Superconductivity Glossary

1G (1st Generation) HTS Wire

BSCCO 2223 is a commonly used name to represent the HTS material Bi_(2-x)Pb_xSr₂Ca₂Cu₃O₁₀.  This wire has a typical superconducting transition temperature around 110 Kelvin.

2G (2nd Generation) HTS Wire

Second generation wire has a significantly different architecture compared to 1G wire. The architecture of 2G HTS wire, which comprises multiple coatings on a base material, or substrate, is designed to achieve the highest degree of alignment possible of the atoms in the superconductor material. The objective is to achieve the highest possible electrical current carrying capacity. While the high degree of alignment of the atoms in an HTS material can be achieved by many different laboratory techniques, the business challenge is to achieve this alignment with a high volume, low cost manufacturing methodology. The AMSC wire’s “three-ply” architecture — consisting of slit 2G tape sandwiched between thin copper or stainless steel strips — is similar to that of AMSC’s commercial 1G HTS wire. AMSC’s 2G HTS wire is called 344 superconductors, as it relates to the 3-ply, 4.4 cm width of the finished wire.

AC

Alternating current. An electric current that flows back and forth, typically changing direction 50 or 60 times per second.

Amp

Ampere. The standard unit for measuring the strength of an electric current.

Coated Conductors

Ribbon-shaped wires that show promise as a next generation wire technology.  These wires are made by depositing a thin layer of HTS material, along with a protective coating, onto a flexible metal ribbon. 

Coil

A wound spiral of wire that forms an electromagnet when energized. Specific coils are often named for their particular geometric design. For example, a "racetrack" coil is oval shaped.

Compressive Stress

Compressive stress arises from forces that attempt to condense or shrink the material or wire (squeezing as opposed to pulling).  A critical compressive stress for HTS wire is the amount of compressive stress that can be applied to the wire before the critical current is reduced below a specified level (such as 95% I_c retention).

Compressive Strain

Compressive strain is the percentage change in a material dimension that is under compression.  A critical compressive strain for HTS wire is the amount of compressive strain that occurs before the critical current is reduced below a specified critical level (such as 95% I_c retention).

Critical Current

The ideal physical definition of critical current is the current where a material has a phase transition from a superconducting phase to a non-superconducting phase.  For practical superconducting wire, the transition is not infinitely sharp but gradual.  In this case, the critical current is defined as the current where the voltage drop across the wire becomes greater than a specific electric field, usually 1 microvolt/cm.  Sometimes, for low-loss magnet applications, a lower electric field criterion is used.  The critical current is represented by the variable I_c. 

Critical Current Density

The critical current density, J_c is the critical current of a superconductor divided by the cross sectional area of the superconductor material.  The critical current density is useful when characterizing the quality of a superconductor material.  The critical current density, J_c should not be confused with the engineering critical current density, J_e (see Engineering Critical Current Density)

Critical Current Retention

The critical current retention is the relative ratio of the critical currents before and after a specific process or test.  For example, assume the initial critical current, I_co, of a wire is 100 amps.  After the wire is subjected to a certain test, the post-test critical current, I_c, is measured at 95 amps.   The critical current retention, I_c/I_co, is 95/100 = 0.95 or 95% for this wire under the test conditions.

Cryogenics

The branch of engineering that pertains to materials and equipment that are used at very low temperatures.

Current Leads

Conductors that carry large amounts of power but minimal heat into ultra-low temperature cryogenic environments. Current leads address the critical problem of heat leak.

Current Limiter

A device used to instantaneously limit the flow of excessive electrical current (fault current) in a circuit, thereby protecting expensive electrical equipment. Fault currents are typically caused by short circuits, lightning or common power fluctuations.

Dielectric

An insulating substance such as oil, liquid nitrogen or paper.

Dirty Power

Fluctuations in the voltage or current on a power system that can affect the performance and operation of electrical equipment.

Electric Motor

Equipment that converts electrical energy into useful mechanical power.

EMF

Electric and magnetic fields surrounding any wire that conducts electricity.

Energy Storage

Reserving electric energy for later use to avoid blackouts or fluctuations in power. Storage methods include batteries, flywheels, pumped hydro-power and Superconducting Magnetic Energy Storage (SMES).

Engineering Critical Current Density

The engineering critical current density, J_e, is the critical current of the wire divided by the cross sectional area of the entire wire, including both superconductor and normal metal materials.  This is different from the standard critical current density, J_c, which is the critical current divided by the cross sectional area of just the superconductor material of the wire.  The engineering critical current density is an important parameter used in the design of applications based on HTS wires.

EPRI

Electric Power Research Institute. Founded in 1972, EPRI identifies and pursues advanced technology for the U.S. electric utility industry to improve power production, distribution and use. EPRI serves approximately 700 member utilities.

Fault Current

The momentary flow of excessive electrical current in a circuit. It is usually caused by short circuits, lightning or common power fluctuations.

Four Probe Resistance

A four probe resistance measurement is a technique for measuring the resistance of low resistance material.  Unlike two probe resistance measurements where the voltage drop is measured by the same probe that does the current excitation, the four probe technique separates the voltage drop measurement probes from the current excitation probes.  This separates the probe resistance from the actual sample resistance measurement.  For low resistance measurements, such as for highly conductive metals and especially for superconductors, it is necessary to perform the resistance measurement using a four probe technique.

Generator

Equipment that converts rotational mechanical input power, such as that from a steam turbine, into electricity by using electromagnetic force.

Grid

The electric power industry infrastructure of interconnected electrical systems and services that provide power to all users.

Hermetic Seal

HTS wire can be hermetically sealed to prevent ballooning issues (see Ballooning).  To establish a hermetic seal, American Superconductor has created a special lamination technique.  This HTS Hermetic Wire prevents liquid nitrogen from entering the system even under a pressurized environment.

High Temperature Superconductor (HTS)

Resistance-free conductors made of ceramic materials that exhibit superconducting properties at temperatures between 20 to 130 Kelvin (-423? to -225?F), therefore requiring less expensive cooling systems than those needed for low temperature superconductors (<10 Kelvin, -441? F). The first high temperature superconductor was discovered in 1986.

hp

Horsepower. A measurement of power used to rate motors. HTS technology will be most effective at first in motors rated 1,000 hp and higher.

Inductive Winding

When wire is wound into a coil configuration, a current flowing in the coil produces a field around the coil, and the coil can be characterized by an inductance determined by the stored energy in that magnetic field.  Such a coil is termed an inductive winding, in contrast to special windings which cancel out the field and are thus “non-inductive.”    A flat inductive pancake coil is a convenient configuration for measuring n-values of long wire lengths at extremely low voltage levels.  Inductive winding is also the standard technique used to make superconductor magnets using HTS wires.

Inductor

A device that stores electrical energy in a magnetic field.

Inverter

A type of power converter that converts dc power into ac power.

Ion-Beam Steering Magnets

Devices used for guiding particle beams in systems such as particle accelerators for physics research or ion-implantation equipment for semiconductor manufacturing.

I-V Curve

The I-V curve or the “Applied Current vs the Voltage Drop” curve is the standard curve measured to observe a material’s superconductivity and critical current.  The voltage drop across the superconductor material is measured as a function of the applied current.  The I-V curve is used to determine physical parameters such as the superconducting critical current, the critical current density and the n-value of the material.

Joule (kJ+MJ)

A unit of measurement of energy equal to a watt-second. kJ represents a kilojoule (1,000 joules); MJ is a megajoule (one million joules).

Kelvin

Kelvin is a unit of temperature starting at absolute zero and having the same scale as Celsius.  Thus zero degrees Celsius is 273.15 Kelvin.  Similarly -489.67 degrees Fahrenheit is absolute zero on the Kelvin scale.  Under ambient pressure, Helium liquefies at about 4.2 Kelvin, Nitrogen liquefies at about 77 Kelvin and water freezes at about 273.15 Kelvin.  The scientific symbol for the Kelvin unit is K.

kVA

Kilovolt-amperes. A unit of measure of apparent power.

kW

Kilowatt. A unit of power equal to 1,000 watts or about 1.34 horsepower.

Liquid Nitrogen

An inexpensive, inert and non-toxic liquid cryogen formed by chilling gaseous nitrogen to 77 Kelvin (-320? F). Liquid nitrogen is used to cool HTS wires and components to achieve superconducting performance in applications such as power cables and transformers.

Longitudinal

For our HTS wire the longitudinal direction is the direction along the length of the wire.  For example the longitudinal tensile stress is produced by a force stretching the wire along its length.  The longitudinal axis is also referred to as the axial, or rolling direction for HTS wire.

LTS

Low Temperature Superconductivity

Metallic Precursor

HTS materials are compounds of metals with oxygen. A metallic precursor is an alloy of the metallic components that is formed into a desired wire shape and then reacted with oxygen to create the HTS compound.

MVA

Megavolt amperes. A unit of measure of apparent power.

N-Value

The n-value describes the relationship of the voltage drop across the wire to the applied current.  For the transition from zero resistance (zero voltage drop) to a finite resistance (finite voltage drop), the I-V curve of HTS wires can almost always be fit with the power law

    E_(j) = E_c (j/j_c)ⁿ

Here E_(j) is the longitudinal voltage drop across the superconductor, E_c is the electric field criterion (see Critical Current), j is the applied current density,  j_c is the critical current density and n is the exponent.  For a sharper transition, the I-V curve has a higher n-value.  The n-value is often used to determine the quality of a bulk superconducting material (see inductive winding).

Power Converter

Devices that are used for converting power from dc to ac or vice versa. They are used in motor controllers, chemical processing, energy storage and large industrial processes.

Power Quality

A smooth, even flow of electricity – free of variations in voltage, current or frequency that provides a competitive advantage for utilities and their industrial customers.

Power Semiconductors

Solid-state devices, such as transistors, used for the conversion and control of electric power.

Resistance

An obstruction to the free flow of electrons in a material. Resistance causes electric current to lose energy in the form of heat.

SMES

Superconducting magnetic energy storage. Devices based on the principle that an electric current introduced into a resistance-free superconducting coil lasts indefinitely and never dissipates energy – which provides a backup power supply that's always charged and ready to respond instantly to power fluctuations.

Tesla

A unit of measure of magnetic field strength. For example, an ordinary kitchen magnet has a magnetic field strength of less than 0.05 Tesla. American Superconductor has produced magnet coils surpassing 2 Tesla, the threshold rating for commercial HTS motors and generators.

Transformer

A device that converts power from one voltage and current level to another. Transmitting energy at higher voltages is more efficient, but consumers need low voltage power. Electricity experiences several voltage changes en route to an end-user.

Self Field

The self field is the magnetic field that is induced when there is finite current flowing in a wire.   In the case of critical current measurements, the self field is the magnetic field that is induced in a straight piece of wire that is being measured.

Splice

A splice is a joint between two wire segments.  AMSC's High Strength HTS wires can be spliced together using a special soldering technique.  Typical splices have an electrical resistance less than 200 nOhm at 77 Kelvin. 

Superconducting Transition Temperature

The superconducting transition temperature T_c is the temperature at which a material undergoes a thermodynamic phase transition from a superconducting state to a non-superconducting or normal state.  High temperature superconductors are usually defined as those with transition temperature above 20 K - 40 K, when measured with small currents and no applied magnetic field.  It should be noted that T_c is suppressed by both applied current and magnetic field.

Tensile Stress

The tensile stress results from a force that would result in elongation of a material.  The critical tensile stress for HTS wire is the amount of tensile stress that can applied to the wire before the critical current is reduced below a specified level (such as 95% I_c retention).

Tensile Strain

The tensile strain is the percentage change in dimension that results from tensile stress on a material.  A critical tensile strain for HTS wire is the amount of tensile strain that occurs before the critical current is reduced below a specific critical value (such as 95% I_c retention).

Transverse

For HTS wire, the transverse direction is usually designated as being orthogonal to the length of the wire.  The two major transverse axes are along the width of the tape (also known as the ab-trans, lateral, or long transverse directions), and perpendicular to the face of the tape (also known as the c-axis, normal, or short transverse directions).

Next Steps…

• » Request more information
• » Request a quote
• » Contact us
• » Attend an event/ webinar