Conventional solid-density laser-plasma targets quickly ionize to make a plasma mirror, which largely reflects ultra-intense laser pulses. This Fresnel reflection at the plane boundary largely wastes our e
orts at ultra-intense laser/solid interaction, and limits target heating to nonlinear generation of high-energy electrons which penetrate inward. One way around this dual problem is to create a material with an anisotropic dielectric function, for instance by nanostructuring a material in such a way that it cannot support the material responses which generate a specularly reflected beam. We present linear theory for metallic and plasma nanowires, particle-incell simulations of the interaction of ultra-intense femtosecond pulses with nickel nanowires, showing penetration of laser light far deeper than a nickel skin-depth, helping to uniformly heat near-solid material to conditions of high energy-densities, and XFEL experiments giving insight into their ionization and excitation.
The development of second-generation short-pulse laser-driven radiation sources requires a mature understanding of the relativistic laser-plasma processes as e.g. plasma oscillations, heating and transport of relativistic electrons as well as the development of plasma instabilities. These dynamic effects occurring on femtosecond and nanometer scales are very difficult to access experimentally.
In a first experiment in 2014 at the Matter of Extreme Conditions facility at LCLS we demonstrated that Small Angle X-ray Scattering (SAXS) of femtosecond x-ray free electron laser pulses is able to make these fundamental processes accessible on the relevant time and length scales in direct in-situ pump-probe experiments [Kluge et al., Phys. Rev. X 8, 031068 (2018)]. Here we report on a recent follow-up experiment with significantly higher pump intensity reaching the relativistic intensity domain, improved targetry, XFEL shaping and particle diagnostics. We give an overview of the new capabilities in combining a full suite of particle and radiation diagnostics including ion-, electron-, bremsstrahlung- and K-alpha-spectrometer, proton beam profile imager and SAX scattering. Especially probing at resonant x-ray energies can give new insight into the ultra-fast ionization processes, plasma opacity and equation-of-state in non-equilibrium plasmas.
Respresenting the collaborations of the latest two MEC SAXS experiments we will give an overview of the experimental setup and the technical implementation of radiation and particle diagnostics as well as imaging methods. We will exemplify the capabilities on the specific example of probing the correlation of thin layers under high-intensity laser irradiation and its consequences for modelling the heating of buried layers and rear surface expansion.
The x-ray free electron laser facility at SLAC National Accelerator Laboratory (named as LCLS) will be upgraded to LCLS-II in the near future. The high repetition rate light source makes the x-ray optics or components exposed to trillions of pulses over years of operation. Material fatigue properties of x-ray optics are essentially important for their lift-time prediction, optics optimization and opto-mechanical design. In this work, the fatigue properties of typical x-ray optics materials such as single-crystal silicon are experimentally measured by using laser pulses. The laser source can have an average power of 50 W at wavelength of 1.03 μm and repetition rate of 0.928 MHz with pulse duration of ~230 fs. The SHG crystal is used to generate 515 nm laser beam for the test to get an equivalent absorption length to soft x-rays. The maximum single-pulse energy is more than 16 μJ. The numbers of pulses that the optics can survive are measured for different pulse energies (fluences). The definition of the damage of x-ray optics is the significant reduction of reflectivity, which is premonitory of damage, and much more stringent than the ablation threshold.