Description
Spherical Tokamak for Energy Production (STEP) is a programme to design, construct and operate the UK’s first fusion power plant, which will be constructed in the 2030s. STEP is designed to have a large vacuum vessel, as the plasma volume should be large enough to produce 1.5 GW of fusion power. On the other hand, due to the spherical torus geometry, the central stack space for the central solenoid is limited, and the inducible loop voltage and total flux in Volt-seconds are limited. It is essential to carefully assess the feasibility of plasma initiation and the uncertainties related to the flux necessary for completing the plasma initiation. In this contribution, we report on the STEP plasma initiation assessment that has been conducted over the past few years by the STEP plasma initiation group. The electromagnetic machine description was developed based on the latest STEP hardware design. Operational coil current waveforms were calculated using YFactory, which solves the state-space representation to produce the target magnetic field configuration, including a poloidal field null, and loop voltages, while satisfying radial force balance, and ensuring vertical stability of the plasma current, while adhering to engineering constraints such as coil current limits and power supply voltage limits. Using the input data, predictive simulations were performed with the full electromagnetic plasma initiation modeling code, DYON[1][2]. The operational scenario that successfully achieved plasma initiation in DYON was selected as the reference case. Based on this reference case, various parameters—such as effective vacuum volume, initial impurity levels, passive structure conductance, plasma current position, and absorbed ECH power—were examined to identify the upper and lower limits of each parameter necessary for successful plasma initiation. The uncertainties in central solenoid flux consumption were quantified by assuming parameters at their upper or lower limits. The lessons learned from the STEP plasma initiation assessment will be beneficial for future devices such as EU-DEMO and CFEDR, particularly if they encounter similar challenges due to large vacuum volumes and superconducting central solenoids.
REFERENCES
[1] Hyun-Tae Kim et al 2022 Nucl. Fusion 62 126012
[2] Hyun-Tae Kim et al 2024 Nucl. Fusion 64 126010