Self-heating effects on the transition to a highly dissipative state at high current density in superconductingYBa2Cu3

Thermal behaviour of high-temperature superconducting fault current limiters: Application to microlimiters

“…However, the current is effectively limited only for electric fields or, equivalently, applied voltages well above that at which the limiter is triggered from normal operation (sample in the superconducting state) to current fault mode. In addition, once J * is attained, a thermal runaway [12,13] can be provoked which causes the reduction of the circulating current well below the nominal value (i.e., the current in normal operation without a fault) or very important damage on the microbridge (that can be even burnt out).…”

“…However, the current is effectively limited only for electric fields or, equivalently, applied voltages well above that at which the limiter is triggered from normal operation (sample in the superconducting state) to current fault mode. In addition, once J * is attained, a thermal runaway [12,13] can be provoked which causes the reduction of the circulating current well below the nominal value (i.e., the current in normal operation without a fault) or very important damage on the microbridge (that can be even burnt out).The superconducting microbridge exchanges heat mainly with its substrate, because at the operation temperatures, around 90 K, the heat transfer coefficient between YBCO films, for instance, and their substrate is h bs % 10 3 W cm À2 K, whereas through liquid nitrogen or between the substrate and the copper holder (h sr ) they are 1000 times less [9,14,15]. Hence, the conditions for a good refrigeration and, therefore, an optimal operation of the limiting device, depend very much on the relative dimensions of the microbridge and their substrate.…”

Thermal Behaviour and Refrigeration of High-Temperature Superconducting Fault Current Limiters and Microlimiters

“…However, the current is effectively limited only for electric fields or, equivalently, applied voltages well above that at which the limiter is triggered from normal operation (sample in the superconducting state) to current fault mode. In addition, once J * is attained, a thermal runaway (Viña et al, 2003;Maza et al, 2008) can be provoked which causes the reduction of the circulating current well below the nominal value (i.e., the current in normal operation without a fault) or very important damage on the microbridge (that can be even burnt out).…”

“…The measurements were made in a cryostat with the sample submerged in a forced flow of helium gas. The temperature of the copper holder of the microlimiter was measured with a platinum thermometer and regulated with an electronic system which ensures a temperature stabilization better than 0.05 K. Two examples of the I − V curves obtained in this R L − R b circuit (with R L = 4.9 Ω, this value taking already into account the resistance of the circuit electrical wires, of the order of 1.9 Ω) by using the electronic system described elsewhere (Ruibal et al, 2007;Viña et al, 2003) are shown in figure 25. In these curves the voltage was imposed and acquired during pulses of 1 s, a time much longer than the one needed by the microlimiter to reach the stationary state.…”

“…with n = 3/2 for I * (Ruibal et al, 2007;Viña et al, 2003) and 1/2 for I min (Skocpol et al, 1974;Gurevich & Mints, 1987;Paulin et al, 1995). Therefore, if the reduced temperature increases both the discontinuity at V * and the ratio I * /I min will decrease, in agreement with the results of figure 25 The "optimal" reduced temperature for the microlimiter operation, T op /T c , will be then given by the condition I * (T op )=I min (T op ).…”