Stanford Linear Accelerator Center

Upon its completion in 1966, the Stanford Linear Accelerator was the longest and straightest structure in the world. The facility, known in early planning stages as “The Monster” (or “Project M”), stretches for almost three miles across the picturesque rolling hills of a 480-acre section of former pasture land located near the campus of Stanford University. Glimpsed from the window of a car speeding along the I-280 highway, which crosses directly over the site, the linear particle accelerator’s housing achieves architectural distinction more for its impressive dimensions and uncompromising linearity than for its formal details. Inside the utilitarian sheet-metal structure, however, resides a landmark in the history of elementary particle physics as well as the backbone on which the diverse research activities of today’s Stanford Linear Accelerator Center (SLAC) National Accelerator Laboratory have evolved.

The linear accelerator’s basic layout, comprising two distinct volumes running in parallel, is visible only in section drawing or in photographs taken during construction. The main element of the accelerating structure is an evacuated copper tube four inches in diameter and 10,000 feet in length. The entire span of the “linac,” as it is called, is housed below grade in a concrete tunnel covered by a radiation shield made up of 25 feet of highly compacted earth. At ground level, a continuous steel frame “Klystron Gallery,” measuring 13 feet high and 20 feet wide, stretches above the path of the accelerator tunnel. This is the portion of the apparatus that is visible from the exterior. A series of side alcoves for control and signaling units, as well as for water and power distribution equipment, project from the sides of the shed building, creating a rhythmic counterbalance to the structure’s dominating axis.

From its inception, SLAC was dedicated to observing the properties and behavior of matter at the most fundamental levels. This was achieved with a high-energy electron beam capable of reaching energy levels of around 20 billion or 20 Gigaelectron volts (GeV). The energy was gradually increased later to around 48 GeV. The above-ground gallery contained 240 high-frequency power tubes, called klystrons, spaced out at 40-foot intervals. Each of these power sources delivered precisely calibrated pulses of microwave energy directly into the accelerator pipe (linac) beneath it. Almost like surfers on an ocean wave, electrons introduced at one end of the machine were sped through the pipe on an accelerating crest of radio waves. At the end of the 2-mile conduit, they reached a separate structure called the “beam switchyard.” Covered by 32 feet of shielding earth, the switchyard housed an array of powerful magnets that distributed specific beam impulses to individual experiment areas (called “End Stations” or “Target Buildings”), where the electrons would be directed to a target. One of these areas, End Station A, was almost 7 stories high and contained 25,000 feet of floor space that originally accommodated 3 massive magnetic spectrometers.

While the SLAC facility was notable for its unparalleled bigness, both in terms of its physical size and its $114 million construction budget, the main intention behind its conception was the observation of the vanishingly and almost undetectably small. As the Center’s first director put it, “In high-energy physics you have to have bigger and bigger machines to see smaller and smaller things.” Soon after the linear accelerator reached full operation, a team of scientists from SLAC and MIT conducted a series of scattering experiments with the beam to discover that protons in the nucleus of an atom were made of even smaller units of matter called quarks. For this discovery, the leaders of the team, Richard E. Taylor (SLAC), Jerome I. Friedman (MIT), and Henry W. Kendall (MIT) shared the Nobel Prize in Physics in 1990.

The completion of the SLAC accelerator was the culmination of a long line of technical achievements at Stanford reaching back to the research program of W.W. Hansen in the 1930s and 1940s. In 1937, a small team led by Hansen and Russell and Sigurd Varian was responsible for the invention and refinement of the klystron microwave tube. While klystron devices proved critical for wartime applications such as radar systems, their ability to generate microwave power subsequently helped drive the creation of increasingly powerful accelerator devices for fundamental physics research. These included the 2-foot “Mark I” machine, completed in 1947; the 14-foot “Mark II” machine, completed in 1949; and the “Mark III,” first operated along a 30-foot segment in 1950. Three years later, the Mark III had grown to 210 feet, allowing for pioneering studies of electron scattering in atomic nuclei, for which Stanford’s Robert Hofstadter received the 1961 Nobel Prize in Physics.

Early on, success of the Mark III experiments led to a series of discussions that set the stage for The Monster. Beginning in 1956, a group of Stanford professors led by Wolfgang Panofsky (the son of the art historian Erwin Panofsky and a participant in the testing of the first atomic bomb) outlined the basic parameters of the new device and examined the feasibility of various sites. In order to think through the many unique issues that accompanied planning and construction, Stanford brought on the joint venture firm Aetron-Blume-Atkinson. The group was made up of Aetron (an architecture, engineering, and management division of the Aerojet Corporation), John A. Blume (a leading firm in the field of earthquake engineering), and the Guy F. Atkinson Construction Company (specialists in large infrastructure projects such as highways and dams). Throughout the process, Charles Luckman Associates (also responsible for the design of major buildings such as the Prudential Tower in Boston, the new Madison Square Garden in New York City, Aloha Stadium in Honolulu, and the NASA Manned Spacecraft Center in Houston) served as the architect for the project.

In contrast to the first generation of electron linear accelerators, which could be lifted by a few people, Project M transcended the scale of laboratory equipment. It quickly became an architectural—and even infrastructural—problem. The accelerator’s 3-kilometer span (in addition to another kilometer for the beam switchyard and the experiment halls) was the longest practical straight path that could be identified on maps of the Stanford grounds. Since the machine crossed the projected route of what would later become I-280, a highway overpass was built over the klystron gallery in anticipation of the eventual construction of the road itself. In digging the trench for the linac tunnel, workers were faced with bringing a variety of soil conditions into a consistent density. During the complex concrete pour that created the tunnel’s rigid shell, a special formula was devised to minimize shrinkage. Ice manufactured in a dedicated facility on the site was added to the mix on warm days to mitigate the effects of temperature change as the concrete set. After all of the joints were grouted with epoxy, the tunnel assumed the form of a continuous monolith. Before work proceeded on the underground portion of SLAC, the project’s designers were also faced with the challenge of modeling the construction of the klystron gallery above it. In order to study the transfer of power from the klystrons, a full-scale prototype focusing on a single section of the linac was constructed in the vicinity of the Stanford football stadium. When the team’s coach spotted the helicopter used to build the 25-foot tall mockup on the day before a game, he thought that an opposing team was spying on their practice.

At the same time that the accelerator’s extreme size demanded new approaches to design and construction, the exacting precision necessary for it to function introduced yet another set of challenges. For example, the linac tube was fabricated from over 80,000 copper cylinders alternated with 80,000 disks. The electron beam, measuring less than a half inch in diameter, had to pass directly through a 7/8–inch hole in the middle of each disk. In order for the beam to reach its target, the entire length of the tube had to be aligned to within a few thousandths of an inch from center. These parts were machined, annealed, stacked, and finally brazed together by a group consisting for the most part of women hired as part-time workers. The final alignment of the 2-mile structure was accomplished by mounting it on a 10,000-foot hollow aluminum girder that was 2 feet in diameter and consisted of 270 independent sections. It was aligned end-to-end with a laser (which had just appeared on the market) and 270 alignment targets mounted inside.

In 1966, several components from the linear accelerator were displayed in the exhibition “Design for Nuclear Research,” held at the Stanford Art Museum and organized by SLAC and the Stanford Department of Art and Architecture. Recalling the Museum of Modern Art’s famous “Machine Art” show of 1934, power dividers, modulator flanges, sections of the accelerator pipe, and other pieces were treated as sculptural elements defined as much by their aesthetic qualities as by their functionality.

However beautiful its components and however groundbreaking its potential discoveries might have been, the SLAC laboratory was not immediately embraced by politicians, nor by some members of the surrounding community. This constituted perhaps the greatest challenge in building the facility. A sticky set of debates transpired in Washington, D.C. Owing in part to the high cost of the project, it was proposed from the beginning as a special joint initiative between Stanford and the Atomic Energy Commission. Stanford first submitted the project to the federal government in 1957. Official funding was then proposed by the Eisenhower Administration in 1959, but not approved by Congress. For the next three years, numerous hearings about the project were held before the Joint Committee on Atomic Energy. Members of Congress expressed concerns about the accelerator’s susceptibility to earthquake damage, as the site was located in close proximity to the active San Andreas Fault. Even more disruptive were reservations held by some politicians about the value of high-energy particle physics, which was difficult to visualize, not easily pinned to specific goals, and seemed remote from everyday experience. All of these hurdles were finally overcome when Congress and the Kennedy Administration funded the project and entered into a formal contract with Stanford in 1962. At the local level, once construction began, the nearby community of Woodside protested the large 220 kilovolt power lines and towers that would be installed along surrounding hills in order to supply up to 80 megawatts of electricity to the facility. In the end, an engineering compromise was reached. Much smaller power poles were designed and painted green in order to visually blend in with the landscape.

Following tests on increasingly long sections of the accelerator in January 1965 and April 1966, the first beam to traverse the full length of the machine was sent on May 21, 1966. In the following years, as instrument fabrication techniques improved and as the character of fundamental physics research continued to evolve, a series of new experiment areas were added to the line. In 1972, for example, the SPEAR (Stanford Positron Electron Accelerating Ring) machine was completed. As opposed to the fixed target experiments facilitated in the original end stations, the ring shape of SPEAR facilitated electron collision research and also ushered in a new era of synchrotron radiation research at SLAC. Findings from experiments conducted at this facility included the discoveries of the J/psi particle in 1974, the tau lepton in 1976, and, at the turn of the twenty-first century, resolution of the machinery that transcribes DNA. Burton Richter (SLAC) and Sam Ting (MIT) shared the Nobel Prize in Physics in 1976 for the J/psi discovery. Martin L. Perl (SLAC) shared the Nobel Prize in Physics with Frederick Reines (UC Irvine) in 1995 for their work on lepton physics.

A larger, high-energy collider machine called PEP (Positron-Electron Project) began operation in 1980, to be followed by an upgraded PEPII in 1997. The world’s first linear collider, the SLC, began operating off of the linear accelerator in 1989; a SPEAR3 upgrade was completed in 2003; and a new free-electron laser facility (the Linac Coherent Light Source) began operating in 2008. These experiment buildings grew alongside an expanding array of office and technical support structures, including the 1963 Test Laboratory and the 1966 Cryogenics Building. At the same time, as the research program became connected to a worldwide network of scientific sites, the SLAC assumed a critical role in the development of a new set of communication technologies that vastly expanded traditional notions of site. In 1991, the first web server outside of Europe was created by SLAC.

The fiftieth anniversary of the first full operation of the SLAC linac occurred in 2016. As the institution’s research portfolio continues to branch into new domains, including applications from biology to art history, a new set of challenges have emerged related to the balance between a need for continuous technological innovation and the imperatives of architectural preservation. In 1983, the facility was designated as a National Historic Engineering Landmark, and while initiatives such as the 1967 “Blue Book,” a comprehensive 1,200-page publication that has now been made available online, established an institutional memory related to the construction of the linear accelerator, the campus’s buildings themselves are currently in the process of historical review with the aim of ascertaining the architectural significance of the spaces at the core of SLAC’s scientific heritage.