Characterizing a molecular beam curtain for non-invasive beam profile monitor

Published on 11 January 2023

Non-invasive beam profile monitors are highly desirable for modern accelerators. They can provide comprehensive information about the particle beam, such as beam sizes and emittances, without stopping the operation. This would be ideal for many applications, including high intensity and high energy applications, such as  the Large Hardon Collider (LHC) and the European Spallation Source (ESS), and proton therapy accelerators, where a beam profile monitor based on a supersonic molecular curtain would guarantee a safe and smooth operation.

The advantages of such a method include minimum disturbance, two-dimensional measurement and versatility. The principle of such a monitor can be seen in the image below.

Concept of using a gas jet as a Beam profile monitor. (Image credit: Zhang, H et al. Vacuum 208, 111701, 2023, CC BY 4.0)

To fulfil the versatility of such methods, different sets of nozzle and skimmers will be used to generate molecular curtains with different sizes and number densities to tailor the need of particular accelerators with the requirement of the vacuum condition preservation and measurement time. Such modifications need to be characterized before their application. Recently in an open-access paper just published in Vacuum, researchers from the QUASAR Group developed a compression gauge method to quantify the molecular curtain. This method uses a hot ion gauge enclosed in a small chamber but with a pinhole opening for accepting the molecular beam for absolute number density measurement as seen in schematic drawing of the movable gauge below. By mechanically scanning the pinhole, the distribution can also be measured.

 

Schematic drawing of the movable gauge (Image credit: Zhang, H et al. Vacuum 208, 111701, 2023, CC BY 4.0)

Previously, laser interferometry techniques, Rayleigh scattering, a common microphone or pressure transducer, multi-photon ionization, and nuclear scattering were used to determine the molecular flow in the number density range of 10²⁰ – 10²² m^-3 or higher but suffer from the signal-to-noise ratio for our application where the number density is much lower.  This research fills the gap in measuring molecular beams in the ranges from 10¹⁴ – 10¹⁷ m^-3  and examples of such measurements can be seen below.  

Measurement of the number density distribution of nitrogen (a) and neon (b) molecular curtains (Image credit: Zhang, H et al. Vacuum 208, 111701, 2023, CC BY 4.0). 

This method does not only pave the way for further improvement of beam profile monitor development but can also be applied in other fields such as nuclear physics, nuclear astrophysics and atomic physics where similar supersonic gas jets or molecular beam sources are widely used.