Variable-RL-cancel circuit for precise JC measurement using third-harmonic voltage method

A precise and reliable non-destructive measurement technique is required for evaluation of critical current density, JC, and electric field E vs. current density J characteristics in the recently developed large-area or long-length high temperature superconducting films. Here, we measured E-J characteristics of a 2-in. φ Y-123 film with an inductive measurement system using the third-harmonic voltage, V3, and adopted a tunable-RL-cancel circuit to decrease harmonic-voltage noise. First, we measured coil current I0 dependences of V3 voltage and obtained the threshold coil current, I0 = Ith, when a certain V3 voltage suddenly arose. And we also measured self inductance of the drive coil, Ld, with changing pressure between the drive coil and film in the range of 0.1-0.6 MPa. The Ld and Ith saturated to constant values at coil pressures above 0.2 MPa, and the saturated Ld value was almost equivalent to the simulated Ld of 134 μH. With application of an adequate coil pressure of 0.25 MPa, we measured Ith with varying drive frequency f from 0.2 to 5 kHz. Harmonic voltage noise was sufficiently eliminated below 1 mV using the variable-cancel circuit. If a constant criterion of third-harmonic inductance V3/(fI0) = 3 μH was applied to define the Ith, obtained E-J curves showed a good fit to the theoretical power-law dependence of E = aJn. The average n value obtained from each E-J curve at five different positions in the film was 21.0, which was slightly lower than the value of 23.5 obtained from transport E-J characteristics.