The Extreme World of Ultra Intense Lasers - with Kate Lancaster

lasers are amazing ever since they flickered into life 60 years ago we didn't really know what to do with them but some 55 years later we have on average three lasers at our home and now they've been useful for medicine and industry entertainment and even digital communications it's revolutionized how we communicate with each other in the world now the story of how lasers are invented is well known I'm not going to talk to you about that today I know what you've come here you want to hear about these dirty great big powerful lasers you don't hear about tiny little laser pointers you want to hear about how we make the world's most powerful lasers and that's the story that I'm going to tell you today now the picture that you see behind you is one of the most intense laser plasma interactions possible today this was taken on an experiment I did probably back in about 2011 using the Vulcan petawatt laser a petawatt is 10 to the 15 watts so that's about 10 to 18 times more powerful than a standard laser pointer so it's absolutely unprecedented and what you see behind you is a lot of physics going on now I'm not going to stand here and explain it right now but I'm going to explain it throughout the talk and hopefully by the end of this talk you'll kind of understand a bit about the physics of laser plasma interactions but behind you can see some of the most extreme conditions that we can create on earth so now you can see why people like me do this for a living it's extremely exciting you know really really exciting but in order to understand how these lasers work we're going to have to sort of dial it back slightly and understand what light is exactly now James Clark Maxwell he was the first person that actually realized that lights and electricity and magnetism were manifestations of the same phenomena and actually light is a electromagnetic wave is a sort of Mickey Mouse picture of an electromagnetic wave behind you here and basically you've got an electric field waving around and then you got a magnetic field waving around and there are eight right angles to each other and just to remind you a little bit of physics your wavelength lambda here is the difference between the two peaks and the amplitude is just the distance between the zero line and the top peak okay now the light that we can actually see visible light is only an extremely small part of the full electromagnetic spectrum that we can actually have in the universe so we only see this tiny little bit 400 to 700 nanometers visible light and the rest is anything from these extremely long radio waves over here all the way up to gamma rays that have wavelengths smaller than the atomic as distant atomic radius so it's absolutely a full spectrum extremely energetic at this end now it's also useful to think about light not just as a wave but actually a packet of energy a packet of energy called a photon and you cannot understand lasers unless we use this concept of a photon now we all know what an atom is I think I hope we're all made of them it's a nucleus that has the protons and neutrons in and then we have electrons orbiting around the nucleus and these electrons can actually move to different energy levels by absorbing light or if they want to lose energy they can emit light and basically if there's that jiggery-pokery going on in the aten absorbing and emitting and we can call this atom as in an excited state and it's these excited atoms that give rise to the fundamental interaction that we use to give rise to lasers so I just want to tell you a little bit about some of the atomic processes that can happen that absorb or emit light so the first thing we have is we say we have an excited atom it's somehow absorbs energy from somewhere it's just sitting around in its excited state it wants to lose energy so it emits a photon of light that's called spontaneous emission we then have the other situation where we have an atom just sitting around in comes a photon the atom can gobble that up and be excited now the really special thing and the real key to lasers is something called stimulated emission we have an atom that's already excited and then in comes a photon and that atom then can drop down in am it a photon but the photon is identical to the photon that came in so what it basically is is a photon photocopier okay so you just make lots of identical photons it's this stimulated emission that's the s and the e in the laser acronym light amplification through stimulated emission radiation so we want to make a laser out of this so we need to do this lots and lots and lots okay so how do we do this well we take some medium it can be a solid liquid gas plasma and we need to give it lots of energy why do we need to do that well we want all the atoms to be in an excited state because we want lots of stimulated emission which requires us to start at a point where the atoms were excited then we need to pump the energy in so what we do is just we can flash lots of white light in for example we can use laser diodes we can use electric fields say for the example of this sir this talk I'm just going to talk about white light being flashed in so you flashing a lot of white light and you excite lots of atoms what we want to get to is a position where there are more atoms in the excited state than in the ground state and this is called population inversion okay so that's the fundamental thing that we want for lasers to happen but of course we need some kind of feedback mechanism otherwise the lights just going to be lost and no lazing can happen so what we do is we put two mirrors around the medium that's going to emit our light and they're mirrored surfaces so you get some photons emitted to start with they start bouncing backwards and forwards you start getting more stimulated emission happening more and more and more and more and that bill up and this is our laser but you might think Katie talking rubbish you've got two mirrors how's the light going to get out well one of these surfaces is partially reflective so the light can actually escape okay so that's there's the kind of very basic principle about how a laser can be made now on the next four slides I've got here are some lessons about the properties of lasers the first thing you need to know which give rise to most of the special properties of lasers is that laser light is actually coherent and travels long distances as such so say for example with lot white light that you can see around you or this little torch here is that the light spreads out in all directions there's multiple wavelengths multiple frequencies although all the ways are just kind of messing around and spreading out everywhere so not concentrated energy lasers the the waves are all lined up they're all in what we call phase where the peaks and troughs are lined up the same frequency and they're all going in the same direction and that is one of the most fundamental and excellent properties of lasers which means we can use them for all sorts of different things that brings me to the the second lesson which is that laser light is monochromatic what does that mean well it's around one color it sort of spreads around a certain central wavelength but but for the purposes of this talk we can think about it as as monochromatic and I just want to show you that this is the case so what I have here in front of me a tiny little box here is a spectrometer and what that does is the same thing as as the prism it basically disperses its light out into its constituent colors and this is a rather more expensive version a few thousand pounds sitting here on the but it is wonderful and so what I'm going to do and I'm just basically along the bottom there you've got wavelength and up the side is some arbitrary intensity okay so first of all I'm going to shine some white light in okay what you can see is although it's a bit lumpy it's just a quite broad spectrum of light around here between sort of 420 and around 5:20 say let's say just for argument's sake so it's quite broadband and we know this light spreading out everywhere now I'm going to shine I've got a green laser from Maplin if anyone's interested it tells me it's 532 nanometers it tells me as I say if I believe it I hope less than a milli Watt if anyone doesn't know what the I save them is and and I'm going to shine it into this sir this is a fiber optic by the way just let it and what you can see is it extremely narrow we it's quite powerful and you can see an extremely narrow beam peaked around five-30 so I do believe although I haven't calibrated this so I don't know exactly and so laser light monochromatic okay all right let's get to talk now the next one is that laser light is extremely directional and it can travel long distances as such now it's an extremely boring demonstration if I go oh look laser it's going in one direction so I thought I'd just take this opportunity to show you my sisters laughing quite like this and basically you can get light to travel over much longer distances if you use a fiber optic okay so because the prop because the laser light is so directional you can bounce it anywhere and do anything with it so what I've got here I think I'm going to need the lights down slightly is a tank of water now I'm going to shine a laser in it okay now this is going to start tiddling out I'm going to shine a laser through what you should be able to see is that the laser light is now actually following the path is that going in someone's eye of the laser beam to do tell me if it is and what's happening there if you can enough working isn't it is that the laser light is doing the same thing or you can raise the lights now the laser lights doing the same thing as the fiber-optic so the laser light is coming in at quite a high angle and what happens is it reflects off the inner surface of that water and something called total internal reflection happens and this is exactly the same thing as a fiber optic this light just thing continues to bounce along inside this stream of water sorry if anyone needs the low that's going to be dribbling out for the rest of it and but exactly the same principle this means that we can transport lasers over thousands of kilometres for modern-day digital communications right and the most fundamental thing about high-power lasers is the intensity right we can focus lasers down so that they can become extremely intense what you need to know for this talk is intensity is power over area so if we get a lot of power concentrate it in a small area we get an extremely intense laser and that's sort of fundamental principle of how we build these extremely intense lasers now I'm going to need a fairly robust assistant anyone yet want to come down all right let me just switch this on okay what we have here is a wonderful demonstration that has been lovingly lent to me by Imperial College is called the popper Matic so you might be able to understand what might be happening soon right I'm just going to prepare this for you now what I want you to do is put the put the balloon in this slot yeah like so and put it coming in and then get in there right well that's not one right is it there you go right right then I want you to pump it up yeah to a certain size then pull it off the end and just keep it at the bottom for now because we've got to a laser focus somewhere in here and we'll see what happens all right so if you want to pump it up keep going oh oh no right here we go hold on that's not it all right now keep going oh no no that's not right we'll get there eventually right you will do this right okay right it's in there now let's ER I'll just keep her at the end there pump it up keep going I need to get to a certain good size where you can see it lighting up a bit hold on a minute yeah that's good and a little bit more right that'll do right now I'm going to pull it off now what I want you to do is just hold that now move it slightly up so we can see keep moving it up and we can see the laser lights getting smaller on the top can't we and it's getting very very near the focus of the laser beam keep going keep going and going through okay so if you want to let the air out of that no one doesn't laugh at that come on right we want to do the same thing again but with a red balloon now the reason nothing happens with a green balloon is because it's green right this green light and it reflects mean light into our eyes so now we're going to do with a red balloon and I hope you're not easily scared right right George blow up again alright that will probably do now OOP alright you want to hold that now bring it close to the focus of the laser keep going keep going I don't like balloons popping so thank you very much you want to go sit down so what we see here is that the red balloon because it doesn't reflect green lights absorb the green light it's absorbed enough energy that the balloon actually bursts I assure you this is I safe because it's completely contained it wouldn't be if it didn't have this plastic around it but anyway so focusing lasers is is the way forward for for getting these high intensities now we want to make extremely powerful lasers then one of the things that we can do is make pulsed lasers okay because intensity is power over area okay so we need to get the the power up power is energy over time so rather than just having a laser operating continuously why not continue white not concentrate all the energy that you had but within the smallest time that you can do so the push towards really high intensity lasers would actually make pulse lasers and there were three really important developments that helped us get to extremely intense pulse lasers now don't worry about a lot of the stuff that's on this plot behind you all I want you to see is along the bottom is year 1960 that's when lasers were invented and this is focused intensity in units of watts per square centimeter okay so that's a really important thing three different things helped us get these intense pulse lasers the first thing was called cue switching the second thing was called mode locking and then the third thing is called chirped pulse amplification okay and and on the next few slides I'm just going to kind of briefly go over the principles of these as to how we actually got to post lasers so key switching we need to rather than having a continuous wave laser we want to make a big pulse of energy okay so what we do is we do all the pumping first we pump the rod so it's got lots of stored energy before we get lazing to happen okay so we pump and pump and pump the rod so it's like blowing up a crisp packet and then bursting it that's kind of the idea of Q switching now what we do is we pump and pump and pump the rod and then somehow we switch the cavity so that lazing can work and then you get an extremely rapid amplification of the laser light and that means we get a giant pulse I'm necessarily short impulse lengths but it's better than a than a continuous wave laser the next thing we can do is actually we can do something called mode locking now the laser is a cavity so it supports standing waves so if you think about waves on a string like you can see behind you you've got your fundamental linear your higher modes and because laser light there's not just one wavelength there is a tiny little band around the central wavelength multiple modes can oscillate inside our laser cavity now those modes don't have any fixed relationship with each other so they don't really do anything interesting but we can make them have a fixed relationship with each other and on the time of one round-trip of the cavity they can actually constructively interfere with each other and produce a giant pulse but extremely short pulse of light so what you end up with is a train of extremely short pulses okay so mode locking enables us to not just have one giant pulse but a train of extremely short pulses and why this is useful will actually become clear in the next slide this is where we get to the real nuts and bolts of ultra powerful lasers the laser itself is not just a cavity okay it's not just the laser cavity that I showed you really powerful lasers the laser cavity is just the seed to the rest of the laser so what we end up with is just a chain of different amplifying materials so so that the material that you use in your laser whether it's neodymium glass for example or some other medium in a cavity it's got the mirrors in that they reflect back and forth but then we have an amplifier chain where it's just single pass so the lights just passing through the lasing medium and you can do that over and over again okay so you go from say extremely small thin rod amplifiers to slightly bigger rod amplifiers and the reason we have to increase the diameter as we go along is because the lasers getting more intense so we have to make the laser bigger so that it doesn't damage the optics you go through because this is this is what we're contending with that with high intensity lasers then at a certain point we have to go from Rod amplifiers because they're not particularly efficient after a certain size because the middle doesn't amplify as much as the outside so you get funny doughnut shapes or weird beam we go to something called a disk amplifier which is something I have here I have a disk amplifier here from the Vulcan laser that's based at the river Appleton Laboratory and in Oxfordshire you might wonder why I've got this well I worked there for ten and a half years and when I left they very graciously gave me a laser amplifier that was engraved that's where you can see my weird face behind here looking a bit sort of perturb and I have been told on the proviso that if they do need the disk back I will have to give it back to them and they'll have to uninvite for me but anyway so then we go from these rod amplifiers to the disc amplifiers and these discs are placed within a sort of box with flash lamps which pump it Pump the energy and these are sort of sitting at Brewster's angle and then you go through those as well so so these are the biggest biggest they're amplifiers that that we have on that laser anyway so obviously increasing the size is one way of decreasing the intensity but another way is to actually because a power is energy over time why don't when we do the amplification stretch the pulse out and only make the pulse extremely short at the end and that's what chapter pulse amplification does so we take the seed pulse that was generate through mode locking and we take it towards what we call a grating pair it's essentially the same thing as what a prison does it disperses the light out into its constituent colors and in doing so it creates a delay between the the front and the back of the pulse so we got this stretched pulse then we pass it through our amplifier chain so our rods and our discs chuck as much energy energy as we can into it and then only finally at the end we take a completely matched pair of gratings that do the opposite thing and compress that pulse down to extremely short pulses and then you get this extremely intense laser that you can see this massive spike here so that's a fundamental thing that drives these super intense lasers there's another another innovation which I'm not going to talk about which is called optical parametric pulse amplification I like to think it works on the tides in the moon but I don't really have time to describe it here but that takes you even further so yeah really exciting stuff now I call this slide all the physics ever because it's it's a rather busy slide the lasers that we're dealing with what happens when you actually focus them down to something like five microns onto a piece of metal so it's kind of like taking all of the light that's falling on the Earth from the Sun and focusing it onto the head of a pin it's incredible okay so you take these lasers you focus on one two retargeting is a certain intensity right if you check it onto the material the electrons in the atoms can start absorbing some of those photons and and get excited out of the atom in a process called ionization now the laser electric field that's associated with this laser is so high that it can actually modify the atomic potential keeping the electrons in the target and I've laces of a certain intensity can start modifying this atomic potential so actually the electrons can do weird things like quantum mechanically tunnel out of the atoms so they're actually escaping slightly easier now that lasers are I'm going to tell you about now which are lasers above 10 to the 18 watts per square centimeter so 10 to the 20 10 to the 21 watts per square centimeter their electric field to mean that you're completely suppressing that atomic potential so actually the electrons just kind of fiddling out everywhere so what does that mean well you're driving a whole bunch of free electrons into a target free electrons that's a current and that current is mega amp current all right that's serious just to put that into perspective this lightning here carries a current 3030 kill amps right mega amp currents going into the target um and this is a making temperatures of you know millions of degrees on the front surface approaching the temperatures of the Sun that's incredible and also because you're driving these massive currents and you've got these temperature gradients and things you get enormous magnetic fields approaching Giga Gauss level Giga Gayle's fields something like neutron star atmospheres that we're talking about the most extreme conditions on earth it's absolutely amazing now because of all these physics happening it gives rise to a sources of lots of different particles so for example some of the the electrons that we're driving well their relativistic has in it you know some fraction of the speed of light right so we've got a basically a military particle accelerator here I mean they're not beams as such but they're doing the do okay so these these electrons can escape the target stuff happens at the back of the target which means that draws our ions ions get accelerated as well so we've got another particle accelerator there we're producing copious amounts of x-rays because we've got electrons going through target and also it's hot so we've got atomic physics happening gamma rays and also we can do interesting nuclear physics as well okay so fascinating tons of physics going on in this target at any given time it's extremely complicated to understand as well I have to say well that's why we keep coming back basically and so back to this plot and we're sort of firmly sitting in what we call the the relativistic optics regime okay so we're the lasers that we can produce at the moment around 10 to the 21 10 to the 22 watts per square centimeter that's it's about is intense we can get at the moment if we start pushing lasers even more intense and lasers are being developed at the moment that we'll be on in the next decade or so they're going to start pushing us towards this kind of ultra relativistic rate II that's the sort of regime that starts melting my brain this sort of regime where you might be able to do quantum electrodynamics and all these crazy things so we're not there at the moment and we're we're squarely here okay so what can we do with these types of lasers well as I said before we can accelerate ions and protons they're useful for physics alone just just understanding physics that can be generated with those particles some groups are actually looking at those those protons for example when thinking can you use them further for proton cancer therapy and instead of using these big fussy cyclotrons and the gantry's associated with them maybe you can kind of maybe miniaturize and maybe reduce the cost of a laser proton therapy with with lasers and we're nowhere near there yet but but lots of work is happening at moment driving huge currents through the target and they're getting hot you get lots of an interesting atomic physics happening you can knock out electrons in in really really tightly bound orbits and that makes things called hollow atoms and you do all sorts of crazy atomic physics and that's really interesting from the fundamental perspective but it means that we can create these states that are not possible anywhere else we can only do these in the lab so six well universe I guess but in the lab that's that's what we do and talking about the universe we can do laboratory astrophysics experience in miniature okay so we can make things like miniature astrophysical jets and that tells us something about what's happening on the universal scale so these are kind of scalable experiments they're extremely exciting we can do experiments which try and understand the origins of magnetic fields in the universe as well it's absolutely fascinating stuff I'm gonna tell you a bit more about how we use these lasers to make fusion that's quite a significant part of this too actually and those sorts of lasers sit down here they're actually not in the relativistic optics regime they're in tech but they're still considered to be extremely intense lasers all things being equal okay but they're not the most intense lasers that we can deal with so I won't talk about that so much here cuz I'm going to talk about it later we can do interesting nuclear physics although the nuclear physics that the lasers that we have now is slightly pedestrian no offense when we get to the more intense lasers we're going to be able to start doing extremely interesting physics really extremely interesting physics and also the physics of neutral nuclear fusion will help us reach really interesting nuclear physics as well and finally we're not just shooting these lasers at solids we can shoot them into gas and when you shoot a laser into a gas it forms a wake behind it which is called a laser wake field and that can trap electrons and accelerate them to extremely high energies and the idea behind that is that we can try and make the LHC rather than over tens of kilometers over tens of meters so that's an extremely interesting area of study which is only going to get better with a more intense lasers so you have some friendly local lasers and what I've got up here is two lasers that are sitting at the central laser facility row was ten and a half years and you've got the the Vulcan laser and the Estridge M&I laser now the Vulcan laser is a is a petawatt system 10 to the 15 watts is one of the most intense lasers that there is and just to give you an idea of the scale of this is about two Olympic swimming pools in size is pretty sizable and most of these rooms are filled with laser amplifiers and these rooms are filled with the interaction areas where we do the physics and that sort of my domain and we have to fire these lasers under vacuum because they're so powerful they would break the air down and turn it into a plasma which we want to actually turn whatever we're doing into a plasma rather than rather than the air itself okay um now the Vulcan laser itself delivers around 500 joules in around half a picosecond second is ten to the minus twelve of a second so the aim there is when they only fire it maybe every half an hour or so so no repetition rate but it's delivering a lot of energy in a fairly short time the other approach to extremely intense lasers it's to actually deliver a moderate amount of energy but in an extremely short period of time and that's what the Astra Gemini laser does and in doing so means it can fire a few times a minute at the highest intensity so they can fire rather than a you know a few times a day so Astra Gemini delivers around ten to fifteen it's got two beams hence the name Gemini 10 10 to 15 joules in 30 femtoseconds so you've got two peda watt beams opposing each other you can do lots of really interesting physics the physics that you tend to do on this is the the electron acceleration laser wakefield physics so extremely exciting stuff there's also another friendly local laser and this is sitting down at a wel de Marston it's called Orion it's been operating for a few years now basically that has five kilojoules of long pulse energy which is extremely special that's one to ten nanoseconds in length that's useful for for fusion studies and kind of high-energy density physics and then they have two petawatt lasers and the architecture for that is largely based around the architecture of the Volcom petawatt laser because it's reliable so so it's a kind of a sister laser of the Vulcan laser and it's all looking extremely shiny and new and the academic community can use this as well so that's good so I'm going to dial it back very slightly and tell you about how we use these intense lasers to actually generate an energy source on earth that's clean and that's kind of basically what I've spent most of my research career thinking about bits and pieces related to this and what do we do well what we're trying to do is recreate the energy source of the Sun which is fusion which I'll go into a detail about on the next few slides but in a controlled way in the laboratory now with lasers the idea behind it is you just compress the hell out of an extremely small pellet made of two types of hydrogen deuterium and tritium until it kind of self ignites so that's kind of like a diesel engine let's say sort of nuclear-powered diesel engine um there's two main ways of doing this with lasers one here you take whole bunch maybe a couple of hundred of lasers and you fire them into a gold can called a whole run and that gold can remix those lasers as x-rays and the x-rays come in compress this tiny pallet and and it and then it ignites basically what do we mean by that you get more energy out than you've put in that's that's the idea the other thing is you can do away with the whole room and that that bit and just fire the lasers directly onto the capsule and that that's the way that I have been working on than the interrupt right so what is exactly fusion you might have heard about it but just just for the sake of argument you haven't let me fill in the gaps so we're taking two types of hydrogen deuterium and tritium now deuterium is present in seawater in in large amounts for every six thousand hydrogen's there's this there's one deuterium so you get lots of deuterium's from seawater now for every 10 to the 17 hydrogen's there there's one tritium so tritium is not naturally occurring so you might think Kate that's pretty stupid why you guys build basing an energy source on on something you can't get occurring naturally well I'll tell you a bit later we can actually make the tritium in laboratory using these reactions now um you take the determined tritium hate them to something like 100 million degrees Kelvin credibly hot ten times hotter than the Sun without touching it so that's the crux of fusion you have to keep it all together without touching it and then what you end up with is helium and a neutron so not producing any carbon or or long live radioactive isotopes now how do we get energy out of that well it all hinges around this equals MC squared most famous equation in the world energy equals mass times the speed of light squared for the purposes of this C as we will know is a constant in all reference frames because Einstein told us so I see is 10 to the 8 bits per second it's a big number so C squared is enormous so what we know is if C squared is a constant energy and mass are somehow interchangeable and that's and that's the crux of its what does that mean in reality well if we take you to him and tritium and we add their masses together weigh them and add them us together do the dew and then we get the the interactions at the end and we get helium and neutron we also add those masses together as well there's a mass loss between the first and the second situation right so you type that tiny little bit of mass stick into that equation ties it by an extremely large number you get lots of energy it's an extremely efficient way of generating energy now I mentioned about tritium not being naturally-occurring what you end up doing is you can surround this reactions the reactor with lithium and the lithium can gobble up the neutrons which carry away most of the kinetic energy of the interaction and that has two functions one you can heat the lithium and that in turn can heat water to make steam to drive turbine so it's a depressingly kind of Victorian back into - what's a rather space-age front-end and the other thing is that we can use that lithium it gobbles up neutrons and makes tritium and we can extract the tritium so I make it sound extremely easy I'm sure we can do it we haven't any one new killer physics works right so how hard can it be right um so it's extremely efficient way of getting energy so 40 tons of coal gives you the same amount of energy that you can get from fusion from half a bar full of seawater and a lithium in a laptop battery what's also extremely expect any is that half a bar full of seawater and that lithium in a laptop battery will cover just one of your lifetimes energy needs it's an extremely small amount of fuel so it's an extremely efficient way of getting energy it's also you know doesn't produce carbon long-lived radioactive waste you can't have like a Chernobyl style meltdown it's extremely safe so and we know lots about how to make it not work so you just turn it off rather than it run away so now offense everyone who does fusion in the room but there are some of you and the thing about the thing about Fusion is that you have to heat it to high temperatures so we end up getting something called plasma so when you add energy solids liquids gases what you end up with is the electrons are able to escape the atoms so you end up with a kind of soup of charged particles so you've got the the negatively charged electrons and the positively charged ions all kind of digging around together and that stuff's called plasma and we need to kind of deal with that plasma a hundred million degrees Kelvin without touching it in order to make fusion work and and that's kind of the crux of how how fusion is quite difficult we can use because it's made of charged particles we can use electric and magnetic fields we can use magnetic fields to control it magnetic confinement fusion does that on the other hand you could use lasers just to smash the hell out of these things in and hope the best and that's and that's what we do so I can't bring a fusion reactor here but can I have the lights down just slightly if you're still there yep um yep I think bit more yeah that's good what we've got here instead of a fusion reactor we've just got a simple plasma ball here and low temperature bits still kind of cool right you know has the power to mesmerize even adults so um what you do is there's the charge builds up in the center and Annis of current flows through the gas that's in here which is noble gases I think it's argon and neon I think probably and the current gives energy to the gas the gas becomes ionized turns into a plasma now you can see that plasma because some of the electrons that escaped can recombine and when they do so like we saw on the earlier slides they have to emit light so that's where you can see it so actually your lit you probably don't know this but your lip by plasmas every day okay you may not know this normal fluorescent stripped tube filled with gas if a current flows through it and in this case I'm just going to use the the plasma ball to help us here and I heard a little girls blur that's quite satisfying and the current can flow through here it gives energy to the gas inside the tube that turns into a plasma and then the plasma is really light but unfortunately plasma inside here you can't see that light but there's a fluorescent coating hence the fluorescent tube that shifts the wavelength to something you can see so so every day when you go in if you're lit by these things it is plasma sitting above your head so there we go and plasmas kind of abacus it makes up 99.9% of the visible universe which of course is a very small part of the whole story which I won't go into because that's not my field and thanks very much for the lights down okay so that's that's what plasma is and that's kind of a fundamental part of fusion and kind of part of what makes it great but part of what makes it quite difficult as well so so how do we actually make fusion work I went on a huge digression there but we were talking about inertia confinement fusion so how does it actually work well we take this pellet of deuterium and tritium that I was telling you about and we fire a whole load of lasers on it and what happens is that heats up the outside layer now that laser before it gets heated up it's absorbed at what is called the critical surface and I'm just going to tell you this the sake of completeness because some people might like this and the laser the critical surface is where the laser frequency equals this plasma frequency now particles inside a plasma oscillate with it with a certain frequency and when that laser frequency equals that plasma frequency the laser could be absorbed and then that energy is transported to higher density material because that happens far out now that higher density material is is heated up extremely rapidly and flies off and that process is called ablation and just for the more squeamish in the audience ablation is also what happens when a medical person fires a laser into your eye to reshape your cornea so that's why I don't put my eyes anywhere near lasers because I've always been told as a laser physicist to not look down a laser beam luckily I've got 20/20 vision so I'll never have to consider that hopefully but um laser ablation this this the outer layer of the capsule is flying away extremely rapidly and so via Newton's law the rest a stir fly inwards and compress now rather than heating up just a big solid density but a piece of DT we have to limit the amount of yield that we get in energy right if we just heat it up a solid lump of DT we blow up the reactor yeah that is not desirable okay so in order to reach the conditions that you actually need the density and radius conditions that we need we rather than hit up a big lump we just heat up a few tens of milligrams of fuel but we can press it a lot so that we reach the same conditions as we would if we just heated up a joint great big lump of DT and that way we can have a react without blowing it up which is extremely useful and how does it get hot and so how did it heat these fusion temperatures well as the laser smack in they launched shocks and those shocks start to converge into the center of the fuel and that actually deposits a lot of energy because shops carry a lot of energy deposits its energy and heats it up to the 100 million Kelvin degrees Kelvin that you need for fusion to occur now of course we're not going to do a lot of work just heating the whole thing up we're just going to heat a tiny little bit in the center because the alpha particles can help us with the rest of it which are produced during the fusion reaction so we heat up the center alpha particles would reduce they travel a little bit out heat up more fuel produce more fusion and so forth and so forth so this kind of fusion burn wave propagates through the fuel and that's basically that in a nutshell I make it sound extremely simple it isn't and let's think about some of the numbers here the ablation velocity so that the outer layer flying off that's 10 to the 7 centimeters per second and the fastest man-made object that we know of well that we know of of course we know it's man-made isn't it silly me and that's the Helius to probe so wikipedia tells me and that travels at 7 7 million centimeters per second in its eccentric orbit around the Sun so we do better than that with these at outer layers and and by rote because of a Newton's third law the implosion velocity is extremely similar as well the pressure at which the layer ablates is hundreds of mega barn that's that's pretty decent pressure but actually because the thing converges you start to get this pressure amplification you end up getting two hundred a Giga bar and the Sun the pressuring the Sun is around 250 gigabytes so it's similar to that so this is incredible stuff we saw recreating the conditions of the Sun on earth in miniature so we are kind of building miniature Suns I think um and the energy frig nishan so ignition is getting more energy out than in we don't really know what it is yet because we haven't done it but just for the sake of arguments about a mega Joule maybe more and there's about a mega Joule contained in a KitKat a four-bar KitKat not to bar none of that skimping um but we deliver that energy over about 10 nanoseconds and and I imagine there's no one in the audience here that can scoff a four-bar KitKat in 10 nano set my sister's putting a hand up I really don't believe her I really don't believe so yeah big numbers so what are the targets look like so what I've got here is two types of target got a sort of rubbish EMS paint drawing of what might be a point design of a direct drive so firing the laser straight on and then we have a luscious beautiful diagram of indirect drive and what does this tell you well where's most of the work on at the moment well most people are doing indirect drive at moment that's the that's what people are doing which is why we don't really know we don't have a point sign for direct drive yet you might as well try and do one before you do anything because you know if you can't do one then well why bother but anyway and so just to explain a little bit this is it this is a pizza slice of a full sphere right and this is for direct ride so when you're firing the lasers directly on the capsule so first of all you have a thin layer of plastic and that's your ablator layer and that's the thing that blacks off and flies away because you don't wanna waste DT for that and then you have a layer of solid DT which is there for cryogenic and the and that we have to do that is for for reasons of entropy I won't go into but we need to start with the highest densities we possibly can with with deuterium tritium which means we need its solid which means it needs to be cryogenically cold that throws a spanner in the works somewhat but we can do it that's fine and then within the capsule just in the empty space we put two deuterium and tritium gas as well now on the converse we know much more here about point design for for in direct drive what we have here is you can see what is called a high Zed mixture cocktail whole room this is not a drink this is just your small gold can made up of different sort of heavy heavy metals and those lasers will come in and those metals have to be extremely good at converting your laser light into x-rays because that's the thing that's doing the compression in this context now the pellet itself rather than having a plastic outer layer needs an outer layer that's good at absorbing x-rays because of course that's what we're driving it with it has but a beryllium outer layer and then it has its normal solid solid cryogenic determined tritium and then a hollow center filled with gas as well and as the daughter of a precision engineer one cannot deny the extremely beautiful technology and engineering that has gone into making these targets and just along the bottom here you can see exquisitely small but exquisitely precision Li precision Li that's not a word precisely engineered targets right here that's the whole whole room assembly you can see the laser entrance windows there and a little diagnostic window to see what's going on inside this is the the capsule itself now the cryogenic DT layer has to be grown has to be grown to a smoothness of less than one micron RMS which then gets coated in brilliant incredible actually it's the target makers that I'm most impressed with in all honesty they have the patience of a saint construct these things which we then blow the hell out of intend nanoseconds they do feel extremely sorry for them these things cost a million dollars in order for a fusion energy to work they need to cost 20 since so yep but we're at the science phase right now okay so and just to give you an idea tiny tiny tiny little pellets now there's places to do the fusion science right but really at the moment there's only one place in the world which has the potential to reach ignition which is extremely exciting it's the National Ignition facility and it's based at the Lawrence Livermore National Laboratory in California it's the biggest laser ever built it's the most interview tafolla laser ever built I think it's incredible I've money I've actually gone to see it it doesn't look like any lasers I've ever seen what you can't see any of the bits it's all covered up normally I'm just used to seeing optics on a bench right this is this is another level it's like a factory it delivers 350 nanometer lighting of 192 beams total energy that they can deliver to target is a couple of mega joules pulse lengths of around 20 nanoseconds and now the beam configurations poller that doesn't mean it's cold it means it's going in at the poles okay because we're using whole room so it has to come in at each end and as I said scheme is in direct-drive which is why you can see what there's been lots of work going on to indirect drive because that's the thing that's driving the National Ignition facility at the moment now it's been operating since around 2008 and there was a concerted campaign to try and get ignition between 2010 and 2012 and didn't quite get there it's a science project but they have done something unprecedented they've delivered a laser no one's ever managed to deliver before they fielded 50 instruments to look at what was going on inside the plasma you know on a typical experiment you're normally fielding about 10 50 instruments surrounding it is just beautiful what they've done is absolutely incredible and after a bit of head-scratching which I'll tell you about in a bit they've managed to actually produce record-breaking numbers of neutrons so that that's good news we might have recognition yet but there's some beautiful science going on there at the moment at this slightest in total my friends are clever because I've got a few friends at NIF and they've managed to look at the problems that have been happening in quite a limited timeframe with quite a limited amount of shots to try and understand what's been going on two of the major problems that were pointed out at the time one was of symmetry and one was of something called mix okay now with the symmetry and you can see this is a sort x-ray picture of what was happening inside the whore arm and they were driving as you can see the poles much more strongly than the equator so you kind of get like a parent cake shaped implosion which is not ideal and they're so clever that they managed to work out that you can actually transfer energy from one set of beams that are doing the compression here to an inner set of beams or vice versa cartman which way around it was and and actually even out the implosion symmetry and in tweaking that a little bit they've managed to get back to a situation which is much better than the kind of pancake they had at the beginning so clever kudos to them the other problem is is one of mix and they were getting much colder implosions than they were expecting and the reason is because they were driving something which is called the rally Taylor instability so what that is if it stays up here this is a bunch of goop in here water and syrup it's an interface instability between two fluids right the lighter fluid and heavier fluid and the interface starts this might take some time so it might not even start and by the time I finish talking but basically that surface becomes unstable and ripples and so when their capsule and what it looks like is as you're imploding the capsule surface starts rippling and what happens is you start mixing some of the beryllium ablator in with your cryogenic DT fuel and that actually acts to radiatively cool the fuel so it's much colder than you'd have expected and so in order to get over this we have to look at the laser pulses that we use so we have to go from a situation of a low fit to a high foot laser pulse that's going to be nothing to you until I point out and it doesn't mean loafer - it's that it's the year that the foots of the laser poles so this is the the profile of the laser pulses that we're using to drive over a certain set of nanoseconds this spiky shape again is for reasons of entropy I'm not going to go into this is the low fat pulse as you can see the foot of the pulse is is lower than this one so what they're doing is they're employing what's called high foot pulse and what that does is it increases the ablation velocity and we know that the ablation velocity if we take it up actually stabilizes rally tailor and in doing so they've managed to increase the neutron yields and then the stabilize the implosion now it's a trade-off between stability and how much you can compress it so we're not going to get to ignition with that kind of pulse length that kind of pole shape but it does tell us something about what was happening physics wise so is extremely interesting so where's new fat um well this is a plot of all the all the shots the ignition type shots taken along the bottom is shot number and along the side is the yield in kilojoules now you're thinking well you're putting mega joules of energy and ultimately and you're only getting kilojoules coupled to the fuel well it's it's inefficient and one eventually we'll have to get enough gain that it takes that into account but we're not at that point yet so what we see here the black lines are the energy that you've managed to couple to the fuel so it just gives you an idea of what you're actually delivering so again it's in on the orders of tens of kilojoules now blue tells you the yield just coming from when I say yield I mean the amount of neutrons which tells you how good your fusion reactions are doing because they produce neutrons right and basically that tells you the the yield that you get just from the compression part right the pink shows you the you that you get from self-heating ie when the alpha particles are produce they start hitting the fuel and pitiful down here no disrespect not really doing very well that's because we had some problems I say we I wasn't there I'm not going to take any claim for that um and as they start applying the things they know look at what happens right with the high foot pulse they start getting implosions back to the yields that they're understanding amazing amounts of energy here they start exceeding the yield that the energy that they put in now that does not mean we've exceeded the you that we've actually delivered from the laser there's lots of inefficiency right so let's get that clear it's not a energy gain in that sense but what it is is unprecedented no one's ever seen this before in the laboratory so the scientific endeavor that is evident here is is something to be applauded frankly it's a it's a beautiful thing and also just this is this is a plot taken from a paper that was written by a guy called Omar hurricane is another best name ever right I'd love that name brilliant so what about the future okay the lasers like NIF fire maybe a few times a day we need to fire a few times a second in order to make fusion energy work with lasers so we need to develop lasers that are more efficient and we can deliver lots of high energy at a higher repetition rate and so rather than using white light to pump her lasing medium we're going to pump it with diode lasers because that a lot of energy with white light is wasted as heat in the rod and actually what you want to do is just shine the wavelength thing that's relevant let's do it with a diode laser now there's lots of projects around the world looking at these diode pump lasers in the UK I believe where some of the best laser builders in the world I'm biased but we if you look at the upper echelon of NIF there there are some people homegrown let's say here and we're hoping to try and combat that problem there's a there's a project called dipole that said that the central laser facility and that's looking into these diode pump solid-state lasers they're already generating these laser heads and selling them around the world so the technology that will hopefully enable Fusion to actually be an energy source rather than just an interesting science project is also enabling us to push lasers into the highest intensities ever that's the thing I find extremely exciting the sort of overlap between stuff that might deliver a terrestrial practical energy source and also get us to do some physics that just kind of melts my mind even if I think about it this project sold a laser head to this project this project is called extreme light infrastructure it's going to be one of the most intense lasers ever built there are three arms of it in Romania Hungary and Prague it's extremely exciting it will be online in the next few years and you're going to be able to do all sorts of interesting physics colliding basically the laser with a beam of already generated electrons from a linic collide them together you get Compton scattering you get ginormous numbers of gamma rays and you can get pair production you can all sorts of crazy stuff you can generate Neutron rich isotopes in the lab it's not getting too excellent nuclear physics that you'd never have been able to do actually with with experiments currently available so this is where nuclear physicists actually start to want to talk to us because at the moment not so much but in the future yes please they they want to start accessing the things that we're able to to generate also you know I've mentioned that we can create these Wakefield's which accelerate electrons will start to be able to drive more energy into those interactions and really see that as a realistic kind of source of of particle acceleration and then this is the bit that I find difficult to get my head round because it's an array deme I don't even think about if we push the lasers even further we can accelerate electrons to the sort of regime they would be in surrounding a black hole so they might we might be able to look at Unruh radiation for example and we might even be able to fire a laser into a vacuum and get something pair productions so imagine that I can't even imagine that I don't really know anything about could actually learn quickly because these lasers are moving fast so that's really exciting and I think on that note I'm going to finish talking and I'm just going to leave you with a family photo album of what it's like to do it firm on super-intense laser so thank you very much for listening

Channel: The Royal Institution

Views: 525,145

Rating: 4.7109499 out of 5

Keywords: Science, lasers, ultra intense, kate lancaster, demonstration, world's strongest laser, technology, lecture, what is a laser, ri, royal institution, girlswithtoys

Id: hcGgaa2mFc4

Length: 59min 18sec (3558 seconds)

Published: Wed Jun 10 2015

Reddit Comments

[u/Nuclear_Physicist]

Very nice talk! Thanks for sharing.

[u/Jasper1984]

So the pulse stretcher changes the timing of different frequencies, probably by sending frequencies in different directions, so they have different path lengths? The amplifier is a cavity produces different frequencies in different times? That would be the reason that you cannot simply compress multiple times, because you're just moving different frequemnces by a different time, and doing it twice would just stretch it again.

[u/sschudel]

Soooo, when can I carve my signature into the surface of the moon?