An analytical model for coupling losses in large conductors for magnetic fusion

Abstract

During the operation scenarios of large superconducting tokamaks such as JT-60SA or ITER, disruptions or VDE (Vertical Displacement Events), AC losses are deposited in the superconducting magnets. Due to these AC losses, the temperatures of the Cable in Conduit Conductors (CICC) increase, reducing the temperature margins. It is of highest importance for the operation to correctly estimate AC losses and conductor temperatures. Due to the fast transient nature of those events, an important component of the losses is the coupling losses.The approach for coupling losses estimation, using a unique effective time constant “nτ” is not completely adequate to calculate these losses.For this purpose, since 2010, a heuristic analytical model called MPAS (Multizone PArtial Shielding), has been proposed. It is now currently used by several teams.For each conductor of the coils of a tokamak, the parameters of the MPAS model and nτ are estimated based on the characterization of the conductor AC losses under a sinusoidal excitation at different frequencies. This process is extensively conducted in existing facilities such as the SULTAN test facility at SPC (Swiss Plasma Center), on the dipole and press of Twente University or on JOSEFA dipole test facility at IRFM (Research Institute for Magnetic Confinement) at Cadarache.This characterisation leads to the constitution of a database for the different conductors used in the tokamak. This has been pushed to the uttermost within the ITER qualification and QA/QC (Quality Assurance, Quality Control).The methodology for estimating the MPAS parameters in relation with the measured AC losses is described in details and illustrated for two conductors. Using this database, it is then possible to calculate the coupling losses during ITER module discharges and scenarios. Estimation of coupling losses are compared to experimental results in recent experiments on ITER CS (Central Solenoid) modules.