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.