This week's guest blogger is Kellen McGee,
1st year graduate student in Physics
(specializing in nuclear and accelerator physics) at the National
Superconducting Cyclotron Laboratory/Facility for Rare Isotope Beams at
Michigan State University.
Big particle accelerators and colliders have
long been some of the most visible physics experiments. Many of us going
through graduate school these days remember when CERN, the 27-km around
circular particle collider in Geneva, Switzerland, turned on. This event
brought great excitement over what the new physics experiment was about to
uncover (notably the much anticipated detection of the Higgs boson), and also anxiety
about whether or not it would accidentally create a black hole, This is but the latest example of how various particle acceleration
technologies have, since the turn of the century, been almost entirely
responsible for us having figured out and validated as much as we have about the
laws of the physical world at the subatomic level.
Physicists, both theorists and experimentalists, drove the
hunt for the particles and laws of the Standard Model: physicists’ current, and
beautifully proven, understanding of the fundamental particles and the three
fundamental forces. These forces are the the weak, the strong, and the
electromagnetic forces. Gravity is, alas, not included; if you’d like to try to
incorporate gravity into the Standard Model, there’s at least a PhD in it for
you, if not a Nobel Prize. The particles of the Standard Model are all the
known fundamental particles that can’t be made up of each other–the various
flavors of quarks, gluons, leptons and neutrinos that make up all other particles
we know of.
However, quite apart from knowing the physics that told you
about the particles, and how they would behave, or be identified in detectors,
you also need physicists who know how to speed up the particles to the energies
you would need to collide or smash them into detectors to learn about the
particles’ insides, be they protons, electrons, neutrons, or even atomic
nuclei.
How does this work?
The history of particle accelerating devices is a long and
interesting one. You might not know that you have a few particle accelerators
in your very own house. If you have an older-model television or computer, for
example, (the kind that isn’t flat), the screen is illuminated by something
called a cathode-ray tube (CRT). This is essentially an electron accelerator,
using a voltage source to accelerate electrons that then zoom off and hit a
phosphorescent screen, causing it to glow wherever electrons hit. This is an
example of the simplest type of what’s called an “electrostatic” accelerator-
two plates, one negatively charged, the other one positively charged, cause
electrons to fly off of one, accelerate in the electric field, and land on the
other plate.
This type of accelerator works because electrons are charged
particles, and will get pushed (“kicked” in accelerator jargon) by an electric
field. Now, it’s cool to zoom electrons around, but there’s only so much you
can do with them because they’re light, and to get them going fast enough to smash into things and expect any
interesting particles to get created (or other interesting physics effects) you
have to give them more energy than we really know how to, (really long
accelerators being really expensive and energy-hungry). Fortunately for us,
other particles have charges. Protons, for example, have positive charge, and
were smashed into each other at CERN (technically, it was a proton-antiproton
collision with the antiproton having negative charge) for their famous Higgs experiments.
While there historically has been fame in colliders and
particle accelerators aiming for higher and higher energies, Nuclear
physicists, people who are interested in the structure and behavior of nuclei,
their interrelationship in the periodic table of elements, their relative
stabilities (how easily their clusters of protons and neutrons fall apart) and
other properties, have been turning to accelerator experiments as well. Accelerators,
either linear or circular (cyclotrons) are also used in the creation of medical
isotopes for cancer treatments. Your local hospital might just have one of
these in their basement, and be looking for newer and more efficient ways of
making these medically critical materials.
Nuclei that are missing electrons have a net positive
charge, and thus can be kicked along an electric field just like the electrons
in a CRT, or the protons at CERN. By accelerating whole nuclei, and smashing
them into carefully engineered targets, nuclear physicists can start addressing
some of the questions above. However, after you get beyond electrons at slow
speeds, acceleration becomes much harder than just setting up two plates and
putting one at positive and the other at negative voltage. The only way to make
those electrons go faster in that setup would be to increase the voltage, which
will always, eventually, lead to an electric discharge (boring name for shock!)
before you get your electrons up to interesting speeds.
If you can’t make one “kick” really strong, your next option
is to line up a series of kicks, that each increase the speed of the charged
particle by a certain amount. Imagine being a whitewater rafter, rafting down a
series of waterfalls- this is similar to what happens if a charged particle
travels across a series of kicks- it gets more and more energy, regardless of
how fast it’s going (before relativity starts kicking in, of course!).
We want to build a nuclear science accelerator to
accelerate ions to interesting energies, about 65% the speed of light. CERN, a
high-energy physics facility aims for 99.999…% the speed of light, for
comparison. To do this, we have to figure out how to line up enough kicks to accelerate the particles to the speed we want to study. This is exactly the problem currently being
tackled by FRIB, the Facility for Rare Isotope Beams at Michigan State University.
In the picture below, you see a plan of the FRIB linear
accelerator (“linac”) under construction. Each of the little boxes along the
straight parts make the particles (the nuclei) go faster by a certain amount. The
little boxes are called cryomodules and in real life are taller than a person
and several meters long. These house the true engines of the accelerator- the
structures that allow us to set up and maintain many, extremely strong (2-5
megavolt-per-meter) electric fields that kick the nuclei along.
The best way to set up these electric fields is still very
much a field of open development. For FRIB’s application, making a number of
different kinds of ions go fast, FRIB decided to use pure niobium
superconducting RF (radio frequency) cavities. This is a mouthful. Let’s break
those words down, a little out of order.
RF: radio frequency. An oscillating electric and magnetic
field. It would at first seem counterintuitive that we would be using RF, since
“oscillating” means the direction the fields are pointing changes by 180
degrees every half period. Physically this means if we used RF at 650 megahertz
(650 million cycles per second) to make an electric field in one direction,
half the time the electric field is pointing in the other direction
(backwards). This problem is controlled by making sure that the particles are
timed to be in the electric field when it’s pointing forwards and out of the
electric field when it is pointing backwards.
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Cavities: this RF has to live somewhere. Cavities are metal,
cylindrical objects with special geometric properties that you
can stick an
antenna inside of and pump RF inside.
Image copyright FRIB, P. Ostroumov, SRF Group et. al.
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This is a model of a
five-cell prototype cavity for FRIB. The upper half shows the distribution of
the magnetic field (strongest in the red regions) and the bottom photo shows
the distribution of the electric field. The series of kicks we see now are the
series of spaces that are the orange-yellow color. The isotope we want to accelerate
enters the tube, then sees five electric fields that make it go faster and
faster. The gaps (blue spaces on the axis of the pipe) are where the particle
is traveling during the time that the electric field in the orange region is
pointed backwards.
Superconducting Niobium: The rainbow pictures, again, are
depictions of what happens to the electric and magnetic fields when an antenna
is put into the cavity and RF is piped in at certain frequencies. The RF waving
along the walls of the cavity can generate resistance that has to be dissipated
as heat, and also constitutes a power drain on the RF, causing less energy to
go into moving the particle forward, and more energy to go into heating the cavity
walls. Ideally, we want a low-resistance material. Fortunately, Niobium, an
element, is a relatively workable metal and goes superconducting at 9.6 degrees
above absolute zero. Thus FRIB and many similar facilities have chosen to
engineer their superconducting cavities from either pure niobium, or some
variety of niobium compound, and cool these in the cryomodules to
superconducting temperatures using liquid nitrogen and liquid helium.
Though the above is only the briefest description, it shows
how accelerators demand a variety of specialists- you can receive a PhD in any
number of accelerator-related subfields including cryogenic systems,
superconducting RF, particle beam dynamics, test diagnostic equipment design
and implementation, controls programming…the list goes on and on. The simple
task of accelerating particles, or nuclei, for science is thus, itself, a
science.
