Progress in the Design of a Hybrid HTS-Nb₃Sn-NbTi Central Solenoid for the EU DEMO

Abstract

State-of-the-art high field solenoids make use of hybrid designs exploiting the superior high field performance of High Temperature Superconductors (HTS) in the innermost region. The benefits of a hybrid Central Solenoid in a pulsed tokamak like the EU DEMO can be two-fold: either to reduce its outer radius (which would result in a reduced overall size and cost of the tokamak), or to increase the generated magnetic flux (which could extend the plasma burn time and possibly increase the power plant efficiency). In the framework of the pre-conceptual design studies for DEMO coordinated by EUROfusion, a hybrid Central Solenoid is proposed based on ten layer-wound sub-coils using HTS, Nb3Sn, and Nb-Ti conductors respectively for the high, medium, and low field sections. The design exploits the flexibility of layer winding by grading both the superconductor and the stainless steel cross sections in each sub-coil, which has the potential for space and cost savings. Mechanical analyses have identified fatigue as the main design driver for the EU DEMO Central Solenoid. Possible alternatives to reduce the sensitivity of the proposed design to fatigue are currently under investigation.

Training and Upgrading Tokamak Power Plants with Remountable Superconducting Magnets

“…The toppling forces are also localised, meaning that grading cable conduit thickness is also beneficial. Taking the example of the central solenoid coil in [144] and using a REBCO cost of 30 $/kA m (6 T, 4.2 K), we calculate the graded multi-superconductor solenoid to have a materials cost ≈ 21 % cheaper (equivalent to an overall capital cost reduction of just 0.3 %) than the ungraded REBCO-only solenoid.…”

Radiative pulsed L-mode operation in ARC-class reactors

“…Inductive current drive of course results in finite pulse time, τ pulse . A high-flux HTS solenoid could provide an attractive pathway to long pulse inductive reactor operation, and such solenoids have already been identified as potentially high yield technological developments for EU-DEMO and SPARC [49,50,20]. Estimation of pulse time will not be attempted here.…”

Advance in the conceptual design of the European DEMO magnet system

“…Two variants for the CS coil have been proposed [4] -one based on a Nb3Sn pancake-wound design [28], [29], whereas the other is a hybrid (HTS-Nb3Sn-NbTi) coil based on layer winding [30], [31] and [32]. The latter is more complex from the manufacturing point of view, however provides a more cost-effective design and a slightly higher magnetic flux compared to the Nb3Sn coil of the same outer radius.…”

“…Since 2018, the mechanical analysis performed on the CS coil addresses also the fatigue load [32], which turned out to be the main design driver for the CS coil. For the existing design requirements and assumptions, which include an operation limited to 20,000 plasma cycles [33], the design restriction due to mechanical fatigue translates into an allowable hoop stress of 300 MPa if stainless steel 316LN is used in the jackets [32]. As a consequence, the space allocation for the CS coil in the 2018 DEMO baseline seems not to be sufficient to reach the target magnetic flux of 250 Wb.…”

“…Even if the outer wall cracks, the coil integrity would be preserved. Some of these four options [32] will be subject to further investigations. If no solution is found, the fusion pulse length in DEMO will have to be shortened.…”