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Title: Electric field optimisation for cryogenic nEDM experiments
Author: Thorne, Jacob Aaron
ISNI:       0000 0004 7431 634X
Awarding Body: University of Sussex
Current Institution: University of Sussex
Date of Award: 2018
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This thesis presents details of the design, construction and measurements of an apparatus (Blue Elbow cryostat) for high voltage testing of a full-size cryogenic nEDM cell in liquid helium at 4.2 K SVP. The test cell is cylindrical and of 24 cm internal diameter with stainless steel electrodes and an insulating borosilicate glass spacer. The cylinder axis of the cell is vertical and the insulator is located in grooves in the electrodes. The electrode separation can be varied from 0.2 cmto 2.6 cm and a voltage of up to 260 kV can be applied across the cell. It has long been expected that a nEDM cell immersed in superfluid LHe at 0.5 K should permit E-fields much greater than room temperature experiments. Long et al. (1) showed that over 400 kV/cm was obtainable in a large cell without an insulating spacer at 4.2 K, but that this was reduced dramatically as the temperature, and hence pressure, was reduced to below 2 K in a pumped LHe bath. Subsequent work by Davidson (2) in this laboratory on small spacerless cells showed that the dielectric strength in the superfluid at 1.9 K could be restored to its 400 kV/cm value by pressurising the LHe to 1 bar. Further work in this laboratory by Davidson (2) and Hill (3) shows that the introduction of a dielectric spacer reduces the value of the breakdown field, Ebd , for a given geometry. However, measurements presented here on smaller scales than the Blue Elbow cryostat, overcame the reduced fields through careful groove optimisation and insulator material choice. Ebd data as a function of separation with the Blue Elbow cryostat in LN2 show a clear reduction compared to data from smaller scale cells, due to surface area effects. Breakdown fields in LHe at 4.2 K SVP with this apparatus indicate fields at 120 kV/cm were achievable at 6mm separation but dropped off dramatically as separation was increased to 12 mm then 16 mm. The reason for the drop off is attributed to the geometry of the electrode. This result, together with Davidson's pressure dependence data, should inform the design of a future cryogenic nEDM experiment.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: TP0480 Low temperature engineering. Cryogenic engineering. Refrigeration