FREIA Magnet Test Facility Wiki

3. Valve box

Document information

Tags/Project: GERSEMI-wiki

Metadata FieldDocument Information
StatusUnder Review
Created2026-06-24
Last Updated2026-09-29
AuthorDavide Cuneo
Rev. #1.4
ReviewersDavide

About the document

This document provides a description of the valve box used within the GERSEMI system.

Valve Box Structural Layout

The facility utilizes a dedicated valve box located in close proximity to the vertical cryostat to regulate and distribute the necessary cryogens to the cooling loops, thermal shields, and superconducting equipment. The valve box is enclosed within a stainless-steel vacuum vessel and houses a 50-liter liquid helium tank that functions directly as a 4 K helium buffer for the main cryostat. Internal components are surrounded by a copper thermal shield actively cooled via liquid nitrogen (LN2) circulation. This shield serves as a thermal intercept at 80 K to mitigate solid conduction heat leaks migrating along the support tie rods and cryogenic valve stems. All internal process piping is thermally anchored to this copper shield, and fluid delivery to the vertical cryostat is routed through a specialized four-pipe transfer line exiting from the lower flange of the valve box.

Thermodynamically, the valve box is rated to manage operational regimes between 1.8 K and 4.5 K under experimental heat loads of up to 100 W. For sub-4 K cryostat operations, the assembly integrates a dedicated counterflow heat exchanger coupled with a Joule-Thomson expansion valve to sub-cool the liquid helium at atmospheric pressure prior to expansion. Additionally, the vacuum vessel houses an independent cooling circuit equipped with an extra heat exchanger designed to supply supercritical helium at approximately 5 K to the neck of the vertical cryostat, suppressing conductive heat loads originating from the warm top flange. A representation of the valve box can be observed in Fig. 1.

Figure 1: Schematic view of the valve box (left) and the vacuum and pressure vessels (right).

The Cryogenic System Validation Simulator

After the delivery and commissioning of the valve box, to validate its operational performance and verify the control system sequences without testing the full facility mass simultaneously, a low-mass cryogenic simulator was constructed. This simulator consists of a small vertical cryostat configured to replicate the functions of the main “Liquid Insert” in saturated liquid helium modes between 1.8 K and 4.5 K, while requiring significantly smaller cryogen volumes and cooling times. Mechanically, the simulator bypasses the main cryostat transfer line and connects directly to the lower interface of the valve box. The two vessels share a common insulation vacuum space maintained by a vacuum pump connected to the valve box shell.

The simulator hardware includes a cold vessel for liquid helium storage and a closed thermal loop designed to induce a controllable thermal load on the supercritical helium circuit. Known heat loads are generated via electrical heaters glued directly to the surfaces of the cold vessel and the helium process loop. To manage conduction and radiation losses, a copper thermal shield is installed around the internal vessel and brazed directly to the vessel neck to intercept heat moving from the upper boundary. The top flange assembly is further insulated with a glued foam plug and a series of floating radiative shields to suppress ambient radiation and prevent external freezing of the upper flange structure.

Figure 3: Simulator connected at the valve box.

Control systems across the facility

Automation, data acquisition, and real-time process loops across the GERSEMI facility are managed by an industrial Programmable Logic Controller (PLC) architecture utilizing Siemens S7-300 hardware controllers. The physical control hardware, including the PLCs, circuit breakers, and system power supplies, is housed within electrical cabinets situated adjacent to the vertical cryostat pit. To facilitate rapid commissioning and hardware modifications during insert transitions, all field cabling is routed through standardized plugs and connectors terminating at localized general electrical distribution boxes.

The physical PLC layer interfaces with the sensors and analog current-driven actuators via Profibus or Ethernet communication networks. For system commissioning and initial diagnostic runs, the PLC layer is integrated with a Supervisory Control and Data Acquisition (SCADA) system operating on a WinCC interface, which maps and handles all digital and analog I/O procedures to provide the real-time dynamic data required for plant supervision. During standard experimental operations with the GERSEMI cryostat, the entire data architecture and control sequences run under the Experimental Physics and Industrial Control System (EPICS) framework, allowing for synchronous online diagnostics and comprehensive post-run data logging.

4.4 Process Instrumentation and Sensor Allocation

The control loops utilize distinct sensor technologies depending on the process fluid and thermal regime being monitored:

  • Nitrogen Circuit Temperature Control: Equipment and shield boundaries cooled by liquid nitrogen are instrumented with platinum resistance thermometers (Pt100), using both thin-film and wire-wound configurations connected directly to the analog input cards of the Siemens PLC.

  • Helium Circuit Temperature Control: Low-temperature lines operating with liquid, sub-cooled, or supercritical helium are monitored using calibrated Cernox thermometers (CX1050SD). These sensors interface with a dedicated CABTF controller containing 16 inlets, capable of multiplexing inputs from up to 48 distinct Cernox channels.

  • Liquid Helium Level Measurement: Saturated fluid heads are monitored via superconducting level probes distributed across four specific system nodes:

    • One baseline probe installed inside the primary valve box volume.

    • One baseline probe dedicated to the internal 4 K buffer tank.

    • Two probes connected in series within the “Liquid Insert” assembly.

    • Four independent probes arranged within the “Magnet Insert” to map the three separate helium baths, where two of the sensors are linked in series to simulate a single extended probe geometry.

  • Mass Flow and Thermal Balance: Mass flowmeters are installed on both the nitrogen exhaust and helium gas return lines. The real-time flow data acquired by these instruments is utilized by the control system to calculate the static and dynamic thermal losses across each cryogenic circuit.

  • Pressure Regulation Loops: Saturated pressures inside the 4 K and 2 K vessel spaces are managed by room-temperature VAT butterfly valves. Each valve is regulated by dedicated proportional–integral–derivative (PID) software loops executing within the PLC program. The underlying process pressures within the helium baths are measured via capacitive pressure sensors routing directly to the automation system.

Feedback

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

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