Levitating Trains

Superconductors are the key to magnetic levitation, from people on platforms to entire trains. But what is a superconductor?
30 June 2014

Interview with 

Professor David Cardwell, Cambridge University


A levitating magnet over a superconductor


Superconductors are the key to magnetic levitation, from people on platforms to entire trains. David Cardwell - Professor of Superconducting Engineering at Cambridge University - works on high temp superconductors, and explains to Ginny Smith what a superconductor actually is...

David -   A superconductor is a material that, when you cool it below a certain temperature, it loses all resistance to flow of electrical current.  So, you can either get very big currents down your superconductor or you can use that current to generate very big magnetic fields.  The currents are bigger than you can get down any conventional material and the fields it produces are bigger than any other source.  So, things like, anybody who's had an MRI scan, that field would've been generated by a superconducting coil.  You just can't achieve those fields using conventional metals like copper.

Ginny -   What other kinds of applications are they used for at the moment?

David -   There are three different types of superconductors.  You either make them in long thin conductors where you can pump current down and you can control it.  Because the material is superconducting, for a certain type of  current, there's no loss.  Significant amount of power we generate is actually lost by heating wires - 10% or 11%.  If you could make your cable superconducting then we'd get rid of all that loss.  So, there are big energy efficiency gains.  You can make them in the form of bulk materials.  Some people call them hockey pucks and you can use those materials like you'd use a permanent magnet.  But the field you generate is a lot bigger.  The final is a thin film form.  That's a very small amount of material but with very good properties and you can use those in things like electronic applications, microwave filters, generators, basically, the electronic small scale industry, so different range of material forms, each of which has its own benefits for a specific type of application.

Ginny -   So, you mentioned these bulk ones that look like hockey pucks.  Is that what you've got in front of you there?  Like I see something...

David -   Yeah, absolutely.  So, what I'll do is I'll demonstrate the properties of a bulk material.  So what happens is, if you have a magnetic field and I've got one here.  It's a speaker magnet.  I'm going to bring the superconductor towards the speaker magnet.  We've got this thing called Faraday's Law and that tells you that that will cause the current to flow.  The current will generate a magnetic field and by Lenz's Law, that field will be in a direction to oppose the change it's causing it, GCSE physics.  So, as I bring the superconductor down, we've got this current established.  If it were copper, that current will decay away.  The superconductor has got no resistance so when the current is induced, it continues to flow - it persists.  So, the field it produced persists and therefore, you've got a levitation force or repulsion force.  So, if I just bring the superconductor down now on top of the magnet, you can see it's levitating away, right above the superconductor there.  And it will stay there until it actually warms up through its transition temperature.  This is about minus 196 degrees or when it gets above its transition temperatures, then it becomes non-superconducting and it'll lose its levitating properties.

Ginny -   So, those two magnets are repelling in the same way that if I've got a pair of bar magnets and try to bring these north poles together, I would be able to feel a force.  Is that the same idea?

David -   It's exactly the same force except if you try to bring 2 bar magnets together, you wouldn't be able to levitate one on top of the other.  One would flip around and north would attract south.  So, there's an intrinsic instability there.  So, one property we've demonstrated with a superconductor here is that you get this stable levitation and that's something you can't achieve with permanent magnets.

Ginny -   Now, that's cool, but it's not very useful, levitating a little hockey puck-sized thing.  Can we use that for anything more practical?

David -   Right.  So basically, the properties we're interested in goes magnetic field squared.  So, if you can double the field, you get four times the levitation force, four times the energy density.  Since we've got stable levitation and an obvious thing is a maglev levitating train on a bigger scale.  There's also a very strong field gradient here and you can use that to separate a magnetic species from a non-magnetic species. Down the line using these materials and motors and generators so you get much bigger fields then you can generate steel for example or iron.  And therefore, you can throw the iron and steel away.  Some people say you can get about...

Ginny -   Lucy is getting quite cross when you're saying that.

David -   ...half a volume or weight.

Ginny -   So, we don't have a train that we can levitate today, but we do have a Kate.

David -   We do.

Ginny -   Can we have a go at making her levitate?

Kate -   There's a big box in front of me which is white and there's a lot of smoke coming out of it and I can see sort of ice crystals forming around some of these screws that are holding it together.  It looks quite intimidating, David.  What's going on in this box you are asking me to stand on purely from faith.

David -   We've got every confidence in you there.  So, we've got an array of these bulk superconductors and the platform on the top at the back of it, we have a ring of permanent magnets.  When the permanent magnets meet the superconductor, they're levitated.  It's just the inverse of what we've done here.  Now, if we try to bring the magnets closer to the superconductor, additional currents will be induced and that will be resisted.  If we try to move the plate from side to side, we're changing the field of surface of the superconductor.  It will try to resist that.  However, the plate has got circular symmetry.  So, if we were to rotate the plate, the field at the centre and the surface of the superconductors is constant.  So, there's no resistance to that motion whatsoever.  So the bottom line is, the plate won't move up or down very much.  It won't move from side to side very much, and if we try to spin it, there's no resistance.

Kate -   So, I'm not going to float across the room, but I could get dizzy as what you're saying.

David -   We can make it into a linear motor, not tonight, but another time, then you might float across the room.

Kate -   If only.  Is there any chance that I could be too heavy for this?  Is there a reason that I'm doing it rather than say, Dave?  No offense, Dave.

David -   I'm glad you said Dave and not me actually.  Like all forces, if you apply forces equal and opposite, the forces supporting you, you will overcome it and the thing will collapse.  But I think it's probably about a thousand Newtons at least that you'd have to apply, so 100 kg in order to support.

Kate -   Okay, so basically, I shouldn't be too fat for this.

David -   I think you should be okay.  My money's on you.

Kate -   I'm having to get Dave to hold the mike so I don't spin around.  Okay, so immediately, I'll spin.  So, I did promise the people levitation tonight, but Dave is going to have to follow me around in circles with the microphone here.

David -   You need to push on something.

Kate -   It's not a very good magic trick.  It's not like I'm basically being Dynamo off the telly and I'm floating metres off the ground.  By how much am I levitating currently?

David -   It's probably about a centimetre.

Kate -   About a centimetre, okay.  Let me give it a spin.  Okay, I'm getting really, really... So, there's no way of it stopping basically.  It's what we've established with that experiment.

David -   Well eventually it will stop.

Kate -   How long is it likely to take?

David -   We could wait until it warms up.

Kate -   We could wait until it warms up.  How long is that going to take...

David -   About 40 minutes.

Kate -   40 minutes of spinning, I'm not sure I can last that long.  I might have to push in the opposite direction.  I'm genuinely dizzy right now.  We're going to have to just wait a moment.  So, this is awesome, but we said that trains might be useful.  Do trains use exactly the same system or is it adapted slightly for humans?

David -   It's the same principle.  A train might use four of these and a linear railroad in a circular track.  So, you get linear motion just in the same way that you've got circular motion here.

Kate -   My feet are currently getting cold because there's a lot of liquid nitrogen underneath me right now.  Now, you mentioned it wouldn't work when it warmed up.  So, are we having to permanently pour liquid nitrogen into train tracks to get maglev trains to work?

David -   Well, you can cause things electrically, so you need a power supply rather than a vat of liquid nitrogen.  So, there are other ways to cool it, but they're not as spectacular as this way.

Kate - It's not as spectacular.  I'm not sure how am I ever get off this.  I could be stuck here for the rest of the show.

Ginny -   A round of applause for Kate as she attempts to climb back down.  So, we've just said that you need a big vat of liquid nitrogen to cool this.  I guess that's probably one of the big limitations that's stopping us from using superconductors in more things nowadays.  Is that something you're working on?

David -   I guess the question I'm asked more than any other is, at what temperature do these materials superconduct at?  Will we ever get room temperature superconductivity?  There are two solutions.  One is, if you're a physicist, it matters because that tells you about the pairing mechanism, what's causing superconductivity.  If you're an engineer, you're not really bothered about that.  You're bothered about how much current can it generate, how much fuel can it produce.  So, we work with the best materials we have.  It just happened to be the ones that we need to cool to these low temperatures.  But in a real application, as I said, you wouldn't need a liquid cryogen.  You'd use like a high tech fridge.  I mean, you don't pour a cryogen in the back of your fridge to keep your milk cool.  Yet, you can maintain a freezer at minus 25.  So, you can use a glorified fridge.  It's called a Stirling cycle cooler.  You can actually achieve 20 Kelvin, so well below this temperature.  But it's not something you'd find in a vacuum cleaner for example.  But it might be something you'd find in an industrial plant that involves moving heavy masses across a workshop floor. Boeing may levitate their aircraft on their production line using this technology one day.  We don't know.

Ginny -   Now, we're going to go over to some questions in the audience in just a second so get your thinking caps on.  But first Kate, have you got a question that came in on social media?

Kate -   Gerald (McMullen) on Facebook and we've just mentioned maglev and things like that, he said, "Whatever happened to levitating trains?"  He doesn't think they're still around.

David -   No, there are.  There's a fairly well-documented train in Japan using low temperature superconductors.  Not liquid nitrogen temperatures but liquid helium temperatures superconductors.  There's one in Shanghai from Pudong Airport to downtown Shanghai.  There's a couple in Germany.  So, they're around.  The problem is, the tracks are expensive as you can imagine.  So, you're looking at, at least an order of magnitude more expensive than a normal train, but you can travel at speeds of getting up to 500 km per hour.

Ginny -   So, who's got a question about superconductors and high speed travel?  We've got one down the front here.

Anastasia -   Hi.  I'm Anastasia.  I'm originally from Russia.  I wonder how do they break those trains.

David -   Essentially, there's a jet on the train.  The superconductors give you levitation.  If you have a clever arrangement, you can get a bit of propulsion.  But essentially, all you're using the superconductors for is a frictionless platform.  It's low temperature flying.  So, like an aircraft, how does an aircraft go forward?  Well, thrust is provided by the jet engine so you'll need some on-board mechanism of propelling.  A big fan on the back of the train might do it.

Sam -   My name is Sam.  I'm 12 and I'm from St. Ives.  You said if it warms up, it will stop spinning.  If you had a train going straight, if it warmed up, will it just suddenly stop half way through a journey if it warmed up?

David -   We wouldn't allow that to happen, would we?  That would be a big problem.  So, what you do is you have a failsafe mechanism.  So, you wouldn't just rely on levitation.  You'd have a wheel base system.  So, if it warmed up and it doesn't warm up quickly as you saw with the demonstration.  It warms up slowly.  As it warmed up slowly, the train will get lower and lower, and lower, and then it would land on its rails, and it would carry on.  So, it would then go in its wheels rather than being magnetically levitated.  At that point, you would've slowed the train down.  It would be a conventional system.

Gareth -   I'm Gareth.  I'm 12 from Huntington.  How many years do you think it will be before we actually get levitating trains or other vehicles?

David -   We've been saying 5 years for about 15 years, but I think reality is, that we are getting very close to applications.  I don't think they'll start with levitating trains.  I think they'll start with smaller devices that do very unique, very niche things that we can't do using anything else.  I think levitating trains may be where we get to eventually, but to begin with, I think we need to do things that use our imagination or that we haven't thought of before.  We're working on a number of devices that do exactly that right now and the turnaround time for those devices are about 2 to 3 years.  So, I really believe that in that timescale, we're going to get commercial applications of these materials.  Whether it's a levitating train, I doubt, but sometime in the future, almost certainly, levitating trains as well.

Danny -   Hi.  My name is Danny and I'm also 12, originally from Syria, but I moved to England.  I'm asking, roughly, how cold is liquid nitrogen and what material are they planning to use for levitating trains?

David -   Okay, so liquid nitrogen is 77 Kelvin.  If you look at the Kelvin scale, absolute zero is minus 263 degrees C.  So, it's pretty cold.  So, 77 Kelvin is minus 196 degrees C.  I gather, if you go to the very north of Finland, you're approaching room temperature superconductivity because it's pretty cold up there, but not quite that cold.  So, it's minus 196 degrees C.  That's the temperature that we operate these materials at or lower.  The lower in temperature you go, the better the performance.  Your second question was, what material.  Well, these materials, they look black.  They're ceramic in nature in terms of properties and they're a complex mixture of elements.  So, this one is yttrium, barium and copper with a bit of oxygen in there.  So, yttrium barium copper oxide.  That's the basic material.  You may change the yttrium, but you will have the barium, the copper and the oxygen there.  So, yttrium barium copper oxide, we make them in a furnace at about a thousand degrees C.

Ginny -   That's pretty extreme.  So, I think we've got time for more questions.  This gentleman in the white shirt had one.

James -   Hi, there.  James, 28, a bit older than the rest of the guys.

David -   You don't look it.

James -   I'm from Cambridge.  I think you almost solved your own question about how you might do levitating trains because it's expensive, but on the long sections of the track in the countryside, you could levitate the train and then as you said, when you get into train stations or near cities, you just lower the temperature down and then they could just run on their tracks.

David -   Yeah, you could.  The problem is, you've got a thermal mass and with transport, you got to have instant changes.  So, you can't wait 3 minutes for something to warm up.  I think the way to do it is not to warm it up, but it's to divert the field.  So, you do something else to the field or in this particular case, if you're moving at a high speed, you've got a high field rate of change.  That induces another field and therefore, you get levitation.  So, when you slow down, it will land automatically.  That's the best way of doing it, but the cost is still in the track and is hugely expensive.  If anybody has ever been on the Shanghai Maglev, just looking at the track, it just oozes Yen.  You can see it and it's hugely expensive.


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