Beamstrahlung monitoring at SuperKEKB upgrade 2023

TL;DR

This work documents a 2023 upgrade of the Large-angle beamstrahlung monitor (LABM) at SuperKEKB, replacing PMTs with Basler CMOS cameras to image the beamstrahlung spot and background simultaneously. By applying two processing pipelines—a full-image CNN and a reduced-image NN based on OpenCV descriptors—the authors predict beam parameters, including the specific luminosity $\mathcal{L}_{spec}=\frac{f_0}{2\pi \Sigma_x \Sigma_y}$, and IP beam sizes, achieving improved accuracy over the previous PMT configuration. The results favor the FI CNN approach, while highlighting timing and synchronization limitations; the authors propose hardware and algorithmic refinements and outline a path toward autonomous beam-parameter estimation via beamstrahlung simulations, with potential applicability to future colliders such as EIC and FCC-ee.

Abstract

Beam monitoring is crucial for particle accelerators to achieve high luminosity. We describe an upgrade of the LABM, a beam monitoring device utilizing observation of the beamstrahlung, radiation emitted by a beam of charged particles when it accelerates in the electromagnetic field of another beam of charged particles.

Figures (21)

• Figure 1: Two vacuum mirrors are used to extract the beamstrahlung light from the vacuum beam pipe through small windows. The beamstrahlung light coming from the IP is indicated with arrows. The light is then guided towards the detector by a system of pipes and mirrors, which is not shown here.
• Figure 2: Optic box with cameras. The light enters the box as indicated by an arrow, the Wollaston prism (1) splits the light polarizations, the lens (2) focuses the light on the cameras (3).
• Figure 3: The results of the scans for x- (PMT1) and y- (PMT2) polarizations for Oho down (upper left), Oho up (upper right), Nikko up (lower left), Nikko down (lower right).
• Figure 4: Examples of images of the spot for three color components (R, G and B) for Oho down x-polarization (upper left), Oho down y-polarization (upper right), Oho up x-polarization (lower left), Oho up y-polarization (lower right).
• Figure 5: Examples of images of the spot for three color components (R, G and B) for Nikko down x-polarization (upper left), Nikko down y-polarization (upper right), Nikko up x-polarization (lower left), Nikko up y-polarization (lower right).