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Apr 27, 2023

SLAC National Accelerator Laboratory

It is one of the least aesthetically pleasing buildings imaginable. Painted a

It is one of the least aesthetically pleasing buildings imaginable. Painted a blah institutional beige, it stretches on and on and interminably on, resembling a long and obsessively straight storage shed. This blight on the bucolic Peninsula landscape may not exude the romance of the Hanging Gardens of Babylon or the Colossus of Rhodes, but the linear accelerator at Stanford is one of the wonders of the world — a monument to humanity's ceaseless quest to understand the universe.

The particle accelerator is the backbone of the SLAC National Accelerator Laboratory, a 426-acre complex on Stanford land off Sand Hill Road, near the main university campus. The seeds of what was to become SLAC were planted on April 10, 1956, when Stanford's Wolfgang "Pief" Panofsky hosted a group of fellow physicists at his home to propose an audacious project: the world's largest and most expensive physics research instrument — a $114 million, 2-milelong linear electron accelerator. Officially called the Stanford Linear Accelerator Center, or SLAC, but affectionately referred to by Stanford scientists as "the Monster," at that time, it would be the biggest U.S. government–funded civilian science project.

Linear accelerators are essentially enormous rifles that fire electron bullets — whose speed is jacked up to 99.999 percent of the speed of light by powerful microwave machines called klystrons — down a long, straight barrel at subatomic targets such as protons. When the electrons collide with the target, spectrometers using massive magnets measure the particle debris generated. This allows scientists to study the most elementary objects in existence, and the forces that hold them together and keep them apart.

In 1962, construction began on two structures, each 2 miles long — one above ground housing 245 klystrons, and one 25 feet below ground housing the accelerator. Precision was required in their construction, with the curvature of the Earth taken into account (a 20-inch vertical adjustment over 2 miles).

In May 1966, the first electron beam shot down the accelerator and crashed into a target proton. Two years later, the Monster was used to slay a theoretical dragon that had long vexed physicists. A series of proton-scattering experiments proved that the particles inside protons were not just a mathematical convenience, as previously thought, but actually existed. They were named quarks, after a word in James Joyce's Finnegans Wake. SLAC physicist Richard E. Taylor and his MIT collaborators shared the Nobel Prize for this research on quarks.

Building on the success of the linear accelerator, scientists then began crashing particles directly into each other, using a circular structure called the Stanford Positron Electron Accelerating Ring, or SPEAR. When electrons and anti-electrons (aka positrons), collided in the ring, new particles were revealed: the charmed quark and the tau lepton. These discoveries revolutionized high-energy physics and led to two more Nobel Prizes for SLAC scientists.

The linear accelerator at Stanford is a monument to humanity's ceaseless quest to understand the universe.

SLAC researchers also made creative reuse of their machines to build new cutting-edge instruments. A side effect of SPEAR spurred the first. Scientists knew that the electrons circling the ring emitted powerful X-rays, known as synchrotron radiation, that most regarded as a wasteful and hazardous nuisance. But a few far-sighted scientists realized that the X-rays could be used to carry out research no other machines could perform. Thus was born the Stanford Synchrotron Radiation Project, later called the Stanford Synchrotron Radiation Lightsource, or SSRL. The world's most powerful X-ray machine, it allows scientists to study the world at the atomic and molecular level.

The second repurposing was even more dramatic. By 2008, with SLAC's original linear accelerator outmoded, a pivot was made to a new, hitherto untested technology: X-ray lasers. Scientists proposed utilizing the final one-third of the accelerator to produce an electron beam, as before, and adding a revolutionary innovation: Using powerful magnets, they would wiggle the electrons, producing X-rays that then form into laser pulses. This would yield X-rays 10 billion times brighter than those from SSRL, permitting researchers to record images of extremely small objects and processes, in real time. In effect, it would allow scientists to make movies of chemistry and biology in action.

Many in the field were skeptical. "A huge fraction, maybe half, of the community did not believe (it) was going to work," said Dr. Persis Drell, former SLAC director, in a SLAC documentary. But one night in 2009, she was awakened with the words, "We have a laser." The Linac Coherent Light Source, or LCLS, was up and running, initiating a groundbreaking new phase of light-based research at SLAC. Scientists have used LCLS’ lasers to uncover the molecular structure of proteins involved in the transmission of diseases; study the extremely hot, dense matter at the core of stars; and develop next-generation painkillers.

Since it opened in 1966, SLAC has been one of the world's most productive scientific projects, shedding (literal) light on fundamental aspects of the universe. That long storage shed under I-280 may be unsightly, but what it has contributed to the realm of human knowledge is as soaring as the Golden Gate Bridge.