Tokamak Energy’s advances in “Partial Insulation” HTS magnet technology
Tokamak Energy has recently announced a new and potentially transformative magnet protection technology that could improve the commercial viability of fusion power plants, delivering higher performance for lower cost than other magnet technologies currently available.
The new method enables the manufacture of superconducting electromagnets with higher energy density that are more physically robust than currently available alternatives. The new magnets can tolerate significant operational damage (making them durable to radiation damage and fatigue effects, or damage from plasma disruptions), they don’t need the same level of high voltage insulation (which is incompatible with a long lifetime future fusion power station environment) and they have simpler cryogenic cooling, further saving cost and construction time.
Read on to find out about the problems that necessitated entirely new thinking on magnet design, and how we have solved them.
A new approach to making superconducting magnets
Since 2016 Tokamak Energy has been following a fast-track magnet technology development programme designed to enable construction of a compact spherical tokamak fusion demonstrator using high temperature superconductors (HTS).
One of the problems we set out to solve was a better way to detect and manage “quench”– the name for a loss of superconductivity in part of the coil. Quench is potentially the Achilles’ heel of HTS: if any part of the superconducting coil develops a resistance, for example in the event of localized damage arising from a plasma disruption, it creates a weakness that can overheat and burn out. The stored energy of a large magnet will be dissipated as heat, potentially causing damage to the magnet which would require replacement.
If an imminent quench is detected it is therefore necessary the tokamak magnets in a matter of seconds before this damage occurs. .
The Tokamak Energy HTS team successfully tested non-insulated coil technology in 2019, innovatively using regular electrical solder to consolidate and join the coil turns together, producing a This magnet technology was shown to be extremely tolerant to defects in the coils – in fact we drilled holes in an early version and it still worked! This magnet also demonstrated a remarkable resilience against quench, a key advantage of this technology.
Back to the traditional way?
Traditional methods used to protect superconducting magnets from quench require the cable from which the coils are wound to be encased in high voltage insulation (which is incompatible with the radiation environment), and to have significant quantities of soft copper to temporarily carry the current in the event that the superconducting layer quenches (i.e. stops conducting). The copper provides no benefit unless a quench occurs, so just wastes valuable space. It also weakens the coil structure, and forces the addition of a bulky high strength steel jacket, using up more space and making cooling of the superconductors more difficult (because steel is a poor thermal conductor).
Remember, a tokamak’s magnetic coil is an electromagnet, so it only generates a magnetic field when electric current is flowing through the spiral coil winding. In a non-insulated magnet it takes time to build up the spiral current because the current can take a shortcut via the radial path between turns. Traditional, insulated coil technology doesn’t have this problem because there is no alternative path for current than to flow along the cable, like a coiled-up pipe. Non-insulated coils are like pipes with lots of connections between the spiral turns of the coil.
So, we asked ourselves, is there a way to find a balance between the two opposing technologies (insulated and non-insulated coils) so that we can retain the enormous benefits of non-insulation coils without a charging delay?
It turns out that there is.
The Middle Way – partial insulation
This problem has been solved using Tokamak Energy’s patented “partial insulation” technology, which allows the resistance between turns of the coil to be selected. This allows us to choose a middle way—enough insulation to get the benefit of faster charging time, but not so much that it stops the current from swerving around defects in a turn and avoiding the problems of hot-spots going into thermal runaway (i.e. burning out). “Partial insulation” refers to a novel and patented method to introduce a small but finite resistance between the turns of the coils.
The net result is a very robust coil technology that delivers far more compact plasma confinement coils which don’t need high voltage insulation and have simple cryogenic cooling. The technology solves a big problem for scaling up HTS tokamak magnets and therefore further enhances the commercial viability of fusion power.
Our latest magnet, , demonstrates the benefits of our novel “partial insulation” quench protection method in a large magnet with significant stored energy. It has all the benefits of a non-insulated coil but without the charging delay.
The technology has already been incorporated into the toroidal field coils of our next HTS demonstrator, Demo4, a mid-scale full tokamak magnet set, which is currently under construction and planned for testing in 2022. Demo4 includes a full set of partially insulated HTS toroidal field (TF) coils, plus two poloidal field (PF) HTS coils. As the PF coils need to be energized quickly to initiate plasma pulses in a tokamak, these are regular insulated coils. Demo4 is designed to reach a peak field of over 20T and emulate the stress conditions the HTS tapes will see in a full sized tokamak. Further, it will test the interplay of all the magnets together and validate the use of HTS magnets within a full tokamak system for the first time.
When this happens, it will be an important step in delivering fusion power plants with higher performance for lower cost.
The benefits of partial insulation technology for a fusion scale tokamak are explained in Tokamak Energy’s contribution to the recently published roadmap for fusion magnet technology in Superconductor Science and Technology journal.
This blog article was written by Rob Slade, Advanced Technology Applications Director at Tokamak Energy, and Rod Bateman, Magnet Technology Manager at Tokamak Energy.