Mauro Pinheiro wrote:
I am trying to build a superconductor switch using supercondductive
wire or tape material, and containing in the air core a superconductor
ceramic. But I am trying to understand more clearly what happens when
the electrical current flowing through the ceramic is "cut-off" by the
magnetic field generated by the superconductor coil.
According to the Meissner effect, magnetic fields are excluded from
superconductors. Also, magnetic fields are expelled from materials
previously magnetized and them subjected to their low temperature Tc.
If that is so, then superconductivity will cease in the presence of a
critical magnetic field Hc.

Hc for Type 1 superconductors and Hc2 for Type 2.

According to the above, there can be no magnetic fields generated by a
superconductor coil under superconductivity conditions!

No, you are forgetting the penetration depth.

Will the magnetic field generated by the current in the coil kill the
superconductivity of the coil?,

If the field is strong enough
or the coil will not be able to
generate a magnetic field strong enough to "cut-off" the current in
the ceramic, because the coil is a superconductor?

No, it is the critical field or the critical current that limits the
field strength that can be obtained in a superconducting magnet. That
depends on the design.



Will the electrical current flow in the ceramic turn OFF and ON when
the superconductive coil is turned On and OFF?


I can't clearly see what you are talking about but I am guess you have a
piece of ceramic superconductor inside an A15 or Niobium superconducting
coil and you're passing a current through it. You're trying to drive the
ceramic superconductor normal through an applied field. The current in
the ceramic superconductor in a constant voltage mode will be greater
when the ceramic superconductor is in the superconducting state than in
the normal state. The current will not be zero in the normal state as
the ceramic material will now become just a resistor. In a constant
current mode then the voltage across the ceramic will be zero in the
superconducting state and something finite in the normal state. Of
course the voltage you will actually measure will be 0 + the thermal
voltages or V + the thermal voltages. To get zero you must reverse the
current and average the two measure voltages together. Then the thermal
voltages will be cancel each other out. How close you come to zero will
depend on the sensitivity of your voltmeter.

How fast (frequency
rate) can the coil/ceramic switching interaction can be attained?

That depends on your circuit. The rate will be limited by the circuits
inductance, capacitance and resistance and not by the Meisner effect.
For ceramic superconductor I think the shortest pulse width of a signal
I've seen has been 10 ps. The rise time had to much faster than that.

Joseph.D.Warner wrote:


Ok Ok, before anyone flames me there is a critical frequency too that
will limit the speed of switching. But I believe for good YBCO that
frequency is about 400 to 500 GHz. So if the pulse width of the field
approaches 2 to 3 ps then the switching will become problematic.

"Joseph.D.Warner" wrote in message ...


I can't clearly see what you are talking about but I am guess you have a
piece of ceramic superconductor inside an A15 or Niobium superconducting
coil and you're passing a current through it. You're trying to drive the
ceramic superconductor normal through an applied field. The current in
the ceramic superconductor in a constant voltage mode will be greater
when the ceramic superconductor is in the superconducting state than in
the normal state. The current will not be zero in the normal state as
the ceramic material will now become just a resistor. In a constant
current mode then the voltage across the ceramic will be zero in the
superconducting state and something finite in the normal state. Of
course the voltage you will actually measure will be 0 + the thermal
voltages or V + the thermal voltages. To get zero you must reverse the
current and average the two measure voltages together. Then the thermal
voltages will be cancel each other out. How close you come to zero will
depend on the sensitivity of your voltmeter.

Joseph,
Thank you very much for answering my questions. They helped me to
understand that "critical" Tc2, Hc2, and Ic2, are the critical
elements when designing for superconductivity.
Rephrasing my question to your answer above, regarding turning OFF and
On the current flow in the ceramic, I meant to ask, that I would like
to "interrupt" or "cut-off" the current flowing through the ceramic
during its superconductive state. That current feeds a Load "A". I
would use the coil's magnetic field to do it. Now, that interruption
would be cyclical, that is, obeying a variable frequency rate. By
"pulsating" the coils's magnetic field, I should be able to "pulsate"
the ceramic current, thus turning OFF and ON Load "A".
Now that you mentioned, I am concerned with the lack of "zero" voltage
or zero current when the ceramic is not superconducting (acting like a
resistor). This lack of zero, means that there is a "leakage" that
will prevent a successful operation of the switch! Reversing the
current to compensate for the leakage could represent a problem.