SSC

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SSC

The Superconducting Super-Collider Project, SSC, was a hadron colliding-beam accelerator which was first proposed by the United States in 1982. It was named Super-Collider because its beam energy of 20 tera electron volts (TeV) was sixty times the energy of Europe's proton-antiproton collider, then beginning operation at the European Laboratory for Particle Physics (CERN) in Switzerland. It was intended to re-establish U.S. supremacy in the field of high-energy particle physics, but although funding was approved in 1987, and construction commenced soon after in Texas, its cost escalated from an initial estimate of $3 billion to almost $12 billion. The project was terminated by the U.S. Congress in 1994.

The SSC consisted of a pair of synchrotron accelerators installed in a single tunnel, 54 miles in circumference. The synchrotron rings were designed to interlace and cross at a number of collision points where large-particle detectors would record and analyze the products of collisions between the two beams of protons. These beams circulated in opposite directions in a guide field provided by electromagnets—dipoles and quadrupoles—excited by coils wound from an alloy of niobium titanium that becomes superconducting when cooled to within 4 degrees of absolute zero.

Anticipated Outcomes of SSC

At the time that the SSC was first proposed, the Standard Model was emerging as the underlying explanation of what had seemed a large number of sub-atomic particles. In this model, three generations of quarks and leptons interact through particles, bosons, which carry the forces of nature. The existence of three of these bosons, the neutral Z boson and the two charged W bosons, was about to be verified and this, together with the earlier discovery of the J/ψ, provided final confirmation of the model. Nevertheless, some very important questions remained unanswered, notably an explanation of the masses of the quarks and leptons and the particles they comprise.

A theoretical concept based upon spontaneous symmetry breaking of the vacuum field—the Higgs phenomenon—predicted the existence of another boson at an energy less than 1 TeV. If discovered, the Higgs boson would confirm the theory, explain the masses, and indicate a threshold in energy beyond which strong and weak interactions would become of comparable strength.

Unfortunately, to produce a particle with a mass of 1 TeV it is not sufficient to collide two protons with an energy of 500 giga electron volts (GeV), otherwise Fermilab's Tevatron collider (completed in1985) would have been powerful enough. In such a collision only one of the three quarks in each proton interact and together the pair will have less than one sixth of the total beam energy. Ten, or preferably twenty, tera-electron-volt protons are needed to be sure to create the Higgs boson.

Special Aspects of the SSC

The most obvious special feature of the SSC is its size, due simply to the difficulty of deflecting high-energy particles in a circle. The magnetic rigidity of a beam of particles is Bρ = e/p , where B is the magnitude of the deflecting field in Tesla, ρ is the radius of curvature of the machine, e is the particle's charge, and p is its momentum. Early synchrotrons and storage rings used conventional magnets whose iron yokes saturated at a field of 2 Tesla, limiting the peak energy of the machine. The quest for higher-energy collisions has led inevitably to larger rings—roughly an extra kilometer of circumference for each 70 GeV of energy. A 20-TeV ring of conventional magnets would be about 300 kilometers in circumference.

It is possible, by passing large currents in the coils, to drive magnets to even higher fields, beyond saturation. In this regime the coils determine the field shape, and the iron yokes are merely there for mechanical stability and to contain stray fields. However, such magnets are only feasible if their windings are superconducting. This reduces the resistive losses to virtually zero. The coils must be very precisely constructed, and moreover, at low temperature the thermodynamic efficiency of refrigerators is so low that 40 megawatts of electrical power is still required to remove the few kilowatts of heat that leak into many kilometers of the SSC magnet.

The SSC was not the first such collider to use superconducting magnets. An earlier 1-TeV superconducting ring (the Tevatron) was completed at the Fermi National Accelerator Laboratory (Fermilab) near Chicago in the early 1980s and was fed with protons and antiprotons. While the guide field of the Tevatron was twice that of a conventional synchrotron magnet, the field of the SSC magnets, 6.6 Tesla, was three times more than a warm magnet.

In spite of the higher field, the SSC circumference was 87,120 meters, almost fourteen times larger than the Tevatron, and three times as large as the largest tunnel available in Europe, which was then under construction for the electron-positron collider LEP at CERN and later destined to house the LHC. Its two semicircular arcs formed a racetrack with two straight sides, one of which accommodated three major experiments. The other side was principally dedicated to a chain of three boosters: injector synchrotrons of 11, 200 and 2,000 GeV, fed by a 600-MeV linac. There were also two more experimental halls. Boosters are used to feed such a large synchrotron because as a proton beam is accelerated, it shrinks and needs a smaller magnet aperture. The chain of injectors exploits this fact so that expensive, wide-aperture magnets are only needed for the smaller, low-energy rings.

FIGURE 1

High beam intensities were needed. In a collider each particle in one beam passes through the other oncoming beam at each collision point and has an opportunity to interact with all the particles in the oncoming beam. As it does so, a particle presents a certain cross section—depending on the nature of the interaction under study. The probability of a collision between any two particles is small, but when multiplied by the number of particles in each beam and by the revolution frequency of the beam in the machine, many interesting events per second may be expected. A quantity called the luminosity, the measure of the probability of such events per second and per unit interaction cross-section, and is typically in the range 10³⁰ to 10³³. Processes for which particles present a cross section of 10^-33 cm² will appear once per second if the luminosity is 10³³. Cross sections of interesting processes may be many orders of magnitude smaller.

The Tevatron, like CERN's proton-antiproton collider before it, collides protons with antiprotons. Antiprotons, being of the same mass but opposite charge as protons, will circulate in the opposite direction in the same ring of magnets, thus avoiding the construction of two distinct rings. However, antiprotons are difficult to produce, and the Tevatron reached a luminosity of at most 10³¹ cm². The de Broglie wavelength of a 20-TeV proton is twenty times smaller than at the Tevatron's 1-TeV hadron, and hence the detail it will reveal in structure is twenty times finer. But such detail is 400 times smaller in cross section, and the luminosity has be over 10³³ to produce an acceptable observation rate. To reach this luminosity one needs to collide two proton beams of very high density—hence the SSC's twin rings.

To reach the highest luminosity both beams must be focused down until a limit is reached when the electromagnetic field from the oncoming beam becomes large enough to disturb the precise magnetic focusing properties of the ring. Other intensity limits come from fields produced by the particle's neighbors and their images reflected in the walls of the vacuum chamber. Any sudden change in the transverse dimensions of the vacuum chamber will be excited by the electromagnetic wake field of the beam passing through it as if it were a parasitic accelerating cavity. The fields set up in the cavity act back on the beam and like an amplifier with a feedback system can become unstable if the current is too high. Yet another potential limit to the luminosity comes from the fact that protons at 20 TeV are beginning to radiate significant flux of synchrotron light just as electrons at much lower energy. This falls on the inner, cold, surface of the vacuum tube adding to the heat load of the refrigerators. Finally, another difficulty with such large machines is the precision required for the magnetic guide field. The largest computers cannot simulate the beam's behavior in these fields for more than a million or so turns, a small fraction of the required lifetime. Such studies stretch the predictive power of nonlinear mathematics to the limit.

A practical concern is that considerable care must be applied to the design of the protection devices and energy dumping circuits, which must safely dispose of the energy stored in the magnets' field if their superconducting properties are suddenly lost due to a mishap.

Reasons for the Cancellation of the SSC

In 1982, the Snowmass Study, organized by the American Physical Society, first proposed the SSC. Their initial cost estimate was $2.9 to $3.2 billion, a figure that was confirmed in 1983 by the U.S. Department of Energy (DOE).

Subsequently, detailed design issues were studied by a Central Design Group set up under Maury Tigner at Lawrence Berkeley Laboratory—a body of experienced accelerator designers who produced a convincing design, building on experience gained in constructing the successful 1-TeV Tevatron at Fermilab. By 1986 the conceptual design study was complete, and in 1987 President Reagan set in motion the search for a site. In 1988 Waxahatchie, Texas, was announced as the successful candidate. This decision was perhaps influenced by Vice President George Bush of Texas; Jim Wright of Fort Worth, then Speaker of the House of Representatives; and a powerful senator, Lloyd Bentsen, also from Texas.

In the past, the management of large accelerator projects, once approved, had been entrusted to their designers. However, in this case the DOE

FIGURE 2

judged the SSC to be too mammoth an undertaking to be constructed without the aid of industrial firms with considerable expertise in the management and operating of large projects—but, it must be said, with very little knowledge of accelerators. The management and operation of the project was contracted to EG&G, Inc., which had managed the Nevada Test Site and the other DOE facilities, and the Sverdrup Corporation, which was involved in defense-based contracts. The Central Design Group Team leader was replaced by Roy Schwitters as director who reformed a design team in Waxahatchie alongside the contractors. It was said that communication between the two communities had its problems and that this contributed to the escalation of cost.

The designers made a number of costly but necessary design modifications. They increased the magnet aperture to ensure the beam was further away from uneven fields near the coils. The strength of focussing magnets was augmented, the energy of the accelerators in the injection chain was increased, and the experimental areas enlarged. Meanwhile, it seemed to some that contracts were placed, not always to the lowest bidder, but with a view to giving every state in the Union a stake in the project. The cost rose steeply from the Central Design Group's estimate of $3.9 billion in 1986 to $5.3 billion in 1987, which was estimated by the DOE for a construction period that was longer by one year. This estimate became $5.9 billion in 1991, but review teams, taking into account the site-specific costs, adjusted this to $7.2 billion and then to $8.2 billion. The final estimate by an independent cost estimating team of the DOE, which added $2.5 billion for peripheral expenses that would not be incurred if the SSC had not been there, was $11.8 billion.

The result of this cost escalation was to trigger the U.S. House of Representatives to cancel funding in 1993. There was a rival project—the space station— which many in the House preferred. There were also those who believed both projects should be sacrificed in order to balance the budget. The Senate restored funding for one year, but in 1994, after some unseemly maneuvering by both sides, Congress finally canceled the SSC.

Those who regret the demise of the SSC blame the way in which DOE set up the project and particularly its choice of contractors. Its management team was headed by a procession of able project managers who left or were replaced. It was said they were frustrated by the lack of sound DOE leadership and were never in the saddle long enough to restrain the rising costs. Another factor was the choice of the site that discouraged many experienced people from joining the team. For those who opposed the SSC and its funding, it was just too much money for the general public to provide to support a science that they were convinced was irrelevant to their everyday life. At that time it was even becoming intellectually fashionable in some circles to question whether science had enhanced the quality of human life at all. Meanwhile, it was left to Europe to construct a more modest collider, the CERN LHC, with beams of 7 TeV, and to the Tevatron at Fermilab to hunt the for Higgs boson, hoping that it might be found below its rather limited reach in energy.

Bibliography

Huson, R., et al. "20 TeV Colliding Beam Facilities: New, Low-Cost Approaches." Proceedings of the DFP Summer Study on Elementary Particle Physics and Future Facilities, 315–322(1982).

Jackson, J. D., ed. Conceptual Design of the Superconducting Super Collider SSC (Central Design Group, SSC-SR-2020, 1986).

Report of the DOE review Committee on the Baseline Validation of the SSC (DOE/ER-0594P 1993).

Report of the HEPEP Subpanel on Future Facilities (DOE/ER-0169, July 1983).

Report on the SSC Cost and Schedule Baseline (DOE/ER-0468P, 1991).

Edmund J. N. Wilson

Building Blocks of Matter: A Supplement to the Macmillan Encyclopedia of Physics