.J. Suehiro

M. Hara

M. Kosaki

M. Nagao

M. Takeo

N. Shimizu

S. Nishijima

S. Tanahashi

T. Satoh

Y. Mizuno

National Institute for Fusion Science (NIFS-Repository)

§32. Electrical Insulation of Superconducting and Cryogenic Devices Kosaki, M., Nagao, M., (Toyohashi Univ. of Technology.) Hara, M., Suehiro, .J ., Takeo, M. (Kyushu Univ.) Shi1nizu, N. (Nagoya Univ.) Nishijinw., S. (Inst. of Industrial Science, Osaka Univ.) tviizuno, Y.(Nagoya Inst. Technology) Tanahashi, S., Satoh, T. 1. Surface discharge in cryogenic nitrogen gas The cryogenic nitrogen gas phase is ap-proaching the condition of the rninirnurn break-down voltage in Paschen curve of gaseous nitro-gen. Thus the voltage induced at the quench-ing of HT superconducting coils would cause discharges. HT superconducting coils are de-signed by ernploying spaces between conductors and turns and therefore, the surface discharge characteristics are very irnportant. This year we have investigated the G FRP spacer perfor-rnance between the parallel rnetallic electrodes in the cryogenic nitrogen gas. The new find-ing is the breakdown voltage of nitrogen gas in the relevant state being chiefly detennined by the product(p·d) of nitrogen gas density(p) and thickness of spacer( d) rather independent of the diameter and thickness of spacers. Further-rnore, when p·d is relatively large, surface dis-charge along the spacer predorninates and the breakdown voltage turns out lower than that of nitrogen gas itself. When p·d is small, how-ever, the surface discharge voltage surrnounts the breakdown voltage of heliurn gas itself and the breakdown takes place preferentially at the gap. These results surnn1arized in Fig. 1 are different frorn the generally accepted concept on surface discharge and therefore, rnight be an in1portant knowhow for the insulation design of the supercon- ducting magnets with abundant srnall gap spacers.[l] 20 > 10 .lll:l 1 -5 0 > ~ .lll:l m .... o:l 1 2 3 5 10 0.1 0.2 0.3 0.5 1 Nitorogen gas density*Gap distance[(kgfa3)•] Figure 1: Breakdown voltage of cryogenic nitro-gen gas with GFRP spacer 2. Bubble rnotion in liquid heliurn and nitrogen . The analyses of bubble rnotion in the cylin-der and plane electrode systern in liquid heliurn and nitrogen revealed that it follows gravity, viscosity and electric force and its trajectry fol-lows the electrode allign1nent. When the bub-ble size in diameter of liquid helium being one third of that of liquid nitrogen, the bubble rno-tion in both liquids are very sirnilar. These re-sults are beneficial in the proper designing of the superconducting coils since they are sirnulating the quenching of pool cooling superconducting rnagnets. 3. Breakdown characteristics of organic thin fihns in cryogenic region The breakdown strength of polyimid thin fihns which has excellent mechanical properties in the cryogenic region for insulation of super-conducting magnet wire were found to be rnore than 2MV / ern and free frorn the effects of elec-trode metals and polarity reversal of applied de voltage. Reference [1] Hoshino,M.,IEE.J 115-A(1995)1243 37 

Electrical Insulation of Superconducting and Cryogenic Devices

Electrical Insulation of Superconducting and Cryogenic Devices

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)