Stability of Superconducting Wire in Magnetic Field

A fundamental experiment was carried out to study the motion of superconducting wire under the influence of electromagnetic force. Experiments were conducted at 4.2 K by varying the experimental conditions such as applied tension to test superconducting wire, insulating material used at the interface of superconducting wire and base material. The experimental method and the test results are reported in this paper. INTRODUCTION The main factor of instabilities of the high field superconducting magnet coils is the abrupt and local wire motion and thus generating frictional heat during the current ramp. The wire motion occurs where electromagnetic force to conductor exceeds frictional force on surface of the conductor. Hence, frictional properties of the conductor and winding structure are important parameters for characterizing stability of the superconducting windings [1]. Kinsley and Iwasa [2] have made a detailed study to observe sliding behaviour at 4.2 K, 77K and 295 K for a number of polymers, laminated composites and coated pieces sliding against either copper or aluminium. Largest and potentially most harmful source of frictional heating is at the position where superconducting wire rests against structural insulating material. A fundamental experimental setup was prepared to observe the effect of insulating material used at the interface of superconducting wire and base material under the influence of varying electromagnetic force. A high magnetic field of 6 T was provided by a superconducting solenoid magnet and current up to 128 A was feed to the test superconducting wire. The voltage generated due to single wire motion under the electromagnetic force is reasonably enough to measure. Thus, enable us to measure directly the wire motion. The wire motion is detected by spike in voltage V of the test superconducting wire. The test set up consists of a high field superconducting magnet, power supplies, data recorder and sample holder for test superconducting wire with the later installed inside the superconducting magnet. A tension unit is attached to give tension to superconducting sample wire. In this work, two different types of insulating material were used. They are Polyimide film (125μm thick) and high strength polyethylene fiber cloth sheet Dyneema ® SK-60 with 1320 dTex. The used cloth was a plain wave having 15 yarns/inch with 165 g/m. Dyneema ® is a registered trademark in Japan. Dyneema has unique property of low coefficient of friction [3] and negative thermal expansion on cooling from RT to 4.2 K [4]. The preliminary results indicate that use of Dyneema reduces the sudden and big wire motion. Hence, use of Dyneema may reduce the frictional heat generated due to wire motion and can make the magnet performance more reliable. Table 1 shows the measured coefficient of friction of Dyneema cloth and Polyimide film at room temperature. Table 1: Frictional Coefficient Measurement Test Data at Room Temperature Sample Frictional coefficient