Frequency-domain harmonic fitting for robust pulse amplitude extraction in noisy Langmuir probe environment

Traditional Langmuir probe diagnostics, employing direct current (DC) bias sweeps with averaging over tens to hundreds of milliseconds, are susceptible to power-line interference and random outliers, leading to significant dispersion in repeated measurements. In this work, under a measurement mode of a DC sweep superimposed with periodic rectangular pulses, we propose a frequency-domain harmonic envelope fitting method. Exploiting the theoretical fact that the harmonic amplitudes of a rectangular pulse follow a sinc envelope, the method combines segmented outlier repair, optimal phase alignment, and weighted least squares fitting of low-order harmonics to accurately reconstruct the pulse amplitude while suppressing power-line interference. In fixed-bias repeated measurements, the frequency-domain method reduces the coefficient of variation from 8.34% (time-domain difference method) to 0.52%, and the measurement spread (the difference between the maximum and minimum values across repeated measurements) shrinks by over 90%; in bias sweep measurements, the average stability improves by 76.8%. Requiring only software post-processing, this method provides a simple and efficient anti-interference solution for pulsed probes.