Experimental Safe Extremum Seeking for Accelerators

TL;DR

This paper demonstrates Safe Extremum Seeking on a large particle accelerator, using a safety filter based on an unknown control barrier function to ensure safe operation while optimizing beam current.

Contribution

It introduces the application of Safe ES with a safety filter to real-world accelerator tuning, including experimental validation and simulation-based tests.

Findings

Safe ES successfully tuned accelerator parameters without safety violations.

The method improved beam current optimization while maintaining safety constraints.

Experimental results validated the effectiveness of the safety filter in real accelerator operations.

Abstract

We demonstrate the recent designs of Safe Extremum Seeking (Safe ES) on the 1 kilometer-long charged particle accelerator at the Los Alamos Neutron Science Center (LANSCE). Safe ES is a modification of ES which, in addition to minimizing an analytically unknown cost, also employs a safety filter based on an analytically unknown control barrier function (CBF) safety metric. Accelerator tuning is necessitated by the accelerators being large, with many drifting parameters due to thermal effects and degradation. At the same time, safe operation (the maintenance of state constraints) is crucial, as damage brings astronomical costs, both financially and in operation downtime. Our measured (but analytically unknown) safety metric is the beam current. We perform multivariable Safe ES on three accelerator applications, in which we adapt 4, 6, and 3 magnet strength parameters, respectively. Two of the three applications are for validated simulation models of beamlines at LANSCE: the first for the Proton Radiography (pRad) beamline of 800 MeV protons for spot size tuning; the second on a high performance code, HPSim, for tuning the low energy beam transport (LEBT) region of of 750 keV protons. The third is an experimental tuning of the steering magnets in the LEBT at LANSCE.