Nonlinear Ionization Dynamics of a Laser Plasma Amplifier

From fusion dynamics in stars to novel light sources, hot dense plasmas are of importance for an array of physical phenomena. An insight of highly-ionized matter is crucial for understanding and controlling the generating processes. High-density plasmas are turbulent and opaque for radiation below the plasma frequency and allowing only a near-surface insight into ionization processes with visible light. Here, the output of a high harmonic seeded laser-plasma amplifier using eight-fold ionized krypton as gain medium operating at 32.8 nm wavelength is ptychographically imaged. A complex wavefront is observed in the extreme ultraviolet beam with high resolution. Ab initio spatio-temporal Maxwell-Bloch simulations show excellent agreement with the experimental observation revealing an overionization of krypton in the plasma channel due to nonlinear laser-plasma interactions. This constitutes the first experimental observation of the laser ion abundance reshaping the laser plasma amplifier. This finding has direct implications for upscaling plasma-based XUV and X-ray sources. Moreover, the presented approach shows the possibility to directly model light-plasma interactions in extreme conditions, such as present in early times of the universe, with direct experimental verification.