Application of RFSoC Technology for Beam Position Monitors at the SuperKEKB Storage Rings Injection Points

TL;DR

This work addresses the need for precise, independent beam-position measurements at SuperKEKB injection points under a two-bunch injection scheme. It implements a flexible BPM readout based on an RFSoC platform, including custom front-end conditioning, firmware, and software with a rogue-based network stack for data handling and processing. ENOB measurements indicate near state-of-the-art ADC performance, and beam-position resolution tests at the KEK linac suggest target performance is achievable with proper signal conditioning and calibration. The deployment at SuperKEKB demonstrates the feasibility of RFSoC-based readout for high-rate beam instrumentation and outlines a path toward broader RFSoC adoption for accelerator diagnostics and feedback systems.

Abstract

In order to achieve its ambitious luminosity target, the SuperKEKB collider must achieve and sustain high beam currents on the order of Ampere in its storage rings. This requires continuous top-up injection and operation with a two-bunch injection scheme, injecting two 96 ns spaced bunches in a single injection cycle. An important input for tuning the injection beam is the position reading from a dedicated beam position monitor (BPM), located after the septum magnets, slightly upstream of where the injected and stored beams converge. Previously, the readout electronics used for these special BPMs were not capable of independent measurement of both bunches in the two-bunch injection mode and modification of the concerned devices and their firmware was not feasible. The opportunity was taken to develop a new readout device based on the AMD/Xilinx RF System on a Chip (RFSoC) platform with the goal of not only providing a sufficiently flexible and performant readout solution for the concerned BPMs, but also to evaluate and gain experience with the platform for beam monitor electronics applications. This paper is concerned with the details of this development as well as evaluation and operation of the developed device.

Figures (11)

• Figure 1: Simplified 3D model of the injection point BPM vacuum chamber for the electron ring.
• Figure 2: Signal maps for all electrodes of the used BPM chambers as determined by BEM simulation. The used chamber outline is shown in white. Only the map values within the outline are of relevance. Black lines represent logarithmically spaced contours.
• Figure 3: Photograph of the used RFSoC 4x2 evaluation board.
• Figure 4: Difference between expected and measured signal power (up to a constant offset) as a function of the set attenuation of the step attenuator in each of the four channels available on an RFSoC 4x2 evaluation board.
• Figure 5: Raw and shaped waveform from one of the electrodes. The former is attenuated by $20dB$ and captured using a Rohde&Schwarz RTO 1024, 10 GSPS oscilloscope. The latter is captured using the RFSoC 4x2 board. Note the large difference in time scales.