Particle accelerators are a mere slice of the pie of the full range of applications of Faraday's science of electromagnetism in the general economy. We can only guess how much revenue particle accelerators contribute today to the US GDP. They are ubiquitous, found in most hospitals, laboratories, universities, and most high-tech industrial centers. They are a direct spin-off of basic research in the twentieth century of the study of atoms, nuclei, and elementary particles. We suspect that the federal revenues derived from the taxing of these facilities are far greater than those being spent on R&D of new and larger particle accelerators and their technologies. In fact, most of the federal money spent on particle physics is for the research, construction, and operation of accelerators and the physics experiments that use them.
We're going to survey the basic kinds of machines that are used in particle physics today, and that can be construed as the world's most powerful microscopes. Bear in mind our microscope analogy: the accelerator produces the beam that we shine on our target to scatter into the detector, which makes an image for our brain to comprehend. The beam particles must be smaller than the things we wish to observe, i.e., they must have smaller quantum wavelengths, and this demands high-energy particle accelerators to produce the beam. The collisions in “colliders” occur among two opposing beam particles themselves. The eyepieces of the microscope are large detectors, as large as a mansion, which are literally wrapped around the point of collision. Rather than one guy in a white coat staring through an eyepiece at a bacterium, high-energy particle experiments are a legion of physicists staring into computer screens and tweaking computer code. But it's all still basic microscopy.
LINACS
With the electron microscope we encountered our first “linear particle accelerator,” often called a linac. A high-energy linac accelerates charged subatomic particles, such as electrons or ions (ionized atoms) in a straight line.2
The energy of a particle determines its quantum wavelength, which we want to make small. To shrink by half the wavelength of a particle traveling near the speed of light, we need to double its energy. To do so requires either a longer linac or a stronger accelerating field. But it is impossible to make arbitrarily strong electric fields. A really strong electric field will yank electrons out of the materials that produce the electric field, such as copper plates. This produces a flow of electric charge that neutralizes the electric field. Since our particle accelerators already use the largest electric fields we can possibly make, to double the beam energy of a linear accelerator we therefore need to expose the particle to the same electric field for twice the distance, that is, we need to double the physical length of a linear accelerator to double its energy.3
We measure electric fields in terms of volts per meter. An electric field always exists in the air on a warm, humid day, pointing upward to the sky, typically of about 100 volts per meter. You don't even notice this field, but when a big thundercloud rolls by, the field (e.g., particularly around metallic, pointed objects) can become many thousands of volts per meter. Electrons are then ripped out of materials. This is, in fact, the principle of a lightning rod—it is designed to allow electrons to drain out of a metal point because this tends to neutralize the surrounding electric field and thus avoids a lightning strike. If the electric field should fluctuate up to hundreds of thousands of volts per meter near an object, then enough air molecules can become ionized to create an electrically conducting channel—and then you get a spectacular lightning bolt.
In modern particle accelerators the accelerating electric fields are almost always generated in copper or alloy superconducting radio frequency (RF) “cavities.”4 These cavities are to electric fields essentially what a bell or a guitar string is to sound. We can fill the cavity with a rapidly oscillating electric field, which is like the sound of a ringing a bell or the plucking of a guitar string (the cavity-filling fields are generated from sophisticated devices, which are themselves mini-accelerators, called “klystrons”). The electric field then vibrates, or resonates, within the cavity. The walls of the cavity simply contain the electric field and offer essentially zero electrical resistance, which would otherwise damp out the field (like putting your finger on a vibrating bell or guitar string). The wavelength of the resonating electromagnetic field is then typically twice the size of the cavity, like the wavelength of the sound emanating from a vibrating guitar string.
If an electrically charged particle enters the filled RF cavity at just the right moment in the ringing cycle, it will experience an electric field that accelerates it through the cavity. We say that the charged particle is “in phase with the electric field.” The particle is then pulled along by the field and will absorb kinetic energy from the electromagnetic field in the cavity (“out of phase,” and the particle would be pushed back). Often people make the analogy to a surfer catching a wave. This analogy is picturesque, but there's a big difference. In our case the electron acquires more and more (kinetic) energy as it experiences the pulling of the electric field, while the surfer, once in motion, rides along at the peak of a water wave at an approximately constant kinetic energy.
The highest useful electric fields achieved in microwave cavities are about 30 million volts per meter. At still higher fields, the physical limits are reached, and copper becomes damaged by the ripping out of electrons, or the superconducting state of the material is destroyed. This presents a physical limit on electric fields that we can achieve in the lab or use in a device.
To build a high-energy linac we can use the clever idea attributed to Leó Szilárd and patented in 1928 by Rolf Widerøe, who built the first linac. We simply string a large number of RF cavities together in a line.5 We arrange for these to resonate so that a charged particle passing through the sequence of cavities always “feels” an electric field that pulls it along the way. We inject particles, usually from a simpler smaller accelerator that serves as an injection device, into the first cavity. After getting a kick of energy from the first cavity, the particle then enters the next one to receive another kick, and so on down the line. In this way, through a sequence of energy kicks, we can create a very high-energy beam of charged particles.
A linac can be configured to accelerate different kinds of particles, which may be electrons, protons, ions (heavy atoms with a net electric charge), or even unstable elementary particles such as muons. The only requirement is that the particles we are accelerating must have electric charge so they “feel” the electric fields. If we desire high-energy neutral (uncharged particles) particles, like neutrons or neutrinos or neutral π0's, we can make them by colliding our high-energy charged particle beam into a block of material, like aluminum, tantalum, lead, or even uranium or a spray of liquid mercury. The collision of our primary beam particles with the atoms within these materials will produce these various other types of particles.
Linacs range in size from the cathode-ray tube in an old TV picture tube, to a meter-long electron microscope, to the 100-meter-long proton linac at Fermilab, way up to the 2-mile-long electron linac at the SLAC National Accelerator Laboratory in Menlo Park, California. Like their relative, the electron microscope, linacs also have many practical applications, such as generating X-rays and gamma rays and high-energy electrons for many material science studies and medical radiation therapy.
Today, the 2-mile-long accelerator at SLAC has been retired from particle physics, yet it continues to serve science as a Linac Coherent Light Source (LCLS). The LCLS uses part of the former linear accelerator and is the world's first “X-ray free-electron laser.” It produces pulses of X-rays each only a billionth of a second long yet a billion times brighter than any other X-ray source. This can be used to take instantaneous pictures of atoms and molecules in motion, such as in a chemical reaction, perhaps in a living cell. This provides crucial information on fundamental processes of chemistry, biochemistry, and technology.6
Linacs at Fermilab and CERN accelerate protons (or the H– ion) and are used as particle injec
tors for the higher-energy accelerators. A single RF cavity, about a meter long, can be a stand-alone source of about 30 MeV electrons that can be used for medical and material industrial applications. Linacs, unlike other circular accelerators, are capable of an output of many, many particles, i.e., a very high-beam current or intensity, producing a nearly continuous stream of particles. The potentially high intensity makes the proton linac an ideal accelerator for studying rare processes—of elementary particles, nuclei, or atoms, where the energy per particle is not as important as the sheer number of accelerated particles.7
Fermilab has plans to build a linac that will be the world's most intense source of protons in the world, called “Project X.” Each proton in Project X will have a mere 8 GeV of energy, (compared to 7 TeV at the LHC; recall that 1TeV = 1,000 GeV), but the machine will have many, many protons and will produce a beam of very high power. We'll have more to say about Project X in chapter 9.
SCRFS: THE NEXT BILLION-DOLLAR THING FOR THE ECONOMY?
After the Superconducting Super Collider (SSC) perished, many US particle physicists set their sails to try to convince the US government to build an enormous linear accelerator that would collide electrons and antielectrons head-on. Despite shockingly high cost estimates and a lukewarm reception by the Department of Energy to undertake such a project, the International Linear Collider (ILC) community has forged on with their dream, creating an official management structure called the “Global Design Effort.”
In 2004, the ILC Global Design Effort made make a monumental decision: The ILC would be based upon the new technology of superconducting radio frequency cavities (SCRFs.) This decision was risky because these cavities had not yet been developed for reliability or to a large-scale manufacturing capability. While this concept had, a priori, a number of technological advantages, it was also fraught with the risk that it required a long-term and expensive R&D effort to learn how to make SCRF cavities. But this was a brave decision that has enormous spin-off potential for the betterment of humanity. The US Department of Energy (DOE) has now invested an order of a half billion dollars on the development of this new technology.
The R&D has been a spectacular success in leading to the development of reliable SCRF cavities. SCRFs are very compact and efficient particle accelerators in terms of the energy they consume. The “wall-power” efficiency, defined as the fraction of energy that ends up in the electron beam compared to the energy consumed from the power source, is many times greater than conventional non-superconducting cavities. To give a sense of required reliability of SCRFs in this R&D program, a 500 GeV ILC requires 18,000 nine-cell RF cavities.8
Whether or not there will ever be an ILC (and there is currently talk of a collaboration between the US and Japan to do so), the potential benefits to humanity of the development of SCRFs are enormous. Though the SCRF accelerates an electron (or proton) beam, the output beam can readily be converted to gamma (high-energy photons) or neutrons. Potential applications include many aspects of medical imaging and medical therapy.
For example, an urgent need exists for the onsite hospital production of the isotope technetium-99 (Tc99). This is the most-often used imaging isotope in medical diagnosis. Tc99 has a half-life of a few hours and must usually be prepared at the hospital site from other radioactive sources, but this presents a challenging and somewhat unreliable supply chain.9 SCRFs are a logical candidate to provide a solution to this problem, which would probably eventually pay back many times over a year the cost of the entire R&D program that led to SCRF development. Electron beams can also be used for the removal of noxious gases in flue gas (chimney smoke) such as SO2 and NO2, surface treatments of materials, novel tunneling techniques, etc. It's “blue sky” for the future of SCRFs, and we expect to see a multi-billion-dollar industry in the not-too-distant future.
And once again, Mr. Gladstone, you will tax it.
MAGNETIC FIELDS MAKE PARTICLES MOVE IN CIRCLES
As we've seen, a charged particle can be accelerated in a line by an electric field. This acceleration imparts energy to the particle and shrinks its quantum wavelength to enable the world's most powerful microscopes, aka particle accelerators. However, like electric fields, magnetic fields also accelerate a charged particle, but only if the particle is already moving and only perpendicular to the direction of motion.10 The net result of this perpendicular acceleration is that the charged particle in a magnetic field will move in a circle, but it will acquire no net increase in energy, i.e., it will only be “deflected” from the original direction of motion.11 Therefore, a magnetic field cannot be used to impart energy to our beam particles and shrink its quantum wavelengths, but it can be used, indeed it is essential, for steering and focusing charged beam particles. Magnetic fields, by deflecting a particle's motion, allow us to create an electromagnetic lens and to make a focusing lens system for a beam of particles. This is precisely what is done in electron microscopes to focus an image.
However, there is another and very important thing we can do: by cleverly using magnetic fields to bend charged beam particles in a circle, we can build a much higher energy particle accelerator than a linac that is more compact in size and therefore usually less expensive. Such circular machines, called cyclotrons and synchrotrons, exploit circular orbits of charged particles moving in magnetic fields to cause the particles to pass many times through the same RF cavity and receive many kicks in energy. The magnetic field simply holds the particle in its circular orbit.
As the beam particle's energy increases, and if the magnetic field is held constant, the particle will stray into a larger and larger radius orbit. To hold the particles in the same orbit, the magnetic field must also increase with the beam energy. This is the principle of a particle accelerator called a synchrotron. As there are limits to the highest electric fields we can create, there are also limits to how large a magnetic field we can create. This translates into the requirement of very large-diameter circular orbits for very high-energy particles. For example, the Tevatron achieved a 1 TeV (one trillion electron volts) beam energy for protons and antiprotons, and was a circular ring about 1 mile in diameter; the LHC is designed to achieve a 7 TeV beam particle energy, uses slightly stronger magnets, but is about 5.3 miles in diameter.
ELECTRIC CURRENTS PRODUCE MAGNETIC FIELDS
As we've seen, electric fields are generated by electric charges and cause electric charges to accelerate. Likewise, magnetic fields are generated by electric currents, and they in turn cause electric currents to deflect in direction or move in a circle.
Moving electric charges are called electric currents. André-Marie Ampère discovered that magnetic fields are produced by electric currents, and that electric currents are affected by magnetic fields.12 Electric charges that are not moving do not produce magnetic fields and are not affected by them. Electric currents in matter, such as in copper or aluminum wire, consist of the most loosely bound electrons moving through the material to produce the current. As far as the electric charge goes, this is an electrically neutral situation—all positive electric charges are sitting at rest, such as the atomic nuclei with most of the electrons remaining attached to the atoms. The looser electrons can be coaxed to move through the material by a battery and become an electric current. The stationary charges associated with the atoms do nothing but keep the material electrically neutral, so that the net electric charge of the material is zero even though it may be carrying a large electric current.
In a simple experiment, two wires with constant electrical currents flowing in them, when placed parallel and near to each other, will be seen to attract or repel each other.13 If the currents are moving in opposite directions, the wires repel each other; if they are moving in the same direction, the wires attract each other. This is a direct observation of the connection of magnetism to electrical currents. The wires are electrically neutral (no net electric charges, hence no electric fields are present), but it is the electric currents that produce the magnetic forces betwe
en the wires. One wire carrying a constant electrical current will also cause a compass needle to deflect.
The ancients, like many people today who are not familiar with magnetism, viewed it as a mysterious, almost magical, property of certain materials, such as iron or “lodestone” (which contains iron). The Chinese were the first to note the existence of magnetism and apply it to build a compass.14
An electron by itself, unattached to an atom, produces its own tiny magnetic field due to its intrinsic spin. In most materials the electrons are paired, in opposite spins and opposite orbital motions, and the magnetic field of one electron is canceled by an opposite magnetic field produced by the other electron in the pair. The atoms in materials such as iron, cobalt, and nickel have unpaired electrons. As a result, though each atom of these elements acts like a very small magnet, if all the magnetic fields of all the atoms are aligned together, the effects can add up to make a large macroscopic magnetic field. In iron, the little individual atomic magnets within the material interact with one another in such a way that they have a tendency to line up in a common direction. This forms a “magnetic domain,” in which clusters of millions of atoms align to produce a common magnetic field. Finally, with a little more coaxing, usually an external applied magnetic field, the domains themselves can be coaxed into alignment, and then you have a powerful magnetic field emanating from a bar of iron. The physics at the atomic level of all of this is quite subtle, complex, and an interesting subject to study—so, yes, in a sense magnetism is a mysterious and special property of certain materials, but it isn't magic.
Beyond the God Particle Page 18