Relationship between Terahertz and X-ray signals Generated from Laser-Induced Plasma on gas targets

Abstract

We have observed terahertz (THz) radiation, by using liquid-helium-cooled THz detectors, from gas targets irradiated by ultra-intense laser pulses of 30 fs and 200 ∼ 300 mJ. The irradiating intensity of the Ti:Sapphire laser pulse was more than 10 W/cm. For a helium gas target, it is incident on the gas jet at 500 ∼ 900 μm above the gas nozzle of 1 mm in diameter by using an off-axis parabolic (OAP) mirror with a f -number of 6.8. The relationships between the THz and the X-ray signals are different in the target materials. In a solid target, the aluminum (Al) target shows a linear dependency between them but no relationship is observed for the Mylar target in the case of linear mode conversion. This may be due to different mechanism of transport of hot electrons in the plasmas from conducting and dielectric materials. The relationships between THz and X-ray signals from the helium gas target was measured, analyzed, and compared with the results from the solid targets. The THz signals were measured from coherent transition radiation that could be generated when short electron bunches passed through a plasma-vacuum boundary. We also observed the dependence of THz signals on total charges of accelerated electrons by changing the electron plasma density. We adjusted the experimental conditions by changing both the backing pressure of the gas and the interaction position from the gas nozzle. In this paper, we discuss the relationship between THz signals, X-ray signals, and the yield of laser-accelerated electrons by analyzing electron charge measured using a Lanex film.