DEGRADATION AND FLUX JUMPING IN SOLENOIDS OF HEAT-TREATED Nb-25% Zr WIRE

Abstract

We report here what we believe is conclusive evidence that the excessive degradation of critical current which occurs in solenoids wound from heattreated Nb-25% Zr wire is due to the simultaneous presence of transport current and flux jumping. Previous unpublished work in our Laboratories had shown that, while the critical current in a 5-cm short sample of Nb-25% Zr wire heated to 70boc for one hour was about 150 A in a transverse field of 20 kG, the same wire wound into solenoids went normal when producing 5 to 10 kG at critical currents less than 10 A. This same type of degradation, in a less severe form, is of course well known for solenoids wound from heavily cold-worked wire.The present experiments were performed with insulated 0.025-cm diam Nb-25% Zr wire which had been heat-treated at 700°C for one hour after the final cold-worked reduction. It has been previously reported that heat treatment of Nb-Zr alloys increases the critical current density of short samples. 2 -4 The short sample critical current vs field for this wire is listed in Table I. The wire was wound into a solenoid of approximate dimensions: 0.3-cm i. d., 1.3-cm o. d., 2.5 cm long (calculated G/ A = 434). The solenoid was cooled to 4.2°K, and a uniform magnetic field was applied to its axis. As this external field was increased smoothly from zero, with no transport current (I = 0) in the solenoid, a sequence of very narrow «1 msec CONTENT ANALYSIS

Research progress on the mechanical behavior of the cable in conduit conductor for the international thermonuclear experimental reactor project

“…早在20世纪60年代末, 人们采用NbZr低温超导 材料 [6] , 并对使用内冷式导体(internally cooled conductor, ICS)绕制的超导磁体进行了研究 [7,8] . ICS是将 NbZr超导线焊接在铜管外侧, 其内部通有液氦, 通过 接触传热的方式使超导股线降温达到超导态 [9] . 这种 冷却方式效率低下, 超导材料很容易发生磁通跳跃致 使磁体系统失超.…”

Superconducting magnet coils for thermonuclear research

“…on single wires [32], and has been identified and associated with degradation in solenoids [33]. The phenomenon occurs when the energy liberated in flux creep, or more generally when the energy liberated by any fluctuation associated with flux motion, exceeds the heat-dissipating capabilities of the material.…”

“…Through several recent investigations it is becoming increasingly clear that flux jump phenomena [33,49] (see section 3) are very closely related to the degradation effect [50].…”

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“…Subsequently, the superconducting magnets were wound with the structure of an internally cooled conductor (ICS) [4]. The superconducting strand is cooled to the superconducting state by the heat transfer copper tube with liquid helium in it [5]. But, the contact cooling method of ICS is inefficient, and the superconducting material is inclined to have a magnetic flux jump which will make the magnetic system to be quenched.…”