Fusion Diary: The Magnet Wizards

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.

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.

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.

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.

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More importantly, however, partial insulation provides for a
“soft landing” of the whole coil. When a quench process is detected, automatic systems interrupt the flow of current through the power supply, creating an open circuit.

As I noted above, in analogy to the momentum of a 10-ton truck, the self-inductance of the coil forces the current to continue circulating. Since its normal spiral pathway through the whole coil is blocked, the current, in order to complete its circuit, has no alternative but to cross over from one winding to an adjacent one.

In doing so, it passes through the partial insulation, where part of the energy is converted into heat. As this occurs throughout the coil, the heating is uniformly distributed, while the current gradually drops off.

The key benefit of this method is that it avoids the damage that might occur if some areas in the coil were to become much hotter than neighboring ones.
Also, the“soft landing” occurs through a natural process within the coil itself, with no need to connect it to the outside.

Tokamak Energy has pioneered other innovations, that render the HTS magnets so robust that some of these magnets have retained their operating characteristics even after holes have been drilled into them. (View a technical presentation on Tokamak Energy's magnet technology.)

Demo4

Construction is scheduled to be completed next year on the next big step forward: Demo4 , an integrated structure containing 44 high-temperature superconductor magnets. The structure imitates the magnet configuration of a real tokamak but without other tokamak components such as the reactor chamber.

Schema of Demo4.
Image: Tokamak Energy Ltd

Demo4 is intended to demonstrate the full range of Tokamak Energy's innovations in HTS magnet technology, to inform the design of the company's prototype HTS reactor, STX and the subsequent demonstration fusion power plant, ST-E1.

Among other things, Demo4 will emulate the mechanical stresses that the HTS tape and the entire magnet structure will experience in the operation of these devices.

In parallel, Tokamak Energy magnets are presently undergoing irradiation testing to optimize their operation in conditions of intense gamma radiation.

In the future, experiments with neutron irradiation will be carried out.

All in all, my visit to Tokamak Energy left me in a very optimistic mood. One can never be sure of the future but Tokamak Energy's strategy and projected timeline seem quite credible. In the meantime, the company has already contributed significantly to the worldwide fusion effort.

Jonathan Tennenbaum , PhD (mathematics) is a former editor of FUSION magazine and has written on a wide variety of topics in science and technology, including several books on nuclear energy.

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