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Superconductors

Matter in which electric current can flow without hindrance.

Superconductor
Image from Steve Bowers

Superconductors

A superconductor is a phase of matter that occurs when an attractive force arises between electrons. While the force between electrons is always repulsive in vacuum, the interaction with condensed matter can bring about correlations in the motion of electrons with each other that result in a net attractive force. When this occurs at low enough temperatures, bound electron partners can fall into a macroscopic quantum state where it becomes difficult to jostle the electrons out of their path. Consequently, the electrons do not lose energy to resistance and electric current can flow without hindrance.

Superconductivity was discovered in the pre-information age. However, these superconductors had electron pairs that were only weakly bound, and thus required extremely low temperatures to maintain the superconducting state. It was not until the 5th century that forms of matter were engineered to maintain superconductivity at room temperature and above. The original room temperature superconductors were time consuming and expensive to produce. In addition, the materials were not initially well suited to real world applications as they tended to be brittle and difficult to work with, typically exhibiting problems with magnetic flux pinning and migration that made them not quite as resistance-free as advertised. Their original applications were restricted to individual micro-scale rigid components, such as magnetic field detectors. In time the problems of manufacturing flexible and robust room temperature superconductors were solved, but the process was sufficiently laborious that the cost remained high until well after the nanoswarm era. This restricted their application to components for small, high value devices - typically sensors or small scale energy storage where a high specific energy or specific power was critical. Eventually methods were developed to rapidly print wide swaths of patterned superconductive films, an advancement which enabled room temperature superconductors to enter the mainstream in the form of cheap consumer goods, bulk energy storage devices, frictionless bearings, high powered microwave devices, and lossless power transmission lines.

Limits

High temperatures, magnetic fields, and currents can all cause superconductivity to break down. The earliest room temperature superconductors could tolerate temperatures of up to 350 kelvin and magnetic fields of 500 tesla (although not both at the same time - at room temperature of 300 K, the critical magnetic field at which superconductivity is quenched occurred near 200 T). Modern superconductors can handle temperatures near 500 K and fields over 700 T at room temperature.

Applications

Power Transmission and Electric Devices

Supercurrents flow through superconductors without any resistive losses. This leads to the obvious application as wires or cable waveguides. These allow electric power in the form of supercurrents from DC to far infrared frequencies to flow without losing energy to resistance or heating the wires. When superconductors form the backbone of a power transmission grid, electricity can be efficiently transferred from the generating stations to the consumer.

In time, metal and carbon wires were largely replaced by superconductive ribbons for nearly all electrical and electronic applications. This led to inductors, transformers, rotary and linear electric motors and electric generators with both higher efficiency and lower mass for a given power.

Energy Storage

If a superconductive path forms a complete loop, a supercurrent can flow around the loop forever. In theory, the migration of lines of magnetic flux across the superconductor can cause some resistance, but for superconductors in which these magnetic flux lines are either sufficiently strongly pinned or are sufficiently difficult to form, the supercurrent can flow for tens of thousands of years without any noticeable decrease (calculations for the more popular superconductive materials show that the supercurrent will not change appreciably over the entire lifespan of the universe, but such claims are clearly impossible to measure).

A flowing current will create a magnetic field, and magnetic fields contain energy. Thus, a device that produces a strong magnetic field will store large amounts of energy. The usual method of doing so is to wind a superconductive ribbon tightly around a tube to create an electromagnet. If the ribbon is connected to itself end to end, a persistent supercurrent can be set up that sustains a powerful field. If the current is interrupted, as by flipping a switch to redirect the current through a load, the inductive backlash creates a surge of voltage that will ram the current through the load and power any device it is attached to. The electromotive force that pushes the current is so strong that, for all practical purposes, a superconductive solenoid can discharge all of its energy instantly if needed.

There are several engineering issues with this approach. If the magnetic field spills beyond the electromagnet, it can pose a danger when loose ferromagnetic objects are violently accelerated toward the electromagnet. Further, eddy currents in conductive (not necessarily ferromagnetic) nearby objects can exert an additional drag if the electromagnet and the conductor are moving with respect to each other. The usual solution is to wrap the superconductive ribbon around a torus (a donut or bagel shape), which keeps the magnetic field entirely within the torus. A variety of other shapes are possible, so long as the tube shape around which the electromagnet is wrapped curves back around so the ends connect up with each other.

Another issue that must be addressed is that a magnetic field exerts forces on currents, including the very current that created it. These forces put tension on any superconductor carrying a supercurrent; if it carries enough current, the tension exceeds the tensile strength of the material and the superconductor breaks. If this occurs when it is storing a lot of energy, the break quickly turns into an explosion as the electromagnet violently tears itself apart. Superconductors do not themselves have a high tensile strength - however, they can be backed up by carbon nanotube fabrics or graphene sleeves. This allows the high strength carbon materials to take up the tension so the device can store much higher energies than if using a superconductive ribbon alone. Since supercurrent flows only over the surface of a superconductor, the thin superconductive ribbon can have negligible mass compared to the thick carbon sheath. It is the tensile strength of the sheath that determines the specific energy (energy per unit mass) of the superconductive solenoid: Specific Energy (in joules/kg) = Tensile strength of sheath (in pascals) / mass density of device (in kilograms per cubic meter).

For nanoconstructed carbon materials, with a tensile strength on the order of 100 GPa and a density of around 2000 kg/m^3, this leads to specific energies of near 50 MJ/kg. This is at the theoretical limit for what matter held together by chemical bonds can withstand, no other energy storage device that relies on chemical bonds for energy or support can exceed the specific energy of a carbon-backed superconductive toroidal solenoid.

The energy density (energy per unit volume) of a superconductive solenoid is determined by the magnetic field strength it can sustain. Energy density (in joules/cubic meter) = 400,000 x (magnetic field (in tesla))^2. Since modern superconductors can withstand fields as high as 700 tesla, this leads to energy densities of 200 GJ/m^3. This is the energy stored inside the "empty" tube of the torus, and does not include the volume occupied by the supporting sheath. Comparing the energy density of the tube to the mass of the sheath, we see that one third of the volume of a well engineered superconductive solenoid is the interior of the tube, while two thirds is the surrounding carbon sheath. This leads to a net maximum energy density (energy stored divided by the total volume of the device) of around 70 GJ/m^3.

For safety reasons, superconductive batteries are not typically energized up to the maximum that their superconductors or sheath can sustain. This risks small jostles or temperature fluctuations pushing the materials past their limits, causing the solenoid to explode. With typical safety margins, a superconductive solenoid will store 15 to 25 MJ/kg.

Once low cost superconductors come on the scene, solenoids become competitive with other forms of consumer energy storage which have as their ultimate limit the strength and energy of chemical bonds, such as torsion batteries and flywheels. Solenoids have the highest specific electrical power of any energy storage device, and are preferred in applications where high power pulses of electricity are required.

Structural Support

Superconductive solenoidal tubes of the sort described for energy storage cells, consisting of an inner lining of superconducitve sheeting and an outer sheath of high tensile strength carbon backing, will become rigid when fully energized as if the magnetic field is exerting a pressure on the superconductor. This allows structures to be engineered such that they are entirely under tension. The superconductor will "inflate" solenoidal tubes until they become rigid, while cables or carbon sheets can constrain the flexibility and extent of the structure. Keeping a structure entirely under tension has certain benefits - for example, under compression you need thick supports to resist buckling while tension can be supported by a slender cable. Further, the tensile strength of graphene-like carbon (including carbon nanotubes) is higher than the compressional strength of the strongest known (normal matter) materials.

Using solenoidal tubes for structural support leads to a number of unique attributes in those structures that use them. One of the most dramatic is that the structure can be de-energized and it will deflate, becoming easy to fold up and pack away. When re-energized, the structure will inflate and become turgid, ready for use. Further, the energy stored in the solenoidal tubes can be used to power the device, combining the function of energy storage and structure. However, if too much energy is drawn from the structure during operation without re-energizing, the structure will droop or wilt as it loses magnetic pressure.

Since the materials used for tension-only structures are flexible, they tend to be resistant to impacts. The structure will compress and bend from the force of the blow, then bounce back. This can be used to cushion occupants or delicate machinery or instruments required for operation. However, the energized tubes present an additional hazard - if stressed beyond their capacity they can rupture, and the resulting explosion can tear the structure apart and injure occupants or nearby bystanders. Consequently, much like energy storage solenoids, structural support tubes are typically energized to only a fraction of their maximum capacity - where high rigidty is needed, they may take up to 1/3 to 1/2 of the maximum energy but for applications where softer structures are allowable or desirable much lower fractions of the maximum capacity may be used.

Because vehicles commonly use an internal store of energy, because they often carry relatively fragile operators or passengers, and because any other form of compact, high specific energy storage has a similar explosion hazard, vehicles very commonly make use of solenoidal tubes and tension cable/sheet construction for much of their frame and body. This leads to personal passenger transport that can be deflated and easily stored in a compact volume, negating the need for spacious parking lots and structures.

Explosives

If a superconductive solenoid is broken while carrying a supercurrent near the limit of what it can withstand, it tears itself apart in a violent explosion. A modern superconductive explosive energized up to near its limit of 50 MJ/kg will have over ten times the explosive energy as an equivalent weight of chemical high explosives. Because fully energized superconductive explosives are dangerous to transport, they are typically stored partially energized along with an energized storage cell. To prime the explosive, the energy storage cell is discharged into the explosive. In propelled munitions the energy storage cell must often necessarily be sacrificed as a fully energized and primed explosive would not withstand the rigors of launch, although there are engineering workarounds involving soft launch or in-flight charging. However, for construction, demolition, and blasting for engineering purposes the explosive is typically remotely primed and the energy storage cell can be reused.

Magnetic Levitation

Superconductors will exclude all magnetic fields from their interior by setting up surface supercurrents that exactly cancel the external fields. This is called the Meisner effect. One consequence of this is that a superconductor will be repelled from sources of magnetic field, and the field sources will likewise be repelled from the superconductor. This effect is strong enough to levitate magnets over a superconductive surface (or, conversely, levitate a superconductor over a magnet).

Magnetic levitation is commonly used for transportation along fixed paths. When superconductors are inexpensive, a track covered with superconductive film is constructed and vehicles with superconductive electromagnets hover above it. They may pull themselves along using a linear electric motor with the coils or the armature embedded in the track, or they may use turbojets, turbofans, or propellers. When superconductors are more expensive than magnets, the track is made of magnets and the vehicle has a superconductive bottom.

Another use for magnetic levitation is frictionless bearings. These find uses in any application that requires rotating parts, from wheeled vehicles to space habitats with stationary docking sections coupled to spin gravity sections. Although the magnet-superconductor interface is frictionless, there are other inevitable losses that cause drag on rotating systems, so while such bearings allow rotation with little intrinsic resistance, such systems will always need external torque to keep them at their desired rotation for long periods. The spin imparted to a flywheel in vacuum suspended on frictionless bearings does last for weeks or even months, however, before slowing appreciably.

Superconductors with defects can pin magnetic flux rather than excluding it. A magnet with its flux lines pinned to a superconductor will tend to remain in the same relative position with respect to the superconductor - if originally levitating and the superconductor is flipped over, the magnet will end up suspended underneath the superconductor by nothing more than its magnetic field. Magnets which are levitated in this fashion do not slide frictionlessly over the superconductor, they tend to stay pinned in place and tend to return to their original position if displaced.

Microwave, Radar, and Optical Applications

Because superconductors exclude both electric and magnetic fields from their interiors, they reflect electromagnetic waves. As long as the frequency of the EM waves is not too high, this reflection is nearly perfect. The cutoff frequency is roughly where the energy per photon exceeds the binding energy of the electron pairs, or about 10 THz (30 micron wavelength) for superconductors with a 500 K transition temperature. At higher frequencies (or, equivalently, shorter wavelengths) the superconductor rapidly loses reflectivity.

For those frequencies where superconductors are perfect reflectors, they are commonly used for resonant cavities. This allows powerful sources of radio waves, microwaves, and far infrared to be produced. These have applications ranging from radar beams to drivers for particle beams and free electron lasers.

A resonant cavity for electromagnetic waves has many of the same limitations as a solenoid for energy storage. If the energy density gets too high, the magnetic field will exceed the critical field for the superconductor and the material will lose its superconductivity. Further, the radiation trapped in the cavity exerts forces on the walls of the cavity, which tend to force the walls apart and will burst the cavity if the force overcomes the cavity's tensile strength. The result is that these resonant cavities can store the same amount of energy as a superconductive solenoid of the same size and weight, and can explode just as disastrously if damaged or overloaded. However, they do tend to leak, and will lose their stored energy over a period of seconds or minutes if not continuously driven with an external source of EM waves. Their application is in the generation and control of microwave and far infrared beams, not power storage.

Sensors

If two superconducting regions are placed closely adjacent to each other, a current flows between them that depends sensitively on the magnetic field going through the space between the superconductive regions. This effect can be exploited to produce exquisitely sensitive magnetic field detectors called Superconducting QUantum Interference Devices (SQUIDs). SQUIDs enable a wide range of sensor and detector technologies, such as ultra-sensitive medical and industrial MRI scanners. The name is occasionally a source of confusion, and it should be noted that SQUIDs have little to do with squids, which are slimy, rubbery predatory mollusks overly endowed with tentacles and suckers (or provolved squids, for that matter, which are fierce, noble and intelligent sophonts).

Computation

The same coupled superconductors that can be used for magnetic sensing can also be used for computation and computer memory. Their high switching rate and low energy per switch make them attractive for certain applications. They are bulkier than computers built out of nanoscale carbon components, and the extra distance the signals have to cross to reach other components means that for many applications these nanoscale carbon computers are faster. Superconductive computers have two advantages, however. First, they can be reversible, meaning that energy dissipation is kept to a minimum. Second, they can be made into quantum computers, allowing them to solve some classes of problems (such as factoring large numbers) much faster than traditional computers. Consequently, most computers built by S0 engineers make use of superconductors to compliment nanoscale carbon. Superconductors do not seem to be used much in the phonon-based computers of transapients, such as the ultimate chip.

Magnetic Confinement

The strong magnetic fields that a superconductive wire can generate and indefinitely sustain with no energy draw make them useful for systems that require magnetic confinement or direction of plasmas. Examples of such systems include magnetic confinement fusion, where a diffuse plasma of exotic isotopes of hydrogen or helium is trapped and heated until fusion occurs between the nuclei; plasma thrusters, where plasma from an onboard source is vented and channeled away from a spacecraft to provide thrust; and pulse drives, where an exterior explosion of plasma (such as might be created by inertial confinement fusion processes) is directed away from a spacecraft to provide thrust. Certain engineering considerations are required for such systems. In particular, the temperatures of the plasma will be far above the superconductive transition temperature, so the superconductive magnet loops mush be heavily insulated and often actively cooled. This can be a particular concern for high performance fusion pulse "torch" drives, which can liberate on the order of a terawatt of power for several million newtons of thrust - the radiated energy of such drives brings the superconductive exhaust bells to near their limit, and the cooling available to the drive components is critical to keep the drive functional.

Failures can be spectacular - as previously discussed, magnetic fields contain energy and produce stresses on the supercurrents that create them. When magnetic confinement units lose superconductivity even in small regions, the resulting current being rammed through a suddenly resistive medium explosively flashes that section of the magnetic coil to plasma. The resulting violent expansion of the vaporized debris is sufficient to compromise structural integrity of the carbon fiber support, allowing the magnetic stresses to rip the coils apart and fling the fragments apart with brutal energy that can cause severe damage to nearby equipment and injury or death to personnel in the area.

 
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Development Notes
Text by Luke Campbell

Initially published on 12 June 2009.

 
 
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