Swept-frequency analysis of a nonlinear HTS coil performed with the measurement bench of Fig. 2. At low incident power , a classical Lorentzian shape is observed (doted line) corresponding to a linear behavior of the HTS coil. At high incident power huge distortions due to the nonlinear behavior are evidenced: the solid and dashed lines correspond, respectively, to down-up and up-down frequency sweeps; the gray and black lines have been acquired with respective integration filter bandwidths of 300 and and emphasize complex out-equilibrium effects possibly including long-term thermal drifts.
Modeling of the measurement bench. The nonlinear HTS coil properties are inductively probed by reflectometry using a linear measurement bench.
Experimental setup. The HTS coil (a) is a multiturn transmission line resonator (Ref. 16). The pickup coil is used to probe local (b). The high-power setup (c) involves a dedicated probe made of a single-turn copper-wire loop and a high-power load connected using a coaxial cable. The probe is connected to the rf amplifier and the multireceiver MR console through the dual-directional coupler.
Experimental quality-factor drop (a) and resonance-frequency shift (b) expressed as a function of the power dissipated in the HTS coil. The low-level resonance frequency setting the shift reference is . The lowest-power data point is obtained in the linear regime using swept-frequency analysis under weak coupling condition, whereas higher-power data points are extracted from single-frequency measurements under strong coupling conditions.
Experimental real and imaginary parts of the HTS coil equivalent impedance expressed as a function of the current flowing in the windings. These characteristics are extracted with and the same data set as in Fig. 3.
Experimental amplitude (filled data points) and phase (empty data points) as a function of the power dissipated in the HTS coil. The amplitude is for from the center along the coil axis. This characteristic is extracted from the same data set as in Fig. 3. For comparison solid (amplitude) and dashed (phase) lines represent the ideal linear case.
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