The magnetic cage

Abstract

The Spherical Tokamak for Energy Production (STEP) requires high-field magnet designs and has therefore adopted the REBCO-based high-temperature superconductor (HTS) as its current carrier. The HTS enables the toroidal field (TF) coils to be remountable, which unlocks STEP's vertical maintenance approach; however, remountable joints, approximately 18 GJ of stored energy and limited space down the centre of a spherical tokamak, make the TF coils the most challenging. STEP has pursued a passive approach to TF coil quench protection in order to limit coil terminal voltage. Initial results suggest that a solution may rely on tuning internal coil resistance coupled with actively powered heaters. The pre-conceptual inter-coil structure demonstrates acceptable stresses and deflections under steady-state operating conditions and preliminary fault scenarios, and loads are distributed to limit the tensile force on the TF centre rod. Finally, the HTS must operate reliably in a high radiation environment and endure high neutron fluences, ensuring commercially relevant magnet lifetimes. Initial experiments indicate that instantaneous gamma irradiation of HTS has no negative impact on current carrying capacity. Experimental programmes are underway to cold irradiate HTS to fusion-relevant fluences and to develop a method of assuring tape irradiation tolerance using oxygen ions as an analogue for neutrons.This article is part of the theme issue 'Delivering Fusion Energy - The Spherical Tokamak for Energy Production (STEP)'.

Keywords: HTS; REBCO; STEP; magnet; quench; superconductor.

Figure 1.

Cross section of the TF cage indicating segmentation of TF coil (a) and representation of the centre rod being retrieved (b). The CS (in light blue) can be seen wrapped around the inboard TF limbs.

Figure 2.

Cross section of a cable comprising a copper channel with a built-in cooling pipe and a stack of HTS tapes (left). Schematic of 40-turn TF winding pack showing four pancakes in a radial plate configuration (right).

Figure 3.

Temperature distribution along a cooled cable at various times after quench initiation.

Figure 4.

Raccoon model of a toroidal field coil. Hotspot evolution is shown. The local temperature rises to over 800 K in approximately 340 s, at which point the simulation is terminated.

Figure 5.

Dump switches are used to transfer stored magnet energy to heaters embedded in the coil structure.

Figure 6.

PF coil set and summary of plasma control functions of the various coils.

Figure 7.

Overview of the inter-coil structure, highlighting the five distinct modules.

Figure 8.

Total deformation plot and von Mises stress plot under thermal and flat-top electromagnetic loads.

Figure 9.

Force distribution through the inter-coil structure showing top end of a TF coil (middle). Simple representations of winding pack cross sections shown on inboard (wedge-shaped) and outboard TF limbs. Mid-plane strain analysis of inboard winding pack (containing four radial pancakes) showing maximum principal strain on HTS cables at <0.2% (left).

Figure 10.

Fault load overview: exaggerated radial deflection of centre rod (left); stress plot of whole structure (cross section through the centre of the machine) (right).