Metrosil Varistors during high energy discharges

IEEE publish the latest technical paper on Metrosil Varistors during high energy discharges

Co-authored with scientists and engineers from CERN and DTT, our latest paper published on IEEE Xplore covers the thermal effects on the electrical properties of Metrosil Varistors during high energy discharges.

This work is key to understanding the Metrosil performance during a discharge from a large superconducting magnet, such as those found in fusion tokamaks, and the paper continues a series of related articles on quench protection and energy extraction.

View the related papers available on IEEE Xplore using the links below.

Characterisation of SiC Varistors at Ambient and Elevated Temperatures for Protection of Superconducting Magnets

Publish date: May 2024 | Authors: Metrosil, CERN, DTT
High-energy varistors comprised of a silicon carbide (SiC) composite material have been used as an alternative to linear dump resistors for quench protection/energy extraction for superconducting magnets. However, characterisation work is required for accurate simulations, in particular at elevated temperatures. For magnets with a high inductance or large stored energy, energy extraction can take several seconds, during which time the varistor temperature increase can affect the voltage characteristics throughout the discharge. This work presents the variations of the parameters of SiC varistors at ambient and elevated temperature and include their effects to model the energy extraction and discharge, using the scenario of the DTT toroidal field coil quench protection and energy extraction, which will use varistors.

Evaluation of the Parallel-scheme Varistors as Energy-extraction System for a Test Facility of Superconducting Accelerator Magnet

Publish date: January 2024 | Authors: KEK, CERN, Metrosil
A varistor is a non-linear V -I resistor and its feature helps to extract stored energy of the magnet quickly while suppressing the magnet voltage. High Energy Accelerator Research Organization (KEK) decided to install the varistor for the test facility in order to conduct quench training of a superconducting magnet safely. The new energy extraction (EE) system consists of varistors made of silicon carbide (SiC) disks and is designed to extract energy up to 5 MJ. These varistors were installed prior to the test of the HL-LHC D1 magnet in 2023 and validated throughout the power test. A current imbalance and turn-on time (tON) among the varistor units, which could affect performance of the magnet protection, are monitored and measured to be within ±4% and 300 μs, respectively.

Characterisation of SiC Varistor Quench Protection Operating at 4 Kelvin for Use with Superconducting Magnets

Publish date: February 2023 | Authors: Metrosil, CERN, Uppsala University, Oxford Instruments
Silicon carbide (SiC) composite high-energy varistors have been demonstrated as a viable alternative to linear resistors as energy extraction devices during an abrupt loss of superconductivity in a magnet, called a quench. These have typically been installed external to the cryostat at ambient temperatures, but for some superconducting magnets it may be beneficial to mount the varistors within the cryostat in vacuum, a gaseous environment, or submerged in liquid cryogens. Varistors are semiconductors and therefore exhibit a temperature-dependent voltage–current relationship, so characterising their behaviour at low temperatures is important to predict their energy extraction behaviour. In this paper we present characterisation data of SiC varistor devices from 4–300 K: voltage-current characteristics, thermal conductivity, specific heat capacity, thermal expansion, and flexural strength. These varistors are a candidate for protection at 1.9 K of the MCBY magnets, currently being built at Uppsala University for CERN.

Experimental Examples of Quench Protection With Varistors to Reduce Quench Voltages and Hot-Spot Peak Temperatures

Publish Date: July 2022 | Authors: Metrosil, CERN, IMPCAS, IHEP, KEK
Superconducting magnet protection must address two main areas of the magnet and circuit performance, namely conductor hot-spot temperatures and circuit voltages. The maximum hot-spot temperatures and voltages occur during the superconductor’s transition called a quench and the subsequent energy extraction. As has been previously demonstrated, by using a varistor more energy can be extracted from the coil for the same maximum voltage, or the peak voltage can be reduced compared to a resistor for comparable extraction times. The principles of operation and theory behind the varistor are covered and three case studies are presented. Case 1 presents experimental data on energy extraction with varistors for the CERN Hi-Lumi MCBRD magnets under test at IMPCAS. Case 2 presents experimental data on the FECR sextupole magnet, also under test at IMPCAS. Case 3 covers simulated data for the CERN Hi-Lumi MBXF magnets, under test at KEK.

WATCH: Metrosil & CERN webinar on quench protection

Varistor Insulation for HTS Magnets

Publish Date: April 2022 | Authors: CERN, Metrosil
A variable resistance thin dielectric insulation coating for REBCO tape HTS coils has been developed. This new type of insulation system switches between high and low resistance, after an increase in inter-turn voltage. Non-Insulated (NI), fully soldered, HTS coils have proven to be very reliable; NI coils are achieving high magnetic fields above 25 Tesla and are almost impossible to quench. Over-current operation simply redirects the excess current out of the superconducting tape, to flow radially through the coil then back to the power supply. The internal coil resistance can then run the current down when the power supply is switched off. The disadvantage with NI coils is, as the coil volume and inductance increase, the charging / discharging time can take many hours, even days. This is not compatible with magnet systems that need accurate and fast current to magnetic field control, such as accelerators or other systems. With the Varistor Insulation (VI) we aim to achieve both robust performances as seen in NI coils and fast ramping with controlled current to field transfer functions. In this paper we present the electrical characterization of the insulation at room temperature and cryogenic temperatures, along with simulated magnet operation during ramping, normal operation and failure modes. We discuss other features of the VI insulation such as, application methods to provide thin layers, and alternative formulations to tune its properties. Its ability to act as a distributed quench heater when the voltage threshold is exceeded is also discussed.