The STEP Fusion programme is a major project driven by UK Fusion Energy to deliver a first-of-a-kind fusion powerplant that can achieve net electricity generation (> 100 MWe), targeting first operations for 2040 and planned for construction in West Burton, Nottinghamshire, UK.
The STEP Cryogenics System will provide cooling to systems crucial to plasma confinement and tritium fuel cycling. STEP will require a total refrigeration load equivalent to approximately 100 kW at 4.5 K. Supercritical helium is required at a range of temperatures, approximately 80 K, 20 K and 5 K. The STEP Cryogenic System will be optimised for energy efficiency, minimising the parasitic load the Cryogenic System imposes on the STEP Prototype Plant is essential to net electricity generation.
The Cryogenic System will face challenging dynamic refrigeration loads. The STEP tokamak machine is a form of magnetic confinement fusion and will use cryogenically cooled high-temperature superconducting (HTS) magnets to contain the plasma. These magnets are essential for keeping the plasma stable, generating dynamic refrigeration loads. Plasma control requires the magnet current to change quickly, which creates heat losses in the superconducting materials that must be removed by the cooling system.
The Cryogenic Distribution System will isolate the cryogenic users from the cryogenic plant to prevent disturbances to refrigerator operation. Liquid helium buffer tanks could be used to provide rapid increases in refrigeration load in response to transient increases in heat load. Conventional machines typically use heat exchangers immersed in liquid helium baths, but generating liquid helium at 4.5 K to cool HTS magnets is inherently inefficient. The performance of liquid helium buffer tanks could be improved by heating the helium vapour produced in an externally fed heat exchanger.
This work examines the STEP Cryogenic Distribution System concept design and the challenge of handling dynamic refrigeration loads, whilst ensuring energy efficiency of the cryogenic system. The study presents an investigation into helium buffer systems for STEP, which are critical for managing transient thermal loads.

Keywords
STEP|Fusion|Helium Buffer