APPLICATIONS OF SUPERCONDUCTORS.
FEW WORDS ABOUT APPLICATIONS OF SUPERCONDUCTORS:
The gate may be biased in the "zero-voltage" state.
The junction may be switched to the "non-zero" voltage state by one of
The voltage rise-time on the junction is associated
with parasitic junction capacitance and junction resistance.
Increasing the gate current (Ig) above the critical (Icr) level.
Decreasing the Icr level below the existing gate current level.
Generally, the superconductive state corresponds
to a logic state of "0", the resistive state to a "1". The nonzero voltage
level is essentially the threshold voltage Vg, which will range from about
1 to 3 mV for classical superconductors.
Vg is related to the superconductor energy gap (D)
The symbol on the left is commonly used to represent
the Josephson junction in circuit diagrams. On the right is the Stewart-McCumber
equivalent circuit for a Josephson junction. The sinusoidal variation of
current as a function of junction voltage V.
Gates may be placed into two general
categories by the way they are driven:
that must transport relatively high current densities in high magnetic
field have a variety of applicatrons:
Coils for windings in motors and generators (Utility,
automotive, marine propulsion applications).
High-field magnets for research applications (Particle
accelerators, material research).
Magnetic Levitating (MAGLEV) coils for high-speed
Superconducting Magnetic Energy Storage (SMES)(Electric
utilities, Military applications).
Magnetic containment fields for thermonuclear fusion
MHD (Magnetohydrodynamic) EMT (electromagnetic thrust)
systems for marine propulsion applications.
MRI (Magnetic Resonance Imaging) which requires extremely
uniform magnetic fields at the 10-20 kgauss level (formerly known as NMR,
nuclear magnetic resonance).
Twisting of superconducting
An eddy current may be induced
in adjacent strands of superconductor that are embeded in a normally-conducting
substrate. This current, if substantial, can lead to quenching of the superconducting
state. The eddy current is reduced by a practice familiar to electrical
engineers who wish to reduce external magnetic fields around lines carrying
alternating currents, that is, twisting of the individual pairs of superconductors.
The shorter, the twist length L, of course, the more effective the technique.
An approximate formula for the induced eddy current Jec is:
Characteristic decay time t
of the eddy currents after the source (B):
In practice, strands are spaced rather closely together
within the normal matrix material.
The primary reason that superconducting
generator configurations are being considered for utility applications
is the reduction in size and weight, along with the capability of higher
The torque tubes serve as torsional supports, but also
retrain the field windings from the centrifugal forces and the enormous
magnetic forces resulting from intense field currents. The armature winding
(placed in stator-slots on the conventional rotor) is now an air-core winding
coaxial to the torque tubes. The elimination of magnetic iron provides
a number of advantages:
A pair of magnetic shields are used on the superconducting
generator. The external shield serves the conventional purpose of shielding
nearly metallic objects from eddy current induction as well as preventing
forces from being exerted on magnetic objects. The rotor shield (which
turns with the rotor) serves to prevent alternating voltage induction in
the rotor structural components and in the superconducting field winding.
The inherent inductive reactance of the armature is greatly reduced, resulting
in improved dynamic machine perfomance and voltage regulation.
Space for the armature winding is increased, which increases potential
power density and generator efficiency.
By elimination of the interleaved stator iron (at ground potential) insulation
requirements on the armature are reduced and/or much higher voltage may
be delivered at the armature terminals. Higher terminal voltage can eliminate
the need for a step-up transformer.
It is important to note that superconducting generator,
while benefiting enormously from the higher current densities that may
be achieved with conventional superconductors, do not require the current
densities that are commonly necessary for a variety of high-field
The new high-temperature superconductors
may have advantages in antenna applications. It is important in many applications
to have antennas that are small compared to wavelength of the energy to
be transmited or received. This is not a particularly efficient approach,
with metals in the normal state, since the ohmic losses in the antenna
may be large compared to the effective radiation resistance. Clearly, any
means of reducing this inherent resistance would tend to improve radiation
efficiency, and this observatin leads quite naturally to the consideration
of superconducting antennas where such would be practical.
Low-temperature superconductors have been used to construct
fraction-al-wavelength antennas, leading to a significant improvement in
radiation efficiency. Obviously, the use of liquid helium as a cryogen
tends to limit the application of such antennas.
Another potential application for superconductors
is in the construction of electromagnetic waveguides. The advantage over
conventional metal waveguides would be at the higher frequencies. In the
case of mm-sized waveguides, attenuation becomes prohibitive except for
applications where the guide length is very short, that is, usually less
than a meter. At mm wavwlengths, conventional metal guides have attenuations
on the orther of 10 dB/m due the high value of surface resistance (Rs)
of the metal walls at ~200 GHz.
Shielding from high-frequency
electromagnetic fields and low-frequency electric fields is relatively
straightforward with conventional materials. Alternating magnetic flux
in the incident wave induces voltages in the conductor; the induced voltages
create currents that generate magnetic fields to cancel the incident H
fields. This is, of course, a statement of Lenz's law. As long as the thickness
and radius of curvature of the conducting surface is large compared to
skin depth shielding using ordinary conductors such as copper or aluminum
is relatively straigthforward and inexpesive.
In contrast, shielding of
low-frequency (or especially dc) magnetic fields normally involves the
use of relatively expensive (and sometimes very thick) magnetic materials.
One may consider these materials to be "short-circuiting" the applied magnetic
flux. The reduction of these low-frequency or dc fields to arbitrarily
low values becomes essentially a problem of cost and, the mass of shielding
that can be tolerated for a particular application.
The concepts of zero (dc)
resistivity and the Meissner effect which have already been intraoduced
are, of course, of criticall importance in the use of superconductors for
electromagnetic shielding. Limiting factors in the use of superconductors
for shielding applications involve the critical field (Hc) above
which the material "quenches" and behaves like a normal conductor and the
fact that superconductor resistivity increases with frequency of the applied
electromagnetic field. The subjects of flux pinning and flux creep are
also critical to shielding for low-frequency or dc magnetic fields.
Recall Ohm's law and Maxwell
E must be zero since dc conductivity (s)
is infinite, but if E is zero then B cannot change with time
in the superconductor. If external magnetic fields (at the superconductor
surface) are changing in time, and B cannot change in the superconductor.
Lenz's law requires that screening currents must develop superficial fields
to cancel the external H-field effects inside the superconductor. Since
the conductivity is infinite, the currents are persistent. The arguments
so far only take infinite conductivity into account and this is quite sufficient
for shielding at high frequencies. For dc magnetic shielding Meissner effect
and flux quantization must also be considered.
the surface resistivity (Rs) tends to increase with frequency, since skin
depth is inversely proportional to the square root of frequency:
If one is primarily concerned with the required thickness
of the shield, superconductors are superior to normal metals until a frequency
is reached where skin depth for a normal metal becomes comparable to London
penetration depth for the superconductor (l).
A comparison of skin depth and London penetration depth
is not the dominating consideration. The superconductor will become normal
at a cutoff frequency (fc) where the "photon" energy hfco
is equal to the full energy gap (2D):
There is interest in using the high-temperature ceramic
superconductors is shield applications for the usual reasons: the availability
and low cost of liquid nitrogen.
In the normal state, surface resistance is a function
of the skin depth (d) and conductivity (s)
of the conductor, that is,
DC and Low-Frequency Shielding:
A fixed amount of flux is trapped by the deflated
shield, but when the shield is expanded (below Tc), the internal flux density
is reduced due to the greater affective area of the expanded shield. This
method can (theoretically) be repeated, that is, expanding shields within
expanding shields, to eventually provide a net shield where no trapped
lines will exist in the innermost shielded volume. By this method, fields
have been decreased on the order of 500 per folded enclosure. Such configurations
are also referred to in the literature as "bladders" and "baloons".