Alternate View Column AV-25
Keywords: superconducting, super collider, high-energy physics, acceleration
Published in the March-1988 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 8/21/87 and is copyrighted © 1987, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.
Modern high energy physics is the study of quarks and leptons and their interactions. The experimental physicists who work in this area have never been troubled by a lack of imagination. In 1954, on the occasion of the 200th anniversary of the founding of Columbia University, Enrico Fermi proposed his vision of the ultimate particle accelerator, a machine that would encircle the entire Earth, a truly world-class machine. Interestingly enough, while no accelerator approaching the scale of Fermi's vision has ever been built, the Tevatron accelerator at FermiLab near Chicago has now reached collision energies which exceed those that Fermi's machine would have produced.
Now the visionaries of the high energy physics community are ready to take the next step, the Superconducting Super Collider or SSC. By now you must have heard or read something about this machine. It is being pushed by President Reagan and the Department of Energy. It will have a total cost in 1987 dollars of $4.4 billion and will be built at a location now in the process of selection. It will be a big machine, the largest proton synchrotron ever planned, "the greatest public works project in the history of mankind", as one congressman put it. The tunnel in which it will be constructed does not circle the Earth. It is more modest, with a circumference of "only" 52 miles. Yet the SSC will produce proton-proton collision energies 40 times higher than those of the FermiLab Tevatron or Fermi's dream machine.
This AV column is an overview of the SSC: what it is, why the high energy physics community wants to build it, and a bit about the political side effects of a scientific program of this magnitude. I do not, by the way, do high energy physics or have any vested interest in the SSC.
We'll start with some questions and answers, including some very basic ones.
Q: What's a particle accelerator?
A: A particle accelerator is a machine that applies electromagnetic forces to particles, usually protons or electrons, to give them a very high velocity and a very large energy of motion or kinetic energy.
Q: What's a synchrotron?
A: A synchrotron is a particular sub-species of particle accelerator. It uses magnetic fields to bend the paths of the particles accelerated so that they travel in vacuum in a large circle or oval, retracing the same path over and over again as the particles are accelerated. Most of the hardware in a synchrotron is devoted not to acceleration but to the dual functions of bending the particles in the desired path and "focusing" them, restoring to the particle herd those that have strayed from of the main group. As particles are given more and more kinetic energy from the acceleration process they become "stiffer", so that stronger and stronger magnetic fields are required for bending and focusing. The magnetic fields of the synchrotron must be progressively ramped up to higher field strengths as the particles acquire more energy.
The actual input of energy in the synchrotron is performed by a few resonant electromagnetic cavity units that develop large alternating electric fields. The peaks in the electric field cycles are timed to match the arrival of a group of particles and to give them a maximum push. As the particles pass through these accelerating structures over and over again, their energy builds until it reaches the desired value, usually the highest energy the magnet system can bend.
Q: How fast are protons travelling when they reach maximum speed in the SSC?
A: The particles at full energy in any accelerator useful for high energy physics are travelling at only a small fraction less than c, the speed of light. The real issue is not their speed but their kinetic energy and their mass. This is because of the effects of relativity: as a particle is accelerated and given more and more kinetic energy, it's velocity and it's mass both increase. But while the velocity must hover just below c, the mass can continue to increase without limit. By the time protons reach full energy in the SSC they will be about 20,000 times more massive than at rest. The energy of their rest mass becomes a negligible fraction of their total mass-energy.
Q: What's a collider?
A: Imagine an accelerated proton striking a second proton that's standing still. The important energy in this collision for producing new particles, etc., is for complicated reasons the energy as viewed from a moving coordinate system where the protons collide head-on with equal relativistic masses and equal but opposite velocities. The available energy in that system is only a small fraction of the kinetic energy of the accelerated proton.
But now imagine instead that we accelerate both protons in opposite directions so they collide head-on in the coordinate system of the laboratory. Then the energy of the two protons combined is available for making particles, etc. This is what's done in a collider. It's really two accelerators, one accelerating particles clockwise, the other accelerating counter-clockwise. Several collision regions are provided where the particles have overlapping paths and high collision probability.
In a collider using particles of opposite charge, protons hitting antiprotons or electrons hitting positrons or protons, only one set of magnets is needed to bend both particles because the opposite direction of the two beams is compensated by the opposite particle charges. However, in the case of the SSC particles with the same charge are to be collided, protons with protons. This requires two acceleration paths with opposite magnetic fields, one with field direction up and the other down. This makes the design of the machine more complicated, but also more versatile.
Q: Why does the SSC have to be 57 miles in circumference?
A: As protons are given more energy they become harder to deflect with magnetic fields. This effect can be compensated by simply making larger magnetic fields. The SSC is designed to use magnets that run at about 6 tesla, about the largest fields feasible with conventional superconductors. After the highest fields possible are used in the design, the only means left of going to a higher energy is to make the radius of the circular path of acceleration bigger. At 20 TeV, the proton beam energy to be reached by the SSC, magnets with a 6 tesla field can bend the beam in a circle with a circumference of no less than 57 miles. To reduce the ring size, one would have to either reduce the energy or use larger magnetic fields. It is likely, however, that the accelerator tunnel would be bored deep underground and could lie under farmland, or even houses or towns.
Q: Why not use the recently discovered warm superconductors for the SSC?
A: This is a deceptively simple question which requires a rather detailed answer. The discovery in early 1977 of warm superconductors created an ongoing revolution in physics. These new materials become superconducting at temperatures between room temperature and the boiling point of nitrogen (77o K), and can sustain much higher magnetic fields then the older "standard" superconductors.
Many have suggested that the SSC project should be delayed until warm superconductors can be incorporated in the design. This, from my point of view, is a reasonable-sounding bad idea. There are three characteristics needed for the superconducting magnets: (1) ability to sustain high magnetic fields, (2) ability to carry large currents, and (3) ability to withstand the mechanical stress produced by magnetic forces. It is clear that the new materials when cooled to near absolute zero would do very well on criterion (1). Critical fields of up to 200 tesla have been estimated for them under these conditions. On (2) the issue is less clear. Physicists have reported observation of large current densities in single crystals, but a method is not yet in hand for sustaining large currents in bulk superconducting material. But the "crunch" comes, quite literally, with (3). The most troublesome aspect of superconducting magnet design is that, when carrying large currents in large magnetic fields, wires experience a large mechanical force. For this reason, magnets for the SSC design are made with massive quatities of iron on the outside of the superconducting wire windings that act both to channel the magnetic flux away from the wires and to hold the wires in place. The real field limits of the SSC magnets are imposed by the mechanical forces they can withstand while carrying current.
The new superconductors are not ductile metals easily made into wire. They are quite brittle, with a consistency more like powdered rock. Even if they can be formed into superconducting wire capable of carrying high currents, their tolerance for mechanical stress will remain a very serious problem.
FermiLab was built in the late 1960's with the proven magnet technology of the time, "warm" magnets with copper windings. Later it became possible at a relatively low cost to convert the same basic machine into the Tevatron, using high field superconducting magnets that more than doubling the machine's energy while reducing electrical power consumption. It seems that this might be a model for the SSC: build it now with conventional superconducting technology, but at the same time develop the new high field superconductors into very high field magnets that could later be used to increase the capabilities of the machine.
Q: Why do we need the SSC?
A: In the past 15 years there has been remarkable progress in particle physics. The quark has gone from a curious might-be particle to the central focus of attention, with the well grounded quantum chromodynamics theory to describe quark interactions. But there remain some mysteries. The "top" quark, the missing member of the 6-quark family, (down, up, strange, charmed, bottom, top) is still unaccounted for. The "higgs" particle, the keystone of the broken symmetries underlying quantum chromodynamics likewise remains without convincing experimental evidence. In the new energy domain that the SSC will open may be a rich territory with new particles and unsuspected new effects, or it may be the beginning of the "energy desert", a vast energy domain where nothing of interest happens until the Planck energy of 1023 GeV is reached. The only way to find out what the territory holds is to go there and look. It is a maxim of physics that if you would see what no one has seen before, you must look where no one has looked before. The SSC will make this possible.
Q: Where will the SSC be built?
A: That remains an open question. Originally the SSC site proposals were to be submitted by August 3, 1987, with final site selection in January of 1989. Congress, sensing the great "slice of pork" implicit in placing a $4.4 billion facility with an annual $370 million operating budget in just one lucky state, voted to extend the deadline a month to September 2, 1987, giving slow starters a better chance to compete. It is expected that a large number of states will submit site proposals. In the 1960's when what became FermiLab was being considered there were 135 site proposals. There will probably be less for the SSC, but more than 25 are expected. Some of the leading contenders for the SSC site are Illinois (the FermiLab Tevatron could serve as an injector), New York (congressional clout, close to many East Coast universities and labs), Colorado (good site near Denver airport), Texas (congressional clout, much effort on proposal), Washington (good site near Spokane airport, cheap electric power), Arizona (good desert site, room for expansion), and Tennessee (big state commitment, TVA power).
The Department of Energy is playing the siting very carefully, like a card
game. It's all rather like a lottery. There will be great enthusiasm in
Congress as long as most states have a proposal "ticket" that is a potential
winner. As soon as the lucky winner is announced, however, enthusiasm in other
quarters will be greatly diminished. Therefore the DOE must see that the SSC
is past most of the congressional hurdles and well on track before the site
selection process is completed or even before the field is significantly
narrowed. It's going to be interesting.
Followup note: The SSC project was cancelled by the United States Congress in late 1993. At the time, prototypes of the superconducting magnets had been demonstrated successfully, and large boring machines had completed the excavation of about 25% of the tunnel at the site near Waxahacie, Texas. My forthcoming novel, Einstein's Bridge (Avon, June-1997) deals in part with the cancellation of the SSC project.
Irwin Goodwin, "Reagan Endorses the SSC, a colossus among colliders", Physics Today 40, #3, 47, (March-1987).
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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