Observation of rapid energy transfer from relativistic, short pulse laser generated electrons to solid material

With the advent of high intensity ultra-short pulse lasers, relativistic electron generation in laser-solid interactions is common in laboratory plasmas. The coupling of these electrons to the surrounding matter plays a critical role in short pulse laser applications such as harmonic generation [1], short pulse x-ray production [2], and fast ignition [3]. We present the experimental observation of the energy transfer and damping rate of relativistic electrons in solid density matter. In general, this data provides a unique window into the time scale for collisional relaxation between relativistic particles and cold classical particles in a dense medium. \\[0pt] [1] Dromet, B., et. Al, ``High hamonic generation in the relativistic limit,'' \textit{Nature}, \textbf{2}, 456-459, (2006). \\[0pt] [2] Salzmann, D., et al, ``Theory of \textit{K-$\alpha $} generation by femtosecond laser-produce hot electrons in thin foils,'' \textit{Phys. Rev. E., }\textbf{65}, 036402-1-5, (2002). \\[0pt] [3] Kodama, R., et al, ``Fast heating of ultrahigh-density plasma as a step towards laser fusion ignition,'' \textit{Nature}, \textbf{412}, 798-801, (2001).