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Description
Electron energy spectra featuring multiple quasi-monoenergetic peaks with uniform, narrow spacing are observed when a few-cycle (~9 fs), multi-terawatt laser pulse is used to drive laser wakefield acceleration in a helium–nitrogen gas mixture. This comb-like energy structure results from periodic ionization injection triggered by successive half-cycles of the laser electric field. Crucially, the evolving carrier-envelope phase (CEP) during laser propagation through the plasma enables this sub-cycle injection process. Our experimental findings provide the first direct observation of CEP-driven energy bunching in a plasma accelerator, establishing a scheme where electron injection and beam shaping can be precisely synchronized to the laser’s optical waveform. This work opens a new pathway toward attosecond-scale control in laser-plasma acceleration.
