Femtosecond laser and x-ray streak camera probe matter in extreme conditions
At the Center for Ultrafast Optical Science, University of Michigan (Ann Arbor, MI), a team led by Don Umstadter has analyzed high-density/low-temperature strongly coupled plasmas with a 100-fs Ti:sapphire laser. Such plasmas are found in inertial-confinement-fusion reactors, white and brown dwarf stars, and Jovian planets. Laser pulses of 50 mJ at 780 nm were focused with a magnesium fluoride lens to an intensity of 1017 W/cm2 on rotating solid-carbon-disk targets. The emission spectra from the resulting plasma were recorded with a grazing-incident, flat-field-imaging extreme-ultraviolet spectrometer coupled to a subpicosecond x-ray streak camera.
With a technique used for the first time in spectroscopy, the researchers extended the range of the streak camera and increased its signal-to-noise ratio by coupling to the camera a jitter-free averaging swee¥system comprising two photoconductive switches triggered by part of the femtosecond beam that sweeps the plates of the camera at 10 GHz, synchronized with the target emission. The technique greatly improved laser contrast and enabled the researchers to derive the first experimental confirmation of the predicted behavior of atoms in superdense environments.