Accelerator Science: Why RF?

I’ve made quite a few videos about particle
accelerators and why we use the ones that we do. But there is one thing that I haven’t mentioned
and is really pretty cool. And that is just exactly how we accelerate
particles. And if you make it to the end, I’ll let
you know why physicists are at least as cool as surfers. I’ve talked about how we make electric fields
to push electrically charged particles around and that’s right to a degree. In the simplest case, we could take two metal
plates and connect them to a battery. That creates an electric field between the
plates. That electric field can then push an object
that carries an electric charge. But that simplified picture isn’t how we
REALLY do it. It turns out that the electric field in most
high energy particle accelerators isn’t static like that. In fact, the electric field in a particle
accelerator is actually oscillating. It starts out as zero, then pushes a charged
particle in one direction, then pushes it in the opposite direction. The pattern repeats itself over and over and
over again. So why do we do it that way and what consequences
are there for that choice? Well why we do that is perhaps the easiest
thing to explain. It’s just hard to make very strong static
electric fields without a spark occurring. It can be done, but there is an easier way. And it turns out to be pretty cool instead we shoot radio waves into a volume
in our accelerator. We call that volume a “cavity” and its
exact shape and size are very important. We’ll get back to that in a minute. Now you may recall that radio waves are just
oscillating electromagnetic fields. Focusing only on the electric side, we get
an electric field that gets bigger, then smaller and reverses itself. So that doesn’t sound so amazing. What’s the cool part? Well it turns out that there is a very interesting
phenomenon called resonance, which we can demonstrate using an ordinary swing. You’ve done this as a child. You sit on the swing and move your feet back
and forth as you lean at the same time. If you do that, you eventually find yourself
swinging in big arcs. What you know from experience but maybe never
really thought about is that there is a particular rate at which you have to rock back and forth
to get the swing moving a lot. Rock back and forth too quickly or too slowly
and you don’t the full swing experience. The particular frequency at which you have
to rock is called the resonant frequency. Little motions at that frequency build up
over time. Particle accelerators also have a resonant
frequency. If you shoot in radio waves of all frequencies
into the accelerator, only a certain frequency will grow. And which frequency is the resonant one depends
on the shape of the cavity in your accelerator. Some cavities prefer higher frequencies and
some prefer lower ones. To get an idea of how that works, think about
what happens when you blow air out of your lungs. It just sounds like wind. But blow across the top of a two liter soda
bottle and you get a characteristic tone. Blow across the top of a smaller soda bottle
and you get a higher tone. The soda bottle stands in for the accelerator
cavity and you see how different size cavities prefer different tones which means different
frequencies. And that characteristic resonant tone of a
cavity allows us to put in a relatively small amount of energy in the form of radio waves
but that energy will get amplified into a very strong electric field because of the
effect of the resonant cavity. By the way, the technical name for those resonant
cavities is an RF cavity. That’s because the cavity selects a particular
radio frequency or RF. Okay, so that’s how we make electric fields
in an accelerator. What are the consequences of that choice? Well, to begin with, it means that your particle
beam can’t be continuous like water coming out of a garden hose. A particle beam is choppy. So why is that? Remember how electric fields interact with
charged particles. And let’s pick a positively charged particle
just to make things easy. Suppose you want to make that particle go
to the right. Then you have to have your charged particle
pass through that electric field when it is pointing to the right. Then the electric field pushes the particle
and makes it go faster. However, if the charged particle comes through
the accelerator when the electric field is pointing to the left, it slows the particle
down. So that means that there are certain times
you want your particle in your accelerator and other times when you don’t. In the simplest way of thinking, this means
that you could put your particle beam in the accelerator half of the time. But it’s actually more complicated than
that. After all, even when the electric field is
pointing to the right, sometimes the electric field is bigger than other times. So to get the most bang for your buck, you’d
really only want to put the particle beam in the accelerator when the electric field
is the biggest. The result is that there is actually only
a very short time in which you’d want to put the beam through the accelerating cavity. It also means that a particle beam would be
a series of very short groups of particles separated by a long distance. We call these groups of particles a “bunch.” Now what I’ve told you is true in the simplest
and most ideal world. And that would be good enough for most people. But you’re not most people, are you? You’re the kind of person who would watch
a video about particle accelerator technology. So I’m going to show you one more cool thing. In the words first uttered in the movie “This
is Spinal Tap,” what I’ve shown you thus far is a ten. But I’m going to show you an eleven. Funny movie by the way. You should watch it. Maybe they’ll give us a kickback. OK…so what is this insight that takes us
to eleven? It comes down to the fact that a particle
bunch doesn’t have a single charged particle in it. It could have millions, billions or even hundreds
of billions of particles in it. And these bunches don’t consist of all particles
in exactly the same space and they don’t pass through the accelerator at exactly the
same time. And…and this is important…they don’t
all have exactly the same energy and velocity. What would be the consequence if we tried
really hard to make the center of this bunch arrive at the time when the electric field
is the highest? Well some particles that had more energy would
move a little faster and they would arrive when the electric field isn’t at a maximum. So they wouldn’t get pushed quite as much
as those particles that arrived a little later. So they’d speed up some, but not the maximum
amount. On the other hand, the particles that were
moving a little slower would arrive when the electric field was bigger. So these particles would get pushed harder
and they’d catch up with the faster ones. So far so good. The problem arises when we talk about the
really slow particles…the ones that arrive after the electric field is a maximum. These slow moving particles encounter a weaker
electric field and get pushed less. So they can never catch up with the faster
guys. And the consequence is that they fall farther
and farther behind and the bunch blows apart. However, if we arrange the timing so the middle
part of the bunch isn’t at the maximum of the electric field, we can use the exact same
phenomenon to make stable bunches. The faster particles encounter the weakest
field and get a small push. The slowest particles encounter the strongest
field and get the biggest push and, voila, stable bunches. So that’s your eleven. It’s a counterintuitive part of accelerator
design, but just like surfers have to find the sweet spot on a wave, so do particles
in real particle accelerators. So there you have it. Now, if you’ll excuse me, I have to get
back to the lab. Surf’s up.


  1. Sahin Kupusoglu

    December 26, 2016 at 11:26 pm

    Dr. Don Lincoln rocks! 11!!!

  2. Jeff Orford

    December 27, 2016 at 6:46 pm

    Thanks so much for this, I found this vid to be really illustrative and enlightening for me.

  3. Linx Domgátic

    January 6, 2017 at 9:17 am

    The only part that gave me hope was when these guy said that the cavities "prefer". I hope they leave old and stupid dogmas like fields… gl

  4. John Christian

    January 6, 2017 at 10:51 am

    Nice video guys, you should do more content!

  5. Franky Jones

    January 7, 2017 at 4:26 am

    Good ole surf on Michigan lake in december!

  6. Marcin Lysakowski

    January 7, 2017 at 8:17 am

    you are awesome! Thank you for the great video 🙂

  7. Aazu

    January 8, 2017 at 9:37 am

    Nice surfing man, great video!
    Thanks, keep em coming.

  8. cosmosgato

    January 20, 2017 at 2:15 am

    No one make physics more accessible then Dr. Don Lincoln.
    This guy is one of the greatest teacher ever.

  9. ' fizicks

    January 27, 2017 at 8:56 pm

    Another great video from Dr. Don Gotta love the Spinal Tap shout out lol

  10. TheyCallMeNewb

    February 5, 2017 at 3:28 am

    Whoa! That must be extraordinarily, exasperatingly, interminably challenging; finding the right field timing. Surf's up.

  11. Sam tron

    February 12, 2017 at 5:05 pm

    just blows my mind that a bunch of wankers got together and worked this all out. It's unbelievable. The thousands of minds and hours that went into getting the large accelerators to work….and the fact that they do…is awesome.

    Thanks so much for your time Don. You explained it so well to the layman.

  12. Douglas Dorman

    February 21, 2017 at 3:59 am

    thank you for the great work you are all doing,Doug.

  13. mattmilladb8

    April 10, 2017 at 12:06 pm

    Should talk a bit about the how the uncertainty principle comes into play when you need to time the RF field in just the right way so that particles are in a well defined position (but consequently ill-defined velocity) at the top of the RF 'arch' so to speak. Good video, thanks!

  14. YCCCm7

    April 26, 2017 at 12:32 am

    Why not just make 10 louder?

  15. Dave B

    September 30, 2017 at 10:56 pm

    11 exists in professional sports too. In an interview with the new england patriot's running back on how he scored the super bowl winning touchdown he explains that he gave it 110%. The next year when they lost the superbowl the same player gave reasons why they lost but he never said he gave it 110%. I actually made up that story because I couldn't name any of the players but watch any sports interview ever and the winning team/player will always say how they gave it 110% but only when they won. Whats the real reason? If there is one the players certainly don't know.

  16. guitarans

    October 17, 2017 at 8:47 pm

    Awesome videos… Thanks.. whats the name og the song at the end?

  17. Reevee

    December 17, 2017 at 4:27 am

    Would a spark really form in a vacuum?

  18. Robert Lunsford

    January 17, 2018 at 5:13 am

    Very similar to how our accelerators work for radiation therapy.

  19. jeff medvin

    January 28, 2018 at 2:55 pm

    I'm looking for the video that explains how a torrent of photons create the appearance of a coherent wave.

  20. zaiks0105

    April 12, 2018 at 4:44 am

    I still don't get it … at least I am honest 😉

  21. Görkem Seven Vids

    July 18, 2018 at 6:10 am

    Surfing on em wave huh? Pretty sure its cooler than water surfers

  22. EclipZe Muzik

    July 18, 2018 at 7:25 am

    wonderful work!!

  23. Ahmed Omar

    July 25, 2018 at 5:29 pm


  24. PartVIII

    August 21, 2018 at 5:05 am

    So cheesy. So informative. I can't get enough Dr. Don

  25. Indra di Lab

    August 23, 2018 at 1:08 am

    The explanations very helpful to tell non scientist friends of mine. Thank you !!

  26. Antony Palmer

    September 1, 2018 at 9:22 pm

    Thank you for such a clear explanation. I work on a particle accelerator and your explanation will really help me explain to others how our system works.

  27. John Edwards

    September 12, 2018 at 8:28 pm

    A big coupled-cavity travelling wave tube. Like a radar amplifier valve except absolutely gigantic.

  28. Alamgir Kabir

    September 30, 2018 at 11:01 pm

    Good job Dr L

  29. Võ Thái Sơn

    October 2, 2018 at 4:50 pm

    "We call these groups of particle .. a bunch" 🤔

    Meanwhile Apple calls its LCD "Liquid Retina".

  30. RME76048

    December 13, 2018 at 5:16 am

    So (budget and space permitting), could a number of accelerators be shifted out of phase relative to each other such that when you wish to have the particles strike a target, they would be combined into a continuous beam as opposed to a single accelerator providing bunches of particles with gaps between?

  31. Rick B

    January 28, 2019 at 9:55 pm

    it's dangerous to surf into an oncoming wave… you become the fixed target.

  32. Rick B

    January 28, 2019 at 9:55 pm

    it's dangerous to surf into an oncoming wave… you become the fixed target.

  33. Mezzo Edbey

    February 3, 2019 at 4:54 am

    As an Electrical and Electronics engineering student, I'm very fascinated by your videos. Well done Fermilab especially yo Dr. Don Lincoln.

  34. Kevin g

    February 20, 2019 at 12:41 pm

    why not direct current capacitor discharging?

  35. betaneptune

    March 3, 2019 at 7:22 am

    How can a particle bunch have not a single charge in it? Aren't we accelerating charged particles? How would you even accelerate a neutral particle?

  36. PrivateSi

    March 11, 2019 at 1:41 pm

    Sounds like the world's messiest experiment using the worlds most precise equipment and understanding of physics.. They smash so many particles together so quickly at such close range due to this bunching I'm not convinced of (all of) the experiment results.

  37. Brian Lee

    April 5, 2019 at 6:52 am


  38. japhet ozogbuda

    April 16, 2019 at 2:59 pm

    does this mean that radio signals can be made stronger using particle accelerator?

  39. Jp Ruz

    August 2, 2019 at 2:41 am

    I have a question, and I hate to ask it here but I can't seem to find the answer. I understand how standing EM waves are created inside the cavity and this is the oscillating electric field that accelerates particles. What I don't understand is the geometry of how these standing waves are created in such a way that the E field points in the direction of the particle's motion. I imagine the standing waves being created in the longitudinal axis (along the length of the cavities), but in this case, the E field would be oscillating vertically and not horizontally. Can somebody please explain? Thanks!

  40. betaneptune

    September 9, 2019 at 2:56 am

    Did I hear you right? Bunches don't contain a single charge particle? If there's no charge, how can an electric field accelerate it?

  41. betaneptune

    September 9, 2019 at 2:56 am

    Why are you suring into the big wave? You should be riding it, moving in the same direction as it.

  42. Nigel cockburn

    September 9, 2019 at 12:45 pm

    So the problem for future accelerators is their diameter or length depending upon the design of circular or linear acceleration type and the law governing where it can be built under peoples properties etc.
    Where to build it.
    Ahhhh, so thats why CERN are so interested in Elons Boring company.
    If 1 or more accelerators get designed to be larger than existing ones then the measurable deformations of planet Earth will become an ever more impinging factor – moving tectonic plates, tidal, magnetic non-liniarity etc.
    May-be building the accelerator underground has a few advantages in respect of screening rf & other forms of radiation but has anybody considered building a new "SuperMassive" and more importantly EXTENDABLE Linear accelerator in space?
    Space would appear to be the ultimate place for such a project as it has effectively no size constraints and less vibrations and less EM interference from mankind etc.

    The space station works, satellites work people are venturing for longer terms into space and soon the moon so why not build a linear accelerator in space and each time its technical capabilities are scientifically exhausted, simply bolt on another accelerator or 5 just before the colider and theres the next generation LHC done with minimal financial or time or management costs.
    To add finance to the equation, I wonder if Elon could launch the hardware for many times less cost than boring the tunnel?
    The accelerator needs high power but of course such an accelerator could be positioned to have 24 / 7 / 365 sun charging a wall of batteries – looks like a couple more visits to speak with Elon are needed by CERN staffs.
    What would the technical problems with a space based accelerator / cpllider be and what technical advantages could a terrestrial tunnelled accelerator have?

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