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Title: Understanding the plasma and improving extraction of the ISIS Penning H⁻ ions source
Author: Lawrie, Scott
ISNI:       0000 0004 6498 5689
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2017
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A Penning-type surface-plasma negative hydrogen (H-) ion source has been delivering beam at the ISIS pulsed spallation neutron and muon facility for over thirty years. It is one of the most powerful and well-renowned H- sources in the world. Although long-term experience has allowed the source to be operated reliably and set up in a repeatable way, it is treated as something of a 'black box': the detailed plasma physics of why it works has always been unclear. A vacuum Vessel for Extraction and Source Plasma Analyses (VESPA) has been developed to understand the ISIS ion source plasma and improve the beam extracted from it. The VESPA ion source is operated in a completely new regime whereby the analysing sector dipole magnet housed inside a refrigerated 'cold box', presently used on ISIS, is replaced by an on-axis extraction system. The new extraction system incorporates a novel einzel lens with an elliptical aperture. This is the first demonstration of an elliptical einzel being used to focus an asymmetric H- ion beam. With the dipole magnet removed, the ion source has been shown to produce 85 mA of H- beam current at normal settings; of which 80 mA is transported through the new einzel lens system, with a normalised RMS emittance of 0.2 π mm mrad. Optical emission spectroscopy measurements have shown a plasma density of 1019 m–3, an H2 dissociation rate of 70%, an almost constant electron temperature of 3.5 eV and an atomic temperature which linearly increases above the electron temperature. In support of these principal measurements, rigorous particle tracking, electrostatic and thermal simulations were performed. In addition, a suite of new equipment was manufactured by the author. This includes a fast pressure gauge, a temperature controller, a high voltage einzel lens circuit, a fast beam chopper and a caesium detection system.
Supervisor: Letchford, Alan ; Faircloth, Dan ; Seryi, Andrei Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Particle accelerators ; Ion sources ; Plasma (Ionized gases) ; Ion extraction ; H-minus ; Cesium ; Accelerator ; Extraction ; Einzel ; Ion source ; Plasma ; Negative hydrogen ; Ion beam ; Spectroscopy ; Caesium