N. Yanagi

S. Yamaguchi

T. Senba

National Institute for Fusion Science (NIFS-Repository)

§19. Thermal Behavior of the Helical Coil Conductor Senba, T. (Grad. Univ. Advanced Studies) Yamaguchi, S., Yanagi, N. Disturbances in superconducting magnets are generally distributed in actual magnets, b~cause the major origin of disturbances ts an electromagnetic force, which should move rigid conductors in some length at the same time, not in a specific point. This movement causes plural separated disturbances by friction at insulating spacers at the same time in pool-cooled magnets. We carried out the experiment to investigate the difference between the Minimum Quench Energy (MQE) with point disturbances and that with distributed disturbances. We installed several heaters along the conductor to simulate distributed disturbances. The heaters were placed beneath insulation spacers in order not to change the exposure rate of the conductor. Figur~ 1 shows. a schematic drawing of the confguratton of thts experiment. Experimental parameters w~re operating current of the conductor sample, ?•as mao-netic field, heating power, and pulse duratton. Total input energy at which the transition to a normal state occmTed by heat pulse were plotted in Figure 2. The ratio of the energy was constant at 1:2:3, when the number of heating spots is changed as 1:3:5. The minimum energy density per heating spots to cause the transition to normal state, (MQEd;s), decreased with the numbe~ of the heating spots. The dependence can be estimated from the experimental results as following n + 1 MQEpoint : MQEdis = 1:-- (1) 2n where MQEpoi!Ir is the minimu~ quench energy against potnt dtsturbance and n ts the number of heating spots. When the n is sufficiently large, MQEdis converges as following, . 1 hm MQEdis ==- · MQE point (2) n~oo 2 Assuming the characteristic length lrh of temperature variation along the conductor, and the exposed length lex of the conductor per one cooling channel, the relation between lrh and lex determines the relation between MQEdis and MQEpoinr. Case ( 1) ~ when l!h << lex : M9Edis = MQE · 1 because one coohng channel ts large potn ' enough to remove the heat energy from two 24 adjacent heating spots. Since the cooling effect is very large, each heating spot can be considered to be completely independent on each other. Case (2) ~ when lrh - lex : MQEdis = ( 1/2)•M Q EJJOi~t, because one cooling channel has comparable stze to remove the heat energy from one heating spots. The experimental condition corresponds to this region; temperature rise from bath temperature to Tcs at one heating spot is effectively cooled by one cooling channel. Case (3) ; when lrh >> lex : MQEdis- ( lln)•MQEpoint, because one cooling channel does not have the size large enough to remove the heat energy from even one heating spot. Since the cooling effect is small, the heating effect accumulates according to the number of heating spots. . .. These results can be used to predtct a mtntmum quench energy when distributed disturbances occur in actual magnets. Liquid Helium Superconductor (KIS0-32) Thermal Insulation (GFRP) .. .... .. .... .... .. 3 5 Number of Heating Spots Fig. 1 Concept of distributed pulse heating. 7T 16.0kA 100 ~: ::. ,.-., 2 >. O'l 10 I... .. l • ....... ,. ..... l ,tQ •.... // /; ' ' .· / : (1,) c: UJ ' -7 v .... :::J a.. .s : --;..- y 7 • p-;.--.:-- ·y ro .... 0 I-: 0.1 l l 0.001 0.01 0.1 Pulse Duration (s) Fig. 2 Total input energy to the conductor subjected to distributed disturbances. 

Thermal Behavior of the Helical Coil Conductor

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