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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