The Superconducting Super Collider

I was employed by University Research Association during the early 1990's. URA was the primary contractor for the U.S. Department of Energy's Superconducting Super Collider project. I worked for the Instrumentation Group of the Accelerator Systems Division. The groups job was to design and build the electronics that would be the "eyes and ears" of the particle accelerators that made up the labortory. SSC was a neat place, filled with neat and very intelligent folks. It's mission was a cool one. The purpose of this section is to give you an idea of what the project was all about.

The Superconducting Super Collider Laboratory (SSCL or SSC for short), what an odd name. Let me start by explaining the name. Superconducting refers to the technology used to produce the enormously strong magnets needed to build a particle accelerator capable of producing the energy levels of interest to high energy particle physicists. The superconducting magnets simply made up a huge "racetrack" for protons to travel around as they gain energy.

Super, refers to the size of the labs main accelerator known as the Collider. The Collider was an oval shaped accelerator the circumference of which was 54 miles. The Collider oval was located about 200+ ft underground and surrounded the city of Waxahachie, Texas. Click to see a map.

Collider refers to the main purpose of the laboratory, colliding protons into each other so scientists and physicists could study the resulting shower of smaller particles. Click here to see a computer representation of a collision event.

Laboratory, of course, refers to what the project was all about, experimentation that would lead to understanding the fundamental constituents of matter.

Much of what follows comes from the booklet, To the Heart of Matter - The Superconducting Super Collider. This booklet was produced by the laboratory during the early 1990’s. A bibliography follows that contains other references. I happened across the booklet that preceded this one during a visit to the Irving Public Library in 1991. I was fascinated by the project. A few months later I was employed by the laboratory. Even today, after the project is dead and gone I am still impressed by its mission and feel that telling it’s story may be of interest to others.

Human curiosity about nature has surely been a major force in the progress of our civilization. History has been shaped by the irresistible urge to find out how the world works, and how we can use that knowledge to better our lives. Among the most basic questions that have always been asked are: What is the ultimate structure of matter? What are the forces by which matter interacts? How did the universe begin? Will it ever end?

These questions particularly the first two, but increasingly the last two as well fall within the domain of high energy physics upon which our knowledge of the other sciences ultimately rests. As biology and medicine are founded in chemistry, and chemistry in physics, so physics is founded in the study of the elementary particles and the forces that govern their behavior. Although the tree of knowledge will continue to be explored fruitfully at many levels, it is only by uncovering its deepest roots that we can fully comprehend the branches above.

Particle physics is perhaps the most fundamental of sciences, but astronomy is the oldest. Throughout history, nothing has so stirred our imaginations, scientific and poetic alike, as contemplation of the heavens. Until very recently, it could scarcely have been foreseen that some of the age-old mysteries of astronomy might be solved by looking in the opposite direction, into the world of elementary particles. Yet this is precisely what is now happening. Particle physicists in company with nuclear and atomic physicists, astrophysicists and cosmologists, are beginning to understand not simply what matter is, but where it came from, and when, and how.

Finding answers to such questions is important because everything else we want to know more about, space and time, energy and entrophy, life and death, the rocks of Earth and the fire of the stars is bound up with them. As our knowledge advances, so must the power and precision of the scientific instruments upon which further advances will crucially depend.

Imagine two rings of metal pipes, eighty seven kilometers (fifty four miles) in circumference, running through a concrete tunnel several meters below ground. The pipes themselves, separated vertically by seventy centimeters (about two feet), are only a few centimeters in diameter. They are under high vacuum and encased in powerful electromagnets held at an ultra low temperature.

Inside the two pipes, narrow beams of protons whirl around the tunnel in opposite directions at nearly the speed of light. The particles in these beams have been accelerated to an energy of twenty trillion electron volts. This is a huge energy for a single particle to carry: particles emitted by radioactive minerals reach energies less than one millionth as great.

At a few special points around the ring, in cavernous underground experimental halls, the beams are made to intersect. Although most of the protons simply pass by each other, there are so many protons in the beams that head on collisions occur a hundred million times every second. In each collision, energy of motion is turned to enormous heat in a tiny fireball.

Click here to see a proposed layout of the SSC

From within this minute cataclysm, a shower of subnuclear particles among them, perhaps, a new and exotic one speeds fleetingly outwards. Sophisticated electronic detectors catch these evanescent particles, recording their speeds, directions, and types; and physicists around the world analyze these records for clues to the innermost nature of matter and the forces that hold it together

This is (would have been) the Superconducting Super Collider, the world’s premier particle accelerator. It will boost its protons to energies twenty times higher than ever before, enabling physicists to search out the elementary constituents of matter with unprecedented precision, and to explore distances one thousandth the diameter of a proton. For physicists , the SSC will be a microscope of unparalleled power.

To probe the inner structure of smaller and smaller objects, physicists must use probes of increasingly high energy. This century’s great advances in accelerator technology have allowed fundamental science to make tremendous strides. Each new machine has revealed new particles and deepened our understanding of the universe. The SSC, accelerating protons to previously unreached energies, continues this quest.

The Proton is the tiny nucleus of the hydrogen atom. In size, it is to a mosquito as a mosquito is to Mercury’s orbit around the sun. Though a twenty TeV (trillion electron volt) proton has about the same energy as a mosquito in flight, that energy is squeezed into a much smaller volume. It is this density of energy, not the energy as such, that makes the collision of protons so incisive a probe.

Because protons carry electric charge, they can be influenced by electric and magnetic fields and by electromagnetic radiation such as radio waves. In the SSC, the protons are confined to their oval bath by strong magnetic fields, and on each of their many millions of circuits around the ring, bursts of radio waves give them a carefully timed boost.

In principle, a single large ring could accelerate protons from a standing start to their final velocity near the speed of light. In practice, it is far more efficient to use a cascade of accelerators, each designed to cover a particular range of energy. In the SSC, protons will first pass through a linear accelerator, and from there will enter a series of three booster rings. Each of these will take in protons at one energy and feed them at a higher energy to the next stage. Finally, the main ring will receive protons at 2 TeV and push them to 20 TeV to await collision.

Ten thousand powerful electromagnets will keep the proton beams tightly focused on their oval track. The SSC will use superconducting magnets, in whose coils electric current will flow unhindered and without the power loss that would make the operation of conventional magnets prohibitively expensive.

In 1983, came proof that superconductivity, long a laboratory curiosity, could be successfully used in a large accelerator. After years of research and development, superconducting magnets were installed in the main ring of the Tevatron, the proton synchroton at the Fermi National Accelerator Laboratory (Fermilab), near Chicago. The outstanding performance of the Tevatron assures us that this new technology, at the frontier of our experience, can be used with confidence in the search for new fundamental physics

Accelerators are to particle physics what telescopes are to astronomy, or microscopes are to biology. These instruments all reveal and illuminate worlds that would otherwise remain hidden from our view. They are the indispensable tools of scientific progress.

The earliest accelerators were simple vacuum tubes in which electrons were given a kick in energy by the voltage difference between two oppositely charged electrodes. From these evolved the Cockroft-Walton and van de Graaff machines, larger and more elaborate, but using the same principle. The modern example of this type of device is the linear accelerator, a sophisticated machine used in many scientific and medical applications. All such straight-line accelerators suffer from the disadvantage that the finite length of flight path limits the particle energies that can be achieved.

The great breakthrough in accelerator technology came in 1920 with Ernest O. Lawrence’s invention of the cyclotron. In the cyclotron, magnets guide the particles along a spiral path, allowing a single electric field to apply many cycles of acceleration. Soon unprecedented energies were achieved, and the steady improvement of Lawrence’s simple machine has led to today’s proton synchrotrons, whose endless circular flight paths allow protons to gain huge energies by passing millions of times through the electric fields that accelerate them.

Until twenty-five years ago, all accelerators were so-called fixed-target machines, in which the speeding particle beam was made to hit a stationary target of some chosen substance. But early in the 1960’s physicists had gained enough experience in accelerator technology to be able to build colliders, in which two carefully controlled beams are made to collide with each other at a chosen point. Several colliders exist around the world today, and the technology for them is by now well established.

Colliders are more demanding to build, but the effort pays off handsomely. In a fixed-target machine, most of the projectile particle is locked up, after impact on the target, in continued forward motion of the debris. In a collider, on the other hand, two particles of equal energy coming together have no net motion, and collision makes all their energy available for new reactions and the creation of new particles.

A look inside Fermilab's tunnel

To the physicist, the idea of a huge particle accelerator as a kind of microscope is reasonable enough: both devices reveal things invisible to the unaided senses. But to the non-scientist, the analogy may be puzzling. Most of us have looked down a microscope, perhaps seeing grains of pollen or the cells of a living organism. How do such pictures compare with the mysterious tracks of unknown particles that are the images we get from a particle accelerator?

We can see objects in the world around us because light bounces off them and enters our eyes. The rays of light create an image on the retina and stimulate nerve cells, sending electrical impulses to the brain. There, those signals are analyzed to construct a mental image of the original object The microscope increases our power of sight by magnifying a visual image through glass lenses that, in effect, enhance the acuity and light gathering ability of our organic lenses in our eyes. But still it is our minds that do the analysis, creating a mental image of the object from its pattern of scattered light.

The electron microscope uses magnetic instead of optical lenses to direct a beam of tiny particles, electrons, onto the object to be studied. The scattered electrons are then focused by another set of magnetic lenses onto a photo phorescent screen, as in a television set, and we see an image multiplied many thousands of times. The electron microscope is clearly a cousin to the conventional kind, but in its use of particles instead of light it also has something in common with the accelerators of high energy physics.

In 1911, Hans Geiger and Ernest Marsden performed an experiment that was the foindation of modern particle physics. At the urging of the great physicist Ernest Rutherford, they used a radioactive source to shoot alpha particles at a wafer thin gold foil, and detected the scattered alphas by watching them with a phosphorescent screen. At the time alpha particles were known to be related to helium atoms, and to be much heavier than electrons, but the nature of the atom was the subject of speculation. J. J. Thompson, the discoverer of the electron, believed that the negatively charged electrons in an atom were embedded, in an unknown way, in a cloud of counter balancing positive charge.

By studying, how the alpha particles scattered off the gold foil, scientists hoped to learn something of the nature of the gold atoms. According to Thomson’s model, the alpha particles should pass through with only small changes of direction, because neither the electrons, which were too light, nor the positive charge, which was too diffusely distributed, could exert enough force on the alphas to knock them noticeably off-course. But what Geiger and Marsden found instead was that some of the alpha particles were deflected through large angles, and a few actually reversed direction altogether. It was as if, said Rutherford in a famous remark, "you had fired a fifteen inch shell at a piece of tissue paper and it came back and hit you."

By analyzing the distribution of scattered alpha particles, Rutherford arrived at the modern picture of the atom, in which the electrons orbit a tiny central nucleus, as in a miniature solar system. In the Geiger-Marsden experiment, most of the alphas go straight through one of those empty spaces, but occasionally one will get close enough to a dense, heavy nucleus that electrical repulsion between the two will push the alpha off its path. This revelation of the atom as a tiny mechanical system, consisting of electrons and a nucleus was the beginning of modern atomic physics.

This alpha-particle microscope allowed Rutherford to "see" the atomic nucleus, and the same general principle behind high energy physics experiments today. With machines that accelerate particles to ever higher energies and smash them into different targets, physicists have seen ever smaller objects. Such experiments have revealed protons and neutrons within the atomic nucleus, and then quarks within protons and neutrons. The SSC will look closer yet, perhaps within the quarks themselves.

We have come a long way from the first simple microscopes, and as the worlds we look within have become more distant from the realm of human senses , so our microscopes have become more elaborate. In the SSC, proton beams will serve as light rays, huge but finely built detectors as our eyes, and arrays of computers as our brains. Though it will probe the smallest objects in the universe, the SSC is fundamentally a tool for increasing the power of our sight: it is a microscope.

More to come soon
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