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

TL;DR

This work addresses the need for high-temporal-resolution timing of keV electrons over an extended time window. It introduces a two-frequency, phase-locked helical RF deflector to convert arrival time into detector position, producing spiral scans via a beat envelope with period $T_b=1/|f_1-f_2|$ and enabling picosecond-scale timing across a wider dynamic range. A first-principles model yields analytical expressions for transverse velocities and exit radii in terms of $a$, $ riangle extvarphi$, and $(k_1,k_2)$, predicting controllable spiral traces. Experimental validation with 2.5 keV electrons confirms spiral patterns and close agreement with theory across $f$ in the $400-1000$ MHz range, demonstrating substantial time-range extension while maintaining ultrafast precision and compatibility with MCP-DL readout and high-rate detectors.

Abstract

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. Spiral scanning will allow measurements with picosecond resolution in a time range 1-2 orders of magnitude larger than the period of the circular scanning.

Figures (8)

• Figure 1: Beat phenomenon for frequencies of 500 MHz and 600 MHz.
• Figure 2: Schematic of the deflector.
• Figure 3: Schematic of the experimental setup. 1- UV photons; 2 - quartz window; 3 - magnet; 4 - collimator; 5 - accelerating electrode; 6 - photoelectron; 7 - cathode; 8 - electrostatic lens; 9 - RF deflector; 10 - MCP detector; 11 - delay line anode.
• Figure 4: (a) typical amplified signals from the DL anode; (b) 2D image of the focused electrons (RF is OFF); (c) 2D image of the scanned electrons (500 MHz RF is ON).
• Figure 5: Photograph of the half-period deflector.