Relativistic wakes produced by intense laser or particle beams propagating through plasmas are being considered as accelerators 1, 2 for next generation of colliders and coherent light sources 3 . Such wakes have been shown to accelerate electrons and positrons to several gigaelectronvolts (GeV) 4-10 , with a few percent energy spread 8-10 and a high wake-to-beam energy transfer efficiency 7 . However, complete mapping of electric field structure of the wakes has proven elusive. Here we show that a high-energy electron bunch can be used to probe the fields of such light-speed wakes with femtosecond resolution. The highly transient, microscopic wakefield is reconstructed from the density modulated ultra-short probe bunch

An efficient method for generating extended plasma waveguides is developed by using the axicon lens in conjunction with the ignitor-heater scheme. The short-pulse ignitor generates the seed electrons by multiphoton ionization, and the long-pulse heater expands the plasma by inverse bremsstrahlung heating and builds up the plasma density barrier by collisional ionization. A 1.2-cm-long plasma waveguide is generated in pure Ar gas with a total energy of only 100 mJ. Evolution of the plasma density profile is measured by time-resolved interferometry to show the waveguide forming process and how it can be optimized.

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Phase-contrast imaging using X-ray sources with high spatial coherence is an emerging tool in biology and material science. Much of this research is being done using large synchrotron facilities or relatively low-flux microfocus X-ray tubes. An alternative high-flux, ultra-short and high-spatial-coherence table-top X-ray source based on betatron motions of electrons in laser wakefield accelerators has the promise to produce high quality images. In previous phase-contrast imaging studies with betatron sources, single-exposure images with a spatial resolution of 6–70 μ m were reported by using large-scale laser systems (60–200 TW). Furthermore, images obtained with multiple exposures tended to have a reduced contrast and resolution due to the shot-to-shot fluctuations. In this article, we demonstrate that a highly stable multiple-exposure betatron source, with an effective average source size of 5 μ m, photon number and pointing jitters of <5% and spectral fluctuation of <10%, can be obtained by utilizing ionization injection in pure nitrogen plasma using a 30–40 TW laser. Using this source, high quality phase-contrast images of biological specimens with a 5- μ m resolution are obtained for the first time. This work shows a way for the application of high resolution phase-contrast imaging with stable betatron sources using modest power, high repetition-rate lasers.

A Ti:sapphire laser system has been constructed with two synchronized main beams of 110 TW and 13 TW, and a 5-TW wavelength-tunable synchronized auxiliary beam for versatile control of laser-plasma interaction. The first main beam provides 3.3-J, 30-fs, 810-nm pulses, and the second 450-mJ, 34-fs, 805-nm pulses. The auxiliary beam comes from amplified spectral windows selected from a supercontinuum of high spatial coherence and provides 38-fs pulses with tunable wavelengths (870-920 nm). The two main beams can be focused down to M 2 = 1.2 and 1.1, with 77 and 81 % energy enclosed in the focal spots, respectively. The energy fluctuations are 1.1 and 1.8 %, and the pointing fluctuations are 4.5 and 4.8 lrad, respectively. By using a preamplifier and saturable absorber before the pulse stretcher to suppress amplified spontaneous emission, the temporal contrast of the 110-TW main beam reaches 4 Â 10 À10 at the -100-ps timescale. Even though the auxiliary beam is generated from a highly nonlinear process, by confining the supercontinuum generation in a single self-trapping filament, a spatial coherence close to the main beams can be achieved. It can be focused down to M 2 = 1.3, with 72 % energy enclosed in the focal spot. The energy fluctuation is 2.6 %, and the pointing fluctuation is 4.7 lrad. The versatility of synchronized multiple-beams with tunable wavelengths, good energy and pointing stability, and the spatiotemporal quality of the laser system has been essential to our experiments in high-harmonic generation, extreme-UV lasers, and laser-wakefield accelerators in which precision control of laser-plasma interaction is facilitated by a concerted sequence of driving pulses.

We report a significant enhancement of betatron radiation from a laser wakefield accelerator (LWFA) by inserting a density-depressed plasma structure. By using a technique of transverse laser machining, a longitudinally density-depressed plasma structure with tunable length and position has been fabricated to increase the betatron amplitude of the electron beam in the LWFA, leading to an enhanced photon number and critical energy of the betatron x-ray beam. By adjusting the length and position of the plasma density depression region, the photon number of the betatron x-ray is enhanced by a factor of 3, and the critical energy is enhanced by a factor of 1.4.

By adding a transverse heater pulse into the axicon ignitor-heater scheme for producing a plasma waveguide, a variable three-dimensionally structured plasma waveguide can be fabricated. With this technique, electron injection in a plasma-waveguide-based laser wakefield accelerator was achieved and resulted in production of a quasi-monoenergetic electron beam. The injection was correlated with a section of expanding cross-section in the plasma waveguide. Moreover, the intensity of the X-ray beam produced by the electron bunch in betatron oscillation was greatly enhanced with a transversely shifted section in the plasma waveguide. The technique opens a route to a compact hard-X-ray pulse source.

Programmable fabrication of longitudinal spatial structures in an optically preformed plasma waveguide in a gas jet was achieved, by using laser machining with a liquidcrystal spatial light modulator as the pattern mask. Fabrication of periodic structures with a minimal period of 200 µm and density-ramp structures with a minimal slope length of 100 µm was attained. The technique is useful for the optimization of various laser-plasma-based photon and particle sources.