X-ray Line Ratio Study of Bulk Plasma Heating by Laser-induced Relativistic Electrons*

Short-pulse laser pulses deposit significant fractions of their energy into relativistic electron beams. The mechanisms by which electron beams heat bulk solid material have long been under study since they play a central role in the ultimate conversion of laser energy into x-rays or fast particles. We build on past studies that measured x-ray line ratios to study bulk electron temperatures at different locations in short-pulse-heated solid targets, here limiting the impact of fast electron refluxing by deploying targets with thickness 20–400 μm. We construct targets with Ti tracer material deposited in layers and micro-stripes at different depths under the laser-facing surface, then study the variations in line emission from Ti Heα and Lyα complexes as a function of tracer position. Our strategy improves on past investigations in several ways: (1) by using laser pulses with high temporal contrast, we reduce uncertainty in bulk plasma density and improve shot-to-shot repeatability; (2) by relying on high repetition rate, we gather ample statistics and include sophisticated error propagation into our analysis; (3) by comparing multiple line ratios via multiple collisional-radiative models, we develop a consistent understanding of the dependence of line ratios on plasma parameters and profiles. We will compare measured electron temperature profiles to models of relativistic electron beam transport to clarify what physical mechanisms are most critical for explaining the data.