High voltage breakdown in the Ramsey cell of the CryoEDM\ud experiment: an experimental study of some relevant parameters
Davidson, Andrew John
A pressure cell apparatus has been designed, constructed and commissioned to measure the dielectric strength of liquid helium as a function of pressure for various temperatures in He I and He II. Breakdown experiments between a set of stainless steel parallel plate Rogowski profile electrodes with a separation of 1.27 mm have been made for temperatures ranging between 1.7 K to 4.2 K and applied pressures of SVP to ~ 2.2 bar. All pressure data taken above and below the λ-point exhibit similar features. At low pressure, near the SVP, breakdown voltage rapidly increases with applied pressure. This behaviour is linear for all temperatures and has an average gradient of 0.176 ± 0.0096 kVtorr-1. This regime changes at a pressure dependant kink point. Above this point a more gradual increase in breakdown voltage with pressure is observed. Data for all temperatures normalised to 50 kV and 1200 torr have an average gradient of 0.01693 ± 0.00092 kVtorr-1 with an average intercept of 29.69 ± 0.88 kV. A linear relationship is observed between the normalised kink pressure and the density of the helium (-34.4 ± 1.4 kg m-3torr-1 and intercept of 5130 ± 200 kg m-3). The normalised kink pressure as a function of SVP produces a linear relationship with a gradient consistent with unity (0.97 ± 0.04 torr2 with an intercept of 88 ± 13 torr). High voltage breakdown initiating at the cathode triple junction (CTJ) has been investigated in He I, He II and LN2 under SVP. Breakdown tests were made between parallel plate Rogowski profile electrodes with an Al2O3 ceramic spacer between them. Chips, cracks and tracks across the ceramic have been observed. LN2 breakdown tests cause catastrophic damage. Calculations of the E-fields in any gap between the ceramic and electrode show E-field enhancements of up to a factor of ~ 4.5. The end of a ceramic sample was sputtered with Gold in an attempted to prevent any E-field in the gap. This work has been carried out within the UK CryoEDM Collaboration and was aimed at understanding better the parameters which will ultimately limit the E-field in the Ramsey Cell of the main experiment.
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