Accelerating Your Particles: So Where Does My Beam Come From Anyway?

This week's guest blogger is Kellen McGee, 1^st 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.

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.

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.