Measurement of Position Resolutions of L-band Cavity Beam Position Monitors

TL;DR

This work presents an L-band cavity BPM prototype tailored for the ILC main linac to achieve sub-μm-to-nm position resolution. By designing a TM$_{110}$ dipole mode at $2.040$ GHz and deploying a multi-channel downconversion and correlation framework, the authors quantify position resolution via SVD-based channel correlations, FFT-based amplitude calibration, and geometrical corrections. Initial 2024 measurements revealed gain and phase incoherencies that degraded performance; subsequent 2025 tests employing a 45 MHz IF and a common LO configuration achieved robust linear correlations and improved resolutions, approaching the ~300 nm level at $1.6~\mathrm{nC}$ beam charge. The study highlights critical hardware synchronization issues (LO phase, downconverter stability) and outlines concrete steps to reach even better BPM performance for collider-scale beam control and feedback.

Abstract

Beam position monitors (BPMs) are indispensable components of modern particle accelerators, providing real-time diagnostics to ensure precise beam control, stability, and quality. As accelerators such as the International Linear Collider (ILC) aim for nanometer-scale beam sizes at the interaction point, stringent requirements on position resolution arise. Specifically, the main linac of the ILC demands a BPM resolution better than 5 μm to support stable beam transport and minimize emittance growth. To address this, we have developed an L-band cavity BPM optimized for the beam conditions of the ILC. In this paper, we introduce a prototype of an L-band cavity BPM and its signal processing system, describe the methodology for position resolution measurements, discuss the problems and solutions encountered in the past experiment, and report the projected position resolutions of about 300 nm at best.

Figures (10)

• Figure 1: Three L-band cavity BPMs installed in the ATF. The beam enters from the left side, and the cavity BPMs are labeled as BPM-A, BPM-B, and BPM-C from upstream to downstream. The electronics for the signal processing are placed under the BPM-B covered by a black lead sheet and lead blocks for a radiation shielding.
• Figure 2: The signal processing chain for the L-band cavity BPMs illustrated along with other components of the ATF such as the steering magnets and stripline BPMs. The red rectangles (labeled as "ZH1P", "ZV1P", "ZH2P", and "ZV2P") indicate the steering magnets (H and V for horizontal and vertical, respectively) and the green rectangles (labeled as "ML2P", "ML3P", and "ML4P") indicate the stripline BPMs. The blue rectangles indicate cavity BPMs. The first cavity BPM labeled as "Ref. Cavity" is a reference cavity which was not used in this work. The downconverter is located inside the beam tunnel whereas the LO source and the oscilloscopes are placed outside the tunnel.
• Figure 3: The downconverter used for the signal processing of the L-band cavity BPMs. One channel out of 8 identical channels is shown.
• Figure 4: The correlation between the measured and predicted signals (scatter plot), and the residual distribution (histogram) from the 2024 experiment for BPM-A horizontal (top left), BPM-A vertical (top right), BPM-B horizontal (middle left), BPM-B vertical (middle right), BPM-C horizontal (bottom left), and BPM-C vertical (bottom right). FFT values and residuals are shown in arbitrary units throughout the paper. Blue dashed line in the scatter plots indicates the ideal correlation. Red line in the histograms is a fit with a Gaussian function.
• Figure 5: Simulated correlations between the measured and predicted signals (scatter plot), and the residual distribution (histogram). The upper figures are from the simulation with the gain incoherency of which was measured as Fig. \ref{['fig:gain_fluctuation']} whereas the lower figures are from the simulation with the phase incoherency of $2\pi$. Only BPM-A cases are shown here for brevity. Blue dashed line in the scatter plots and red line in the histograms have the same meanings as in Fig. \ref{['fig:correlation_2024']}.