Current distribution monitoring enables quench and damage detection in superconducting fusion magnets

Abstract

Fusion magnets made from high temperature superconducting ReBCO CORC^® cables are typically protected with quench detection systems that use voltage or temperature measurements to trigger current extraction processes. Although small coils with low inductances have been demonstrated, magnet protection remains a challenge and magnets are typically operated with little knowledge of the intrinsic performance parameters. We propose a protection framework based on current distribution monitoring in fusion cables with limited inter-cable current sharing. By employing inverse Biot-Savart techniques to distributed Hall probe arrays around CORC^® Cable-In-Conduit-Conductor (CICC) terminations, individual cable currents are recreated and used to extract the parameters of a predictive model. These parameters are shown to be of value for detecting conductor damage and defining safe magnet operating limits. The trained model is then used to predict cable current distributions in real-time, and departures between predictions and inverse Biot-Savart recreated current distributions are used to generate quench triggers. The methodology shows promise for quality control, operational planning and real-time quench detection in bundled CORC^® cables for compact fusion reactors.

Figure 1

Ribbon CICC (bottom) and 6-around-1 (top) wound with CORC^® cables. Both are high-current cables relevant for fusion with limited current sharing between sub-elements.

Figure 2

Circuit diagram for the CORC^® CICC triplet of Ref.. Hall probe positioning and field distribution are shown in Fig. 4. Purple arrows and circles show the direction of magnetic field produced by the current in each cable branch.

Figure 3

Experimental setup from Weiss et al. with cable and Hall probe labels corresponding to Fig. 4.

Figure 4

Geometry and Biot-Savart field calculation of CORC^® triplet CICC data in Ref. with 1 kA in each cable. CICC configuration corresponds to the network schematic in Fig. 2.

Figure 5

Methodology of this work. Cable currents are recreated from experiments using inverse Biot-Savart techniques, which are used to extract electric circuit parameters. These parameters can be used for quality control and test planning. The second phase compares the trained model predictions of cable currents with inverse Biot-Savart recreated cable currents for quench detection.

Figure 6

Processing of I-V curve with Hall probe measurements (top plot, see Fig. 4) into recreated cable currents with model fits (bottom plot).

Figure 7

Simulated performance of the triplet cable during a fast trapezoidal ramp to 3900 A at 10,000 A/s using the extracted parameters in Tables 1, 2, 3. The top plot shows the current, the second plot shows the termination voltage, the third plot shows the superconductor voltage, and the bottom plot shows the inductive voltage of each cable (see Fig. 2).

Figure 8

Current redistribution-based quench detection in CORC^® triplet test with dynamic ramp of 2000 A/s. No quench is induced in this case; this is correctly identified.

Figure 9

Current redistribution-based quench detection in CORC^® triplet test with dynamic ramp of 2000 A/s. Heater on C1 (middle cable) induces a quench that is correctly identified.

Figure 10

Current redistribution-based quench detection in CORC^® triplet test with dynamic ramp of 2000 A/s. Heater on C2 (right cable) induces a quench that is correctly identified.