Laser intensity dependence on relativistic electron isochoric heating in a solid density copper foil*

Understanding the dynamics of relativistic electron isochoric heating in an intense short-pulse laser-solid interaction is essential for electron-driven warm dense matter, laboratory astrophysics, and fusion energy research. Recently, an experimental platform combining a high-power laser and an x-ray free electron laser (XFEL) has been used for measuring the laser-solid interaction with femtosecond temporal resolution. At the SACLA XFEL facility, we demonstrated spatiotemporally resolved X-ray transmission imaging to visualize the fast electron heat front propagating in a solid-density copper target [1] and to diagnose the target’s plasma conditions [2]. Here, we report an experimental investigation of the laser intensity dependence on isochoric heating of a solid copper foil. A high-power femtosecond laser irradiated on a 2 µm thick Cu foil at an intensity ranging from 1x10¹⁸ and 5x10¹⁹ W/cm², and a 10-fs collimated x-ray beam imaged the entire foil to measure changes in x-ray transmission caused by target heating. In addition to the XFEL imaging, escaping fast electrons and electron-induced K-alpha x-rays were measured with a magnet-based electron spectrometer, an x-ray spectrometer, and a monochromatic x-ray imager. We found that the size of an enhanced transmission region increases with the peak laser intensity, indicating that a higher mean kinetic energy of the fast electron spectrum is generated. However, the electron temperature inferred from the smearing of the K-edge profile in XFEL imaging is comparable. Details of the experiment and a shift of the K-edge to higher photon energy we observed will be discussed.