In Thermonuclear Experimental Reactor, Superconducting Busbar is used for current transmission between magnet
coils and current leads. The work temperature of the Busbar is about 4K because of liquid helium via inside. The large
temperature grad from 300K to 4K could lead to the defects and damages occur on the insulation layer, which is made of
glass fiber and polyimide and has a big different thermal expansion coefficient compared with the metal inner cylinder.
This paper aims at developing an infrared transmission non-destructive evaluation (NDE) method for inspecting the
insulation layer of Superconducting Busbar; theoretical model of transient heat conduction under a continuous inner heat
source for cylindrical structure is described in the paper; a Busbar specimen which is designed with three delamination
defects of different depths is heated inside by pouring hot water and monitored by an infrared detector located outside.
Results demonstrate excellent detection performance for delamination defects in the insulation layer by using
transmission thermography, all of the three defects of different depths can be visualized clearly in the thermal images,
and the deeper defect has a better signal contrast, which is also shown in the temperature difference between defects and
sound area vs. time curves. The results of light pulse thermography is also shown as a comparison, and it is found that
the thermal images obtained by the transmission thermography has a much better signal contrast than that of the pulse
thermography. In order to verify the experiments, finite element method is applied to simulate the heat conduction in the
Busbar under the continuous inside heating, and it is found that the simulated temperature vs. time and simulated
temperature difference vs. time curves are basically coincident with the experimental results. In addition, the possibility
of in-service inspection for Busbar insulation layer in ITER item is discussed.