Materials Selection, Qualification and Manufacturing of the In-Vessel Divertor Cryopump for JET
The introduction of a cryopump into the interior of a large tokamak raises several technical problems related to the thermal stresses, eddy current forces and choice of materials. The JET divertor cryopump has been optimised in terms of stresses, flow stability and operation — the liquid nitrogen cooled chevron structure in particular having to fulfil conflicting requirements at cryogenic temperatures. These requirements include good thermal conductivity in order to minimise thermal gradients (to reduce the radiative heat load onto the liquid helium circuit), high electrical resistivity (to minimise eddy current stresses), high mechanical strength and good mechanical formability.
This paper reports on the materials selection based on measurements of properties at cryogenic and elevated temperatures and the development of an optimised thermal treatment combining solution heat treatment, brazing and precipitation hardening. It also reports on the successful development of various manufacturing technologies which have been employed including (a) techniques for brazing of the chosen copper alloy onto inconel and stainless steel, (b) surface blackening of the copper alloy with plasma sprayed ceramic coatings that are vacuum compatible and able to withstand temperatures between 70K and 1135K and (c) plasma spray deposition of copper onto stainless steel in order to produce an anisotropic composite material with improved thermal conductivity, high strength and high electrical resistivity for use at temperatures between 70K and 650K.
KeywordsElectrical Resistivity Vacuum Vessel High Electrical Resistivity Good Thermal Conductivity Vacuum Plasma Spray
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