The Stanford Linear Accelerator Center

The Stanford Linear Accelerator Center (SLAC) is a national basic research laboratory devoted to experimental and theoretical research in elementary particle physics, to the development of new techniques in high-energy accelerators and elementary particle detectors, and to a broad program of research using synchrotron radiation. It is located on 425 acres of Stanford property west of the main campus, on Sand Hill Road. Stanford University operates SLAC under a contract with the US Department of Energy. Established in 1962, SLAC has become a world-renowned elementary particle physics laboratory, recognized for more than four decades of acheivement in the field. Some of the highlights of SLAC’s distinguished history are shown in the timeline.

The Accelerators

As directions in particle physics and synchrotron radiation physics have changed and new technologies have come of age, SLAC has pursued a vigorous program of accelerator improvements and developments combined with the use of new research instruments. These have resulted in an innovative and diverse series of state-of-the-art accelerators and detectors.

The Linac and the SLAC Linear Collider

The first major accelerator facility at SLAC was the two-mile-long linear accelerator, completed in 1966. In the early years, with 240 klystrons along the machine, each with a peak power of 24 MW, it produced electron beams of about 20 GeV and positrons of about 13 GeV. During the 1970s, a radio frequency (rf) energy compression scheme called SLED (SLAC Energy Doubler) was added to each of the 240 klystrons that boosted the electron energy to above 30 GeV. In the 1980s, new, improved klystrons manufactured at the laboratory reached peak powers of 64 MW which, in combination with SLED, boosted the linac energy to 50 GeV.

From the beginning, a “switchyard” of magnetic elements at the end of the accelerator could direct the beams to any of several different experimental areas. These included bubble and streamer chambers and a variety of particle spectrometers. The large spectrometers in End Station A were used in the late 1960s and early 1970s in a series of electron-proton and electron-neutron scattering experiments that demonstrated the existence of quarks inside the nucleon.

Professor Richard Taylor, the leader of the SLAC team, shared the 1990 Nobel Prize in Physics for his part in this work. In addition to primary beams of electrons and positrons, secondary beams of pions, kaons, muons and photons were also produced and used in a variety of other experiments.
With the advent of colliding beam storage rings (SPEAR in 1972) and PEP (in 1980), which are discussed in more detail below, the SLAC linac also served as source and injector for electrons and positrons.

Starting in 1983, extensive modifications and additions to the existing linac enabled it to simultaneously accelerate and collide 50 GeV electrons and positron bunches on a pulse-to-pulse basis. This new machine, the SLAC Linear Collider or SLC, separated the beams at the end of the accelerator, guiding them through two opposing magnetic arcs and focusing them on each other, to micron-size spots at just the correct center-of-mass collision energy (~ 92 GeV) to produce the Z 0 particle, the neutral mediator of the weak interaction. Physics at the SLC started in 1988 with the Mark II detector, replaced in 1991 by the SLD (SLAC Large Detector), a more modern instrument. In 1992, the SLC was first operated with polarized electrons, an innovation that opened a new area of precision physics measurements. Between 1992 and 1996, a series of improvements to the accelerator steadily increased the luminosity of the SLC and brought the electron polarization to an average of 78%. In 1996, the SLD was equipped with a new vertex detector (VX D3) which greatly enhanced its physics capability. In the 1997–1998 run, the SLD logged in excess of 300,000 Zs. Operation of the two-mile linac in the SLC mode has also greatly enhanced its performance for more recent stationary target electron scattering
experiments with polarized electrons in End Station A. Other innovative experiments have been performed in the straight-ahead Final Focus
Test Beam where tiny electron bunches with full heights of 120 nanometers have been measured. With all these contributions and innovations, the SLC is the pioneering embodiment of a new colliding-beam technique that continues SLAC’s leading role in developing the next generation
electron-positron collider. Such a collider, capable of producing TeV-scale center-of-mass energies, is now under design.

The SPEAR Storage Ring

SPEAR, an electron-positron colliding beam facility, began operating in 1972 and eventually achieved center-of-mass energies reaching about 7.4 GeV. The SPEAR facility was used in the discovery of the psi particle, for which Professor Burton Richter, current Director of SLAC, shared the Nobel Prize in 1976. It was also used in the discovery of the tau lepton, for which Professor Martin Perl shared the 1982 Wolfe Prize and the 1995 Nobel Prize. In late 1990, SPEAR’s operation was transferred to the Stanford Synchrotron Radiation Laboratory (SSRL) for dedicated use in synchrotron radiation research. In 1992, SSRL became a division of SLAC. The SPEAR storage ring now has its own electron injector (linac and synchrotron booster).

The PEP Storage Ring Electron-Positron Collider

A second large electron-positron colliding beam facility called PEP began operating in 1980 at enter-of-mass energies up to 29 GeV. At the peak of its use, the PEP machine had five large particle detectors in operation at five of its six electron-positron interaction points, as well as a small detector at its sixth interaction point. The facility supported the research of over 300 physicists from around the globe until about 1990. In 1994, construction of a new machine called PEP-II began in the original PEP tunnel. The purpose of PEP-II is to serve as a B Factory where an unprecedented number of B mesons will be produced to study CP violation. While in PEP-I the electron and positron beams were stored at equal energies in a single ring, PEP-II is equipped with a high-energy ring (HER) for electrons at an energy of 9.1 GeV and a low energy ring (LER) for positrons of 3 GeV. The center-of-mass colliding energy will be about 10.5 GeV. Colliding electrons and positrons at different energies to boost the motion of the center-of-mass and separating the vertices of the decaying B mesons constitutes another major innovation executed by SLAC in collaboration with LBNL and LLNL.