Few-Cycle-Laser-Driven Electron Acceleration

Laser-driven plasma waves were proposed as compact electron accelerators [1] owing to their ability to produce longitudinal accelerating ¯elds several orders of magnitude higher than those attainable in conventional accelerators. A promising implementation relies on "broken" plasma waves driven by a laser pulse shorter than half the plasma period [2]. In the absence of intense laser pulses of the required duration, previous experiments [3] - [13] drew on longer (several-10-fs) driver pulses. Under these circumstances, monoenergetic electron acceleration is preceded by a nonlinear interaction of the laser pulse with the relativistic plasma [14], which shortens its duration into the required domain. Longer-than-optimal driver pulses compromise e±ciency as well as reproducibility, and result in copious amounts of low-energy electrons accompanying the monoenergetic emission with an exponentially-decaying spectrum, forming a "thermal" background.
Here, we report the first electron accelerator based on high-density plasma waves driven with laser pulses ¯tting in one half of the plasma period. Direct excitation of a broken plasma wave permits monoenergetic electron acceleration virtually free of thermal background for the first time. In our experiments, 5-terawatt, 8-femtosecond laser pulses yield electron bunches up to energies of 25 MeV.
The dramatically reduced °ux of low-energy electrons as compared to earlier experiments also manifests itself in a strongly-reduced secondary radiation emerging from the accelerator and o®ers the potential for enhancing e±ciency and stability with more intense driver pulses.