Characterizing low dI/dt X-pinch bright spots through charge-based analysis of saturated soft x-ray photodiode signals.

This study investigates the characteristics of X-pinch plasmas driven under low current rise rate (dI/dt) conditions using soft x-ray spectroscopy combined with the Bennett relation. X-pinch experiments were conducted on the SNU X-pinch device using copper wires at a low dI/dt of 0.2-0.3 kA/ns. The resulting 1-10 keV soft x-ray signals, measured by using an x-ray-filtered AXUV photodiode array, exhibit significant nonlinear effects due to the high intensity of the soft x-ray pulses. This work characterizes the nonlinear behavior of the AXUV-HS5 Si PIN photodiodes under intense pulsed radiation using a pulsed laser. We identified a charge conservation property that the total collected charge remains proportional to the incident pulse energy despite temporal profile distortion. Based on this diagnostic finding, we developed and applied a new framework for plasma parameter estimation. By combining a spherical emission model with the Bennett equilibrium, this approach determines that the soft x-ray source plasma is a "bright spot," characterized by a plasma density ne ∼1021 cm-3, size d ∼30-40 μm, at electron temperature Te ∼1 keV, and an emission duration tB ∼1 ns, rather than an extremely compressed "hot spot."