Advances in understanding vacuum break dynamics in liquid helium-cooled tubes for accelerator beamline applications

TL;DR

This paper reviews experimental and theoretical studies on nitrogen gas propagation during vacuum breaks in liquid helium-cooled accelerator tubes, highlighting the effects of superfluid helium and complex geometries on flow dynamics.

Contribution

It introduces a 1D model for gas dynamics in uniform tubes and discusses plans for 2D modeling to address complex geometries in accelerator beamlines.

Findings

Gas front deceleration is nearly exponential, stronger in superfluid helium.

The 1D model reproduces key experimental observations.

Complex cavity geometries cause anisotropic flow patterns.

Abstract

Understanding air propagation and condensation following a catastrophic vacuum break in particle accelerator beamlines cooled by liquid helium is essential for ensuring operational safety. This review summarizes experimental and theoretical work conducted in our cryogenics lab to address this issue. Systematic measurements were performed to study nitrogen gas propagation in uniform copper tubes cooled by both normal liquid helium (He I) and superfluid helium (He II). These experiments revealed a nearly exponential deceleration of the gas front, with stronger deceleration observed in He II-cooled tubes. To interpret these results, a one-dimensional (1D) theoretical model was developed, incorporating gas dynamics, heat transfer, and condensation mechanisms. The model successfully reproduced key experimental observations in the uniform tube system. However, recent experiments involving a bulky copper cavity designed to mimic the geometry of a superconducting radio-frequency (SRF) cavity revealed strong anisotropic flow patterns of nitrogen gas within the cavity, highlighting limitations in extrapolating results from simplified tube geometries to real accelerator beamlines. To address these complexities, we outline plans for systematic studies using tubes with multiple bulky cavities and the development of a two-dimensional (2D) model to simulate gas dynamics in these more intricate configurations. These efforts aim to provide a comprehensive understanding of vacuum breaks in particle accelerators and improve predictive capabilities for their operational safety.