Abstract
High frequency modulations appearing in the kinetic energy distribution of laser accelerated ions are proposed for retrieving the acceleration field dynamics at femtosecond time scale. Such an approach becomes possible if the laser-cycling field modulates the particle density in the ion spectra and produces quasi time stamps for analysis. We investigate target and laser parameters determining this effect and discuss the dependencies of the observed modulation. Our findings refine a basic mechanism, the Target Normal Sheath Acceleration, where an intense and ultrafast laser pulse produces a very strong electrical field at a plasma-vacuum interface. The field decays rapidly due to energy dissipation and forms a characteristic spectrum of fast ions streaming away from the interface. We show that the derived decay function of the field is in accordance with model predictions of the accelerating field structure. Our findings are supported by 2-dimensional particle-in-cell simulations. The knowledge of the femtosecond field dynamics helps to rerate optimization strategies for laser ion acceleration.
Highlights
Possible applications of laser-accelerated ion beams need tailored energy distribution functions (EDFs) of the ions
High-frequency modulations appearing in the kinetic energy distribution of laser-accelerated ions are proposed for retrieving the acceleration field dynamics at the femtosecond timescale
One can conclude that the ion acceleration itself is a main channel of energy dissipation for the sheath acceleration structure
Summary
Possible applications of laser-accelerated ion beams (cf., e.g., [1,2,3,4,5], and references therein) need tailored energy distribution functions (EDFs) of the ions. We suggest to use a modulation phenomenon in laser-driven proton beams for tracing the acceleration process at a femtosecond timescale This allows comparing experimental EDF results with model functions of the acceleration field dynamics. We use and study in more detail faint density modulations in the kinetic energy spectrum of TNSA protons [12] which become visible if the laser intensity contrast between the pulse background and the peak is at a level of about 1010 At such conditions, even the cycling action of the laser field within the temporal envelope of the laser pulse imprints onto the acceleration field and the velocity distribution of the accelerated ions. We conclude that the process of energy dissipation and the corresponding decline of the acceleration field strength are related to the gain of kinetic energy during the ion acceleration in the field
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