Electric field optimisation for cryogenic nEDM experiments

Thorne, Jacob Aaron (2018) Electric field optimisation for cryogenic nEDM experiments. Doctoral thesis (PhD), University of Sussex.

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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

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.

Item Type: Thesis (Doctoral)
Schools and Departments: School of Mathematical and Physical Sciences > Physics and Astronomy
Subjects: T Technology > TP Chemical technology > TP0480 Low temperature engineering. Cryogenic engineering. Refrigeration
Depositing User: Library Cataloguing
Date Deposited: 16 Nov 2018 08:55
Last Modified: 16 Nov 2018 09:02
URI: http://srodev.sussex.ac.uk/id/eprint/80282

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