Energy-Threshold and Preplasma Sensitivity of a Paired-Control PIC Suprathermal-Electron Number-Fraction Diagnostic

Simulations that track individual particles are widely used to study the energetic electrons produced when an intense laser pulse strikes a solid target, yet the reported fraction of these suprathermal electrons depends strongly on the energy threshold, sampling region, and post-processing rule used to extract it. We define a number-fraction diagnostic that compares each laser-driven simulation with a matched simulation in which the laser is switched off, normalizing the two electron populations separately before subtracting them. Across three independent runs of the reference case, the mean fraction decreases from 0.180 to 0.0662 and 0.0210 as the energy threshold increases from 1.0 to 1.5 and 2.0 keV, respectively. The corresponding 88% decrease from 1.0 to 2.0 keV is substantially larger than the observed seed dispersion; the descriptive CV remains below 7% at all three reference thresholds. A prescribed plasma layer in front of the target lowers the 1.5 keV value to 0.0420 and 0.0328 for scale lengths of 0.2 and 0.5 laser wavelengths. Reconstructing a previously used histogram-based estimator shows that neglecting the unequal widths of its energy bins explains 97% of its eightfold deviation from the particle-resolved value. The diagnostic therefore quantifies how a reported suprathermal-electron fraction depends on its definition and provides a reproducible basis for comparing threshold, preplasma, estimator, and random-seed effects.