J. Suehiro

M. Hara.

S. Yamada

National Institute for Fusion Science (NIFS-Repository)

§14. Study on Electrical Insulation under Practical Condition of a Superconducting Coil Operation Hara. M., Suehiro, 1. (Kyushu Univ.), Yamada, S. A superconducting coil of LHD is exposed to various stresses, such as a thermal, electrical or mechanical one. The LHD coil should be stably operated even under these stresses. In this project, the authors studied electrical insulation under practical coil operation conditions. This year, special effons were made to clarifY behavior of contaminant metallic particle under high electric field in pool boiling liquid helium. Effects ofthe particle shape were investigated in detail using spherical and cylindrical particles as model contaminants. Data obtained in this project would be useful for electrical insulation design of LHD coils. Main results are summarized as follows. I) The metallic particle repeatedly moved between the upper and the lower electrodes before the electrical breakdown took place under high electric filed. This suggested that the electrical breakdown could be considerably influenced by the particle. 2) The particle generated gaseous bubbles and micro discharge when it collided with the electrodes. 3) For a spherical particle, which had a less field-enhancement factor than a cylindrical one, a discontinuity was found in the pressure dependency of the breakdown voltage at the A point (Fig. I). 4) The breakdown voltage of liquid helium contaminated with cylindrical particles was lower than that with spherical ones over the whole pressure range (Fig. 2). This seemed to be attributed to higher field enhancement effect of cylindrical particles with sharp edges. 5) The breakdown characteristics of the liquid helium were considerably influenced by the gaseous bubbles generated by contaminant particle movement. 6) These results suggest that the contaminant particles should not exist in the liquid helium form a viewpoint of the electrical insulation. A serious circumstance may happen for superfluid liquid helium (Hell), which has been a possible coolant in Phase II. Because the breakdown voltage abruptly decreases if a phase transition from HeI to Hell accidentally occurs. Reference I) Hara M., Nakagawa H., Suehiro J., Shinohara T.: "Electrical breakdown triggered by a free conducting spherical particle in saturated liquid He I and He II under uniform dc field", IEEE Trans. on. Dielectrics and Electrical Insulation 10, (2003) 1022 2) Maeda, Y., Nakagawa, N., Suehiro, J., Hara, M., Yamada S.: "Breakdown characteristics of HeI or Hell in the presence of a cylindrical metallic particle", Annual meeting ofIEEJ (2004) ~ 60 4S 30 15 o rp [!1m] 320 SOO 1000 D.P.S. • .6. • P.S. 0 !l. 0 , ¥'oO<+ G=6.Smm: o,~.·-, , , , o:>-~ , , f:,. .... -a~.~ oo.~ ~ mSf'!..· . ' ~J,. -.<* •• : • Hell.., III Hel 1 A-print 10 Pressure, p [kPa] ~ 100 Fig. l. Pressure dependency of the breakdown voltage ofthe liquid helium contaminated with a spherical metallic particle with various radius rp' D.P.S.: Data measured during pressure decrease. P.S.: Data measured during pressure increase. so O.Smm 1mm 2mm 40 Df • .... ~ -aJ D 0 V 5 > <Ii Cl co -0 > <: ~ ~ co Q) ~ [D 30 G=6.5mm v G 20 10 , .p , v , v v 0 00 , , 00 0 o '" CD 'bv' 0 0 p <:>e oo 0 ... ..; •• ~If"'~ ... .froI, ... L , ; Hel ~Hell~J, ~O 100 1000 Pressure, p[T orr] Fig. 2. Pressure dependency of the breakdown voltage of the liquid helium contaminated with a cylindrical metallic particle with various diameter D, (free particle) or D, (stationary particle) and constant length 2mm. 113 

Study on Electrical Insulation under Practical Condition of a Superconducting Coil Operation

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