Increasing Fusion Supply Chain Diversity: High-Temperature Semiconductors
HTS can take multiple forms, but the most popular for fusion are HTS tapes (shown above on the right). Let’s look at some more likely use cases for HTS tapes.
Magnetic Resonance Imaging Machines
One of the most common superconductor use cases is magnetic resonance imaging (MRI) machines. These machines utilize extremely high-power magnets (1-7 tesla) and radio waves to excite atoms in the body; when the radio waves are turned off, the atoms emit radio waves. These signals are then used to create an image. In general, superconducting magnets create “cleaner” images with higher magnetic field strength and lower signal-to-noise (SNR) ratios than older machines that did not use superconducting magnets. Today, most MRI machines use niobium-titanium magnets, which are cooled by liquid helium to temperatures <9.4 degrees K.
MRI magnetic systems (and RF coils) using HTS tape-based superconducting magnets have several advantages over classical low-temperature superconductors:
- Lower-cost operation: HTS tape-based magnets can utilize liquid nitrogen instead of liquid helium, reducing operating costs.
- Stronger magnetic fields AND smaller coils: HTS magnets can run at higher currents than niobium-titanium superconducting magnets while being both smaller and lighter than systems using “classical” superconductors.
- Higher coil sensitivity: HTS tape-based MRI systems can provide even better imaging than current technology because of their improved ramp speeds and gradient strength.
HTS based-MRI machines utilizing first-generation HTS based on Bismuth-2223 have been in discussion since the 1990s, with the first magnets being built in 1999. With the advent of second-generation HTS tape-based magnets, even better performance can be realized. The primary challenge today is the cost of the HTS tape-based magnets. Niobium-titanium and niobium-tin alloy materials are generally (today) easier to manufacture because they are not ceramics like HTS materials. However, using HTS magnets in fusion machines will likely help solve these issues over time.
Superconducting Transmission Lines
Temperature is one of the challenges facing the building of high-tension (i.e., high voltage) transmission lines. Several issues affect power lines as they heat up:
- The resistance of the power line increases with temperature, reducing the current that the wires can carry.
- As the lines heat up, the material in the line expands, leading to sagging in the lines.
This is compounded by the inherent resistance in the power lines, which also heats them, as the square of the current being carried by the line. This is typically overcome by running the lines at high voltages, reducing the current needed to transmit a given power level. In the US, transmission lines generally run at 115 kilo-volts (kV), 230 kV, or 500 kV. Transmission lines also utilize high-temperature conductors to reduce the impact of hot weather on conductivity and sag. While the movement to “underground” transmission lines is becoming more popular (especially in fire-prone areas), undergrounding lines make heat removal an even more significant problem. Even with these steps, the increasing electrical demand in the US will only compound this issue in the long term.
One approach to mitigate this is building high-power distribution lines from HTS tape material. With near-zero resistance, experts believe that HTS tape-based lines could carry up to ten (10) times the current that copper-based lines can carry. HTS tape-based cables also generate minimal magnetic fields, simplifying power line siting. On the negative side, HTS tape-based transmission lines require cooling from liquid nitrogen, meaning that the distributed cryogenic cooling infrastructure would need to be deployed. Demonstrator lines have been built in many places globally, including a demonstrator installation of half a mile on Long Island. This is seen as a particularly promising market for HTS tapes despite the higher infrastructure cost and complexity.
Other High-Power Applications for HTS Tape
Experts are looking into several other applications for HTS tapes, including the following:
- Motors, Generators, and Other Rotating Machines: Motors and generators essentially turn rotating energy into current and vice-versa. Using HTS tapes for the windings in these devices would make them more efficient and reduce power loss due to heating.
- Fault Current Limiters: protect downstream equipment from current faults (overcurrent events) by quickly breaking the circuit when the current limit is exceeded. HTS tape-based fault current limiters can be faster and more efficient than conventional current limiters.
- Superconducting Magnetic Energy Storage Systems (SMES): If you saw the James Bond movie “The Man With The Golden Gun”, the villain/protagonist Francisco Scaramanga (played by Christopher Lee) had his own island powered by a futuristic solar cell that stored its power in helium-cooled coils (see picture on right). While there were a lot of aspects of the power storage system were unrealistic (for instance, open containers holding liquid helium and superconducting coils, which would have boiled off almost instantly), the concept of using superconducting coils to store energy is not far-fetched. While these systems would be more expensive to build than battery energy storage systems (BESS), they would not have the battery fire issues that have plagued a lot of “battery farms.”
Conclusion – Extending the Use Cases for HTS Helps Fusion Energy
These alternative use cases for HTS tapes are helpful to fusion energy. The most significant value they provide is to reduce the risk to companies that build HTS tapes by providing an additional market that is likely more stable than fusion energy. As the cost of HTS tapes continues to decline, the potential for additional use cases grows, further spreading the cost to develop and manufacture HTS tapes, benefiting everyone.
