If you’re a fan of my videos, you know that I’ve been on an accelerator kick for the
last few.
And if you’re not a fan, then why the heck not?
You should take a look, ‘cause, well- accelerators are very cool.
Now there are lots of different shapes of particle accelerators, but when we’re talking
about cutting edge research, there are two main ones: one of them is circular and one
of them is a straight line.
So I thought I’d tell you about the pros and cons of the two different choices.
But first, to understand the differences, you need to know how particle accelerators
work.
The principle is actually really simple.
You use an electric field to push a particle with electrical charge and that particle goes
faster.
Now the concepts of electric charge and electric fields aren’t the most common ideas, so
let me use a more familiar situation to help me make my point.
Now you all know about gravity and how it works.
There is a gravity field that points downward.
That gravity fields interacts with objects that have mass and those objects get pushed
downwards.
That, by the way, is a techno-geek way to say that things fall when you drop them.
But now imagine if you were to change the direction of gravity.
We’re going to cheat a little bit and pretend that there is a cartoon kind of gravity that
is a little different than ordinary gravity.
I’m going to hold this cartoon ball and see what happens if we have cartoon gravity
point to my left.
Maestro- can you point cartoon gravity to the left and give me a cartoon ball?
Super.
Now, if I let the ball go, what happens probably won’t surprise you.
The ball falls to the left.
If cartoon gravity points in some other direction, then the ball will fall in that direction.
Here, let me show you.
Maestro, can you pick a direction?
Now, I’m going to cover my eyes.
Don’t tell me what it is- surprise me.
Now materialize a ball in my hand.
When I let it go, let’s see what it does.
So it went that way.
So that means that cartoon gravity was pointing that way.
Now, cartoon gravity isn’t a real thing.
We can’t arbitrarily point gravity in any old direction.
However we can do that with electric fields.
Let me show you how.
The easiest way to make a simple electric field is to get two metal plates like these
here.
If we connect the two plates to the opposite sides of a battery, it will set up an electric
field between the plates.
We won’t make that connection just yet.
Now we also need an electrically charged object.
Okay, so now we’ll connect the wires from the opposite sides of the battery to the plates
and let’s see what happens.
So we connect the one wire.
Now focus on the ball.
I’ll do a countdown on what happens as we connect the other wire: 5, 4, 3, 2, 1, connect.
Voila!
The electric field makes the electric charge combine to make a force.
Okay, so that’s the electric field and charge.
What about the circle vs. line thing?
Suppose we wanted to use the very strongest electric fields possible and wanted to make
a beam of particles with the energy of the Large Hadron Collider, or LHC, which is the
highest energy particle accelerator in the world.
What would a straight accelerator look like?
Well, a strong, but technically feasible electric field might be about 25 million volts per
meter.
Since the LHC has a design energy of 7 trillion electron volts, that would take an accelerator
about 280 kilometers or about 170 miles.
That’s about from New York City to Providence, Rhode Island.
So, building an accelerator that long would be hugely expensive.
So that’s where circular accelerators come in.
Circular accelerators have some elements common to straight ones and some new features.
To begin with, a circular accelerator doesn’t have accelerators all around the circle.
Instead, what they have is a relatively short length of accelerator and the rest of the
circle are a series of magnets.
When a charged particle moves through a magnetic field, it moves through a circular path.
In the case of the LHC, the whole accelerator is 27 kilometers around, or a smidge over
16 miles, but the portion of the circle that is an accelerator like we’ve talked about
so far is just over two meters or six and a half feet.
Over those few meters, the electric field is effectively 8 million volts per meter,
although if you look it up, you’ll find different numbers for technical reasons.
The outcome of this design means that when a proton, which is the particle accelerated
by the LHC, goes through the acceleration region, its energy goes up by 16 million electron
volts.
With each orbit, the beam gets more and more energy, cycle after cycle.
It’s like a child’s merry go round, in which an adult throws the same handle over
and over again as it spins.
The merry go round goes faster and faster.
Of course, at the energies involved, the beam is already moving near the speed of light,
so the beam doesn’t actually speed up.
But it does gain energy and the basic analogy is sound.
So let’s think this through.
To get the 7 trillion electron volt design energy of the LHC, the beam would have to
go around the circle about 410,000 times.
Given that the LHC beam moves at basically the speed of light, it goes around the circle
about eleven thousand times a second.
So that means the LHC could, at least in principle, accelerate a beam to design energy in 36 seconds.
Now it actually takes longer than that, but that’s because as the energy of the beam
increases, the strength of the magnets also have to increase to make the beam travel in
a circle of constant radius.
On the other hand, a circular accelerator is harder.
In the case of the LHC, the beam goes around eleven thousand times a second and the beam
can circulate in the machine for twelvish hours.
That means everything has to work to incredible precision to keep the beam inside the accelerator.
So that’s one minus for a circular accelerator.
There’s another consideration about linear vs. circular accelerators.
When you force an electrically charged particle to move in a circle, it gives off radiation,
which makes it lose energy.
This radiation is not the same for all kinds of particles.
Very massive ones such as protons lose energy much more slowly than light ones, such as
the electron.
Thus it is much harder to make a high energy circular electron accelerator than a proton
one.
To hang some numbers on that statement, the tunnel that holds the LHC proton accelerator
used to hold the LEP electron accelerator.
The LHC can accelerate protons to over 150 times the energy that the LEP accelerator
accelerated electrons.
This radiation that electrons emit when moving in a circle is why scientists are thinking
about a linear accelerator for the next big electron collider.
This is called the International Linear Collider or ILC and it’s in the design stages.
There is no guarantee that it will ever be built.
So these are the big differences between a linear accelerator and a circular one.
A linear one is simpler, but it takes a very long one to reach very high energies.
A circular accelerator reuses a short linear accelerator over and over again, but it requires
a lot of precision.
And linear accelerators are potentially better if you want to accelerate high energy electrons.
Which you choose to build depends on your research needs and both techniques can be
used to help scientists to better understand
the world around us.