Collective relativistic electron dynamics in dense matter

Exposing matter to few-cycle relativistic light fields, which will be available for the first time from MAP’s unique sources (LWS-10 and PFS), ionises atoms on the time scale of the light oscillation period and triggers collective, relativistic electron motion with unprecedented efficiency. This collective motion unfolds in a background of inert ions and induces giant longitudinal plasma fields which accelerate electrons and subsequently ions in the forward direction. As an example, Fig. C.1.8 depicts the simulation of a 5-fs, 115-mJ laser pulse (available from LWS-10 shortly) generating a beam of 70 ± 10-MeV electrons in a simple gas target by wakefield acceleration. Such electron pulses have recently been observed¹⁾, but the reproducibility of the process remains to be improved. One goal of this project is to resolve the wakefield structure in space and time for the first time. This is to gain vital information on how to optimise and tailor the acceleration process. Other patterns to be studied occur when laser-driven relativistic beams pass through overdense plasma, exhibiting filamentation and filament coalescence. These dynamics are of great interest for fast ignition of nuclear fusion and also in the context of cosmic jets.

In a first step, we will use sub-50-fs, 1-J, 20-TW pulses from a Ti:sapphire laser focused to intensities of > 10¹⁹ W/cm² to trigger and sub-5-fs pulses to probe relativistic dynamics of low-density plasmas by means of shadowgraphy and schlieren imaging with a temporal and spatial resolution shorter than the plasma period. Later, we plan to use the intense attosecond X-ray pulses from few-cycle-driven relativistically-moving surfaces (A.2.2) for attosecond time-resolved plasma imaging. These pulses will be energetic and short enough to freeze relativistic collective electron motion in solid-density matter, allowing one for the first time to study relativistic transport, in particular filamentation of mega-ampere electron currents, in real time. The experimental work will be supported by dedicated 3D Particle-in-Cell (PIC) simulations.