The energy of a laser-wakefield-accelerated electron [1] is limited by, among others, the distance it must propagate until it outruns the plasma wave by half of the plasma wavelength, i.e. the dephasing length Ld [2]. The dephasing length is given approximately by Ldλp3/λ02, where λp and λ0 are the plasma and laser wavelength, respectively. In order to excite resonantly a plasma wave, the laser pulse duration should match half of the plasma wavelength. Up to now, no direct or systematic measurement of the electron dephasing has been realized due to quality and reproducibility of the electron bunches or the long durations of the laser pulses ( 25 fs) employed to excite the wakefield, which led to dephasing lengths much larger than applied typical acceleration lengths. Taking advantage of the shortness of the pulses delivered by LWS-20 (<5 fs) and LWS-10 (8 fs) [3], dephasing lengths in the order of 100 µm become measureable using a highly precise and tunable injector such as shock-front injection [4]. For a given electron density within the range of 3 15 × 1019 cm3, by changing the acceleration length, energy-tunable quasi-monoenergetic electron bunches were generated (Fig.1 Right-inset). Fitting the peak energy of the electron spectrum with respect to the acceleration length (Fig.1 Right), yields a maximum accelerating field of (100 200GV/m), and most importantly, a dephasing length (80 300 µm) for each electron density. The latter matches quite well the expected values from the linear theory (Fig.1 Left). These results give a solid basis to design higher energy accelerators using longer laser pulses.

© 2015 IEEE

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