Identify the physical origin of PCA mode 4

Identify the physical origin of principal-component-analysis mode 4 in the electron-beam-profile fluctuations, whose core–halo structure has no unique counterpart among the measured conventional scalar observables.

Background

Principal component analysis separated the electron-beam fluctuations into several physically interpretable modes associated with charge, pointing, and beam-width variations. Mode 4 instead exhibited a more complex core–halo structure and could not be linked uniquely to the conventional scalar measurements used in the analysis. Its physical cause therefore remains unresolved by the present measurements.

References

Mode 4 exhibits a more complex core--halo structure and does not have a unique counterpart among the conventional scalar observables. Its physical origin cannot be determined from the present measurements.

— Stable 50 MeV Beams from a 100 Hz Laser-Wakefield Accelerator Driven by an OPCPA Laser  (2609.26390 - Smartsev et al., 22 Sep 2026) in Section IV, “Advanced statistical analysis”