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Updated: Dec 22, 2025

Building Langmuir Probes and Emissive Probes for Plasma Potential Measurements in Low Pressure, Low Temperature Plasmas

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ISIS Penning source extraction studies reveal the 3-dimensional Child-Langmuir effect.

D Faircloth¹, O Tarvainen¹, S Lawrie¹

¹STFC ISIS Pulsed Spallation Neutron and Muon Facility, Rutherford Appleton Laboratory, Harwell OX11 0QX, United Kingdom.

The Review of Scientific Instruments

|May 3, 2020

Summary

A scaled H- ion source was developed to meet higher beam requirements for ISIS operations. Emission area and aperture shape influence space-charge limited current density, following a 3D Child-Langmuir relationship.

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Area of Science:

Plasma physics
Particle accelerator technology

Background:

The ISIS facility has utilized a 1X negative hydrogen (H-) ion source for 35 years.
A new 2X scaled source was developed to achieve higher beam current and duty cycle for the front-end test stand.

Purpose of the Study:

To compare the performance of different emission areas (aperture dimensions) for both 1X and 2X H- ion sources.
To investigate the impact of slit and circular extraction geometries on beam characteristics.

Main Methods:

Experimental comparison of 1X and 2X Penning surface plasma sources with varying aperture sizes.
Analysis of beam current density in relation to extraction aperture geometry and aspect ratio.

Main Results:

The 2X source, with doubled linear dimensions, was developed to meet enhanced operational demands.
A 3D Child-Langmuir relationship was observed, indicating space-charge limited current density is dependent on the extraction aperture's aspect ratio.

Conclusions:

The study provides insights into optimizing H- ion source design for future accelerator applications.
Understanding the relationship between emission area, aperture geometry, and current density is crucial for scaling ion sources.