FREIA Magnet Test Facility Wiki

4. RRR test measurement procedure for superconducting magnets

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Tags/Project: Test-Procedures-wiki

Metadata FieldDocument Information
StatusUnder Review
Created2026-09-21
Last Updated2026-10-01
AuthorDavide Cuneo
Rev. #1.3
ReviewersDavide

Introduction and Context

This document describes the procedure for measuring the Residual Resistivity Ratio (RRR) of the magnet under test. The RRR is defined as the ratio between the electrical resistance, or resistivity, of the conductor at room temperature and its resistance at cryogenic temperature, either 4.2 K or 1.9 K depending on the operating point considered. For this particular magnet type, the RRR value is typically expected to be around 200.

To describe the procedure a test of this kind was run at CERN. The test is performed on a two-aperture magnet, with the two apertures referred to as AP1 and AP2. As in the other documents of this wiki, the four copper leads of the magnet insert are labelled A and B for AP1, and C and D for AP2. The measurement itself consists of monitoring voltage and current at low frequency while the magnet goes through a temperature transient, which is why a low-frequency acquisition system is required: the warm-up or cool-down process that the measurement relies on can take several hours, or even entire days.

The specific example documented here was carried out during the warm-up phase of the magnet. While the test plan does allow this measurement to be performed during cool-down, or after a thermal cycle, warm-up data is generally less convenient to work with, since the resistance plot grows extremely slowly during this phase, which makes it harder to identify a clean point for the RRR calculation. This point is discussed further in the data analysis phase, below.

Acronyms and Terms

TermMeaning
RRRResidual Resistivity Ratio: the ratio between the conductor’s resistance at room temperature and at cryogenic temperature, used as an indicator of conductor quality
AP1 / AP2Aperture 1 and Aperture 2: the two independent magnetic circuits of this two-aperture magnet
VTVoltage Tap: a wire connected at a specific point along the coil winding, used to measure the voltage, and therefore the resistance, of that particular segment
DMMDigital MultiMeter: the instrument, and associated application, used here to log voltage and current during the test
PCPersonal Computer, the computer running the DAQ software. Note: in this document “PC” stands for Personal Computer. In the Control Room overview document, the same abbreviation instead stands for Power Converter, so always check the context.
DAQData Acquisition system, used to record and log the electrical signals from the test
LFLow Frequency: the acquisition chain used for slow, long-duration measurements such as this one
TDMSTechnical Data Management Streaming: the binary file format used by the DMM software to store the raw logged data
DIAdemA data-analysis software package, by National Instruments, used to import and process the .tdms files recorded during the test
Burndy connectorA multi-pin connector (Burndy is the brand name) used throughout the facility to terminate bundles of wires, such as the voltage taps, in an organised and reusable way

Required Equipment

Carrying out this measurement requires the following:

  • an external power supply, an Agilent E3645A in this example, driven in voltage mode;
  • an external shunt resistor (100.828 mOhm in this example) connected in series with the circuit, used for high-precision current measurement and shown in Figure 1;
  • a digital multimeter (DMM) at the measurement station, used to log voltage and current;
  • the PotAims chassis together with its Burndy connectors, used to route the voltage tap signals to the acquisition hardware.
    Figure 1: Shunt resistor used in the RRR measurement campaign.

Phase 1: Electrical Connections & Setup

The main objective of this phase is to connect the power supply, the shunt resistor, and both apertures of the magnet in a single, continuous series circuit.

Series Circuit Configuration: The connections must follow a strict path to close the loop: Positive terminal of the Power Supply Positive of the shunt resistor Negative of the shunt resistor Copper Lead B Copper Lead A (naturally in series with B inside AP1) Copper Lead C (via banana connector from A) Copper Lead D (naturally in series with C inside AP2) Negative terminal of the Power Supply.

Inter-aperture Connection: External banana connectors are used to bridge the two apertures in series (connecting A to C/D as required to close the loop).

Power Supply Settings (Critical): The Agilent E3645A power supply is turned on to supply a constant current, her set to 50 mA (generally will be specified in the magnet test procedure). It is extremely important to set the voltage limit on the power supply to the maximum. As the magnet warms up, its resistance increases significantly; maximizing the voltage ensures the power supply does not become voltage-limited, allowing it to maintain the required probing current all the way up to room temperature.

Phase 1: Magnet Reception and VT Connector Preparation

Before any electrical connection can be made, the wiring coming from the magnet needs to be properly prepared. Upon arrival at the test facility, this wiring is often poorly executed: rather than being organised into a dedicated connector, the wires frequently arrive bare. It is essential that the voltage taps are properly labelled at this stage, since the way these connections are handled at reception matters not only for the RRR test itself, but for the entire testing campaign, and in particular for ensuring that the voltage tap signals can be acquired correctly later on. A picture of a properly prepared Burndy connector, with the voltage taps labelled and installed, is shown in Figure 3.
Fig.3: A picture showing the Burndy connector with the voltage taps wiring installed inside.

To reach this state, the operator has to manually install a Burndy connector onto the incoming voltage tap wires. This means individually crimping a Burndy pin onto each bare wire coming from the magnet; the result of this crimping operation is shown in Figure 4. This work requires a specific set of specialised Burndy tooling: a dedicated crimping tool, handling tools used to insert and manage the connector pins, and a further specific tool used to safely extract a pin from the connector body whenever a correction is needed. These tools are shown in Figures 5 and 6.

Fig.4: Burndy ends crimped to the VTs wires.

  • Tooling: A specific set of specialized Burndy tooling is required for this operation: a dedicated crimping tool, handling tools for inserting/managing the connector pins, and a specific Burndy tool used to safely extract pins from the connector body if corrections are needed. Fig. 5 and 6 show the above mentioned toolings.

Figure 5: Burndy tooling used to push in and out pins from the connector.

Figure 6: Burndy tooling used to crimp ends to the VTs wirings.

Phase 2: Electrical Connections and Setup

With the voltage tap connector ready, the main objective of this phase is to connect the power supply, the shunt resistor, and both apertures of the magnet into a single, continuous series circuit.

The connections must follow a strict path in order to close the loop correctly. Starting from the positive terminal of the power supply, the circuit goes to the positive side of the shunt resistor, then to its negative side, then to copper lead B. From lead B, the current naturally continues in series through the magnet to lead A, since A and B are connected internally within AP1. From lead A, an external banana connector carries the current to lead C, and from there it continues naturally in series through AP2 to lead D, since C and D are connected internally within that aperture. Finally, lead D is connected back to the negative terminal of the power supply, closing the loop. In practice, the two apertures are bridged together using external banana connectors, wired so as to connect A to C, or D, as required, in order to close the series path.

A visual reference for the complete wiring loop can be observed in Fig. 2.
Figure 2: Picture of the setup for the RRR measurement. The connection from the shunt resistor follow the following path: CL B, then banana from A to D and then a final banana from C to the power supply to close the circuit.
Once the wiring is complete, the power supply settings need to be configured carefully, as this step is critical to the success of the test. The Agilent E3645A is turned on to supply a constant current, set in this example to 50 mA, though the exact value is generally specified in the magnet’s own test procedure.

NOTE

It is extremely important to set the power supply’s voltage limit to its maximum: as the magnet warms up, its resistance increases significantly, and maximising the voltage limit ensures that the power supply does not become voltage-limited, so that it can keep delivering the required probing current all the way up to room temperature.

Phase 3: DMM Wiring and Signal Routing

Once the power loop is energised, the voltage taps and the wires coming from the shunt resistor still need to be correctly routed to the acquisition hardware. In this example, at the measurement station, the two wires carrying the voltage signal from the shunt resistor were connected to card No. 1 of the DMM, specifically between pin 1 and pin 2. All the voltage taps coming from the magnet, arriving via the Burndy connector originating from the PotAims chassis, were then wired to the DMM as well.

To make sure every pin ends up in the right place, the operator must strictly follow a dedicated Excel spreadsheet specific to the individual magnet, referred to as the TestDoc file. This spreadsheet maps exactly which pin of the Burndy connector corresponds to which channel on the DMM cards, and should be treated as the single reference for this wiring step.

It is also worth noting how this is handled in the SM18 test hall, where intermediate dispatching panels, placed between the Burndy connector and the DMM, are used to sort and route the individual cables into the correct DMM cards. The same approach should be applied here as well.

Phase 4: Software Setup and Test Execution

With all the electrical and signal connections secured, data acquisition can be started. The dedicated DMM software is launched first. Before starting the log, a specific configuration file has to be uploaded into the software, in order to initialise the channels and the acquisition parameters correctly. Once this is done and the software is running, it will continuously log the low-frequency data for as long as the thermal transient lasts.

Phase 5: Data Analysis and RRR Calculation

The raw data recorded by the DMM is stored in the .tdms file format, and is imported and analysed using DIAdem. When working with a warm-up curve specifically, identifying the exact resistance point to use for the ratio calculation can be difficult, since the resistance plot grows extremely slowly over time; for this reason, cool-down data is technically preferred over warm-up data when calculating the definitive RRR value.

Regardless of which transient is used, before the final RRR ratio can be calculated, the individual resistance segments probed by the different voltage taps must first be summed together, in order to obtain the full resistance of the coil. Only once this total resistance value is available can the RRR itself be calculated.

Quick Reference

ItemValue
Probing current50 mA (check the specific magnet test procedure)
Shunt resistor value100.828 mOhm
Power supply modelAgilent E3645A, driven in voltage mode, voltage limit set to maximum
Pin-mapping reference fileTestDoc (Excel spreadsheet, magnet-specific)
Raw data format.tdms
Analysis softwareDIAdem

Operational Checklist

  • Checked the incoming wiring from the magnet and confirmed the voltage taps are clearly labelled
  • Crimped a Burndy connector onto the voltage tap wires, using the dedicated crimping and pin-handling tools
  • Connected the magnet circuit in series
  • Set the power supply (Agilent E3645A) to the required constant current (50 mA in this example)
  • Set the power supply voltage limit to its maximum
  • Connected the shunt resistor voltage wires to DMM card 1, pins 1 and 2
  • Wired all voltage taps to the DMM following the TestDoc pin-mapping spreadsheet
  • Routed the cables through the dispatching panels
  • Launched the DMM software and uploaded the correct configuration file
  • Started the log and let it run for the full duration of the thermal transient
  • Imported the resulting .tdms file into DIAdem for analysis
  • Summed the individual VT segments to obtain the full coil resistance before calculating the final RRR

Feedback

Found an error or something outdated in this note? Please contact Davide Cuneo (davide.cuneo@physics.uu.se).

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