Measurement of quench back behavior on the normal zone propagation velocity in a CICC

ICEC 15 Proceedings

Measurement of Quench Back Behavior on the Normal Zone Propagation Velocity in a CICC Toshinari Ando Masataka Nishi Takashi Kato Jun Yoshida Noboru Itoh Susumu Shimamoto Naka Fusion Research Center, Japan Atomic Energy Research Institute, Naka-machi, Naka-gun, Ibaraki, Japan 311-01 The normal zone propagation velocity in a cable-in-conduit conductor (CICC) has been experimentally investigated. The CICC used for this experiment consists of eighteen NbTi/Cu composite strands inserted in a stainless steel conduit with 26 m length. The measurement of the propagation velocity of the normal front was carried out by observing the appearance of the resistive voltage on each of taps attached to the conductor. In this experiment the quench back behavior that the propagation velocity is suddenly increased, was observed, which has been predicted from numerical simulations by C. Luongo et al. The behavior was influenced by the initial helium pressure within the conduit and was easy to occur at low pressure. INTRODUCTION Cable-in-conduit conductors (CICC) are used in large superconducting coils such as fusion magnet and superconducting magnet energy storage(SMES) because they are high stable and of low ac loss in large capacity current structure and their fabrication is easy. As one of characteristics of CICC, it has been predicted by Dr. Dresnet that the propagation velocity of normal zone in CICC is increased with the time unlike the propagation velocity in conductors immersed in liquid helium[l]. This prediction has been confirmed experimentally by the authors[2]. After that, Lungo et al. have been presented extence of quench back behavior that the propagation velocity is suddenly increased, by computer analysis[3]. We tried to observed experimentally this behavior. In this paper, the experimental date of quench back behavior will be presented and discussed. EXPERIMENTAL SETUP The cable-in-conduit conductor used in this experiment was composed of 18 NbTi/Cu composite strands and single stainless steel wire inserted into the stainless steel conduit. Each NbTi/Cu strand was 0.98 mm in diameter. The ratio of Cu to NbTi was 3:1. The stainless steel wire was 1 m m in diameter, and was used as a heater to study the pressure rise in the cable-in-conduit conductor. The cable-ir)-conduit conductor had a void fraction of 48.1%. The relevant parameters and a picture of the cross-section of the cable-in-conduit conductor are shown in Table 1 and Fig. 1, respectively. The conductor was formed into a double-layer fifty-turn solenoid. Voltage tapes were attached on each turn, each of length around 50 cm. An inductive heater of length to create an initial normal zone. Both ends of the conductor were connected to a bomb via pipe, to fill the conductor with high pressure helium. A relief valve was installed on the top of the pipe. The solenoid was placed inside a 240 mm-diameter 8T-superconducting magnet. All experiments were carried out under stagnant helium conditions, that is with a helium mass flow rate of zero. When pressure inside the conduit rose, as a result of Joule heating in the section of the conductor which was driven normal upon activation of the inductive heater, helium inside the conduit was released from the relief valve preserving the initial pressure. After a background field of 7 T was established and the solenoid current is set to a certain value, the inductive heater activated. After about 7 see, the solenoid current was decreased to zero. The duration time of the inductive heater was about 0.1 ms. The signal from each voltage tap was monitored by means of a 60-channel digital oscilloscope. EXPERIMENTAL RESULTS AND DISCUSSION Figure 2 shows normal front location (Z) as a function of time (t) for several transport currents at 7 T and the initial pressure inside the conduit was 1 MPa. For every current, Z was approximately proportional to the 1.6th power of the time. Namely, the propagation velocity of the normal front in a cable-in-conduit conductor was

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ICEC 15 Proceedings proportional to the 0.6th power of the time and was increased with transport current. However, in the case of the transport current of 2.0 kA, a slight change was measured on the velocity when the normal front reached to around 10 m. This behavior was dearly observed when the initial pressure inside the conduit was set at 0.3 MPa. Figure 3 shows normal location (Z) as a function of time (t) at 2.0 kA and 7 T for initial pressures of 0.3 and 1.0 MPa, respectively. For 0.3 MPa, when the normal front reached to around 3m, the velocity was rapidly increased. Namely, quench back behavior was observed, which was predicted by Luongo et al [3]. At the same time sudden pressure rise (P) at the center of conductor as shown in Figure 4, The starts of sudden pressure rise and sudden velocity increase were completely same. The behavior occurred when the difference between pressures at the center and the end of the conductor reached to about 7 MPa. As the reason for occurence of the quench back, temperature rise due to friction loss of helium or a large heat generation due to rapid increase of the stabilized copper resistivity in the conductor are considered. However, these considerations were not yet verified. CONCLUSION

Quench back behavior has been observed on the normal zone propagation velocity in CICC. This behavior was sensitive to the initial helium pressure within the conduit and was easy to occur at low pressure. ACKNOWLEDGEMENTS The authors would like to tanks Drs. M, Tanaka and H. Tsuji for their encouragement and support in this work. REFERENCE .

2. 3.

Dresner, L., The growth of normal zone in cable-in-conduit superconductors,In:Proc. 10th Syrup. Eng. Prob. of Fusion Research, IEEE, Philadelphia, (1983) 2043-2046 Ando, T., Nish, M., Kato, T., Yoshida, J., Itoh, N., and Shimamoto, S., Propagation velocity of the normal zone in a cable-in-conduit conductor, Advances in Cryogenic Engineering, (1990) 35 701-708 Luongo, C. A., Loyd, R. J., Chen, E K., and Peck, S. D., Thermal-hydraulic simulation of helium expulsion from a cable-in-conduit conductor, IEEE Trans. on Mag. (1989) 25 1589-1593

Table I Parameters of the cable-in-conduit conductor used in this experiment

Cable configuration Conduit inner diameter Conductor length Void fraction Hydraulic diameter Ratio of Cu/NbTi

3 x 6 x 0.98 mm NbTi/Cu strands + I mm stainless steel 5.97 mm 26 m 48.1% 0.694 mm 3.0

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Figure I Cross-section of the cable-in-conduit conductor used in this experiment

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Cryogenics1994Vol34 ICECSupplement