Siting the Superconducting Super Collider (1988)

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2 The Superconducting Super Collider The NAS and the NAE were asked to assess site proposals for a Superconducting Super Collider (SSC), particularly in light of how a given site might enhance the scientific productivity of the laboratory, and to develop a list of best-qualified sites. The academies were not asked to evaluate the scientific merit, opportunities, or need for an SSC. The focus of the academies' activities was on the site-specific effects that might affect the construction and productive operation of a super collider. Much has been written elsewhere about the scientific promise of the SSC; the interested reader can find discussion of the science in the following publications: 1. Committee on Science, Engineering, and Public Policy, Report of the Research Briefing Panel on Scientific Opportunities and the Super Collider, National Academy Press, 1985. 2. J.W. Cronin, "The Case for the Super Collider," Bulletin of the Atomic Scientists, May 1986, pp. 8-11. 3. J.D. Jackson, M. Tigner, S. Wojcicki, "The Superconducting Supercollider," Scientific American, Vol. 254, No. 3, March 1986, pp. 66-77. 4. L.M. Lederman, "To Understand the Universe," Issues in Science and Technology, Vol. I, No. 4, 1985, pp. 55-65.

5. C. Quigg, R. Schwitters, "Elementary Particle Physics and the Superconducting Super Collider," Science, Vol. 231, March 28, 1986, pp. 1522-1527. 6. R.F. Schwitters, "Super Collider," American Politics, July 1986, pp. 5-7. THE SSC AS A SCIENTIFIC INSTRUMENT The SSC will have two counterrotating, tightly focused streams of protons guided along nearly circular orbits by superconducting magnets (see Figure 1). The protons in each beam will be accelerated to 20 trillion electron volts (TeV),* and the two beams will be brought into collision at several interaction regions around the circumference of the accelerator, yielding at those points a total useful energy of 40 TeV. The energy available for creating new particles of interest at the collision points of the SSC will be 200 times the energy that would be available if only one such beam were directed at a fixed target (see Figure 2). The events resulting from collisions of interest will be complicated, with hundreds of particles flying out from the collision point. These particles must be tracked and measured to reconstruct the underlying physics responsible for the event. One particular challenge to SSC experimenters will be the high rate of collisions—about 100 million per second—of which only a tiny fraction will provide the scientists with new information of interest in specific investigations. Sophisticated detectors of enormous size (see Figure 3) will be installed at the collision points. These detectors employ tracking chambers, analyzing magnets, large calorimeters, and other devices to observe high-energy particles to provide information on the tra- jectories, energies, and identities of the particles in a proton-proton collision. In addition to the detection hardware, extensive and elab- orate high-speed, sensitive electronics is required to register and interpret the large amount of data that results from each collision event. The data are screened by special-purpose, high-performance computers to select events of special interest and then archived for subsequent off-line analysis. The very large scale and sophistication of the detectors are neces- sitated by the high energies of the particles to be detected and by the inherent complexity of the collision events of interest. Materials such *An electron volt is a unit of energy equal to the energy given to each electron flowing through a circuit by a one-volt electrical battery.

The Superconducting Super Collider (SSC) SSC anatomy (For scale drawing. see map at hoi torn right.) Legend v// A Power supply and liquid helium refrigeration unit O Interaction point and associated recording collision detectors [[ 1| Interaction hall housing interaction point ••• Locations of superconducting magnets 15 to 20 building campus - Main laboratory. warehouses. shops and offices Operation of injection system Injection system loading protons into clockwise beam pipe of main ring Injection system loading protons into counterclockwise beam pipe of main ring Main ring sustaining dual proton beams The size and power of the underground SSC would allow narrow beams of protons to collide at almost the speed of light. creating new sub- atomic particles observable only at very high energies that cannot be attained by existing accelerators. This diagram tracks the path of proton beams. Protons are produced by the ionization of hydrogen atoms. The injection system (upper right). composed of a linear accelerator and three progressively larger circular energy boosters. prepares the protons for the main-ring collisions by acceler- ating them to higher and higher energies. Some protons are loaded into the clockwise beam pipe (green). then others into the counter- clockwise one (black). Inside the main ring (see central schematics). accelerating units speed the protons to 20 times their energy. The protons are guided in the high-energy booster and main ring by about 10.000 powerful superconducting magnets. refrigerated by liquid helium to 4.35° Kelvin (about -270°centigrade. -455° Fahrenheit). The magnets maintain the beams on their cir- cular paths; special magnets near the interac- tion points force collisions between protons traveling in opposite directions. Detection apparatuses at each collision site will measure the energy released by the collisions and will trace the paths of particles produced by the collisions. By creating levels of energy similar to those ot the "big bang." scientists hope to learn about the fundamental laws of nature that guided the creation of the universe. Informational graphics- Michael Yanott FIGURE 1 A simplified description of the Superconducting Super Collider from the Bulletin of Atomic Scientist*, volume 42, page 9, May 1986.

as iron and lead are used to absorb the energetic particles. Enough of these heavy materials must be used to assure that all of the particles produced are absorbed. Further, because particles are released in different directions, detectors must fully surround the collision point. The various requirements set the scale of the detectors, some of which may weigh as much as 40,000 tons. Different kinds of detectors will be installed at different collision points in order to allow the investigation of a number of physics problems. Many crucial experimental tests have been proposed and formulated, and conceptual designs of various detectors have been prepared. The detectors will generally require large-scale collaboration— including international cooperation—among many university and laboratory groups. Traditionally, such collaborations evolve from the common interests of experimental physicists, who unite in semiper- manent scientific groups of students, postdoctoral researchers, and university professors supported by the engineering and technical staffs of laboratories and universities. The data collected by the detectors become available to all the collaborating members working in separate smaller groups according to their scientific interests. The proposed accelerator scheme makes use, first, of a proton in- jector system that will accelerate protons to about 1 TeV, at which point the protons will be injected into the main SSC ring for the final acceleration phase. The injector will consist of a series of four separate accelerators: a linear accelerator about 500 feet long that will raise the protons from rest to an energy of 0.6 billion electron volts (GeV); a low-energy synchrotron booster about 820 feet in cir- cumference, using conventional magnets, that will raise the protons to 7.0 GeV; a medium-energy ring about 1.2 miles in circumference, again using conventional magnets, that will raise the energy of the particles to 100 GeV; and a high-energy booster (HEB), some 4 miles in circumference, that will use superconducting magnets to increase the proton energies to 1 TeV. The main ring will be located in a 53-mile-long race-track-shaped tunnel, 10 feet in cross-sectional diameter with its centerline at least 35 feet below the earth's surface. The tunnel will contain two evac- uated tubes with proton beams moving in opposite directions and about 9500 powerful electromagnets—spaced along the beam lines— to keep the proton beams tightly focused in the evacuated tubes and constrained to closed, nearly circular orbits. (About 8000 bending (dipole) and about 1500 focusing (quadrupole) magnets will be used)

>s O) 01 c CD ~o O 20TeV Detector 40Te\ COLLIDER 20TeV FIXED-TARGET ACCELERATOR Beam energy FIGURE 2 The advantage of a collider.

FIGURE 3 A collider detector during its assembly. (see Figure 4). The ring will receive bunches of 1-TeV protons from the injector; the protons will be distributed around the ring and accelerated until they reach an energy of 20 TeV. When the protons are at the desired energy level, it will be possible to deflect the two beams so that they collide head-on with one another in the center of the particle detectors that surround the beams at the interaction points. After acceleration to full energy, the beams will continue to circulate for many hours while the experimental detectors record col- lision events. When the beam intensity falls, a new batch of protons will be introduced into the SSC and accelerated. The main-ring bending and focusing magnets will use supercon- ducting wire to carry the electric current that sets up the magnetic field. The costly and sophisticated superconducting cable as well as the great precision and quality control required in assembly will

10 SUPER INSULATION ELECTRICAL BUSING LIQUID HELIUM PASSAGE SUPERCONDUCTING COIL LIQUID HELIUM RETURN HELIUM GAS RETURN IRON YOKE COIL COLLAR. S.S. HELIUM CONTAINMENT SHELL 80° K SHIELD 20' K SHIELD VACUUM SHELL LIQUID NITROGEN 20° K HELIUM GAS SUPPORT PEDESTAL FIGURE 4 Detail of magnet assembly. make the magnets expensive to build, although they will be relatively inexpensive to operate, because the superconducting coils have essen- tially no electrical resistance. In principle, a 20-TeV accelerator (of considerably larger circumference) could be built with conventional copper conductor electromagnets, but, because of the resistance of the wire, it would consume at least 4000 MW of power (as opposed to a total of 100 MW to be consumed by the entire SSC complex, much of which is necessary to cool the superconducting magnets to their required operating temperature) and lead to unpractically high operating costs. Using superconducting magnets will reduce the total power consumption of the magnetic confinement system and permit the creation of magnetic fields several times stronger than any that could be achieved with conventional electromagnets. A stronger magnetic field will make it possible to confine protons of a given energy (say, 20 TeV) to an orbit of smaller radius and thus reduce the required length of the accelerator tunnel. The design circumference of the SSC—53 miles—is determined by the maximum intensity of its magnetic field—6.6 tesla—and the maximum energy of the protons—20 TeV. The accelerator will also require hundreds of miles of cryogenic

11 plumbing (at the boiling point of liquid helium, 4 K) to establish superconductivity. Such systems (though with a lower magnetic field) have been successfully constructed and used on a large scale in the Tevatron ring of the Fermi National Accelerator Laboratory, but the SSC cryogenic system will be some 13 times larger in scale than anything ever attempted. THE SSC AS A CrVTL WORKS PROJECT The proposed SSC will be the largest scientific instrument ever made. The tunnel, which will be the largest component of the facility, will be out of sight and covered by at least 35 feet of earth to ensure that no significant radiation ever reaches the surface. Approximately every 5 miles along the 53-mile tunnel, a cluster of surface buildings— housing cryogenic refrigerators, helium compressors, power supplies, and support facilities, and providing points of access—will be visible. Additional shafts allowing access to the collider tunnel will be located midway between adjacent service areas. The campus—a focal point of the site—will be a science research center large enough to accommodate a staff of 3000, with a central office building, an auditorium, and various laboratory, support, and industrial buildings. The SSC will consist of five major components: (1) an under- ground injector complex of cascaded accelerators to accelerate pro- tons from rest to 1 TeV; (2) a main collider ring to accelerate, focus, and guide two beams of protons in opposite directions around the tunnel until they each reach an energy of 20 TeV, and then to "store" them in the ring until they are depleted through collisions; (3) collision/experimental areas containing the particle detectors; (4) campus/laboratory areas; and (5) a site infrastructure of roads and utilities. The experimental areas containing the massive particle detectors will be located in two regions clustered diametrically opposite each other on the circumference of the collider ring. Each experimental area will have surface structures and underground collision and access halls. The dimensions of the collision halls will vary to allow a spectrum of possible experimental apparatus. The largest collision halls could be up to 160 feet long, 120 feet wide, and 130 feet high. Because the outside parts of the detectors are very likely to include large assemblies of thick steel plates—making the individual detector components enormously heavy—a thick concrete floor with

12 steel plate capable of supporting loads of up to 9 tons/ft2 will be used in the halls. The campus area may have 15 or more buildings clustered in four major groups—a central laboratory building and auditorium, industrial buildings, warehouses, and auxiliary support buildings. The central laboratory building will provide office and laboratory space for administrative and technical personnel. One building might contain all the major offices of the facility and light laboratories for the development and testing of electronic components. Industrial buildings will house limited component assembly activities, various workshops, and associated offices. Warehouses will serve as receiving and storage facilities. The auxiliary support buildings—fire, rescue, site patrol, visitor services, waste management, and vehicle storage buildings—will provide services to the entire complex. The central laboratory facilities with their associated office and shop buildings, and assembly and staging areas, will be arranged like a small college campus. Roads and utilities, adjacent to the campus, will include a main electrical substation consisting of incoming high-voltage electrical service, transformers, switch gear, and distribution systems. A sec- ond substation will be located on the far side of the ring. Water treatment facilities will process the cooling water used for the SSC. A road network will be needed in the campus, injector, and exper- imental areas, to connect the cluster regions, and to provide access to the service areas and access points located around the ring. A significant part of the project's capital cost will go toward the 9500 superconducting magnets, whose design, construction, and testing will require advanced technologies and precision engineering of the highest order. In addition to the magnets, other advanced systems for the SSC will include the radiofrequency acceleration cavities, cryogenic facilities, particle detectors, supercomputers, and laboratory equipment. The construction of conventional facilities, by contrast, will not require significant innovation as much as a scaling-up of existing methods. Construction of the tunnel and experimental halls, as well as the requisite infrastructure of utilities, transportation, housing, laboratories, offices, shops, maintenance, and so on, will be large in scope but straightforward in principle. It will be possible to excavate the 10-foot-diameter tunnel by cut-and-fill methods or tunnel-boring machines, or by a combination of both. The area enclosed by the ring will be left, for the most part, completely untouched.

13 THE SSC AS A HUMAN ENDEAVOR An essential fact to keep in mind is that the initiation of an SSC is not simply "starting up a new large facility" but the creation of a new international basic research laboratory. This means that the efficient and prompt start-up of the new facility is only the first of several major tasks that will need to be undertaken. The second major effort is the creation of an infrastructure and environment to facilitate creative research through supporting the efforts of both inside and outside users. A third major task is the training of operat- ing personnel to run and maintain the accelerator itself and the large ancillary complex required in support of experimental undertakings. A fourth item is the establishment of the administrative machinery to manage the SSC complex as well as to deal with the external constituencies: government, industry, the public, and the domestic and foreign participants in the work of the SSC. The SSC will be a very large laboratory; its staff—scientists, engineers, technicians, skilled technical and mechanical laborers, and professional administrators—will number nearly 3000, including 500 visiting scientists (many on sabbatical leave from universities in the United States or abroad) and their students, who may participate in its work for periods of weeks to years throughout its operating period. Experience indicates that for each individual employed or working at a laboratory, several additional people (including families) are brought into the community in connection with schools, stores, maintenance, services, and other support facilities. Thus the SSC is likely to create additional employment in the community at large and could generate demand for additional housing, schools, and other services beyond what is required for the SSC staff itself.