how would superconductor on superconductor contacts work? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2023, 09:02:08 pm
content: If you have two superconductors, what is a electrical contact like?I don't mean normal conductor to super conductor, I mean super conductor to super conductor.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 03, 2023, 11:27:00 pm
content: You obviously can't solder them, as the solder would be a huge resistance compared to the rest.And of course each and every superconducting magnet has such a connection to connect the ends of the coil together.From what I have heard (which is not much), making the connection reliable is quite problematic, as the superconductor tends to be brittle and mechanically weak (I think they are usually wrapped with some other material just for mechanical stability. And the weld (if that is what it is) has to be robust and survive huge temperature cycles.From what I know they first join the ends, then cool the thing below it's critical temperature, and then they "charge" the magnet. I guess they throw permanent magnets though the coil or something like that to get the superconducting current going.

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2023, 11:45:20 pm
content: My understanding is that "persistent-mode" superconducting magnets are made by carefully welding the two ends together, and that imperfections in that weld can cause the current to decay over long time scales.To charge the magnet, there is a section of the wire that is heated above the critical temperature, with two copper wires running to a conventional constant-current supply that can reach the required current.The resistance of the unheated magnet winding is zero, and the resistance of the heated section is still small, but non-zero, so the supply current goes into the coil.The external current is ramped up, and when the required field is reached, the heater is shut off and the section goes superconducting, shorting out the current supply, so the supply can be removed.Thereafter, the current "chases its tail" persistently, maintaining the field.Many years ago, visiting my undergraduate college, Dr R R Wilson (before he built Fermilab) gave a lecture about superconducting magnets and passed a 6-inch sample of the wire around the audience.One student asked about the silver coating: he replied it was for electrical insulation (surprising, but silver does not superconduct at 4 K) and thermal conductivity (to the liquid helium).

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2023, 11:49:48 pm
content: I mean if you had two super conductors and touched them together.Like a butt joint.Does the interface between two super conductors conduct superconductively? If I just laid two bus bar on top of each other without a bolt it would conduct electricity but be relatively high resistance. what if you did that with two super conductors?I mean with no interface material.

author: TimFox
author_rank: Super Contributor
timestamp: August 03, 2023, 11:57:46 pm
content: How do you achieve "no interface material"?The purpose of welding (not soldering) is to have the same material (super-stuff) through the connection.With enough pressure, you might get a "cold weld" or "fusion weld".(I don't know myself how to weld superconductors.)Here's a 1976 paper on ultrasonic weldinghttps://www.osti.gov/servlets/purl/7125279-tio1Xn/

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2023, 01:38:12 am
content: yeah but what happens if you just have two perfectly good super conductive blocks on top of each other each connected to a wire ?does it have some kind of contact resistance? not tight enough to chip it. like light pressure with springs on blocks of ceramic super conductor.in welding I can see that just throwing metal on a grounded table seems to provide some kind of contact even if its not fastened. how is it with super conductive materials?would a spring loaded super conductive 'alligator clip' type connector work? (very gentle spring)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2023, 01:40:20 am
content: I suspect (but do not know) there is some interface states shenanigans going on, where supercurrents aligned to one crystal, hop grain boundaries to the next and so on, at sites where their wave functions just happen to overlap. Whether this is tunneling through a barrier, or a resonance effect with width, or magic angles between crystal planes, I have no idea.It might also be that supercurrents flow along most any wave vector, and this isn't actually relevant. Or that it depends on the type of superconductivity.In any case, it happens that a bulk material has enough contact points, that for a given (bulk) contact method, the critical field inside the material isn't exceeded. Mmm, ah, but it isn't critical field inside, it's always on the surface, remember.I wonder, do they manage to fuse that stuff with very precise alignment? Wire is normally made of myriad filaments embedded in a ductile carrier; can that be lined up? Or maybe it doesn't actually matter; you certainly need to avoid making isolated loops of filaments (think: trying to join litz cable by welding individual strands!), but maybe a bulk connection can be made anyway, and critical field is just violated as a matter of course, but nonuniformly across the joint in such a way that current filaments manage to find their way, anyway? ...Does that work? Surely the current filaments would attract and annihilate, resulting in voltage drop? Hmm.Well, anyway. Engineering problems that have already been solved; consulting with a SC supplier would be illuminating. If expensive. But, y'know, if you need it, you need it, and if you don't... you don'treallyneed to know?...On the upside, a lot of SC research will be state labs, i.e. free/public articles. Just gotta go looking for 'em.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2023, 01:42:40 am
content: its infuriating not to know the answer to such a simple question of what happens if you touch two current carrying things.like lets go with optical lap (where optical plate shows good flatness)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 04, 2023, 01:46:53 am
content: For superconducting magnets, the weld is very critical, and the reason is very simple.More current through the magnetwire results in a stronger magnet, but there is a limit to the current that a superconductor can conduct before it looses it's superconductivity.So if the weld is only 80% of the surface area, the whole magnet looses 20% of it's potential.Vacuum welding is a known problem in certain applications. When two metals (especially when they fit well and are very clean) are placed on top of each other, then they can spontaneously weld together even at room temperatures. But that's metals, I don't know about superconductors, maybe you can try it yourself.

author: bdunham7
author_rank: Super Contributor
timestamp: August 04, 2023, 02:01:50 am
content: Quote from: coppercone2 on August 04, 2023, 01:42:40 amIf you did a very good job of butting the ends then I'd say the current might just flow through as if there were no joint. If there were voids in the joint you'd at least have reduced maximum current density and perhaps other effects--you'd have to research that. However, I suspect that you'd find it impossible to get the butt joint good enough to not represent a disturbance in the structure of the superconductor. But if you got them very close but not quite together, you might have a Josephson junction where the Cooper pairs could just tunnel through with a very very small voltage drop and a characteristic oscillation. You shouldn't fret about not getting a simple answer to this question, Brian Josephson received the Nobel Prize for coming up with his answer.

author: BrianHG
author_rank: Super Contributor
timestamp: August 04, 2023, 02:29:21 am
content: I heard that in YBCO superconducting tape, used in an experimental fusion reactor, to open and close the core, the ends of the coil of the flat superconducting tape turns where interleaved put together like a deck of shuffled playing cards, then that mass of coil contacts were squeeze clamped together under hundreds of pounds of pressure in a vice to complete the entire magnetic coil loops all at once.No welding or soldering. The magnet coil was completely superconducting and was also able to be opened and closed to allow installation around the outside of the toroid core of a Tokamak reactor.

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2023, 02:36:47 am
content: where does the voltage drop heating occur in the jospeh junction?its some kind of quantum resistance? whats going on there exactlyif its in air I imagine (total guess) that there is some kind of quantum current flux? thing heating the medium that its moving through, so an air heater? but what if its under vacuum? like in a tube you get ion heating or whatever (thermionic). what is happening if its tunneling?i think of either ion beams or arcs. but what is happening in QTis it like causing a disturbance where it reappears stressing the material surface and making heat ? i assume its not a perfect themal diode either so I guess where its disapearing there is a void that makes turbulance or whatever that also makes some heat?

author: pickle9000
author_rank: Super Contributor
timestamp: August 04, 2023, 03:09:30 am
content: wow super interesting.

author: jbb
author_rank: Super Contributor
timestamp: August 04, 2023, 03:12:06 am
content: I suspect it varies by superconductor type & application.I think that the Large Hardon Collider superconducting magnets were soldered together using low temperature Indium solders, and a long parallel overlap so the effective contact area is very large.But they messed up some of the joints a little, and one got a bit too hot, which [caused a problem](https://home.cern/news/press-release/cern/cern-releases-analysis-lhc-incident).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2023, 03:47:31 am
content: Quote from: coppercone2 on August 04, 2023, 02:36:47 amhttps://en.wikipedia.org/wiki/Josephson_effectIt's a DC voltage drop between superconductors, and an AC current through it. It's not thermal loss, though the AC may result in resistive losses in surrounding materials.Waveform depends on voltage, so, I suppose the AC voltage should be kept low (say, a very low impedance resonator) for a reference application.Power transfer is V*I but the AC impedance is an open parameter (mind that, being superconducting, significant power can be delivered into vanishingly small resistances) so the AC power, and V or I amplitude, is just whatever. I think.Tim

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2023, 03:51:55 am
content: Uhh so I am guessing the AC current is like orthogonal to the DC current right? So you could tap it on the sides to make a reference microwave voltage?But if its not tapped/terminated/whatever, what happens there with that AC voltage?Like, totally lostis the AC just there superimposed on the DC slowly radiating out of the system assuming its a loop?so that means that if you make a dodgy loop with DC energy that has superconductor on superconductor contacts it will actually make GHz noise ? maybe confused with atomic clockthen it will just emit EM radiation?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2023, 03:54:31 am
content: Same terminals. Superposition.

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2023, 03:55:37 am
content: so basically its a noisy contact that loses power to THz radiation (which is like heat)?

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2023, 03:56:34 am
content: does this mean that a tunnel diode might be making THz radiation ? not sure how the quantum tunnel effect ties into super conductor, it seems that that mechanism should work without super conduction?kind of wondering about dense suspensions too, like if you try to energize a tube of oil with alot of silver nano particles in it that have single atom layer thick insulating coatings

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2023, 04:25:29 am
content: Carriers in a TD or Zener, the tunneling is lossless but the excess carrier energy past the barrier is simply dissipated as drift (technically, electron-phonon interaction).Tim

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 04, 2023, 04:30:37 am
content: Quote from: Doctorandus_P on August 03, 2023, 11:27:00 pmOf course you can, get prepared for the new incoming nightmare : Rohs superconducting solder!As if working with rohs wasn't terrible enough...

author: octillion
author_rank: Contributor
timestamp: August 04, 2023, 04:49:36 am
content: To answer the original question, if you have two superconductors in contact and the interface is clean, then you've got a superconducting interface. The geometry and quality of the interface will dictate things like the critical current/magnetic field/temperature.As for the details on how superconducting interfaces are made, this depends on the material, geometry, and use case. Joints don't always need to be superconducting. Superconducting interfaces can be mechanical interfaces, welds or wirebonds, solder joints (ultrasonic soldering is often useful).Superconductors range from soft metals like lead and aluminum, refractory metals like niobium and NbTi, ceramics like YBCO and ReBCO. Here's a database of superconductors (currently in early stages)https://wiki.ieeecsc.org/The superconductor could be a thin film, solid wire, plated wire, multi-filament matrix in a normal metal stabilizer, tape.The superconductor might be used for transmitting high power DC, transmiting tiny DC power, generating large magnetic fields, carrying minuscule RF signals, or providing magnetic shielding.

author: coppercone2
author_rank: Super Contributor
timestamp: August 07, 2023, 04:46:26 am
content: to clarify, is quantum tunneling 2 phenomena1) classic materials - electron phonon interaction2) superconductive materials - something with Joseph junctionsdoes it matter if 1&2 are also in dry gas or in vacuum? (a few atoms of gas can fit in quantum tunneling distances I think)might be interesting too if you can setup a quantum bridge and see what flowing matter through it does (if it has an effect on whatever gas flow might be considered in this range). maybe there is interaction like a rope bridge in a windy canyon or something. I guess it would be rather clogged with anything but atomic hydrogen, which is conductive unfortunately (in the form I know of it?) like glaciers/ice sheet flow or something

author: Bud
author_rank: Super Contributor
timestamp: August 07, 2023, 05:04:50 am
content: So if superconductor has no loss, then my 1mm^2 superconducting wire can transfer unlimited current, like a Gazilion amps. Correct?If not what is the limiting factor ?

author: bdunham7
author_rank: Super Contributor
timestamp: August 07, 2023, 05:15:04 am
content: Quote from: Bud on August 07, 2023, 05:04:50 amSuperconductors will have a maximum current density, if you exceed that they don't superconduct.

author: coppercone2
author_rank: Super Contributor
timestamp: August 07, 2023, 05:23:38 am
content: I wonder how that destabilization looks like, if you just ramp the current up with a very small source until it does whatever the term is.like if there was just a handful too many electrons in the current, what happens? do you get a local hot spot that dances around a superconductive ring? (their not going to be distributed isotropically I think, so the effect should manifest itself in a random region of greater density of electrons).Like if you had a superconductive ring and it was right near saturation and you gave it a pulse, would a spot that is totally random just heat up somewhere on the ring?

author: octillion
author_rank: Contributor
timestamp: August 07, 2023, 11:54:19 am
content: Quote from: coppercone2 on August 07, 2023, 04:46:26 amIn normal metals you can have quantum tunneling of electrons. In superconductors, you've got quantum tunneling of Cooper pairs; a Josephson junction is an electrical element that does this deliberately. The weak link in a Josephson junction can be a thin insulating barrier, a narrow passageway (make the wire very thin, but still connected), or a short normal-metal section.https://en.wikipedia.org/wiki/Josephson_effectQuote from: bdunham7 on August 07, 2023, 05:15:04 amSuperconductors have a critical current, critical temperature, and critical magnetic field, limits beyond which superconductivity will cease. Superconductors are also only lossless for DC, AC has several loss mechanisms. Starting at page 15 of this thesis (not vouching for it, just the first acceptable search result) lists a few AC loss mechanisms with referenceshttps://ris.utwente.nl/ws/portalfiles/portal/6076106/t0000018.pdf. Beyond what the thesis covers, which are losses you would see even in the most ideal conditions, you'll also have AC coupling to nearby objects like bulk dielectrics, normal metals, and dirt on your device. Machined superconducting cavities made of materials like ultra-pure niobium or aluminium that have been thoroughly cleaned and polished (mechanical polishing, buffered chemical polishing, and electro-polishing) can exhibit quality factors in the billions. The Q's are higher at high power, as surface dirt becomes saturated with energy. Here's some examples used in acceleratorshttps://indico.cern.ch/event/817584/attachments/1865543/3101254/SCRF_Cavities2019.pdf

author: coppercone2
author_rank: Super Contributor
timestamp: August 07, 2023, 09:00:03 pm
content: Quote from: octillion on August 07, 2023, 11:54:19 amso when a cooper pair quantum tunnels it makes the microwave radiation from the voltage drop, and the electron quantum tunnel makes heat (conventional)?

author: octillion
author_rank: Contributor
timestamp: August 07, 2023, 10:03:04 pm
content: When below the critical current of the Josephson junction you will have a tunnelling supercurrent with no voltage drop across the junction. In this case the junction acts like a non-linear inductor. This is a "kinetic inductance" where the energy is stored in the kinetic energy of the cooper pairs, rather than in a magnetic field. It's possible to produce microwave inductors of much higher impedance with Josephson junctions than what magnetic inductors allow, as parasitic capacitances are lower.When the critical current is exceeded a constant voltage bias across a junction will produce an alternating current, where the energy from traversing across the voltage drop translates to a photon proportional to the voltage. This is like the band gap of an LED, with larger gaps producing higher frequency photons.