N. "Yanagi

S. Imagawa

T." Mito

National Institute for Fusion Science (NIFS-Repository)

Conductor Position (mm)Fig. 1 Spatial profile of the observed one-sided travelingnormal-zone. The coil current was 10.8 kA.Fig. 2 Schematic illustration of current transferringprocess from the superconducting strands to the metalstabilizer through a thin resistive layer.V19V11HeaterMetal stabilizer (II)....0.............:.........__---.V1 V5t= 0.05 st=0.3si < '?'Ji b t=O.15s ,/ ": QQD .::;'" \\ <>1\::[ co 500 10002520§.Q)0> 15Sas 10c::050>C.30-500References1) N. Yanagi, et aI., IEEE Trans. Appi. Supercond. 9(1999) pp. 1113-1116.2) H. Kaneko, Cryogenics 33 (1993) pp. 1077-1085.3) A.V. Gavrilin, et al., Adv. in Supercond. XI (1999)pp.1447-1450.the longitudinal direction as is illustrated in Fig. 2. In theN-S boundary indicated by (I), the Hall electric field isgenerated in the opposite direction to the intrinsic electricfield applied by the external power supply. Therefore, theeffective resistance of the metal stabilizer is lowered andthe characteristic length for current transfer becomeslonger, and thus, the joule heat loss at this region becomesless. On the other hand, at N-S boundary (II), the powerdensity of joule loss is enhanced due to the longitudinalHall electric field. This model might explain the observedfact that the propagation velocity at the N-S boundary (I)is slower and that at (II) is faster.A one-dimensional numerical code has been developedto simulate the normal-zone propagation in thealuminum-stabilized superconductor for the helical coils[3]. The longitudinal Hall electric field generated at thetwo N-S boundaries are being included into this code andthe normal-zone propagation will be precisely examined.§ 3. Asymmetrical Propagation of "Travel ingNormal-zones" in the Aluminum-stabi Iized Superconductor for the LH 0Hel ical Coi IsAn aluminum-stabilized superconductor is used for thehelical coils of the Large Helical Device (LHD). Dynamicstability is important for this type of superconductors dueto the slow magnetic diffusion process in the purealuminum stabilizer. The transport current can betransferred only partially to the pure aluminum stabilizerat normal-to-superconducting (N-S) boundaries and thenormal-zone propagates with a characteristic time scale ofshorter than the transverse magnetic diffusion. Thus, atemporal and finite-length normal-zone propagation couldbe observed. It has been confirmed in the formershort-sample tests as well as in small R&D coil tests thatthe propagation velocity differs in the upstream anddownstream sides of the superconductor [1]. Thisdifference is observed only with a condition that thecurrent transfer from the superconducting strands to themetal stabilizers occurs in the direction perpendicular tothe external magnetic field. Moreover, it does not dependon the orientation of conductor samples. Thus, it might becaused by some electromagnetic mechanisms.An R&D coil with an inner radius 200 rom and outerradius 400 rom was constructed using the same type ofsuperconductor as that used for the LOO helical coils. Itwas tested in liquid helium at temperature 4.4 K as well asat 3.8 K (which was supplied by cryogenic compressors).The stability test was conducted using resistive heatersattached to the surface of the innermost windings andpropagation of a normal-zone was monitored using anumber of potential taps distributed along the conductor.We observed that the propagation velocity wasasymmetrical with respect to the two longitudinaldirections along the conductor, just as was found in theshort-sample tests. Moreover, up to a certain coil current,the generated normal-zone propagated only in onedirection and it recovered back into superconducting fromthe other end. This was confirmed to be a so-called"traveling normal-zone" which propagates only in onedirection. Figure 1 shows a typical example of theobserved spatial profiles ofa normal-zone.In order to clarify the mechanism for causing thisasymmetrical normal-zone propagation, the currenttransferring process from the superconducting strands tothe metal stabilizers should be investigated. When thecurrent transfer occurs in the direction perpendicular to theexternal magnetic field, a Hall electric field is generated inYanagi, N., Imagawa, S., Mito, T.95

"Asymmetrical Propagation of ""Traveling Normal-zones"" in the Aluminum-stabilized Superconductor for the LHD Helical Coils"

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)