Stability of a cable in conduit conductor under fast magnetic field variations

Abstract

Under fast magnetic field variations, ac losses are deposited in a Cable in Conduit Conductor (CICC). The corresponding power losses are transferred to helium thanks to the high wetted perimeter of the conductor. The critical energy of the CICC can be expected to be proportional to the high volumetric heat capacity of helium and to the temperature margin. To confirm the expectations, stability tests under a transversal pulsed magnetic field were performed in the Sultan test facility on a prototype JT-60SA conductor sample. The shape of the magnetic field variation as a function of time is a truncated sinusoid. The experimental results are not totally in agreement with expected behavior on two particular points: -the deposited energy in the conductor as a function of the pulsed field amplitude, measured by calorimetry, deviates from the expected quadratic behavior for coupling losses. The deviation is also increasingly dependent on the transport current. -at a given Sultan background magnetic field, the critical energies at low temperature margins are reduced in comparison with expected values. An explanation based on the saturation of parts of the cable is proposed.

Void Fraction Influence on CICCs Coupling Losses: Analysis of Experimental Results With MPAS Model

“…We scan sinusoidal field frequencies ranging from 5 𝑚𝐻𝑧 (to get refined data for ntau determination) up to 4 𝐻𝑧 (to deeply investigate ranges where single constant model likely becomes invalid). Our power supply having operating limitations, amplitude of the input current decrease with frequency but in a reasonable range which does not affect the coupling losses while rescaling with 𝐵 𝑖 2 [11]. 𝐵 𝑖 being the internal field of the CICC in response to the applied external field.…”

Modeling electro-magnetic and thermal Stability of Cable-in-Conduit Superconductors for Fusion Magnets

“…The discrepancy at 1 K in temperature margin is probably related to the magnetic field peak value, about 0.31 T. As expected, the difference in applied magnetic field peak value and temperature generates is MPAS analytical model a discrepancy in the calculated energy. This type of behavior was already observed in [73].…”

“…The structure of a CICC is significantly different, and very much more complex than of a strand. The use of a global time constant to represent the AC coupling loss of a multistage cable can be convenient and accurate for gradual variations of magnetic field (dB/dt), but this approximation fails to describe the behavior for faster field variations [73]. With the same methodology, the coupling loss can be fitted analytically assuming the presence of several time constants associated with respective volume fractions in a CICC, a concept which was proposed in [43] and later also adopted in [74], [75].…”

“…Unfortunately, these stages do not necessary reflect the real contribution which every stage gives to the total coupling loss. The parameters of the MPAS are calculated for a given ITER conductor by fitting the measured AC loss data obtained in SULTAN in a given range of frequency, typically 0.1 to 5 Hz [73]. It is important to note that the parameters obtained from a fit to a coupling loss versus frequency curve, is only valid for the specified experimental conditions of selected frequency range and amplitude of the applied magnetic field.…”

“…For the stability test, instead of a continuous sinusoidal pulse, a single sinusoidal magnetic field pulse is applied to the conductor, see Figure 2.3. In terms of power the pulse shape can be expressed as a truncated sinusoid [73]. To model the stability trigger pulse it is necessary to modify eq.…”