Digital Low-Level RF control system for Accumulator Ring at Advanced Light Source Upgrade Project

TL;DR

This paper presents a digital low-level RF control system for the ALS Upgrade Project's Accumulator Ring, featuring high stability, real-time interlock, and integrated cavity emulation for system validation.

Contribution

It introduces a novel digital LLRF system with advanced stability, fast interlock, and integrated diagnostics for the ALS upgrade.

Findings

RF amplitude stability < 0.1%

Phase stability < 0.1°

Interlock latency < 2 μs

Abstract

Currently ALS is undergoing an upgrade to ALSU to produce 100 times brighter soft X-ray light. The LLRF system for Accumulator Ring (AR) is composed of two identical LLRF stations, for driving RF amplifiers. The closed loop RF amplitude and phase stability is measured as $< 0.1\%$ and $< 0.1^\circ$ respectively, using the non-IQ digital down conversion together with analog up/down conversion, under a system-on-chip architecture. Realtime interlock system is implemented with $< 2 \mu$s latency, for machine protection against arc flash and unexpected RF power. Control interfaces are developed to enable PLC-FPGA-EPICS communication to support operation, timing, cavity tuning, and interlock systems. The LLRF system handles alignment of buckets to swap beams between AR and Storage Ring by synchronous phase loop ramping between the two cavities. The system also includes an optimization routine to characterize the loop dynamics and determine optimal operating point using a built-in network analyzer feature. A cavity emulator of 31 kHz bandwidth is integrated with the LLRF system to validate the performance of the overall system being developed.