowed us to reconstruct the temporal wakefield structure and to determine the plasma density. Our results indicate that the fundamental limitation to energy gain presented by laser depletion can be overcome by using staged acceleration, suggesting a way of reaching the electron energies required for collider applications.
Acceleration and manipulation of electron bunches underlie most electron and X-ray devices used for ultrafast imaging and spectroscopy. New terahertz-driven concepts offer orders-of-magnitude improvements in field strengths, field gradients, laser synchronization and compactness relative to conventional radio-frequency devices, enabling shorter electron bunches and higher resolution with less infrastructure while maintaining high charge capacities (pC), repetition rates (kHz) and stability. We present a segmented terahertz electron accelerator and manipulator (STEAM) capable of performing multiple high-field operations on the 6D-phase-space of ultrashort electron bunches. With this single device, powered by few-micro-Joule, single-cycle, 0.3 THz pulses, we demonstrate record THz-acceleration of >30 keV, streaking with <10 fs resolution, focusing with >2 kT/m strength, compression to ~100 fs as well as real-time switching between these modes of operation. The STEAM device demonstrates the feasibility of THz-based electron accelerators, manipulators and diagnostic tools enabling science beyond current resolution frontiers with transformative impact.
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
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