# Maximum AC Frequency in a Super Conductor?

#### BrianConlee

Is there a critical ac frequency for a super conductor?

I know there's a critical current density and critical field, but assume we stay below those levels.

Thanks.

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#### BrianConlee

I figure I might as well ask this question on this post too:

Will ac persist 'forever' in a superconductor like dc will?

#### Pumblechook

I think AC skin depth reduces with lower conductor resistance (and increasing frequency) so in theory with near zero resistance you get near zero skin depth and therefore the gain by using a superconductor is largely wiped out.

#### Bob S

Pumblechook is correct. I have measured the inductance of some large superconducting magnets as they were cooling down and warming up, using ac frequencies between 10 Hz and 1000 Hz. In these magnets, the inductance rather quickly increased about 10% (from 40 millihenrys to 44 millihenrys) as the magnets warmed up between 8 kelvin and 10 kelvin. 44 millihenrys was the room temperature value. The missing 4 millihenrys represented the integral of B H over the coil volume.

The DC currents in a SHORTED superconducting magnets will "persist" like in MRI magnets. The only persistent currents in superconducting magnets are DC currents. AC will not work.

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#### f95toli

Gold Member
Good normal conductors tend to be better than superconductors for passive devices like antennas etc above about 100 GHz, but this partly for practical reasons related to fabrication.

The theoretical limit is simply the frequency corresponding to two times the gap energy divided by Planck's constant; for e.g. niobium this is about 700 GHz. Photons with an energy higher than that will simply break a Cooper pair into two electrons meaning the material starts to behave like a normal conductor.
This is one of the main reasons why it is so hard to make astronmical detectors (mixers etc) that work above about 900 GHz.

#### Bob S

This is one of the main reasons why it is so hard to make astronmical detectors (mixers etc) that work above about 900 GHz.
In a project about 25 years ago, I used GAsFET (gallium arsenide FET) amplifiers cooled to about 10 kelvin for very low noise operation in the 2 - 4 GHz band.

#### f95toli

Gold Member
Yes, similar amplifiers are used to amplify the down-mixed signal from superconducting mixers (I use a 4-8 GHz InP HEMT amplifer operated at 1K in my lab, but for a different application).
You can easily find HEMT amplifiers that operate up to 110GHz, but above that frequency you need to use a mixer.
The nice thing with superconducting mixers is that you can actually get a net IF gain from the mixer itself; although I don't think that works in practice in the frequency range we are talking about here.

#### Pumblechook

I didn't know semiconductors could also be superconductors??

I though the purpose of cooling amplifieres was to reduce noise.. Nothing to do with superconductors ???

Semiconductors conduct more when hot.

#### f95toli

Gold Member
Sorry, what I (and presumably Bob S) meant was that HEMT amplifiers (semiconducting) are often used to amplify the signal (i.e. the IF from a mixer) coming from a superconducting device.
I.e. in most applications one tend to use a combination of superconducting and semiconducting devices, occasionally even on the same chip. I guess we were slightly OT.

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