Studies for Arc Detection in the Radio Frequency Distribution System of the European Spallation Source

  • Norman Turner

Student thesis: Doctoral Thesis


The European Spallation Source includes a linear accelerator that will deliver protons with an energy of 2 GeV and an average power of 5 MW onto a rotating tungsten target. The accelerator is supplied with radio frequency (RF) power by over 3 km of RF waveguides, some of which will be force ventilated. RF power will be supplied in pulses 3 ms long. This thesis describes measurements and modelling relating to optical detection of arcs and the potential causes of arcs in the RF waveguides.

Measured optical losses in straight WR1150 RF waveguides are high (approximately 50% per reflection). As a consequence the waveguides act as optical ray filters, with losses lowest for rays propagating axially. A novel style of optical arc detector head optimised to capture axial rays is described.

Sharp items can increase the likelihood of arcing in a waveguide by enhancing the electric field. The electric field enhancement at WR1150 waveguide longwall bends (E bends) was investigated by modelling. Electric field field strength enhancements by a factor of 3.5 were found for a 10 mm bend radius.

The spatial distribution of 246 pit marks in the E bend of a WR2300HH waveguide was investigated following arcing incidents. It was not possible to conclude that there was a preferred orientation for adjacent pit marks. A preferred orientation would have suggested a direction for arc travel. This finding can be contrasted with reports, of arc pits in waveguides subject to continuous power, which indicate pitting trails towards the RF source.

The forced waveguide ventilation could potentially lead to dust ingress into the waveguides. Heating effects of potential dust contaminants were assessed using published theoretical studies. Heating of contaminants < 1 m is unlikely to cause arcing. The risk of particle heating causing arcing increases with particle size. Iron containing particles can be particularly susceptible to heating by the RF fields.

Potential contaminants from inside the waveguides and from the areas where waveguides will be installed were collected and analysed by scanning electron microscope. Silica was the predominant material but particles containing iron were found. Some of the iron containing particles were near spherical with a diameter of 20 m.

Irradiation studies using 28 MeV protons at dose rates up to 2 Gys􀀀1 and total doses of up to 15 kGy were performed on relevant components. No degradation in mechanical performance was found in a very limited set of tests on nitrile rubber toroidal rings, and ethylene propylene diene monomer (EPDM) rubber toiroidal rings following a dose of 15 kGy. The role of the toroidal elastomer rings is to align cylindrical components but is not to provide fluid seals.

The optical properties of Schott LKS glass optical fibres degraded rapidly during irradiation at dose rates of 0.2 Gys􀀀1. No recovery in the optical properties of these glass fibres was observed over a period of 3 months post irradiation. It is postulated that the poor performance of the Schott LKS glass fibres is related to the presence of lead in the glass. At dose rates of 0.2 Gys􀀀1 polymethylmethacrylate (PMMA) optical fibres also exhibited radiation induced attenuation but at greatly reduced rates compared to the Schott LKS glass fibres. Recovery of the optical properties of the PMMA fibres post irradiation occurred on at least two timescales. Immediately post irradiation at 2 Gys􀀀1 the recovery rate was greater than 2% per minute, later the recovery had a half life of approximately 20 days.
Date of Award28 Sep 2022
Original languageEnglish
SupervisorRobert Edgecock (Main Supervisor) & Roger Barlow (Co-Supervisor)

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