Fig. 4: Ultrafast dynamics of a charged ultracold microplasma.

a–e Snapshots from a CPT simulation tracing the electrons (blue) and ions (red) right after strong-field ionization of an ultracold atomic cloud. An initial homogeneous distribution of ions at a density of ρ = 2 × 10¹⁴ cm⁻³ and electrons at a temperature of T_e ≈ 5250 K is created in a micrometer-sized volume (a). The initial kinetic energy of the ions corresponds to a coupling parameter of Γ_i = 4800 according to Eq. (2). The electrons leave the ionization volume on a picosecond timescale and are subsequently decelerated by the core of remaining ultracold ions (b). While the outer electrons escape, the inner electrons are bound onto orbital trajectories in the ions’ attractive Coulomb potential forming an ultracold plasma (c). On a nanosecond timescale the ionic component expands driven by Coulomb repulsion and further cools the electrons to temperature below 100 K (d, e). f Picosecond time-evolution of the field-free plasma simulation with the total energy per particle for the electrons (light blue) and the ions (red) and the mean kinetic energy of the electrons captured within the plasma (dark blue). g Evolution of the kinetic energy of the trapped plasma electrons and the effective depth of the space charge potential on the nano- to microsecond timescale for field-free expansion (dark blue/red), ±U_ext = 5 V (blue/red) and ±U_ext = 300 V (light blue/red) as well as the theoretically predicted kinetic energy evolution for adiabatic electron cooling (dotted blue line).