A Helical-Deflector-Based Radio-Frequency Spiral Scanning System for keV Energy Electrons

We present the design, modeling, and experimental validation of a radio-frequency based time-to-position conversion system for keV electrons incorporating a helical deflector operating in the 400-1000 MHz range. The device performs circular deflection of the electrons when driven by a single RF frequency and enables spiral scanning when two phase-locked RF voltages with slightly different frequencies are applied. The superposition of the two phase-locked RF voltages produces an amplitude-beating field whose slowly varying envelope modulates the deflection radius, transforming the circular scan into a controlled spiral on the detector plane. A detailed theoretical model describing the electron dynamics under two phase-locked RF voltages with different frequencies was derived, yielding analytical expressions for the transverse velocity and radius-vector components at the deflector exit. The experimental studies demonstrated good agreement with the model predictions. Spiral scanning will allow measurements with picosecond resolution in a temporal dynamic range 1-2 orders of magnitude larger than the period of the circular scanning.