We describe the first demonstration of plasma wakefield excitation by a train of laser pulses, the concept underlying multi-pulse laser wakefield acceleration (MP-LWFA). Measurements of the wake amplitude show a clear resonance when the pulse spacing is matched to the plasma wavelength, and are in good agreement with analytical models. We also take a first step towards energy recovery by showing that a trailing laser pulse can damp the plasma wave.
We recently reconsidered the idea of exciting plasma wakefields by a train of low energy laser pulses, rather than by a single, high-energy pulse. In this approach, which we have called “multi-pulse laser wakefield acceleration” (MP-LWFA), the wakefields driven by the pulses in the train will add coherently, and hence the amplitude of the plasma wave will grow from the front to the back of the train if the pulses are spaced by the plasma period.
MP-LWFA has several potential advantages. Using a train of low-energy laser pulses opens up plasma accelerators to novel laser technologies, such as fibre or thin-disk lasers, which cannot directly deliver Joule-level pulses, but which can provide lower-energy pulses with pulse repetition rates in the kilohertz range at wall-plug efficiencies at least two orders of magnitude higher than conventional solid-state lasers. Further, the pulses in the train do not need to be identical, providing opportunities for optimizing efficiency and controlling the wakefield excitation. Finally, MP-LWFA provides a natural architecture for “energy recovery” – i.e. the removal, and potentially the re-use, of energy remaining in the wakefield after particle acceleration – since pulses at the back of the train can be timed to be out of resonance with the wake driven by earlier pulses.
We will describe the first proof-of-principle demonstration of the MP-LWFA concept. In this work, trains of up to seven laser pulses were generated from single Ti:sapphire laser pulses by inserting a Michelson interferometer prior to the vacuum compressor. The amplitudes of the plasma waves driven by the pulse trains were measured by frequency-domain holography using chirped, 400 nm probe and reference pulses. Measurements of the wakefield amplitude as a function of the plasma density show a clear resonance when the pulse spacing is matched to the plasma wavelength, and are in good agreement with analytical models.
We also demonstrate a first step towards energy recovery by showing that a suitably delayed laser pulse can damp the plasma wave driven by an earlier pulse.
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