Fusion Diary: The Magnet Wizards

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The tapes Tokamak Energy has been using are less than 0.1 mm thick, with a micron-thin layer of the REBCO superconductor material sandwiched between several layers of other materials that protect it and lend it favorable electrical and mechanical properties Incredibly, these thin tapes are capable of conducting thousand of amperes of electric current – with zero resistance!

For experimental and demonstration purposes Tokamak Energy is currently building a full-set magnet coil system similar to those that will be used in future spherical tokamaks. Christened“Demo4 ,”
this system will achieve a magnetic field strength of over 18 Tesla, nearly a million times stronger than the Earth's magnetic field, and over five times the field strength in the Joint European Torus (JET) reactor.

Left: Magnet expert Matt Bristow (l) with the author, who's holding a sample of REBCO high-temperature superconducting tape. Right: HTS magnets, which are used for testing purposes. Photos: Tokamak Energy Ltd

In the discussion with Matt Bristow, I was particularly interested in the problem posed by the phenomenon called“quenching.” This is one of the areas where Tokamak Energy has made ground-breaking innovations.

What is a quench?

Under certain circumstances, a superconductor can lose its superconducting properties and become resistive. When this happens in a superconducting coil, it is called a“quench.” As the current continues to flow, the resistive areas heat up.

Quenching typically begins at a small location and propagates in a kind of chain reaction, referred to as a“thermal runaway.” The heat spreads to the adjacent regions, driving the temperature up and causing them to become resistive in turn.
In this context, one should keep in mind the elementary fact that the inductance of a coil opposes any change in the current. Shutting down the current in a large high-field magnet is like trying to stop a 10-ton truck. One way or the other, the energy will end up as heat; the key question is: How fast does this happen and where does the heat go?

An unmitigated quench in a coil carrying a large current can lead to a rapid conversion of the magnetic field energy into heat. Sudden heating can severely damage the coil, which may then have to be replaced. The danger of severe damage is especially acute when the heating is concentrated in a few locations rather than distributed evenly over the whole coil.

Given the large mechanical stresses in a tokamak, a heat-provoked deformation in one coil might endanger the whole supporting structure.

Preventing or at least mitigating the effects of a quench is essential to the safe and reliable operation of any device using superconducting magnets. That includes not only tokamaks but also NMR medical imaging devices, high-energy particle accelerators, etc.

A quench could be extremely costly for a tokamak fusion power plant with its powerful superconducting magnets, requiring a long downtime for repairs and replacement of components. The ability to prevent or mitigate quenches is thus a precondition for tokamaks to become economically viable sources of energy.

Quenching can occur for a variety of reasons. An obvious one is a failure of the cooling system, causing the temperature of the superconductor to rise over its critical temperature. But quenching can also occur due to anything that affects the internal structure of the material, such as excessive bending, mechanical shock, radiation damage, etc.

Fortunately, a quench typically takes some time to develop. If an impending quench can be detected early enough, measures can be taken to halt it or at least avoid major damage to the coil. Today there are numerous means for quench detection. Sensors can identify tell-tale signs such as small changes in voltage and the magnetic field, temperature increases, mechanical strains and even acoustical signals.

But what do you do if there are signs of an impending quench or if the quench has already begun? The time may be so short that an automatic response is required.

In the conventional approach, the current in the magnet is deliberately“dumped” in a controlled fashion, by discharging (short-circuiting) the current through an external resistor that dissipates the energy as heat. This approach has disadvantages, however, and can risk damaging the magnet.

In one variant of this strategy, multiple resistors are installed, permitting the current to be discharged at several points along the coil simultaneously. Here the aim is to insure a uniform discharge of the coil. Another way to deal with the danger of hot spots is to use heaters, installed inside the coil at regular intervals, to cause the entire coil to quench simultaneously.

The best solution, of course, is to avoid quenches altogether.

Prevention, early warning and mitigation of quenches comprise a key focus of Tokamak Energy's activity. Here the company has made ground-breaking innovations and accumulated valuable intellectual property.

I shall just mention one of them, which is a potential game-changer.

Partial insulation technology ensures 'soft landing '


Winding magnets from REBCO high-temperature superconducting tape. Photo: Tokamak Energy Ltd

Commonly the tapes or wires from which tokamak coils are wound are fully insulated. In case of a quench, the insulation gives the current no alternative but to continue flowing through the entire length of the coil, aggravating any and all hotspots along the way.

The opposite approach is to leave out the insulation entirely. The“non-insulation” tactic permits the current to bypass an initial hot spot by flowing into adjacent turns in the winding. Unfortunately, this process increases the time required to charge up the magnet and has other operational drawbacks.

Tokamak Energy's scientists have developed an alternative approach called“partial insulation” (PI): Instead of being 100% insulating, the material used to separate the coil windings has a small, but non-zero resistance.

During normal, steady-state operation of the magnet, as long as the windings remain superconductive, the current has no compulsion to leak across this partial resistance. But when a quench begins to develop in one winding, the partial insulation permits some current to flow to the neighboring windings. This reduces the amount of current flowing through the hot spot, preventing it from heating up more and thereby decreasing the danger of a thermal runaway.